Design Guide for Bridges for Service Life (2013)

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258 6.1 introduction This chapter is a best-practices guide and discussion for preventing corrosion of ex- posed structural steel for bridges and includes factors to be considered for design through installation, inspection, and maintenance. Various types of coatings, includ- ing painting, galvanizing, and metalizing, are discussed along with other methods of corrosion prevention that include the use of steels with higher resistance to corrosion, such as weathering steel. Figure 6.1 shows the structural steel elements susceptible to corrosion. The focus of this chapter is on superstructure elements; however, much of the discussion is also applicable to deck and substructure elements. Within the superstructure component, structural steel subsystem elements include all configurations of steel shapes and plates that alone or in combination comprise members used as supporting steel on various types of structures, including trusses, beams, haunch parallel-flange welded plate girders, multiple web and single-bottom- flange tub girders, and square or rectangular cross-section box girders. Also included are all angles, channels, fasteners, sole plates, diaphragms, shims, and bearings. There are three primary methods for preventing corrosion of structural steel: 1. Use of coating systems, 2. Use of corrosion-resistant steel (weathering steel) or noncorrosive steel, and 3. Avoidance of corrosive environments or corrosion-prone details. Each of these methods is discussed in this chapter. The use of partially painted weathering steel is also addressed. 6 CORROSiON pREvENTiON OF STEEL BRiDGES

259 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.2 deScriPtion oF methodS For corroSion Prevention This section discusses the corrosion process and describes the three main methods used to prevent corrosion of steel bridges. 6.2.1 Corrosion of Steel: general Discussion In its simplest form, the corrosion of steel results from exposure to oxygen and mois- ture. Corrosion is accelerated in the presence of salt from roadway deicing, salt water, or perhaps salt deposited from other sources. The fact that steel corrodes is one of the few fundamental limitations of steel as a material of construction. Although steel corrodes readily in the presence of oxygen and moisture, the rate of corrosion is accelerated in the presence of chloride ions or other corrosive chemi- cals. Chloride ions result mainly from the use of deicing agents composed of materials with readily soluble chloride ions. These ions create an atmosphere in which unpro- tected steel corrodes very quickly. In order to ameliorate corrosion issues, engineers have used protective coatings as one means of protecting steel from the impact of the environment. Structural Steel Elements Subject to Corrosion SubstructureSuperstructureDeck Steel/Orthotropic Deck Expansion Joints Open/Sealed Drainage Elements Bearings Structural Steel Subsystem Elements Steel Piles Steel Pier Caps or Columns Railings Figure 6.1. Structural steel elements subject to corrosion. Figure 6.1. Structural steel elements subject to corrosion.

260 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.2.2 Description of Coating: Painting In a cost-effective, multicoat paint system, the primary purpose of the coating layer closest to the steel surface is to provide corrosion protection for the steel surface. Any special aesthetic considerations are accommodated in the subsequent coating layers, principally the topcoat. Aesthetics, while important for some applications, is not the focus of this chapter. The performance of protective coatings is dependent on the environment in which they are exposed. In some dry climate areas of the country where corrosion is not an issue, aesthetic considerations can play a more compelling role. The survey of state departments of transportation (DOTs) conducted as part of SHRP 2 Project R19A (final report available at http://www.trb.org/Design/Blurbs/168760.aspx) confirms that a bridge system that could be expected to provide 50+ years of service life in relatively dry states such as Arizona would last only 20 to 30 years in states with a moister climate. It follows, therefore, that a sure way to protect steel from corrosion is to keep it from getting wet, and a key means of accomplishing this is by coating the steel to provide a barrier to the elements, protect the steel from moisture, and keep it dry. The ability of the topcoat, or outmost layer, to shed water is the key to using coating as corrosion protection. When water penetrates the outer coating layer(s) and comes in contact with the steel substrate, the primer acts to inhibit corrosion as the steel surface is subjected to repeated wet–dry cycles. From the earliest years in the steel bridge era in the United States, beginning around 1874 with the Eads Bridge in Saint Louis, Missouri (see Figure 6.2), lead and chromium rust-inhibitive pigments were added to paint to supplement the barrier protection offered by a coating film. For almost 100 years, the use of lead- and chromium-pigmented, multilayer coatings was the norm during new bridge construc- tion, maintenance overcoating, and maintenance repainting. After about 1965, bridge coating engineers began to turn away from coatings con- taining these toxic heavy-metal pigments and instead started using coatings containing metallic zinc as the corrosion-inhibitive pigment. The DOT survey conducted as part Source: Courtesy KTA-Tator, Inc. Figure 6.2. Eads Bridge. Figure 6.2. Eads Bridge. Source: Courtesy KTA-Tator, Inc.

261 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES of SHRP 2 Project R19A indicated that all states responding use a system consisting of a zinc-rich primer. As long as the zinc pigment in the coating is in close metal-to-metal contact with the steel substrate, the coating provides galvanic protection to the steel. Galvanic protection is provided when zinc and steel (iron) are connected (i.e., have a conductive pathway between them) in the presence of air (oxygen) and moisture. In this coupling of materials, zinc (the less noble metal) will oxidize (corrode) in prefer- ence to the iron (steel). The preferential oxidation of zinc provides protection for the steel as long as there is nearby zinc left to be consumed in the chemical reaction, which takes place at the anode. When the zinc is consumed, the steel beneath will be subject to corrosion (oxidation). The method used to resist corrosion attack since about the mid 1960s has been to use a multicoat, “belt and suspenders” approach. In a multi- coat system, the outer layer or layers resist the effects of weather and protect the zinc from being consumed in the atmosphere, while the zinc-rich primer inhibits cor- rosion from occurring at the steel beneath in locations where the coating is breached. Even in instances in which steel is painted with a coating system using a zinc- rich primer, when the protected steel surface is bathed in salt water and is subjected to many wet–dry cycles, discontinuities in the coating inevitably provide a pathway through the coating for moisture to reach the zinc-coated steel surface beneath. As a result, the zinc begins to react to protect the steel from corroding. Eventually, the metallic zinc in the zinc-rich primer is consumed, and corrosion in the form of red rust (iron oxide) results. The corrosion protection offered by the zinc may last many years before there is evidence of corrosion of the steel; the rate of corrosion is dictated by the local factors surrounding the steel (e.g., wet–dry cycles, chloride contamination, humidity). The following general discussion of coatings introduces the use of protective coat- ings in the prevention of corrosion of structural steel. Figure 6.3 identifies the various items discussed. 6.2.2.1 Paint Coating Composition Figure 6.4 illustrates the basic ingredients of an industrial protective coating. The chart divides a coating into two major components: pigmentation and vehicle. The pigmen- tation typically consists of corrosion inhibitors, colorants, and extenders, although other raw materials may also be included. The vehicle typically consists of the resin or binder, solvents, and any additives that may be included in the formulation. The vehicle may also contain other raw materials to provide additional or different perfor- mance characteristics. The vehicle carries the pigmentation to the surface and binds it into the coating film. The ingredients can also be categorized as nonvolatile components and volatile components, indicated in Figure 6.4 by (NV) and (V), respectively. Nonvolatile com- ponents remain in the coating and on the surface once applied. Conversely, the volatile components evaporate from the coating into the air once the coating is applied to the surface. The nonvolatile components typically include the resin or binder, the pigmen- tation, and any additives that may be incorporated into the formulation. The volatile

262 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE component is the solvent system used in the formulation that is a component of the wet film, but not the dry film, of the coating. 6.2.2.1.1 Vehicle Resin The vehicle resin (or binder) portion of the coating vehicle comprises both volatile and nonvolatile components. That is, it is both part of the wet film and the dry film. Often a coating is identified generically by the type of resin used in the formulation. For ex- ample, a two-coat epoxy is a commonly specified coating system. In this case, “epoxy” is used to describe both the coating type and the raw material resin system used to formulate the coating. The resin system is the film-forming component of a coating. It cohesively bonds the pigmentation together and adhesively bonds the coating to the underlying substrate or coating layer. It is essentially the glue of the coating. In many cases, the resin system dictates the performance properties of a coating. 6.2.2.1.1a Pigmentation The pigment is also a nonvolatile component of the coating formulation and is es- sentially an insoluble raw material. It suspends in the resin and solvent rather than dissolving. Although some believe that the pigment merely gives the coating its color, that is only one of several potential functions. Figure 6.3. Paint system general considerations. Bridge Coatings General Considerations Coating Systems Curing Mechanisms Coating Components and Ingredients Pigmentation Vehicle Nonvolatile Coalescence Solvent Evaporation Primer Surface Preparation Volatile Oxidation Polymerization Moisture Cure Intermediate Coat Top Coat Application Methods Conventional Air Spray Airless Spray Plural Component Spray Electrostatic Spray Brushes, Daubers, and Rollers Figure 6.3. Paint system general considerations.

263 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Industrial Coatings Pigment Vehicle Inhibitors (NV) Colorants (NV) Extenders (NV) Resin (NV) Solvent (V) Additives (V) Zinc Chromate Phosphate Borate Red Clay Acrylic Primary Secondary Thickeners White Blue Silica Mica Alkyd Oil Vinyl UV Absorbers Plasticizers Catalysts Epoxy Urethane Siloxirane Wetting Agents Figure 6.4. Industrial coating elements. Source: Courtesy KTA-Tator, Inc.

264 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The pigment in a coating may also provide corrosion protection. If used for this purpose, the pigmentation must be formulated into the primer layer (the layer adjacent to the steel substrate). Inhibitors like barium, phosphorous, and others formulated into a primer inhibit the corrosion process. Zinc powder added to a primer in suffi- cient quantities galvanically protects the underlying steel. Because of the shape of the pigment particles, certain pigments even provide barrier protection; in other words, their inherent shape and the way in which they orient themselves in the dry film create a barrier to moisture penetration through the coating. Examples include micaceous iron oxide and leafing aluminum pigments. These raw materials are lamellar, mean- ing they are plate-like, tend to lie flat in the coating film, and cause any moisture that penetrates the coating film to take a considerably longer pathway to the substrate. Extenders such as silica, mica, and clay may be incorporated into the formulation to improve film build, increase the solids content of the coating, and/or provide added barrier protection. 6.2.2.1.1b Additives Additives formulated into the coating also become part of the dry film. Various quan- tities of additives are used by the formulator to adjust the consistency, flow-out, sur- face wetting, color, ultraviolet light (sunlight) resistance, and flexibility or to prevent settling in the can (suspending agents). For example, an alkyd coating that typically chalks and fades on exposure to sunlight can be formulated with silicone (minimum 30%) to provide better color and gloss retention characteristics. In this case, the sili- cone is an additive. Polyurethane coatings are formulated with hindered amine light stabilizers to help preserve gloss and color on exposure to sunlight, and plasticizers formulated into a coating provide film flexibility. 6.2.2.1.1c Solvents The solvent system in a coating is the volatile component. Although the solvent sys- tem is part of the wet film during application, it is not intended to be part of the dry film once the coating dries or cures. This component is referred to as a solvent system because it is uncommon for a coating to be formulated with just one type of solvent. Typically, a blend of solvents is used, and each type of solvent in the blend may per- form a different function. As a general rule, primary solvents are formulated into the coating to reduce the viscosity of the resin, pigment, and additives so that the coating can be properly atomized through a spray gun or applied by brush and roller. Second- ary solvents typically stay in the wet coating film a little longer than the primary sol- vents, as they are more slowly evaporating solvents that help the coating to flow out to form a uniform, continuous film. 6.2.2.1.1d Volatile Organic Compounds Solvents have been used in coatings for many decades because they have been useful and affordable. Many solvent systems in a coating (and thinners added to a coat- ing by the applicator) are categorized as volatile organic compounds (VOCs) by the

265 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Environmental Protection Agency (EPA). The type and amount of solvent(s) used in an industrial coating may be regulated by the EPA because as they evaporate from the coating film into the air, they can photochemically react with sunlight and become a precursor to ozone (a component of smog). As part of the Clean Air Act, federal and state environmental agencies have developed regulations to control ozone-producing operations. The amount of VOCs that can be legally emitted into the atmosphere varies considerably from location to location. For example, densely populated areas such as Southern California and Houston, Texas, have very strict VOC regulations, but less populated areas typically comply with the federal limit, which represents a considerably higher threshold. California has led the nation in the march toward coat- ings with ever-lower amounts of VOCs. Coatings suppliers have been reformulating and retesting their coatings as VOC regulations have tightened, and eventually, the use of such materials in coatings may diminish to the point that they are not a significant part of coatings used on bridges. The VOC content of a coating is expressed in pounds per gallon (or grams per liter) and is reported on the manufacturer’s product data sheet. Many manufacturers also recalculate the VOC content of a coating after the addition of thinner, and this information is also commonly provided on the product data sheet. When painting a structure in the field, the VOC limit is typically dictated by the specification or the local air pollution agency for the project. Conversely, fixed facili- ties such as painting shops are sometimes required to log the number of gallons of paint used over a specific period (say 90 days) and the VOC content of each type. 6.2.2.2 Curing Mechanisms The method in which a coating converts from a liquid to a solid state is known as the curing mechanism. Many liquid-applied coatings dry by solvent evaporation, but cure by employing a separate reaction. 6.2.2.2.1 Solvent Evaporation Coatings that cure by solvent evaporation actually only dry. That is, the resin, pig- ment, and additives are suspended in a solvent system. When the solvent evaporates from the applied film into the air, the resin, pigment, and additives remain on the sur- face. Because there is no subsequent curing reaction, the resin can be redissolved by the solvent system that evaporated from the coating film. This lack of subsequent curing is why a coating that cures by solvent evaporation should not be overcoated with a coat- ing containing strong solvents, as such solvents may redissolve the underlying coating film. A vinyl coating is an example of a coating that cures by solvent evaporation. 6.2.2.2.2 Coalescence Waterborne acrylic coatings cure by solvent evaporation and form a coating film by a process known as coalescence. Water, the primary solvent in these coatings, first evaporates from the acrylic-containing emulsion coating film. As the water evaporates, a special coalescing solvent (e.g., propylene glycol) aids in fusing the acrylic emulsion

266 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE particles to form a solid film. The coalescing solvent then evaporates from the coating film. Without this coalescing solvent, the acrylic particles will not impinge and fuse together, which can result in a poorly performing coating film. The coalescing process typically requires a minimum 50°F air temperature. Should the air temperature fall be- low 50°F before the coalescing process is complete, curing may stop and may not start again once the temperature recovers. This major concern with industrial waterborne acrylic coatings should be carefully considered by the specifier. 6.2.2.2.3 Oxidation Coatings that cure by oxidation react with oxygen (air) to form a film. This oxidation process never stops as long as the coating is exposed to oxygen. For example, long- used alkyd coatings, which typically contain unsaturated oils, pigments, and driers, cure by oxidation. Many aged alkyd systems, even those that have been in service for decades, become very brittle, as the resin continues to oxidize long after the coating is fully cured. 6.2.2.2.4 Polymerization “Poly” means “many.” Many monomers or “mers” are used to create a polymer. These monomers are formulated into components, and the components are packaged sepa- rately by the coating manufacturer. It is only when these components are blended in the correct proportions that a chemical reaction known as polymerization occurs, gen- erating a very resilient coating layer. Coatings that cure by polymerization are multi- component, typically packaged in two or three containers. Prior to application, these components are blended in the correct ratio. Generally, only complete, pre packaged kits are blended. Once blended, the chemical reaction begins. Coatings that cure by polymerization have a limited pot life. That is, the blended components must be ap- plied before that pot life expires. The pot life will vary from a few minutes to several hours, depending on the formulation and temperature of the coating. Many coatings cure by polymerization; epoxy coatings and aliphatic acrylic or polyester polyurethane coatings are a few of the more common types used on bridges. 6.2.2.2.5 Moisture Cure Hydrolysis is the reaction of a coating with moisture in order to cure. Only a few industrial coatings hydrolyze in the curing process. These include moisture-cure ure- thanes and ethyl silicate–type inorganic zinc-rich primers, which require a minimum amount of moisture to cure. In this process, moisture-cure urethanes release carbon dioxide, and inorganic zinc-rich primers release ethyl alcohol. The moisture cure pro- cess results in a very resilient coating layer, similar to that achieved by polymerization. (Zinc coatings are further discussed in a subsequent section.)

267 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.2.2.3 Coating Systems Defined In many cases, coatings can be combined to create a coating system, which includes both the surface preparation and the application of one or more coating layers. If mul- tiple coats of the same product are specified, contrasting colors are sometimes used to help ensure the coverage of the applicator. Although coating layers usually consist of a primer and topcoat, in many instances an intermediate coat may also be specified. When multiple coatings are used to create a system, they must be compatible. In addition, each coating layer has a function that is performed at a given thickness. Accordingly, adding extra thickness of an epoxy intermediate coat cannot make up for an inadequate zinc-rich primer thickness. Each layer should be applied at the optimum thickness (i.e., neither too thick nor too thin) and verified for proper thickness before the application of subsequent layers. 6.2.2.3.1 Surface Preparation The Society for Protective Coatings (SSPC) and the National Association of Corrosion Engineers International (NACE) have developed standard requirements that include the end condition of the surface and materials and procedures necessary to achieve and verify the end condition. The level of surface preparation to be performed is an integral component to the coating system. For example, there would be little point in applying a zinc-rich primer to a marginally prepared surface, because the zinc must maintain intimate contact with clean steel to provide galvanic protection. If zinc were to be applied over an old coating, the desired galvanic protection would not develop. Conversely, applying a surface-tolerant coating to a surface prepared to SSPC-SP 5/NACE No. 1, White Metal Blast Cleaning, would be excessive, as equivalent performance could be achieved over a much lesser degree of cleaning, usually at a much lower cost. 6.2.2.3.2 Primer Function The function of the primer is to bond the coating system to the substrate. It also pro- vides corrosion protection for the steel substrate by using one or more methods of barrier, inhibitive, or galvanic protection. The primer must also be tolerant of the level of surface preparation performed and must be compatible with the next layer applied. If the primer is the only layer, as with a single-coat system, it must be resistant to the service environment and provide corrosion protection to the steel beneath. 6.2.2.3.3 Intermediate Coat Function An intermediate coat is typically incorporated into a coating system for the purpose of adding barrier protection. It must be compatible with both the primer and the topcoat.

268 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.2.2.3.4 Topcoat Function The topcoat, or finish coat, is the first line of defense in a corrosion-protection system. It must also be aesthetically compatible with the specifier’s priorities and is often required to maintain color and gloss levels for long periods of time. Naturally, the topcoat must be resistant to the service environment and must be compatible with the underlying layer (i.e., primer or intermediate coat, as appropriate). In addition, the topcoat must be able to accept a maintenance overcoat. 6.2.2.4 Coating Application Methods 6.2.2.4.1 Conventional (Air) Spray Conventional or air-atomized spray uses compressed air to transport the paint from a pressurized pot to the spray gun in order to atomize the coating into a fine spray and then propel the atomized coating to the surface. As the ratio of air to paint is quite high (~600:1), compressed air cleanliness is critical, and transfer efficiency is relatively low. The primary reason conventional (air) spray is used to apply industrial coatings is the ability to precisely control the amount of paint that exits the spray gun and to control the shape of the spray pattern. The apparatus used for the application of metalizing spray is a special variation of a conventional spray gun. Conventional air spray equipment consists of a pressure pot equipped with two regulators. The first regulator is used to control the amount of pressure in the pot itself (pot pressure), and the second is used to control the volume of atomization air that is used to break up the stream of paint exiting the spray tip. Coating manufacturers provide recommended pot and atomization pressures, which often have to be adjusted slightly based on project conditions (e.g., amount of thinner addition, temperature, and so forth). Two hoses connect the spray gun to the pot; one contains the paint and the other contains the atomization air. The spray gun has two controls. The lower control regulates the amount of paint that comes out of the spray tip, and essentially adjusts how far the operator can pull back the spray gun trigger, which regulates the amount of paint that exits the spray tip. The upper control regulates the shape of the fan pattern, from a small circle for striping of corners and other small areas to a larger oval for spraying flat surfaces. A conventional spray gun can be half-triggered—that is, the trigger can be pulled back part way—so that the atomization air, without paint, exits the spray nozzle. This compressed air can be used to perform a final blow-down of the surface immediately before the coating application. The conventional spray gun is held 6 to 10 in. from the surface, with variations in distance dependent on the type of coating and prevailing spraying conditions, such as air temperature and wind speed.

269 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.2.2.4.2 Airless Spray Airless spray has long been the most common method used for applying bridge coat- ings. If the equipment is operating properly and the applicator employs good spray technique, the finish quality can approach that which is created by conventional spray, but at much higher production levels. Because airless spray does not use com- pressed air to atomize the coating, the transfer efficiency is relatively higher than conventional spray, reducing airborne emissions. Further, because airless spray does not incorporate compressed air into the paint stream, the cleanliness of the com- pressed air is not critical. Airless spray equipment consists of a paint pump that is operated using an air compressor equipped with a regulator. Coating manufacturers provide recommended airless spray pressures, which frequently have to be adjusted slightly based on project conditions (e.g., amount of thinner addition and temperature). A single hose contain- ing the paint connects the spray gun to the pump. The airless spray gun is held 18 to 24 in. from the surface, with variations in dis- tance dependent on the type of coating and prevailing spraying conditions. 6.2.2.4.3 Plural Component Spray Plural component spray is used for coating materials with a relatively short pot life or coating materials that do not contain viscosity-reducing solvents (e.g., 100% solids materials). Plural component spray does not require premixing the coating compo- nents. Rather, the individual components are pumped to a mixer or manifold at the correct ratio. They are then mixed and delivered to the spray gun using a short mate- rial hose that can be flushed with solvent in a solvent purge system. This is known as an internal mix process. There are also external mix plural component systems that send each component to the spray gun in separate material hoses. The components blend as they exit the spray tip. It is common for the material hoses to be heated for plural component spray in order to reduce the viscosity of the components and allow for easier transport and improved atomization. Plural component spray is available in two basic designs: fixed ratio and variable ratio proportioning pumps. Fixed ratio pumps can only proportion the components in a set ratio (e.g., 1:1), but a variable ratio pump can proportion the materials according to the required ratio (e.g., 2:1, 4:1, 8:1, 16:1). Plural component spray equipment can be complex and typically requires a technician to set up the equipment and monitor the mix ratio so that the coating materials are not applied off-ratio. This equipment is set up to spray similar to airless spray equipment. 6.2.2.4.4 Electrostatic Spray Electrostatic spray is sometimes used to apply bridge coatings. Because of its poten- tially high transfer efficiency rate and the resulting reduction in material usage, it can be an attractive application method. In principle, the paint particles are energized (+) as they exit the spray gun. The electrical charge is imparted by a small wire protrud- ing from the spray nozzle. The surface to be coated is grounded (–). The particles are

270 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE “attracted” to the component or part having the opposing electrical charge, signifi- cantly reducing overspray and material usage. The coating, however, must be able to accept an electrical charge, and the addition of a polar solvent is sometimes required to inhibit the coating’s natural resistivity. Electrostatic spray can also result in a wrap- ping of the coating on the opposite side of the direction of spray, making it an attrac- tive alternative for coating inaccessible areas. Because of the difficulty in achieving a uniform ground on large structures, electrostatic spray has not been widely used to coat bridges. Electrostatic spray equipment is typically set up to spray as an airless operation, and electrostatic spray can be used to coat smaller members and hard-to- reach areas. 6.2.2.4.5 Brushes, Rollers, and Daubers The use of brushes for bridge projects is typically limited to striping, the application of a layer of coating to surfaces where it is difficult to achieve a normal film build. For the same reason, brushes are also used on pitted or rough surfaces around rivet heads, welds, and bolt-and-nut assemblies and to cut-in inside and outside corners. Daubers are often used to coat surfaces within crevices like back-to-back angles. Rollers have high coating-transfer efficiency and can be used to coat large flat surfaces with limita- tions: roller nap can become embedded in the dry coating film and act like a wick to pull moisture into the coatings, and film thickness is hard to control with a roller. Choosing the method of coating application depends on many factors, including the size and configuration of the surfaces to be coated, the proximity to other struc- tures, environmental regulations, and the specification and coating manufacturer’s rec- ommendations. Although airless spray is no doubt the most frequently used method, there are other issues for the contractor or applicator to consider when choosing an application method, including speed and control. 6.2.3 Description of Coating: Hot-Dip galvanizing The material in this section is from the American Galvanizers Association (www. galvanizeit.org) and adapted from AGA (2006). Hot-dip galvanizing (HDG) is a pro- cess in which fabricated steel, structural steel, castings, or small parts, including fas- teners, are immersed in a kettle or vat of molten zinc, resulting in a metallurgically bonded alloy coating that protects the steel from corrosion. HDG is often referred to as simply “galvanizing,” a term that is often used incorrectly to describe steel coated with zinc by other methods such as paint or plating. These other methods of applying zinc to steel for corrosion protection are very different from HDG. The AGA HDG Specifier’s Guide (2006) states, “Galvanizing forms a metallurgical bond between the zinc and the underlying steel or iron, creating a barrier that is part of the metal itself. During galvanizing, the molten zinc reacts with the surface of the steel or iron article to form a series of zinc–iron alloy layers. [Figure 6.5] is a photomicrograph of a galvanized steel coating’s cross section and shows a typical coating microstructure.” The HDG coating consists of four layers. The first three layers above base steel have a mixture of iron and zinc, and the external top layer is typically composed of 100% zinc.

271 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES The Specifier’s Guide continues: Below the name of each layer in [Figure 6.5] its respective hardness, expressed by a diamond pyramid number (DPN), appears. The DPN is a progressive mea- sure of hardness (i.e., the higher the number, the greater the hardness). Typi- cally, the Gamma, Delta, and Zeta layers are harder than the underlying steel. The hardness of these inner layers provides exceptional protection against coat- ing damage through abrasion. The Eta layer of the galvanized coating is quite ductile, providing some resistance to impact. The galvanized coating is adher- ent to the underlying steel on the order of several thousand pounds per square inch. . . . Hardness, ductility, and adherence combine to provide the galvanized coating with unmatched protection against damage caused by rough handling during transportation to and/or at the project site, as well as in service. The toughness of the galvanized coating is extremely important since barrier pro- tection is dependent upon the integrity of the coating. 6.2.3.1 Hot-Dip Galvanizing Process Though the process may vary slightly from plant to plant, the fundamental steps in the galvanizing process are surface preparation, galvanizing, and finishing. 6.2.3.1.1 Surface Preparation Degreasing/Caustic Cleaning. A hot alkaline solution removes dirt, oil, grease, shop oil, and soluble markings. Figure 6.5. Magnified cross section of galvanized coating. Source: Photo courtesy of the American Galvanizers Association.

272 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Pickling. Dilute solutions of either hydrochloric or sulfuric acid remove surface rust and mill scale to provide a chemically clean metallic surface. Fluxing. Steel is immersed in liquid flux, usually a zinc ammonium chloride solu- tion, to remove oxides and prevent oxidation before the item is dipped into the bath of molten zinc. In the dry galvanizing process, the item is separately dipped in a liq- uid flux bath, removed, allowed to dry, and then galvanized. In the wet galvanizing process, the flux floats on top of the molten zinc and the item passes through the flux immediately before galvanizing. 6.2.3.1.2 Galvanizing The article is immersed in a bath of molten zinc between 815°F to 850°F (435°C to 455°C). During galvanizing, the zinc metallurgically bonds to the steel, creating a se- ries of highly abrasion-resistant zinc–iron alloy layers, commonly topped by a layer of impact-resistant pure zinc. 6.2.3.1.3 Finishing After the steel is withdrawn from the galvanizing bath, excess zinc is removed by draining, vibrating, or, in the case of small items, centrifuging. The galvanized item is then air cooled or quenched in liquid. 6.2.3.2 Coating Uniformity The AGA HDG Specifier’s Guide (2006) states The galvanizing process naturally produces coatings that are at least as thick at the corners and edges as the coating on the rest of the substrate. As coating damage is most likely to occur at the edges, this is where added protection is needed most. [Figure 6.6] is a photomicrograph showing a cross section of an edge of a galvanized piece of steel. Because the galvanizing process involves total immersion of the material, . . . all surfaces are coated. . . . Galvanizing provides protection on both exterior and interior surfaces of hollow structures. . . . Hollow structures must be detailed in a way that allows zinc to drain from the interior when the item is removed from the kettle. The inspection process for galvanized items is simple, fast, and requires mini- mal labor. . . . Galvanizing continues at a factory under any weather or hu- midity conditions. . . . The galvanizer’s ability to work in any type of weather allows a higher degree of assurance of on-time delivery. . . . A turnaround time of two or three days is common for galvanizing. ASTM, ISO, CSA (the Canadian Specification Association), and AASHTO specifications establish minimum standards for thickness of galvanized coat- ings on various categories of items. These minimum standards are routinely exceeded by galvanizers due to the nature of the galvanizing process. Factors

273 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES influencing the thickness and appearance of the galvanized coating include chemical composition of the steel, steel surface condition, cold working of steel before galvanizing, bath temperature, bath immersion time, bath with- drawal rate, and steel cooling rate. 6.2.3.3 Effect of Amount of Silicon in Steel on Galvanized Coating The Specifier’s Guide (AGA 2006) explains this effect: The chemical composition of the steel being galvanized is perhaps the most important, [influencing factor]. The amount of silicon and phosphorus in the steel strongly influences the thickness and appearance of the galvanized coat- ing. Silicon, phosphorus, or combinations of the two elements can cause thick, brittle galvanized coatings. The coating thickness curve shown in [Figure 6.7] relates the effect of silicon in the base steel to the thickness of the zinc coating. The carbon, sulfur, and manganese content of the steel also may have a minor effect on the galvanized coating thickness. The combination of elements mentioned above, known as “reactive steel” in the galvanizing industry, tends to accelerate the growth of zinc–iron alloy layers. This may result in a finished galvanized coating consisting entirely of zinc–iron alloy. Instead of a shiny appearance, the galvanized coating will have a dark gray, matte finish. This dark gray, matte coating will provide as much corrosion protection as a galvanized coating having a bright appearance. It is difficult to provide precise guidance in the area of steel selection [for gal- vanizing] without qualifying all of the grades of steel commercially available. [However,] the guidelines discussed below usually result in the selection of Figure 6.6. Cross section of corner of galvanized steel section. Source: Courtesy American Galvanizers Association.

274 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE steels that provide good galvanized coatings: Levels of carbon less than 0.25%, phosphorus less than 0.04%, or manganese less than 1.35% are beneficial; Silicon levels less than 0.04% or between 0.15% and 0.22% are desirable. Silicon may be present in many steels commonly galvanized even though it is not part of the controlled composition of the steel. This occurs primarily because silicon is used in the deoxidization process in for the steel [and is found in continuously cast steel]. The phosphorus content should never be greater than 0.04% in steel [that is] intended for galvanizing. Phosphorus acts as a catalyst during galvanizing, resulting in rapid growth of the zinc–iron alloy layers. [This growth is virtually uncontrollable during the galvanizing process.] Because the galvanizing reaction is a diffusion process, higher zinc bath temperatures and longer immersion times will generally produce somewhat heavier alloy layers. Like all diffusion processes, the reaction proceeds rapidly at first and then slows as layers grow and become thicker. [However,] contin- ued immersion beyond a certain time will have little effect on further coating growth. When galvanizing reactive steels, the diffusion process significantly changes [and proceeds at a faster rate, producing thicker coatings]. The thickness of the outer pure zinc layer is largely dependent upon the rate of withdrawal from the zinc bath. A rapid rate of withdrawal causes an article to carry out more zinc and generally results in a thicker coating. Figure 6.7. Galvanized coating thickness curve. Source: Courtesy American Galvanizers Association.

275 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES ASTM, CSA, and AASHTO specifications and inspection standards for galva- nizing recognize that variations occur in both coating thickness and composi- tions. Thickness specifications are stated in average terms. Further, coating thickness measures must be taken at several points on each inspected article to comply with ASTM A123/A123M for structural steel and A153/A153M for hardware. Figure 6.8 shows a thickness measurement being taken. Fortunately, many grades of steel commonly used in steel bridges meet the chemi- cal requirements and are readily galvanized. When in doubt the owners or engineers should be queried, or the galvanizer’s advice should be sought. 6.2.3.4 Inspection Inspections for coating thickness and surface condition complete the process. Inspec- tion of structural steel will normally fall under ASTM A123/A123M–12, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, or ASTM A153/A153M–09, Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware. 6.2.4 Description of Coating: thermally Sprayed metal Coating or metalizing The material in this section was adapted from Ellor et al. (2004). When sizes and shapes of steel members will not fit in a galvanizing kettle or when the schedule simply does not allow time to transport items to a galvanizer’s plant, there is the option to metalize the item. Figure 6.8. Coating thickness measuring. Source: Courtesy American Galvanizers Association.

276 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Thermally sprayed metal coating (TSMC), referred to as metalizing, is the process of applying metallic zinc in wire form to clean steel by feeding it into a heated gun, where it is heated, melted, and spray applied by using combustion gases or auxiliary compressed air to provide ample velocity. Metalizing may be used on any size steel object, which eliminates limitations due to vat size and awkward shapes. Applying a consistent coating in recesses, hollows, and cavities adds a measure of complexity. Pure zinc can be used, but often zinc is alloyed with 15% aluminum to provide a smoother abrasion-resistant film. 6.2.4.1 Thermally Sprayed Metal Coating Processes Two similar processes are used to apply the metallic zinc to the steel surface. These processes, which are differentiated by the manner in which the zinc metal is melted, are described in the following sections, adapted from Ellor et al. (2004). 6.2.4.1.1 Flame Spray Process The flame spray process can be used to apply a wide variety of feedstock materials, including metal wires, ceramic rods and metallic and nonmetallic powders. In flame spraying, the feedstock material is fed continuously into the tip of the spray gun or torch, where it is then heated and melted in a fuel gas/ oxygen flame and accelerated toward the substrate being coated in a stream of atomizing gas. Common fuel gases used include acetylene, propane, and methyl acetylene–propadiene (MAPP). Oxyacetylene flames are used exten- sively for wire-flame spraying because of the degree of control and the higher temperatures attainable with these gases. The lower-temperature oxygen–pro- pane flame can be used for melting metals such as aluminum and zinc, as well as polymer feedstock. The basic components of a flame spray system include the flame spray gun or torch, the feedstock material and a feeding mechanism, oxygen and fuel gases with flow meters and pressure regulators, and an air compressor and regulator. With wire-flame spraying, the wire-flame spray gun or torch [shown in Figure 6.9] consists of a drive unit with motor and drive rollers for feeding the wire and a gas head with valves, gas nozzle, and an air cap that controls the flame and atomization air. Compared with wire-arc spraying, wire-flame spraying is generally slower and more costly because of the relatively high cost of the oxygen-fuel gas mixture compared with the cost of electricity. However, flame spraying systems are generally simpler and less expensive than wire-arc spray- ing systems. Both flame spraying and wire-arc spraying systems are field porta- ble and may be used to apply quality metal coatings for corrosion protection.

277 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.2.4.1.2 Wire-Arc Process Due to its high deposition rates, excellent adhesion, and cost-effectiveness, wire-arc spray is the preferred process for applying TSMCs to steel [bridges]. In the wire-arc spray process, two consumable wire electrodes of the metal be- ing sprayed are fed into a gun such that they meet at a point located within an atomizing air, or other gas, stream. An applied direct current (DC) potential difference between the wires establishes an electric arc between the wires that melts their tips. The atomizing air flow subsequently shears and atomizes the molten droplets to generate a spray pattern of molten metal directed toward the substrate being coated. Wire-arc spray is the only thermal spray process that directly heats the material being sprayed, a factor that contributes to its high energy efficiency. The wire-arc spray system consists of a wire-arc spray gun or torch [shown in Figure 6.10], atomizing gas, flow meter or pressure gauge, a compressed air supply, DC power supply, wire guides/hoses, and a wire feed control unit. Op- eration of this equipment must be in strict compliance with the manufacturer’s instructions and guidelines. 6.2.4.2 Thermally Sprayed Metal Coating Guidelines Table 6.1 provides a TSMC selection guide for 20- to 40-year life and shows the TSMC thickness typically applied under various environmental conditions. Figure 6.9. Schematic of a typical flame-wire spray gun. Source: Ellor et al. 2004.

278 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Again quoting from Ellor et al., TSMCs should always be applied to “white” metal (SSPC-SP 5/NACE # 1, [White Metal Blast Cleaning]). It is common practice in fieldwork to apply the TSMC during the same work shift in which the final blast cleaning is performed. The logical endpoint of the holding period is when the surface Figure 6.10. Schematic of a typical wire-arc spray gun. Source: Ellor et al. 2004. tABLE 6.1. tSmc coAting guide Environment Coating Thickness mil (µm) Sealer Atmospheric Rural Zinc or zinc–aluminum 6–8 (150–200) No Industrial Zinc or zinc–aluminum 12–15 (305–308) Yes Marine Aluminum or zinc–aluminum 12–15 (305–308) No Immersion Fresh water Zinc–aluminum 12–15 (305–308) Yes Brackish water Aluminum 12–15 (305–308) No Seawater Aluminum 12–15 (305–308) No Alternate Wet–Dry Fresh water Zinc–aluminum 10–12 (250–305) Yes Seawater Aluminum 12–15 (305–308) Yes Abrasion Zinc–aluminum 14–16 (355–405) Yes Condensation Zinc or zinc–aluminum 10–12 (250–305) Yes Source: Ellor et al. 2004.

279 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES cleanliness degrades or a change on performance (as per bend or tensile test) occurs. If the holding period is exceeded, the surface must be re-blasted to establish the correct surface cleanliness and profile. Thermal spraying should be started as soon as possible after the final anchor tooth or brush blasting and completed within 6 hours for steel substrates sub- ject to the temperature to dew point and holding period variations. In high- humidity and damp environments, shorter holding periods should be used. In low humidity environments, or in controlled environments with enclosed structures using industrial dehumidification equipment, it may be possible to retard the oxidation of the steel and hold the near-white-metal finish for more than 6 hours. With the concurrence of the purchaser, a holding period of greater than 6 hours can be validated by determining the acceptable temperature– humidity envelope for the work enclosure by spraying and ana- lyzing bend test coupons, or tensile adhesion coupons, or both. Should the sample fail the bend test, the work must be re-blasted and re-tested. When specified, . . . a flash coat of TSMC equal to or greater than 1 mil (25 μm) may be applied within 6 hours of completing the surface preparation in order to extend the holding period for up to 4 hours beyond the applica- tion of the flash coat. The final TSMC thickness, however, should be sprayed within 4 hours of the application of the flash coat. This procedure should be validated using a tensile adhesion test, or bend test, or both, by spraying a flash coat and waiting through the delay period before applying the final coat- ing thickness. For small and movable parts, if more than 15 minutes is expected to lapse between surface preparation and the start of thermal spraying, or if the part is moved to another location, the prepared surface should be protected from moisture, contamination, and finger/hand marks. Wrapping the part with clean, print-free paper is normally adequate. If rust bloom, blistering, or a degraded coating appears at any time during the application of the TSMC, the following procedure should be performed: 1. Stop spraying. 2. Mark off the satisfactorily sprayed area. 3. Repair the unsatisfactory coating (i.e., remove the degraded coating and reestab- lish the minimum “white metal” finish and anchor-tooth profile depth as per the maintenance and repair procedure). 4. Record the actions taken to resume the project in the project documentation. 5. Contact the coating inspector to observe and report the remedial action to the purchaser.

280 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The materials and application costs of a TSMC system are higher than the cost of conventional liquid-applied coatings; however, the dominant factor may not be either material or application labor cost. Instead, the cost of taking the facility out of service, contractor mobilization, environmental constraints, and monitoring costs are often the major considerations of the total cost. In some complex coating work, the actual cost of materials and application is less than 20% of the total process. Therefore, if service life is increased by the use of TSMC, the process can pay for itself. Neverthe- less, TSMC is used most effectively on broad flat surfaces, and complexities occur when the gun or hoses are maneuvered around elements that are mounted at an angle to the main flat surface. For example, TSMC is most efficient when used on a girder web or flange, but progress may be slower when connection plates or stiffeners are encountered. The current premium for TSMC can be as little as 40% compared with other protective treatments. In many cases, this is a small price to pay for a material that has proven performance and an estimated service life measured in decades. Conditions that can damage a TSMC system should also be considered as part of the overall coating selection process. According to Ellor et al. (2004), Impact and abrasion are significant environmental stresses for any coating sys- tem. Abrasion is primarily a wear-induced failure caused by contact of a solid material with the coating. Examples include foot and vehicular traffic on floor coatings, ropes attached to mooring bitts, sand particles suspended in water, and floating ice. When objects of significant mass and velocity move in a direc- tion normal to the surface as opposed to parallel, as in the case of abrasion, the stress is considered to be an impact. Abrasion damage occurs over a period of time, whereas impact damage is typically immediate and discrete. Many coating properties are important to the resistance of impact and abrasion, including adhesion to the substrate, cohesion within the coating layers, tough- ness, ductility, and hardness. Thermally sprayed coatings of zinc, aluminum, and their alloys are very impact resistant. Zinc metalizing has only fair abra- sion resistance in immersion applications because the coating forms a weakly adherent layer of zinc oxide. This layer is readily abraded, which exposes more zinc, which in turn oxidizes and is abraded; 85:15 wt% zinc/aluminum is more impact/abrasion resistant than pure zinc or pure aluminum. 6.2.4.3 Concerns Related to Performance of Thermally Sprayed Metal Coating Coating selection may be limited by the degree or type of surface prepara- tion that can be achieved on a particular structure or structural component. Because of physical configuration or proximity to other sensitive equipment or machinery, it may not always be possible to abrasive blast a steel substrate. In such cases, other types of surface preparation, such as hand tool or power tool cleaning, may be necessary, which, in turn, may place limits on the type of coatings that may be used. In some cases, it may be necessary to remove the old coating by means other than abrasive blasting, such as using power tools,

281 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES high-pressure water jetting, or chemical strippers. These surface preparation methods do not impart the surface profile that is needed for some types of coatings to perform well. In the case of thermally sprayed coatings, a high degree of surface preparation is essential. This kind of preparation can only be achieved by abrasive blast cleaning using a good-quality, properly sized angu- lar blast media. Thermal spraying should never be selected for applications in which it is not possible to provide the highest-quality surface preparation. . . . Angular blast media must always be used. Rounded media such as steel shot, or mixtures of round and angular media, will not produce the appropri- ate degree of angularity and roughness in the blast profile. The adhesion of TSMCs can vary by an order of magnitude as a function of surface rough- ness profile shape and depth. TSMCs adhere poorly to substrates prepared with rounded media and may fail in service by spontaneous delamination. Hard, dense, angular blast media such as aluminum oxide, silicon carbide, iron oxide, and angular steel grit are needed to achieve the depth and shape of blast profile necessary for good TSMC adhesion. Steel grit should be manu- factured from crushed steel shot conforming to SAEJ827. Steel grit media composed of irregularly shaped particles or mixtures of irregular and angular particles should never be used. Steel grit having a classification of very angu- lar, angular, or subangular . . . by the American Geological Institute should be used [Hansink 1994]. For additional information, see Joint Standard SSPC-CS 23.00/AWS C2.23M/ NACE No. 12, which is discussed in Specification for the Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel (SSPC 2003). 6.2.5 Description of Corrosion-Resistant Steels: AStm A1010 ASTM A1010 steel is a 10.5% to 12.5% chromium structural steel with superior corrosion resistance when compared with traditional weathering or galvanized steels (Fletcher et al. 2005). A1010 is widely used in thicknesses from 1/8 to ½ in. for struc- tures subjected to aggressive service conditions, such as coal railcars and coal-process- ing equipment. Because of its superior corrosion resistance, A1010 is also a candidate for challenging bridge applications. The steel can meet the strength and impact proper- ties of AASHTO M270 Grades 50W and HPS 50W up to a thickness of 4 in., making it an attractive steel for traditional plate girder bridges. 6.2.6 Description of Weathering Steels Uncoated weathering-grade steels contain small amounts of copper, phosphorus, chro- mium, nickel, and silicon to attain their weathering or corrosion-resistant properties. Weathering-grade steels are currently supplied under AASHTO Specification M270, ASTM A709 Grade 50W, and in high-performance steel grades HPS 50W, 70W, and 100W. When used in the right environment, these steels are very cost-effective in both the short and long term as they eliminate the need for shop and field painting.

282 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Weathering steels have been successfully used on coal hopper cars, buildings, and electric transmission towers and began appearing in bridges on a large scale in the mid 1960s (McEleney 2005). Currently, thousands of weathering steel bridges are provid- ing trouble-free service across the United States. According to Bill McEleney in A Primer on Weathering Steel (2005), unpainted weather steel, properly designed and detailed, can realize bridge life cycles up to 120 years with minimal maintenance. This high-strength, low- alloy steel forms a tightly adhering “patina” during its initial exposure to the elements. The patina is essentially an oxide film of corrosion by-products about the same thickness as a heavy coat of paint. The initial corrosion of weather steel depends on the presence of mois- ture and oxygen. As corrosion continues, a protective barrier layer forms that greatly reduces further access to oxygen, moisture, and contaminants. This stable barrier layer greatly resists further corrosion, reducing it to a low value. Under appropriate conditions, weather steel will generally corrode at a rate of less than 0.3 mils per year. Corrosion of conventional steels, on the other hand, forms rust layers that eventually dis engage from the surface, exposing “fresh” metal below, thereby continuing the corrosion cycle. . . . Bridges constructed of weathering steel in suitable environments, and with proper detailing, have all the qualities of conventional steel, plus they offer the following benefits: • Initial cost savings compared to conventional painted alternatives; • Low maintenance consisting of periodic inspection and cleaning, which reduces direct operating costs; • Minimal indirect costs from traffic delays for major maintenance operations; • Faster construction resulting from elimination of shop and field painting; • Good aesthetics, since weathering steel bridges eventually achieve an attractive dark brown color that blends well with the environment and improves with age; • Low impact on the environment, compared to painted alternatives that emit undesirable volatile organic compounds (VOCs); • Minimal health and safety issues relating to initial and future painting; • A good track record for long-term performance based on various state and federal studies. Certain environments with high moisture, salt, or pollution levels can have unde- sirable effects on the performance of weathering steel and can inhibit the proper devel- opment of the protective patina. These environments include

283 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES • Locations of continual or persistent rainfall (or wetting), fog, or high humidity, or where heavy, close vegetation can hold moisture against the steel and prevent drying. • Locations that have contact with salt-laden roadway drainage such as below leak- ing deck joints. • Locations subject to airborne salt water spray such as in marine coastal areas. • Locations that create a tunnel effect such as with recessed, minimum clearance under passes with close, high abutments and where deicing salts are used fre- quently. In these situations, rising salt water spray from the lower roadway can be deposited on the girders above. • Locations that contain high concentrations of pollution and industrial fumes, especially those containing sulfur dioxide. Poor detailing can also have detrimental effects on weathering steel performance. Steel detailing should permit all parts of the steelwork to dry, avoid moisture and debris retention, and promote adequate ventilation (McEleney 2005). The FHWA Technical Advisory T-5140.22, Uncoated Weathering Steel in Structures (FHWA 1989) provides the following recommendations for detailing bridges that contain weathering steel: • Eliminate bridge joints where possible through use of continuous girders and integral abutments. • Control water on the deck near the expansion joints deck. Consider the use of a trough under the deck joint to divert water away from vulnerable elements. • Paint all superstructure steel within a distance of 1½ times the depth of girder from bridge joints. • Locate welded drip bars in areas of low stress. • Minimize the number of bridge-deck scuppers (holes cut near the edge of a deck to drain water below). Fewer scuppers result in a higher amount of flow through each, minimizing the chance for blockage. • Eliminate geometries that serve as water and debris “traps.” • “Hermetically seal” box members when possible, or provide weep holes to allow proper drainage and circulation of air. • Cover or screen all openings in boxes that are not sealed. • Consider protecting pier caps and abutment walls with drip pans and plates to minimize staining. Proper inspection and maintenance are also necessary for achieving desired weath- ering steel performance. “Inspectors should specifically look for leaking expansion joints, blocked drains, buildups of debris and other moisture traps, sealant failure, and bulging joints and overlaps,” according to McEleney (2005). Any of these conditions, if found, should be addressed with appropriate maintenance.

284 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE McEleney further summarizes research performed by the TxDOT that recom- mends the following periodic measures: • Flush debris, dirt, and bird and bat droppings from the bridge structure. • Clear vegetation from pier and abutment areas to enhance air circulation. • Reseal deteriorating joints. • Unblock drains and troughs. 6.3 FActorS AdverSeLy AFFecting Service LiFe 6.3.1 general Discussion One of the most important tasks for developing corrosion-prevention systems is prop- erly identifying the prevailing service environment, for existing structures, or the pro- jected service environment, for new structures. To what will the system and the bridge be subjected? Whether the structure already exists or is being planned, answering this question can be a challenge. Service environments can be both predictable (e.g., deic- ing salt exposure on a bridge in the winter) and unpredictable (e.g., hurricanes and other like storms may bring unexpected conditions). Figure 6.11 shows some of the factors that can influence the service life of steel bridge elements related to corrosion; it is followed by a brief discussion of these factors. Figure 6.11. Factors that can influence service life of steel bridge elements related to corrosion. Environmental Factors Detail Exposure Particle Impact Nearby Industries Exposure Type Coastal Regions Moisture Type Outside Products Temperature

285 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Detail exposure. Effects will vary for details having interior versus exterior expo- sure. In cases of interior exposure, entire structures or parts of structures are sheltered and exposed to a less aggressive environment. Coatings on the interior beams or the interior of box members are examples. Exposure type. Effects will vary for atmospheric versus immersion-like exposure. In cases of immersion-like exposure, it must be determined whether the exposure is constant or intermittent (i.e., splash) or from condensation. Moisture type. For immersion-like exposure, the medium must be considered (i.e., fresh water, salt water). Temperature. Normal operating and extreme conditions must be considered. Coastal regions. Prevailing environment (i.e., coastal airborne sea saltwater mist) must be considered. Particle impact. The likelihood and type of physical damage (i.e., impact damage from traffic or traffic-propelled debris) must be considered. Nearby industries. Surrounding operations (e.g., an adjacent chemical plant) must be considered. Outside products. The type and concentration of product that will be stored or transported in tank cars or vessels over or beneath the structure must be considered. The specifier should consider these and other likely potential environments before selecting a coating system. Any coating manufacturer will almost certainly request this type of information before recommending a coating system. Note also that there may be multiple service environments for a given structure, and interviewing nearby facility owners and plant maintenance personnel may provide added insight into the actual service environment that may be less than obvious. 6.3.2 factors Affecting Service Life of Steel Bridge Elements Specific to Paint Coating Figure 6.12 shows factors that affect the service life of steel bridge elements specific to paint coating. The factors are then described further. 6.3.2.1 Moisture and Debris Traps The creation of moisture and debris traps in new structures is an area of obvious con- cern, as the presence of such areas will certainly shorten the service life of any organic coating system. The design of new structures should focus on eliminating the creation of these corrosion-prone conditions. If such areas are absolutely essential to the design, corrosion-mitigation strategies must be developed. During maintenance recoating or overcoating projects on existing structures, the consideration of debris and moisture traps is even more critical. If residual contaminants remain on the surface and are overcoated or recoated in any area, the service life of virtu- ally any system will be shortened. This effect is especially exacerbated by the presence of chloride-laden residue from seawater or snow and ice removal activities. The service life of any coating will be extended by extra cleaning efforts in these moisture trap locations. In addition, owners should consider the use of zinc spray metalizing in these areas, mak- ing use of the best protection in the most aggressive corrosion locations.

286 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE As a result of this type of exposure, a surface that is normally expected to be dry is in effect an area of severe exposure. These areas are categorized by the SSPC as SSPC Category 2A (frequently wet by fresh water), 2B (frequently wet by salt water), 2C (fresh water immersion), or 2D (saltwater immersion). Coatings for these areas should be chosen with care. SSPC currently recommends that zinc-rich, primer-based materials be used. 6.3.2.2 Roadway Joints The design of roadway joints is discussed in Chapter 9. In the past, leaky expansion joint seals were one of the principle reasons that steel below bridge decks became wet and corroded. These leaks allow water from the bridge deck to cascade from the deck and pour onto the steel members beneath the deck. These leaks change the exposure conditions in such areas from an exposure zone that is designed to be dry (exposure Zone 1B exterior, normally dry) to one of the following: Zone 2A (frequently wet by fresh water), 2B (frequently wet by salt water), 2C (fresh water immersion), or 2D (saltwater immersion). The type of corrosion protection used in these areas is described in Section 6.2. See also the discussion about composite protection in Section 6.5.1.6. 6.3.2.3 Deck Cracks Cracks in the deck and/or in the wearing surface allow water, especially salt water, to penetrate through the deck and pour salt water onto steel surfaces below. When the steel becomes wet, corrosion almost always follows. Figure 6.12. Factors affecting service life of steel bridge elements related to paint coatings. Factors Affecting Paint Coatings Moisture and Debris Traps Back-to- Back Angles Wind- Blown Rain or Salt Roadway Joints Exposed Steel Bearings Deck Cracks Chlorides Splash- Zone Exposure

287 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.3.2.4 Splash-Zone Exposure When traffic travels beneath a steel overpass or through a steel truss or similar struc- ture, an area of the steel above and often beside the traffic is bathed in water from the roadway. This water is deposited on the steel surfaces. Figure 6.13 shows the effect of roadway splash on coated railings. Splash from automobiles and trucks can travel vertically as high as about 20 ft and horizontally 10 ft or more. Painted steel surfaces within that envelope will be in an environmental Zone 2A, 2B, 2C, or 2D (as previ- ously described) and perhaps in a zone requiring special treatment with zinc spray metalizing when possible, galvanizing any steel that is replaced and any steel that can be removed, galvanized, and returned to service. 6.3.2.5 Exposed Steel Bearings Steel bearings are often sheltered and isolated from water or salt water by the steel and roadway deck directly above, as shown in Figure 6.14. On some structures, steel bear- ings can be the target of corrosion. At times bridges must be closed to traffic and entire bearings must be replaced, which can require extensive shoring, as shown in Figure 6.15. Steel bearings can be exposed to an immersion-like environment as a re- sult of leaks from joint areas above. 6.3.2.6 Back-to-Back Angles The use of back-to-back angles should be discontinued when new or replacement steel configurations are encountered. In rehabilitation maintenance, the use of back-to-back angles presents a configura- tion that is very difficult to clean or coat effectively; great care in cleaning and coating such areas is recommended. Special tools, often developed by the contractor for these special situations, are needed for effective protection, and even with their use effective protection is usually unattainable. Figure 6.13. Effect of roadway splash on coated railing.

288 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Figure 6.14. Bearings below roadway joints are difficult to protect. Source: Courtesy KTA-Tator, Inc. Source: Courtesy (left) KTA-Tator, Inc., and (right) District 11-0, Pennsylvania Department of Transportation. Figure 6.15. Shoring to support bridge during bearing replacement operation. Note heavy corrosion on bearings in right-hand photo. Figure 6.15. Shoring to support bridge during bearing replacement operation. Note heavy corrosion on bearings in right photo. Sources: Courtesy (left) KTA-Tator, Inc., and (right) District 11-0, Pennsylvania DOT. 6.3.2.7 Wind-Blown Rain or Salt Spray During rain, water can be blown onto steel surfaces even when there is a bridge side- walk above. The ability of a coating to withstand this occasional exposure to fresh, nonbrackish water should not present a problem for zinc-rich-based coating systems. In fact, rain water can provide a benefit because it can cleanse exposed surfaces.

289 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES During storms, salt water from nearby brackish water or from ice- or snow-melt can be blown onto steel surfaces. Repeated exposure to salt water is very corrosive, and the duration of wetness and the number and length of wet–dry cycles affect the degree and extent of corrosion. In areas with long periods of wetness, such as those caused by frequent wet–dry cycles, hot-dip galvanizing (HDG) or zinc spray metalizing should be considered for primary corrosion protection. 6.3.2.8 Chlorides In a landmark literature survey compiled by Alblas and von Londen published in the Journal of Protective Coatings and Linings in February 1997, the researchers stated that “It has been clearly established that soluble salts on the surface of steel will increase the rate of corrosion and paint breakdown for many [coating] systems now in use.” This conclusion is as true in 2012 as it was in 1997. The authors offered several conclusions: • From available data, it is not possible to establish a definitive allowable level of chloride contaminants. • In relation to the durability of the paint system, a maximum chloride level of 10 to 50 μg/cm2 is thought to be permissible, depending on the use and exposure condi- tions. This is only a rough guideline. • Under specific conditions, higher maximum levels of chloride (up to hundreds of micrograms per square centimeter) are allowed for special, durable paint systems (e.g., zinc silicate). • Exposure to marine conditions and/or industrial environments considerably in- creases the chloride contamination on steel. • Abrasive blast cleaning does not remove all the chloride. • Results of detection methods for soluble chlorides are affected by temperature, mechanical forces, and the chemicals and type of analytical method used. • The effect on steel of the hydrochloric acid generated as a consequence of the cor- rosion reaction is notable. Therefore, the removal of as much chloride as possible during blast cleaning and other surface preparation efforts is crucial. Although there is still not complete agreement as to the precise level of chloride residue that is acceptable, the Surface Preparation Commentary for Steel and Concrete Substrates (Subsection 4.3.6, Soluble Salts) (SSPC 2004a) identifies three levels of chloride removal: – 0 μg/cm2; – Less than 7 μg/cm2; and – Less than 50 μg/cm2. • A level of chloride removal commonly specified for conventional mild steel is 7 μg/cm2.

290 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE An FHWA-sponsored study (Appleman et al. 1995) concluded that the maximum safe level of chloride to remain when cleaning weathering steel is 50 μg/cm2. In accordance with SSPC-SP 10, abrasive blast cleaning on noncorroded areas will reduce chloride levels to an acceptable <7 μg/cm². It will not always do so on heavily rusted, rust scale–covered, or pitted or pack rust–affected areas. In some areas, it is believed that complete removal of all chlorides via only dry abrasive blast cleaning is at best unlikely, and at worst, provides a false sense of secu- rity, even if white metal blast cleaning (SSPC-SP 5, White Metal Blast Cleaning) is specified. Cleaning efforts beyond abrasive blast cleaning are usually needed. The use of high-pressure water cleaning (5,000 to 10,000 psi) has been found to significantly reduce residual chlorides to a very low level. High-pressure waterjetting (10,000 to 30,000 psi) has also been used to reduce residual chloride levels. As noted, the effect of chlorides on the corrosion rate of steel has been studied and is well documented. 6.3.3 factors Affecting Service Life of galvanized or Painted Steel Bridge Elements Specific to galvanizing Coating The corrosion protection of unpainted galvanizing comes from the formation of a thin, invisible layer of insoluble corrosion products. Zinc, an active metal, reacts with oxygen in the air; zinc oxide starts forming within 24 to 48 hours after galvanizing and takes about a year to cover the entire galvanized surface. The zinc oxide converts to zinc hydroxide on exposure to moisture in the form of rain, dew, or high humidity. The final step is the reaction of zinc oxide and zinc hydroxide with carbon dioxide in the air to form zinc carbonate. This reaction requires free-flowing air. Zinc carbonate is the dense insoluble material that forms the protective layer, sometimes called the patina. Zinc oxide and zinc hydroxide are water soluble and not very dense. They adhere loosely to the surface, so painting over zinc oxide or zinc hydroxide does not provide good adhesion of the coating to the surface. The practical problem is that zinc oxide, zinc hydroxide, and zinc carbonate are all white and cause the galvanized sur- face to appear a dull, matte gray, which does not allow a visual determination of what form of zinc compound is present. Knowing the compound is important, because some forms are not corrosion resistant and are unsuitable for painting over. 6.3.3.1 Reactivity of Zinc The reactivity of zinc is well known to galvanizers. For instance, they know that if pieces are closely stacked together for shipment, there will be no access to carbon dioxide in free-flowing air to form the zinc carbonate. In such a case, only loose zinc oxide and zinc hydroxide will form, causing rapid consumption of the zinc. For this reason, closely spaced galvanized pieces should be unpacked, after which the loose white deposit on the surface should be noted. If this reaction process is allowed to continue, it can consume all of the zinc by reaction with the moisture caught between the pieces. Although rare, rusting of the unprotected steel may then occur, resulting in the presence of rust beneath the deposit.

291 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.3.3.2 Prevention or Passivation of White Storage Stain The white deposit described in Section 6.3.3.1 is called wet storage stain. Galvanizers apply a light coating of oil to prevent the stain. The oil forms a barrier to keep mois- ture from reaching the zinc, thus preventing the zinc from being converted to oxide and hydroxide forms. However, as paints do not stick to oil, painting the surface with- out first removing the oil is unacceptable. This is true no matter what type of coating is applied. Another process used to prevent wet storage stain is quenching or passivating with chromates or phosphates. Quenching (i.e., cooling and water bath) is not harmful in itself. However, the quenching bath may contain small amounts of oil and grease on the surface of the water that are picked up when pieces are removed. Coatings also do not stick to chromate-quenched galvanizing, but the phosphate improves adhesion. Although wet storage stain can damage galvanizing, the methods used to prevent it can affect painting results. It is always recommended to consult the galvanizers concerning the process employed, especially if the galvanized items are to be painted. 6.3.3.3 Repair of Defects in the Galvanized Surface The next step in surface preparation is to repair any defects or handling damage. Gal- vanizing can leave high spots and zinc droplets, which occur when a galvanized piece is withdrawn from the bath and excess zinc runs down the edges onto a protrusion or irregular edge. Droplets form at edges where zinc drains from the piece and can be removed with hand tools. High spots are usually ground down with power tools. Care is required to avoid removing so much zinc that the remaining thickness is below the specified minimum. SSPC-Guide 14, Guide for Repair of Imperfections in Galvanized or Inorganic Zinc-Rich Coated Steel Using Organic Zinc-Rich Coating (SSPC 2004b) should be consulted. Unstable zinc oxide or zinc hydroxide may not have been entirely removed during the initial cleaning process. There is no simple method for identifying the presence of either, so the surface must be further treated. Galvanizing can be eroded if exposed to very strong acids or alkali, which may cause the zinc to dissolve as metallic zinc is soluble in very strong acid or alkali envi- ronments. In these unusual circumstances, if regalvanizing is not possible, repairs can be made with coatings in accordance with SSPC-Guide 14 described above. After restoring the zinc protection, a decision can be made as to whether painting is desired. 6.3.3.4 Preparing Weathered Galvanizing for Painting Fully weathered galvanizing (i.e., galvanizing that has been outdoors for at least 1 year and preferably about 2) should have a fully formed layer of protective zinc carbonate. Nothing is required to prepare the surface for normal atmospheric exposure, and its service life will not be limited in the normal course of exposure events. Bare galvanized surfaces will be subjected to the vicissitudes of the local weather environment, and this unknown, complex exposure may mean that it makes sense to combine galvaniz- ing and painting. A so-called duplex system (i.e., galvanizing and paint) should be

292 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE considered. Such systems are said to provide 1.5 to 2.5 times the service life of the sum of both galvanizing and paint if each is considered separately, and an aesthetically pleasing palette of colors is available. If a surface of weathered galvanizing is to be painted, the surface must normally be power washed with clean water at about 1,450 psi. Spot repairs of any damage in accordance with SSPC-Guide 14 are all that is necessary. 6.3.3.5 Surface Preparation of New Galvanizing for Painting The often-made claim that galvanized surfaces cannot be painted is incorrect. In fact, galvanized surfaces are routinely painted successfully. Several steps are described in this section in which errors of omission or commission are routinely made, with rem- edies for each. There is no reason why so-called duplex systems cannot perform for decades. New galvanizing means galvanized steel that is between 1 or 2 days new and up to about 2 years old. Wet storage stain, if present, must be removed before surface preparation. Removal can be done by brushing the stain with a 1% to 2% ammonia solution such as diluted household ammonia. After treatment, ammonia should be removed by rinsing with warm water. The first step in the surface preparation is to wash off oil, grease, and dirt. This cleaning is performed in accordance with SSPC-SP 1, Solvent Cleaning. Water-based emulsifiers or alkaline cleaners work best. A mildly alkaline cleaner should be used. The cleaning solution should be applied by dipping, spraying, or brushing with soft- bristle brushes. A temperature range of 140°F to 185°F works well. Afterwards, the surface should be thoroughly rinsed with hot water and allowed to dry. One helpful tip for determining if oil was applied to the galvanized surface to prevent wet storage stain is to contact the galvanizer; another way is to perform a water bead test in which a drop of water is placed on the surface. If it beads, oil will probably be present. The best advice is that when in doubt, the entire surface should be washed as described. After the surface is washed, it should be examined for zinc ash, a residue that consists of particles of oxidized zinc that float on the surface of the galvanizing bath. The ash can be removed by washing the surface with a 1% to 2% ammonia solution. Common methods for treating the surface in the field before painting are phos- phating by the use of wash primers or sweep-blast cleaning. 6.3.3.5.1 Phosphating Preparatory to Painting Phosphating is often accomplished by using a wash primer, a coating that neutralizes the surface oxides or hydroxides and etches the galvanized surface. The most common wash primer is polyvinyl butyral (e.g., SSPC-Paint 27). These materials are very thinly applied (0.3 to 0.5 mil) by brush or spray. The galvanized surface should shadow through the coating at this thickness. If the galvanized surface is completely hidden, the wash primer is too thick. Wash primers have poor cohesive strength and will split apart if they are too thick, resulting in paint disbondment.

293 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Phosphating is not recommended if a zinc-rich primer is going to be applied. Zinc- rich primers require intimate contact between the zinc particles in the paint and the zinc metal on the galvanized surface. The zinc phosphate acts as an insulator in the same way that iron oxide (i.e., rust) acts as an insulator on steel surfaces. 6.3.3.5.2 Sweep-Blast Cleaning Preparatory to Painting Sweep blasting is a method of light blast cleaning that can remove zinc oxides on the surface and roughen the surface without significantly removing the galvanizing. Sweep-blast cleaning should be performed with abrasives that are softer than the gal- vanized surface. The use of materials with a Mohs scale hardness of five or less is suitable. Sweep-blast cleaning should be performed in accordance with SSPC-SP 7, Brush-Off Blast Cleaning. 6.3.4 factors Affecting Service Life of Steel Bridge Elements Specific to metalizing Coating Metalized coatings provide corrosion protection to steel by both sacrificial and barrier protection. The coating itself provides a barrier between the environment and the steel surface, especially when applied in combination with conventional sealer coatings (e.g., epoxies, polyurethanes, and acrylics) as topcoats. Due to the electro chemical reaction between steel and zinc or aluminum in an aqueous or salt-contaminated envi ronment, these coatings sacrifice themselves to protect the steel at the site of any damage, or holes, in the coating. This sacrificial protection is akin to the protection provided by zinc-rich primers or galvanizing. 6.3.4.1 Metalized Coatings Metalized coatings can be applied in the shop or in the field by using a variety of techniques and equipment. The metal or metal alloy is applied in wire form and is fed through a source and liquefied. The source may be either flame (i.e., oxygen–acetylene) or electric arc. The liquefied metal is immediately propelled onto the prepared steel surface by using air spray in a manner similar to that used in painting. Once on the surface, the liquid metal cools and dries very quickly to form a continuous protective coating over the steel surface. 6.3.4.2 Cost of Metalizing Cost estimates made in 2012 place metalizing as two to three times per square foot the cost of conventional painting. A recent project at a large fabricator queried by the authors indicates that a significant differential currently exists. On a best-case basis, it is estimated that metalizing costs at least 40% to 50% more than painting. 6.3.4.3 Salt-Contaminated Areas Metalized coatings consist of spray droplets that have solidified and overlapped, pro- viding a somewhat coarse matrix. This matrix is a barrier coating, as well as a chemi- cally active one, as a result of the anode–cathode relationship between zinc and iron.

294 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The primary benefit of metalizing over other coating technologies is its durability and corrosion resistance, especially in salt-rich environments. For this reason, metalizing should be considered as an option for bridge structures in salt-rich environments or for areas or components of bridge structures that receive considerable exposure to salt and moisture from drainage and runoff. Although there are cost differences between metalizing and painting, in many cases metalizing should be specified. Based on the performance of metalizing over a long period of time, repairs and renovations on steel bridges would benefit by its use. Metalized coatings have been shown to perform very well in studies when applied over steel that has been blast-cleaned in accordance with SSPC-SP 5, White Metal Blast Cleaning, or SSPC-SP 10, Near-White Blast Cleaning. These coatings have a dull gray appearance with a rough texture as applied, but may be sealed and topcoated with most conventional paints. Sealing is recommended by many existing guidelines as it tends to increase coating life, reduce the deleterious effects of metalized coating poros- ity, and improve aesthetics. Metalized coatings provide the benefit of defect tolerance. The sacrificial nature of these coatings provides corrosion protection to the underlying steel at the site of breaches in the coating film. Metalized coatings, particularly aluminum and aluminum alloys, also tend to be quite abrasion resistant. 6.3.4.4 Bond Strength The bond between the metalized coating and the steel surface is mechanical in nature. As such, the bond is sensitive to surface contaminants and to the shape of the surface profile. Surface preparation should be specified as above (SSPC-SP 10 or SSPC-SP 5) with an angular 2- to 4-mil anchor-tooth profile. Because blast cleaning with rounded steel shot has produced deficient adhesion results, steel shot abrasives should not be used on surfaces that will or may be metalized. As a solventless coating application method, metalizing is less forgiving than con- ventional paint application. Applicators must be properly trained and experienced with the specific equipment and metals or alloys to be used. Because metalized coatings are inherently porous, achieving an adequate coat- ing thickness (6- to 8-mil minimum) in an overlapping spray pattern is critical to coating life. 6.3.4.5 Field Versus Shop Application Metalizing technology may also be applicable to field maintenance coating operations when a long-term, durable corrosion-protection coating system is required. Applica- tions of metalized coatings in the shop, and particularly in the field, require technically sound specifications and practices.

295 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6.3.5 factors Affecting Service Life of Steel Bridge Elements Specific to Weathering and noncorrosive Steels 6.3.5.1 Corrosion-Resistant Weathering Steel With the introduction of steel as a material of construction for bridges in the late 1800s, the industry has sought to find a form of steel that can overcome its most basic limitation: corrosion. It was believed that an answer had been found in the 1970s with the introduction of weathering steel. Since the 1970s, the search for the ideal material has evolved through improved higher-strength weathering steel, or high-performance steel. Each step in this evolution has produced incremental improvements in the per- formance of weathering steel in normal weathering environments. Unfortunately, there has been even wider use of weathering steel in bridges at locations that are not recom- mended for the best use of weathering steel. These areas are often in heavily salted areas or areas where the steel is sheltered or exposed to other conditions so that the corrosion-resistant patina simply does not form. These areas are discussed elsewhere in this chapter. 6.3.5.2 A1010 Structural Stainless Steel Although initial corrosion studies performed on A1010 steel have been favorable, the use of A1010 steel is currently inhibited somewhat by its premium cost. It is believed that with sufficient production, volume costs will come down. Although testing has produced promising results, its performance in aggressive, salt-laden areas is not com- pletely known. Even with these unknowns, however, it is hopeful that a solution to the 125+ year-old problem of dealing with the corrosion characteristics of steel will be determined. As additional bridges are constructed using A1010 steel, time will tell. As of 2012, one bridge has been completed in California and two others are under construction in Oregon. 6.4 oPtionS For enhAncing Service LiFe: corroSion PerFormAnce oF SteeL 6.4.1 general Categories of Solutions for Preventing Corrosion of Steel Bridge Elements In general, there are three options for developing corrosion-prevention systems for steel bridges: 1. The use of a coating system, which can consist of paint, galvanizing, or metalizing systems; 2. The use of corrosion-resistant steel (weathering steel) or noncorrosive steel; and 3. Avoidance of corrosive environments or corrosion-prone details.

296 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.4.2 general Strategies for Producing an Effective Corrosion-Protection System Regardless of the option selected from the list provided in Section 6.4.1, there are five major strategies that can result in an effective corrosion-protection system. These strategies are described in the following sections and listed here: 1. Review design to assure that the best protection is designed into the structure, 2. Use composite protection, 3. Use corrosion-proof materials, 4. Employ superdurable coatings, and 5. Use ongoing engineered maintenance painting. 6.4.2.1 Designing Corrosion Protection into the Structure Through the first 125 years of the steel bridge era, steel bridges have benefitted from the corrosion control foresight of their designers. The elimination or minimization of corrosion on such structures has resulted in a knowledge base that, if systematically applied to every structure, can benefit each one. As these lessons learned are applied, the corrosion-resistance features, principles, experiences, and insights should be de- signed into every new and rehabilitated steel bridge. Actions taken during the earliest project design stages can cause a dramatic lengthening of the coatings part of the main- tenance–repair–replace cycle by eliminating areas likely to corrode early in the ser- vice life of a structure. If corrosion resistance is designed into a structure by carefully managing the configuration and details of bridge design and detailing while using the current coatings systems, bridge corrosion resistance should improve dramatically. As seen in Figure 6.16, lack of attention to the relationship between design and potential corrosion can lead to unwanted exposure of the metal to corrosion. The design review should be considered a design hold point. In this instance, hold point means that fur- ther progress on the design would depend on having a corrosion review performed and a corrosion-resistance control plan initiated. This design phase review is a major means for creating a 100-year life for a new steel bridge. In order to attain 100 years of service life, it will be necessary to develop and use preferred details, which will serve as a way to lengthen the time before any maintenance painting is needed during the structure’s expected 100-year service life. 6.4.2.2 Composite Protection Currently the use of zinc to protect steel from corrosion is the gold standard of care. The use of a composite protection strategy is based on the premise that there is an order of efficacy in terms of corrosion protection provided by zinc as delivered in its various forms. HDG is considered the most efficacious protection because of the iron–zinc alloy that is formed on the steel surface closest to the outside of the HDG part. Even if the HDG surface is later nicked, the alloy layer will afford substantial protection from

297 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES corrosion. Many smaller bridge elements, steel bearings, cross frames, bolts, and expansion devices can be protected with HDG. Metalizing, as noted previously, has been tested for decades and also found to be an excellent means of protecting steel from corrosion. The spray-applied zinc does not form an alloy layer like HDG, but it does provide zinc in intimate contact with steel (iron) in order to provide effective galvanic protection. Metalizing has been tested repeatedly in both the laboratory and field and found to provide a very high level of corrosive protection. Zinc-rich, primer-based coatings systems have been the workhorse of the steel bridge industry for over 40 years, and coating systems based on zinc-rich coatings have a successful track record on countless bridges. Uncoated weathering steel also has a 40+ year history of providing successful cor- rosion protection in certain exposure areas. Structures and parts of structures can be protected using combinations of protec- tive steps. For example, steel bearings or cross frames can be hot-dip galvanized or metalized and then painted. Some fasteners (ASTM A-325 bolts) are available with either an HDG or mechanically galvanized coating and either can be coated or not as required. Figure 6.16. Details difficult to paint. (a) Lack of stiffener clearance to back wall results in an almost unpaint- able detail. (b) Coating failure at splice, especially around fasteners. Source: Courtesy KTA-Tator, Inc. (a) (b)

298 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE A composite approach is often adopted on weathering steel bridges when girder ends are cleaned and coated. Figure 6.17 shows several structural details that are suit- able for composite protection. In the future, ASTM A1010 or a similar material may conceivably be routinely employed in coastal areas or where salt usage is a certainty. It may prove feasible to use A1010 in combination with weathering steel. In such a case, perhaps girder ends could be made of A1010 steel, while the remainder of the girder is composed of weathering steel or painted regular mild steel. Table 6.2 summarizes the comparative functionality of galvanizing, metalizing, and zinc-rich paint in terms of cost, protection, and durability. “Duplexable” in Table 6.2 refers to the particular coating’s ability to be combined with other types to provide a composite coating system. In the Duplexable column, G represents galvanize, Figure 6.17. Details suitable for composite protection. (a) Welded cross frame could not be galvanized. (b) Mill to bear stiffener leaves crack in the coating design. (c) Unique opportunity for composite protection presented by the configuration of a large trunnion girder for a lift bridge. Source: All photos courtesy KTA-Tator, Inc. (a) (b) (c)

299 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES M represents metalize, and P represents a paint layer. For example, galvanizing can be combined with a top paint coat, and zinc-rich paint as a primer can be combined with multiple additional paint layers. 6.4.2.3 Use of Corrosion-Resistant or Corrosion-Proof Materials 6.4.2.3.1 Corrosion-Resistant Steel Weathering steel (coated or uncoated) has been the subject of much research and dis- cussion since its initial use on bridges in about 1970. Weathering steel’s roots lie in the improvement in the corrosion resistance of steel when small amounts of copper, chro- mium, nickel, phosphorous, silicon, manganese, or combinations of these elements are added to carbon steel. When weathering steel is properly exposed, a rusty red-orange to brown or purple-tinted patina forms. When the patina is formed, the corrosion rate of the steel stabilizes within about 3 to 5 years. The formation of the protective patina requires a series of wet and dry periods. In certain situations, the protective patina does not form completely or not at all. For example, if the steel is sheltered from the rain, the dark patina cannot form. In areas with high concentrations of corrosive industrial or chemical fumes, weathering steel may exhibit a much higher corrosion rate. In a saltwater marine environment or in areas heavily exposed to chloride-containing deicing materials, the protective patina does not form. The use of uncoated weathering steel in such locations is not recommended. When weathering steel is used in locations where regular wet–dry cycles occur, the steel is corrosion resistant to the point that no coating is necessary. In some loca- tions, weathering steel enjoys a vastly enhanced corrosion resistance that can render it relatively impervious to corrosion. The exact degree of corrosion resistance afforded is dependent on a number of variables, including climatic conditions, pollution levels, and the degree of sheltering from the atmosphere, as well as the composition of the steel itself. These variables influence the areas in which the use of weathering steel is appropriate. In a survey conducted as a part of the SHRP 2 R19A Project (final report available at http://www.trb.org/Design/Blurbs/168760.aspx), about one-third of the 16 DOTs responding reported the use of weathering steel on over 50% of their steel bridges. tABLE 6.2. three wAyS to APPLy zinc to SteeL Method Efficacy Relative Cost Durability Duplexable? Galvanize Best Best ($1.76/ft2) Best Yes, G/P Metalize Better Better ($4.10/ft2) Better Yes, M/P Zinc-rich paint system Good Good ($2.27/ft2) Good Yes, P/P/P Source: KTA-Tator, Inc. Note: The best (strongest), second-best, and third-best performers are identified as Best, Better, and Good, respectively. Galvanizing costs are from the American Galvanizers Association. Other costs are from experience and research among fabricators.

300 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Many states use Technical Advisory FHWA T5140.22, Uncoated Weathering Steel in Structures (FHWA 1989), as a guidance document. This technical advisory for weath- ering steel use is in the process (as of 2012) of being revised and updated by FHWA. In order to shield the weathering steel in areas likely to experience chloride-laden water exposure, many states paint the end of the weathering steel members for a dis- tance of 1.5 to 2 times the depth of the web. The same zinc-rich, primer-based coating systems used by the various DOTs for nonweathering steel are employed. In other loca- tions, weathering steel is used for its other characteristics and is coated in its entirety for protective and/or aesthetic reasons. 6.4.2.3.2 Noncorrosive Steel The perfect weathering steel would be a material that is corrosion proof in all environ- mental conditions. Such a material would present a desirable solution to addressing the main limitation of steel as a construction material, that is to say, steel rusts. And steel rusts even more in the presence of chlorides. Therefore, the “perfect” material would be a true unpainted, corrosion-resistant solution for any environment. A steel product believed by some to meet these stringent requirements has now been developed. The corrosion resistance has been accomplished by chemically aug- menting the steel’s metallurgical composition. This new grade of weathering steel, ASTM A1010, contains 10.5% to 12.5% chromium and is said by its vendors to be immune to corrosion based on testing performed in Kure Beach, North Carolina, in a 25-m test site (see Figure 6.18). Figure 6.18. Corrosion resistance of A36, 50W, HPS-70W, 100W, and A1010 grades of steel exposed at Kure Beach, North Carolina [25.4 μm = 1 mil (0.001 in.)]. Source: Fletcher et al. 2003. 0 20 40 60 80 100 120 0123456 C or ro si on L os s, m ic ro ns Years Kure Beach 25m A36 50W HPS-70W 100W A1010 A1010

301 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES This “corrosion-proof” steel is just beginning to be used on bridges. One small structure was constructed in 2004 in Colusa County, California. This project was the subject of a report presented at the 2004 Prefabricated Bridge Elements and Systems Conference. The bridge design was a prefabricated lightweight section referred to as a multicell box girder. Less than 23 tons of A1010 were needed to form the structure of this short-span bridge with overall dimensions of 72 ft long by 32 ft wide. Construction of two bridges using A1010 steel is under way in Oregon, and the possibility of constructing other bridges has been mentioned in other states. Although A1010 steel remains a somewhat costly alternate material choice, it does demonstrate that it may be possible to anticipate the use of such a material in the future. 6.4.2.4 Use of Superdurable Coatings The search for the Superman of coatings continues: • FHWA’s Research, Development, and Technology Program on coatings has re- cently focused research on two-coat systems and their ability to perform as well as the traditional three-coat system that has been in use since around 1965. Testing of one-coat system candidate materials for steel bridges has also been under way. FHWA released its final report on that testing, Performance Evaluation of One- Coat Systems on New Steel Bridges, as Report No. HRT-11-046 (FHWA 2011c). Although there were some promising prospects among the materials tested, none of them approached the performance of the three-coat control system in the testing. • Ongoing efforts to identify new resins and pigments for improved coatings are under way in the private sector. • Developing new superdurable coating materials via both basic and applied re- search efforts, including industry-to-industry technology transfer, is under way. The use of nano particles in coatings has begun but has not yet spread to the bridge industry. The potential use of nano-sized (a billionth of an inch) pigment particles that can dramatically alter the performance of a coating is much anticipated. • In new construction, the coating systems and procedures are basically unchanged since the late 1960s. For new bridges the steel is coated using a system consisting of a zinc-rich primer, usually an epoxy midcoat and usually a urethane topcoat, applied over steel cleaned in accordance with SSPC-SP, 10 Near-White Metal Blast Cleaning. Initial cleaning and priming is normally done in the fabrication shop. 6.4.2.5 Maintenance Painting As in many other areas of the construction industry, quality in bridge painting must be built in and cannot simply be added after the fact. Properly selected and applied coatings can often last for many decades with periodic planned maintenance painting. A comprehensive approach to maintenance painting requires considerations of surface preparations, inspection, and proper planning.

302 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.4.2.5.1 Surface Preparation The initial condition of the surface to be cleaned will determine the amount of work, time, and money required to achieve any particular degree of surface cleanliness. It is more difficult to remove contaminants from rusty steel and to remove mill scale from new steel than it is to wash surface film off steel in good condition. Therefore, it is necessary to consider the surface condition before selecting the method of cleaning. The method of cleaning is an integral part of how the coating system may be expected to perform in any given environment. The initial condition of the steel may determine the choice of abrasives. Steel shot is an economical and effective choice for removing intact mill scale. Although their use in the field is not unknown, steel abrasives are usually recycled and therefore find their most common use in the shop. However, if the steel is rusted or pitted, an angular abrasive, such a steel grit or a nonmetallic mineral abrasive, will more effectively scour out the rust. The initial conditions encountered can be broadly divided into three categories as follows: 1. New construction—steel not previously coated; 2. Maintenance—repainting of previously coated, painted, metalized, or galvanized steel; and 3. Contaminated surfaces—common to both new construction and maintenance. Typical contaminants that should be removed during surface preparation are rust, corrosion products, mill scale, grease, oil, dirt, dust, moisture, soluble salts (e.g., chlo- rides and sulfates), paint chalk, and loose, cracked, or peeling paint. Each of these contaminants is discussed in the following paragraphs. Rust contaminants include rust, rust scale, and pack rust. Rust consists primar- ily of iron oxides, the corrosion products of steel. Whether loose or relatively tightly adherent, rust must be removed for satisfactory coating performance. Rust resulting from the corrosion of steel is not a good base for applying coatings because it expands and becomes porous. Ideally, rust and rust scale should be removed, even when using the lowest degrees of hand and power tool cleaning (SSPC-SP 2, Hand Tool Cleaning, and SSPC-SP 3, Power Tool Cleaning). Judgment should be used on an individual project basis whether the cost and effort required to remove the stratified rust, rust scale, and to a greater or lesser extent, pack rust, can be justified by the expected increase in the life of the coating system. To effectively repair pack-rusted joints, it may be necessary to remove rivets, separate the plies of steel, clean, paint, and refasten with bolts. On riveted and bolted connections, bridge management practices are required that cause surfaces to be repaired long before such inefficient, costly repairs are necessary. It is obvious that many square feet of steel can be cleaned and recoated before the cost of disassembly and reassembly of bridge connections is equaled.

303 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES The existence and treatment of rust and particularly pack rust can make bridge repair so expensive that bridge demolition may appear to be a feasible option. When such matters are expected to be at issue, agreement about the extent of removal of these materials should be reached before work begins. There is a trade-off between repair cost and extended service life. For maintenance repainting, the degree of surface preparation required depends on the new coating system and on the extent of degradation of the surface to be painted. The amount of rusting on the surface is based on the numerical scale of zero to 10 given in SSPC-VIS 2, Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces (SSPC 2000), in which a reading of 10 indicates no rust and a rating of zero indicates more than 50% rusting. SSPC-PA Guide 4, Guide to Maintenance Repainting with Oil Base or Alkyd Painting Systems (SSPC 2004c), suggests the minimum surface preparation needed for each degree of rusting. This guide includes a description of accepted prac- tices for recleaning old, sound paint, removing rust, and feathering the edges of sound coating around the area and recoating. Additional information on the subject may be found in SSPC-SP COM, Surface Preparation Commentary for Steel and Concrete Substrates (SSPC 2004a). Mill scale is a bluish, normally slightly shiny outside residue that forms on steel surfaces during hot rolling at the steel mill. Although initially tightly adherent, it even- tually cracks, pops, and disbonds. As a general rule, unless it is completely removed before painting, at some point it will most likely crack and cause the coatings to crack, exposing the underlying steel surface. In addition, steel is anodic to mill scale, and as the anode in the resultant dissimilar metals corrosion cell (with oxygen from the air and moisture) will corrode more rapidly. At least in the short term, mill scale is somewhat unpredictable in its effect on the performance of coatings, although if it remains tightly adhered, intact mill scale may not have to be removed at all for steel exposed to a mild atmospheric exposure. However, if the steel surface is to be coated with primers with low wetting properties or exposed to severe environments such as wetness or immersion in fresh or salt water, then removal of mill scale by blast cleaning or power tool cleaning is necessary. Soluble salts are deposited from the atmosphere onto surfaces. If they are permitted to remain on the surface after cleaning and are coated over, they can attract moisture that can delaminate the coating and cause blisters. Salts, particularly chlorides, may also accelerate the corrosion reaction and underfilm corrosion. Methods for measuring the amount of salt on the surface are described in SSPC-Guide 15, Field Methods for Retrieval and Analysis of Soluble Salts on Steel or Other Nonporous Substrates (SSPC 2005). In some circumstances, it is desirable to remove soluble salts by power washing or other methods employing wet methods before power tool or abrasive blast clean- ing. In other circumstances, salt removal is more efficient after the member has initially been subjected to abrasive blast cleaning. The extra effort required to remove this non- visible surface contaminant will help immeasurably in improving coating durability.

304 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The sun’s ultraviolet light causes exposed organic coating to chalk to some extent. Chalk is the residue left after deterioration of the coating’s organic binder on exposed surfaces. All loose chalk must be removed before coating in order to avoid intercoat adhesion problems. Sharp edges, such as those which at times may occur on some rolled structural members or plates, as well as those resulting from flame cutting, welding, grinding, and shearing, could influence coating performance and may need to be addressed. Additional guidance on the subject of material anomalies and sharp edges can be found in Steel Bridge Collaboration Specification S-8.1, Guide Specification for Application of Coating Systems with Zinc-Rich Primers to Steel Bridges (AASHTO/NSBA 2006). 6.4.2.5.2 Coatings (Paint) Inspection The importance of coating inspection during surface preparation and coating applica- tion cannot be ignored or underemphasized. As a general rule, it is not possible to visually examine a coated surface and know whether the surface preparation and coating application were done in accordance with the applicable specification and good painting practice. Determining whether the coat- ing material was properly mixed; whether a component was substituted, adulterated, or left out completely; or whether an entire coat of paint was simply skipped, can be a tedious, costly process. Once a surface has been painted, it is usually not possible to determine whether each painter in a crew has complied consistently with the specifica- tion. It is important to recognize the value of adequate inspection. Unless trained inspectors monitor the entire operation from start to finish, there is no way to know for sure about the level of specification compliance actually achieved; coating systems can only perform if they are properly installed. Certification programs for bridge paint inspectors are offered by the Society for Protective Coatings. The SSPC Bridge Coatings Inspector (BCI) training course was developed by a committee of more than a dozen DOT representatives. The course is appropriate for any level of worker in the coatings industry, including apprentices, blasters, painters, foremen, superintendents, engineers, and inspectors. Details about the BCI course can be found at www.sspc.org. Attendance at the BCI class is designed to instruct the attendees to be able to do the following: 1. Define the varying professional roles of the inspector, bridge owner, and painting contractor and their relationship to each other at the project site. 2. Identify what preparation the inspector must make before the start of work in order to conduct effective inspections. 3. Recognize common coating inspection and related terms. 4. Identify and properly adjust and operate commonly used coating inspection in- struments and test equipment. 5. Identify fundamental surface preparation and coating application processes.

305 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES 6. Identify key documents (e.g., specification, product data sheets, and technical bul- letins; industry technical standards; and references) required to perform competent inspections. 7. Identify inspection check or hold points. 8. Create inspection documentation, including a basic inspection plan. 9. Identify processes normally inspected and documented. 10. Identify common coating application defects. A variety of other inspection training classes are offered by private organizations and nonprofit societies. Although all these classes have strengths, SSPC-trained and -certified BCIs have received training prepared specifically for the coating of steel bridges. The SSPC-trained BCI is considered by many to be the most experienced and best-trained BCI in the bridge coatings inspection field, and many of SSPC’s coat- ings inspection training courses include the preparation of inspection plans in the curriculum. To provide guidance to those responsible for creating quality control plans and a training document for course participants, SSPC developed a Guide for Planning Coatings Inspection in 2008. The planning guide describes the importance of qual- ity monitoring on a project to reduce the risk of coating failure and the challenges associated with trying to assess quality after the project is complete. It also stresses the importance of planning the inspection to increase the likelihood that inspections are performed and the results properly documented. The intended purpose of the planning guide is to assist coating inspectors, quality control personnel, and owners with a tool to help ensure the coating or lining installation is the best it can be. 6.4.2.5.3 Worst First Versus Engineered Maintenance Painting Many agencies are perennially short of maintenance painting funds. As a result, the bridges that receive coating attention are those that appear to be in the most distress (worst first). By the time a structure appears to need the most attention, it is probably well past the point at which the spot-on zone cleaning can be effectively employed. SSPC Technology Update TU-3, Overcoating (2004), offers guidance when consider- ing overcoating. Subsection A.2.1 (Bridge Painting Using Risk Tables) of Appendix A deals with the percentage of the surface at which the cost of the repairs approaches that of full removal. The percentage identified as the critical percentage beyond which the surfaces are rusted or distressed such that surface preparation is necessary is 16% per ASTM D610, Rust Grade 2 (Rust Grade 10 = essentially no rust, and Rust Grade 0 ≥50% rust). SSPC reports in this document that according to the Guide for Painting Steel Structures (AASHTO 1994), “whenever the surface preparation area exceeds 15% to 20% of the surface area, the economics are such that a total removal of lead paint is the most viable option.”

306 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE From a cost perspective, the difference in cost between spot or zone cleaning and painting versus full removal is dramatic. According to industry sources, a typical lead- paint removal project in the northeastern part of the United States (in 2011) averages ~ $13/ft2. Of that amount, about half ($6.50/ ft2) is attributed to surface preparation (access cost, containment, equipment, abrasive, and labor). If spot or zone cleaning were possible, costs on a comparative basis would be about $3.90/ft2 (30%). If cleaning were performed when the surface affected was >3% (Rust Grade 4) but <10%, costs would be lower still. It is apparent that a timely touch-up would extend the service life of the paint project and lower costs. The reasons for being forced into a worst-first mode vary, but the practice is common. If an engineered approach to maintenance recoating were employed, overall costs could be reduced by a large percentage, and the condition of the coatings on the bridge inventory in a given city, district, or state would, in time, improve. It is recognized that bridge maintenance activities are driven by many factors, not all of which are corrosion related. When the matter of maintenance painting is consid- ered, an engineering approach will help to counter the worst-first approach. If the de facto use of the worst-first practice is unchanged, coatings costs will be at their highest. If every bridge that was repainted, even if completely redone, were placed in a mas- ter schedule of planned paint touch-up in, say 20 years, eventually the worst bridges would be repainted and those structures that were able to be recoated to extend their service lives would emerge as the norm. There are further enhancements to the plan- ning that can be employed. For example, the deterioration of one structure may be faster than another. In those cases, the examination and evaluation of a structure can be scheduled in a different cycle. Early intervention saves money, can stretch the budget to cover additional proj- ects, and can eventually improve the condition of the steel structures across an area, district, or even an entire state. 6.4.2.6 Bridge Maintenance Owner’s Manual Every new bridge and every rehabilitated structure should be delivered with an owner’s manual containing a lifetime maintenance plan that outlines, much like an automobile owner’s manual, when designated corrosion-mitigation activities are to be undertaken. For example, it might be recommended that coating touch-up of minor nicks and scratches be undertaken every 5 years and that every 25 or 30 years, whenever certain conditions exist, the structure be touched up and overcoated. Following this mainte- nance plan based on known, needed activities would allow the owner to achieve the service life inherent in the structure. 6.5 StrAtegy SeLection ProceSS Specifiers can choose among a wide variety of coating systems. Several of these systems are recognized by the coatings industry as having a track record of successful perfor- mance in a given service environment. These systems are assembled by the coating

307 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES manufacturer according to product. In many cases, a given system can be used in a multitude of locations and service environments. For example, a system comprising a zinc-rich primer, an epoxy intermediate coat, and an acrylic polyurethane topcoat can be used to protect bridge steel in most areas and has a +40-year service history. A coating system is selected on the basis of the prevailing service environment, the intended life of the structure, the level or degree of surface preparation possible, the intended service life of the coating, access, and any other constraints. Table 6.3 lists common generic coating systems, common service environments within a given structure, and coating systems candidates for each. This chart repre- sents a cross section of coating systems. tABLE 6.3. coAting SyStemS For highwAy bridgeS (new conStruction And mAintenAnce) Coating System Highway Bridges (New) Highway Bridges (Maintenance-1) Highway Bridges (Maintenance-2) Inorganic zinc-rich primer–polyamide epoxy– acrylic polyurethane X Polysiloxane X Organic zinc-rich primer–polyamide epoxy–acrylic polyurethane X X Organic zinc-rich primer–polyamide epoxy–polysiloxane X Organic zinc-rich primer–polyamide epoxy–fluoropolymer X X Organic zinc-rich primer–polyurea X X Moisture-cure urethane zinc-rich primer–moisture- cure urethane–moisture-cure urethane X X Moisture-cure urethane zinc-rich primer–moisture- cure urethane–acrylic polyurethane X X Inorganic zinc-rich primer–waterborne acrylic X Organic zinc-rich primer–waterborne acrylic X Thermal spray coating–sealer X X Epoxy sealer–epoxy mastic–acrylic polyurethane X Epoxy mastic–acrylic polyurethane X Epoxy mastic–waterborne acrylic X Moisture-cure urethane sealer–moisture-cure urethane–moisture cure-urethane X Moisture-cure urethane–moisture-cure urethane– acrylic polyurethane X Alkyd–silicone alkyd X Calcium sulfonate alkyd (two coats) X Source: KTA-Tator, Inc. Note: Maintenance-1 = recoating (total removal and replacement of existing system); Maintenance-2 = overcoating (spot or zone repair, spot or zone repriming and overcoat); X = coating system is applicable to the types of bridges indicated.

308 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Maintenance overcoating is a process in which new coating is applied over exist- ing coating. Based on industry knowledge and DOT survey information obtained for this project, the coating systems currently in use for this purpose include acrylic, cal- cium sulfonate, epoxy sealer–epoxy–urethane, epoxy sealer–urethane, polyester, and polyaspartic. Maintenance recoating is a process in which a new coating system is applied over a surface from which all old coating has been removed. According to the DOT survey information described in this section, the most commonly used systems consist either of an organic or inorganic zinc-rich primer with an epoxy midcoat and a urethane topcoat. These zinc-rich primer–based systems have proven themselves in the field on thou- sands of structures for over 40 years. In the bridge industry, few materials have this proven track record. No doubt the demonstrated longevity of the systems has contrib- uted to their continued use. 6.5.1 Characteristics by Coating Table 6.4 lists common coating types and their inherent properties and characteristics. Although zinc–epoxy–urethane systems are widely used on bridges, special circum- stances may dictate the use of systems tailored for a specific application. A description of the more commonly used coatings is included in this section. An explanation of Table 6.4 and an example of how it can be used to select a coat- ing material on the basis of the desired performance characteristics follows. The left column of the chart contains industrial coating types. Note that within a coating type category, there can be subcategories that are not shown. For example, the category of organic zinc-rich primer includes an epoxy zinc, urethane zinc, vinyl zinc, and so forth. This list is not exhaustive, but rather contains some of the more common coating types. The top row on the chart contains 17 common characteristics. Once the service environment is identified and the intended use of the coating is determined, the specifier can review which generic categories of coatings are available. For example, if the specifier is considering overcoating, five coatings can be consid- ered for this application (alkyd, calcium sulfonate alkyd, epoxy mastic, moisture- cure urethane, and waterborne acrylic). However, if the overcoat material must also demonstrate abrasion resistance, then only two candidates remain, epoxy mastic and moisture-cure urethane, as the other three do not possess abrasion-resistant proper- ties. If single-pack paint is desirable (i.e., a product that has all ingredients in a single container), then the specifier can select a moisture-cure urethane from these two, as the epoxy mastic is a two-pack product that requires mixing prior to application. 6.5.1.1 Acrylic Acrylic coatings can be formulated as thermoplastic, solvent-deposited coatings, cross- linked thermoset coatings, and water-based emulsion coatings. The acrylic resins, with suitable pigmentation, provide excellent film-forming coat- ings characterized by excellent light fastness, gloss, and ultraviolet stability. Chemi- cal resistance to weathering environments is generally excellent, as is resistance to

309 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES tA B LE 6 .4 . co At in g ch Ar Ac te ri St ic S ch Ar t C o at in g T yp e Color and Gloss Retention Surface Tolerant Flexible Easy to Apply Low Cost Can Be Modified Acid Resistant Caustic Resistant Abrasion Resistant Solvent Resistant Fast Dry Single Pack Low VOC Available Overcoat Material Chemical Resistance Immersion Typical Maximum Service Temperature a A lk yd b X X X X X X X X X 25 0o F Si lic on e al ky d X X X X X X X 25 0o F C al ci um s ul fo na te a lk yd X X X X X X X 25 0o F Ep ox yb X X X X X X X X 25 0o F Ep ox y m as tic X X X X X X X X X X 25 0o F U re th an eb X X X X X X X X X 25 0o F M oi st ur e- cu re u re th an e X X X X X X X X X X X X X 25 0o F In or ga ni c zi nc r ic hb X X X X 75 0o F O rg an ic z in c ric hb X X M cu z X X Bi nd er de pe nd en t W at er bo rn e ac ry lic b X X X X X X X X 25 0o F Po ly ur ea X X X X X X X X X X 35 0o F Po ly si lo xa ne X X X X X X X X X 20 0o F– 1, 40 0o F Th er m al s pr ay c oa tin g X W D W D X X X X X W D X W ire de pe nd en t So ur ce : R ep rin te d w ith p er m is si on o f S SP C : T he S oc ie ty fo r Pr ot ec tiv e C oa tin gs . N ot e: x = c ha ra ct er is tic is a pp lic ab le t o co at in g ty pe . M cu z = m oi st ur e- cu re d ur et ha ne z in c; W D = w ire d ep en de nt . a C on su lt co at in g m at er ia l m an uf ac tu re r’s p ro du ct d at a sh ee t. b M os t co m m on t yp es o f c oa tin gs u se d on s te el b rid ge s.

310 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE moisture. Most acrylic coatings are not suitable for immersion service or strong chemi- cal environments. In some state DOTs water-based coatings are required as topcoats because of local environmental rules banning VOCs or local preferences. 6.5.1.2 Cross-Linked Thermoset Coatings Chemically cured coatings refer to coatings that harden or cure and attain their final resistance properties by virtue of a chemical reaction either with a copolymer or by reaction with moisture. Coatings that chemically cross link by copolymerization include the epoxy family of coatings, including urethanes. Chemically cured coatings that react with water are moisture-cured polyurethanes and all of the inorganic zinc-rich coatings. Coatings based on chemically cured binders can be formulated to have excellent resistance to acid, alkalis, and moisture and to resist abrasion, ultraviolet degradation, and thermal degradation. Chemical and moisture resistance increases as the cross-link- ing density increases within the larger macromolecule. The rate at which the molecule cross links is dependent not only on the reactants, but also on the cross-linking mech- anism. Most importantly, external factors such as temperature and, with moisture reactions, atmospheric humidity, affect the rate and extent of cross linking. Thus, for chemically altered converted coatings, after application the coating must set through solvent or water volatilization and then harden and attain its final cured properties via the cross-linking reaction, which is temperature and/or moisture dependent. A reac- tion that is too fast may lead to an overcured, hard, impervious coating that cannot be recoated or topcoated with a properly adherent subsequent coat. This is always a problem in maintenance repainting when a renewal coat is applied to the original cross-linked coating system after an extended period of time. As a general rule, curing of most chemically cross-linked systems should proceed for approximately 7 days at 75ºF before the coating system is exposed to severely corrosive conditions. In corrosive environments, the structure may have to be enclosed in a containment device, and the coating manufacturer’s guidance on these issues should be sought. Following is a discussion of the more common chemically cross-linked binders or resins used for coatings. 6.5.1.2.1 Epoxies Chemically curing epoxies usually come in two packages: one consists of the epoxy resin, pigments, and some solvent, and the other is the curing agent. For bridge coat- ings, the two packages are mixed immediately before application and, on curing, develop the large macromolecule structure that provides a tough, water-resistant, durable film. However, the film is subject to chalking when exposed to sunlight and is normally topcoated. 6.5.1.2.2 Urethane Coatings Urethane coatings have chemical- and moisture-resistance properties similar to the epoxies, but they can also be formulated in a variety of light-stable colors and hues that maintain their gloss and wet look after prolonged outdoor exposure.

311 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES Acrylic urethanes are perhaps the most widely used corrosion-protection urethanes for atmospheric service on bridges. These coatings, when properly formulated, have excellent weatherability, gloss, and color retention and good chemical and moisture resistance. They can be readily tinted and pigmented to provide a variety of deep and pastel colors at a lower cost per gallon than the next most popular class, the polyester urethane. They have excellent weathering properties. 6.5.1.3 Zinc-Rich Coatings Zinc-rich coatings are a unique class of coating materials that provide galvanic pro- tection to a ferrous substrate. As the name implies, the binder is highly loaded with a metallic zinc dust pigment. After the coating is applied to a thoroughly cleaned sub- strate, the binder holds the metallic zinc particles in contact with the steel and with each other. Thus, metal-to-metal contact of two dissimilar metals is made, resulting in a galvanic cell. In this metallic couple, zinc becomes the anode and sacrifices itself to protect the underlying (cathodic) steel. The major advantage of corrosion protection using zinc-rich coatings is that pit- ting corrosion is eliminated, even at voids, pinholes, scratches, and abrasions in the coating system. This cannot be said of any nonzinc type of protective coating, and it is this protective capability that makes zinc-rich coating so unique and invaluable on bridges. This advantage, however, comes with certain disadvantages. The underlying steel substrate must be cleaned of all mill scale, rust, old paint, and other contaminants that may interfere with metal-to-metal contact. Thus, the degree of surface prepara- tion must be quite thorough: blast cleaning should, at a minimum, be an SSPC-SP 6 (Commercial) blast. For more aggressive, immersion-like exposures, SSPC-SP 10, Near-White Blast Cleaning, or SSPC- SP 5, White Metal Blast Cleaning, is necessary. No material is perfect. Because of the high reactivity of the zinc dust pigment, zinc-rich coatings are not suitable outside a pH range of approximately 5.5 to 11, and most bridges are located in environments that are within this range. (pH ranges from 1 to 14, with pH 7 being neutral.) Strong acids and strong alkalis will attack the zinc dust pigment, and even if topcoated, penetration of the chemicals may occur through pinholes, scratches, voids, or discontinuities within the topcoat, leading to aggressive attack of the zinc-rich primer. However, despite these disadvantages, the advantages that accrue by eliminating pitting corrosion make zinc-rich coatings, either topcoated or untopcoated, one of the most widely used corrosion-prevention coatings for painting steel bridges. Zinc-rich coatings can be used either as primers with topcoats or as complete one- coat systems. Both organic and inorganic zinc-rich coatings are used extensively for steel protection on bridges and highway structures, and any area where fresh or salt- water corrosion, mild fume, and high humidity and resultant corrosion are a problem.

312 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 6.5.1.3.1 Types of Zinc-Rich Coatings Zinc-rich coatings can be subcategorized into two types: those with organic or with inorganic binders. The organic types are similar in many ways to the epoxy and ure- thane coating systems previously discussed, except that sufficient zinc dust pigment is added to provide galvanic protection. The inorganic zinc-rich coatings use a different binder chemistry and are quite unlike the organic zinc-rich coatings. 6.5.1.3.1a Organic Zinc-Rich Coatings Organic zinc-rich coatings are most commonly formulated from epoxy polyamide, urethane, vinyl, and chlorinated rubber binders. The drying, hardening, and ultimate curing of the organic zinc-rich coating is predicated on the type of binder used. The organic nature of organic zinc-rich primers makes them more tolerant of deficient surface preparation, as they more readily wet and seal poorly prepared surfaces where residues of rust or old paint may remain. Sim- ilarly, topcoating with the same generic type of topcoat is more readily accomplished because organic zinc-rich coatings of all types generally have a less porous surface and are more akin to conventional non-zinc-rich coatings than are the inorganic zinc-rich coatings. 6.5.1.3.1b Inorganic Zinc-Rich Coatings SSPC has categorized inorganic zinc-rich coatings for use in the bridge industry in two major groups: self-cured water-based alkali metal silicates and self-cured solvent-based alkali silicates. Although the binder in both cases is an inorganic silicate, essentially the same material as glass or sand, the curing of the binder is different. 6.5.1.3.1c Self-Curing Water-Based Alkali Silicates The most common of these silicate binders is based on potassium and lithium sili- cates or combinations of the two. Lithium hydroxide–colloidal silica and quaternary ammonium silicate binders are also included in this category. Self-curing alkali silicate zinc-rich coatings become hard within minutes and are considered generally resistant to precipitation within half an hour after application. When final curing is ultimately attained, most water-based zinc-rich coatings expe- rience a color change, often from a reddish-gray or light gray color to a darker bluish- gray color. 6.5.1.3.1d Solvent-Reducible, Self-Cured Inorganic Zinc-Rich Coatings The binders for this class of coatings are essentially modifications of partially hydro- lyzed alkyl silicates. Of these, the ethyl silicate type is most commonly used.

313 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES During the condensation phase of the reaction, the partially polymerized silicate combines with atmospheric moisture to eliminate alcohol, which vaporizes. After complete hydrolysis, the cross-linked network forms a matrix to hold the pigment particles. 6.5.1.3.2 Durability of Zinc-Rich Primer–Coated Structures Thousands of zinc-rich paint–coated structures have been built since the late 1960s that are in good condition. In these cases, a third of the desired service life of 100 years has already been met. The ability of the coating to last 100 years or more appears to be achievable. Improved coating systems that have been extensively tested by the National Trans- portation Product Evaluation Program (see Section 6.5.2) can be expected to perform for 100 years. Naturally, periodic systematic planned maintenance painting must be performed. Simply put, “painting it now and coming back in 100 years” expecting to see a coating system in good condition is not believed to be achievable at this time. 6.5.1.4 Hot-Dip Galvanizing Galvanizing is the process in which steel pieces or parts are immersed in a kettle or vat filled with molten zinc, resulting in a metallurgically bonded alloy coating that protects the steel from corrosion. When galvanizing is exposed to the natural wet and dry cycles of the atmosphere, it develops a zinc by-product layer on the surface. This layer is stable and nonreactive unless exposed to aggressive chlorides or sulfides. The protective layer is a key compo- nent in the longevity of the HDG coating in the atmosphere. The American Galvanizers Association projects that HDG items will last 75 to 100 years. Figure 6.19 shows the relationship between time to first maintenance and zinc thickness for various types of environments. The various environments shown in the key are listed in the order that the lines are shown in the graph, from top to bottom. Although there are some important differences between zinc-rich coatings and gal- vanizing, the extensive field performance history of zinc-rich coatings in combination with the American Galvanizers Association data strongly suggests that steel that has been properly coated with a zinc coating and has additional coating layers to extend the service life of the zinc coating beneath can last 100 years or more. 6.5.1.5 Thermal Spray Metalizing Zinc in wire form may be applied to clean steel by feeding it into a heated gun where it is heated, melted, and spray applied using combustion gases or auxiliary compressed air to provide ample velocity. Metalizing may be used on any size steel object, so limitations due to vat size and awkward shapes are eliminated. Applying a consistent coating in recesses, hollows, and cavities adds a measure of complexity. Pure zinc can be used, but often zinc is alloyed with 15% aluminum to provide a smoother film.

314 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Metalizing spray application generates a smaller spray pattern, and application is normally slower than spray painting; accordingly, zinc thermal spray is generally more costly. FHWA funded two studies relative to zinc thermal spray that are reported in Publi- cations FHWA-RD-91-060 (Kogler and Mott 1992) and FHWA-RD-96-058 (Kogler et al. 1997). The 1992 report stated that “metalized test systems . . . performed extremely well in the battery of accelerated tests.” The later study reported on the testing of 13 coating systems for 5- to 6.5-year periods at three seaside sites. Three of the systems were 100% zinc, 100% aluminum, and 85% zinc–15% aluminum metalizing. In most cases, there was virtually no rusting or undercut in the metalized coating, and they were reported to have “near perfect corrosion performance.” In this study, Kogler et al. (1997) noted that the metalizing did not perform any better with or without the epoxy topcoat. In summary, metalized coatings outperform conventional liquid-applied zinc-rich coating systems, but they are less easily applied and they are less aesthetically pleasing without having high gloss topcoats. 6.5.1.6 Composite Protection It is believed that in a given exposure, galvanizing will outperform metalizing because of the alloying effect of molten zinc and steel (iron). Metalizing has been shown to produce impressive corrosion protection, and generally, zinc-metalized surfaces will outperform zinc-rich paint–coated surfaces. Zinc-rich paint–coated members have a Figure 6.19. Service life chart for HDG coatings in various environments. Source: Photo courtesy of the American Galvanizers Association. Figure 6.19. Service life chart for HDG coatings in various environments.

315 Chapter 6. CORROSiON pREvENTiON OF STEEL BRiDGES proven field history. When difficult conditions are foreseen, all three methods can be combined to provide protection from corrosion. For example, steel bearings can per- haps be hot-dip galvanized, and girder ends or cross frames could be metalized. In addi tion, it may be possible and desirable to coat a longer section on the girder end than the traditional 1.5 times the girder depth. The best set of practices should be set forth for the myriad of service conditions encountered, depending on the exposure conditions encountered on both a macro- and microenvironment basis. 6.5.2 Performance Evaluation of individual Protective Coating Systems Products Independent verification of coating system performance based on laboratory testing and/or field exposure is a critical component to selecting a coating system. A given coating system may have multiple manufacturers. It is not safe to assume that all coating systems within a given generic category are created equal. Therefore, careful evaluation of coating system performance is needed before full-scale field application to determine which of the candidate systems will perform the best. AASHTO oversees a materials testing branch—the National Transportation Prod- uct Evaluation Program (NTPEP)—comprising highway safety and construction mate- rials project panels. These panels are made up of state highway agency personnel whose objective is providing quality and responsive engineering for the testing and evaluation of products, materials, and devices that are commonly used by the AASHTO member DOTs. In 1997, the Structural Steel Coatings (SSC) Panel was created to develop a standard specification, a corresponding project work plan, and a reporting system for testing industrial coating systems for use on bridge and highway structures. Data are generated by prequalified independent testing laboratories and uploaded to a central database known as Datamine for DOT access. All testing is paid for by the coating manufacturer. The advantage of this type of performance evaluation is that many agencies within a given industry can access performance data with little or no associated costs. Limi- tations include the ability to keep the database current as new coating systems come to market, the time associated with generating the performance data (SSC requires approximately 10 months), and the application of the same performance criteria to the data for a coating that may be used on a bridge structure in northern Minnesota and on a bridge in Phoenix, Arizona, two very different service environments. Some facility owners and agencies may choose to employ a combination of perfor- mance evaluation methods. For example, a bridge owner may subscribe to Datamine (industry-specific performance evaluation) and may also suspend or mount racks of test panels containing candidate coating systems for a bridge structure.