Rail Base Corrosion Detection and Prevention (2007)

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5 CHAPTER 2: ANALYSIS OF RAIL CORROSION 2.1 Definition Corrosion is defined as the deterioration of a material due to its interaction with its environment (1). Corrosion is affected by several factors, including electrochemical, metallurgical, physicochemical, and thermodynamics (see Figure 1). The presence of hydrochloric acid (HCl) in the reactions shown in Figure 1 is due to the electrolytic decomposition of sodium chlorine (NaCl), potassium chlorine (KCl), and other chlorines present in the salts deposited on the rail. Sulfates, such as sodium sulfate (Na2SO4) or ammonium sulfate ((NH4)2SO4), are also decomposed and form sulfuric acid (H2SO4). The decomposition of the chlorines and sulfates is due to the presence of the direct current (DC) that breaks the molecules. The resultant hydrochloric and sulfuric acids form electrolytes that induce electrolysis and increase the corrosion rates. Most environments can be corrosive, but, generally, inorganic materials are more corrosive than organic materials. For example, in the oil industry, sodium chlorine, sulfur, hydrochloric, and sulfuric acids are the major factors that accelerate corrosion instead of the petroleum. Figure 1. Electrochemical effects and reactions occurring during corrosion of Fe present in steel in aerated systems containing water (H2O) and chlorine ions. HCl is a byproduct of dissolved chlorines + DC. Rust is the substance formed when iron compounds corrode in the presence of oxygen and water. Rust is a mixture of iron oxides and hydroxides and is a common form of corrosion on steel. This corrosion is the result of the oxidation reaction when iron metal returns to a more stable state. The rust forming process is summarized in three stages: (1) formation of Fe2+, (2) formation of hydroxide ions, and (3) the chemical reaction with oxygen to create rust. Hence, rust is Fe3+ oxide that is formed by the dehydration of Fe2+ and hydroxide. The concentration of chlorine ions accelerates corrosion by making the solution (water + salts) more conductive. A magnetic hydrous ferrite, Fe3O4xH2O, often forms a black intermediate layer between hydrous Fe2O3 and FeO. Hence, rust films normally consist of three layers of iron oxides in different states of oxidation (2,3). Steel 2Fe+2 e e H+ H+ H2 H+ H+ H+ Cl- Cl- O2 H2O H+ O2 H+ Cl- O2

6 2.2 Corrosion Principles Rail base corrosion is a combination of corrosion environments; for example, humidity (seawater and highly polluted water) and soil. The corrosion problems of systems with the presence of water have been well studied over many years, but despite published information on material behavior, corrosion is in some cases unpredictable. Most of the elements that can be found on earth are present in seawater, at least in trace amounts, with chloride ions being by far the largest constituent. On the other hand, soils are formed by the combined weathering action of wind and water, and also organic decay. Corrosion in soils is a major concern, especially because much of the buried infrastructure is aging. Rails are expected to function reliably and continuously over several decades. However, corrosion in soils is very complex due to the presence of several elements as well as variations in properties or characteristics across three dimensions resulting in a major impact on corrosion. Chemical reactions involving almost all of the existing elements are known to take place in soils, and many of these are not yet fully understood (2,4). Polluted water in liquid form represents the essential electrolyte required for electrochemical corrosion reactions. A distinction is made between saturated and unsaturated water flow in soils. The latter represents movement of water from wet areas to dry soil areas. Water is usually directed against gravity, and its flow is dependent on pore size and distribution, texture, structure, and organic matter. Figure 2 shows a detailed diagram with the parameters affecting the corrosion rate (2). Figure 2. Relationship of variables affecting the rate of corrosion in soil (2).

7 In addition to water and soil, rail on the transit systems carries electric current that further increases the rail corrosion rate. Stray currents have promoted corrosion damage on North American rail transit systems for more than a century. In the United States alone, there are more than 20 transit systems operating electrified rail systems in major urban centers. The transit systems that show the most severe rail base corrosion effects are the ones located in high-density urban areas with high humidity or underground cables and piping (water and gas) systems, which are susceptible to this form of corrosion damage (2,5). In such transit systems, rails are used to close the electric circuit resulting in a system where corrosion rate can potentially be accelerated in the presence of return DC currents on the rails that transport the return current. Oxygen transport is more rapid in coarse-textured dry soils than in fine waterlogged textures. Excavation can obviously increase the degree of aeration in soil, as compared with the undisturbed state. It is generally accepted that corrosion rates in disturbed soil with greater oxygen availability are significantly higher than in undisturbed soil. Soils usually have a pH range of 5 to 8. In this range, pH is generally not considered to be the dominant variable affecting corrosion rates. More acidic soils obviously represent a serious corrosion risk to common construction materials such as steel, cast iron, and zinc coatings. Soil acidity is produced by mineral leaching, industrial wastes, and city drain leaks. Alkaline soils tend to have high sodium, potassium, magnesium, and calcium contents (2). The effects of chlorines and sulfates on soils are of particular interest to the transit lines because the salt deposits found along the tracks have a high content of a white substance with high concentrations of chlorines and sulfates (6–9). (See Table 1 for corrosivity ratings particular to chlorine and sulfates.) TABLE 1. Corrosivity Ratings Based on Soil Resistivity Soil Resistivity, ohm cm Corrosivity Rating > 20,000 Essentially noncorrosive 10,000–20,000 Mildly corrosive 5000–10,000 Moderately corrosive 3000–5000 Corrosive 1000–3000 Highly corrosive < 1000 Extremely corrosive Chloride ions are generally harmful, as they participate directly in anodic dissolution reactions of metals. Pure water and oxides (e.g., SiO2, Al2O3, and CaCO3), usually present in soils, are nonconductors. However, the presence of salt decreases the soil resistivity allowing the transfer of DC resulting in an electrolyte. In some cases, the level of chloride ions in soils is comparable to those of seawater. The main sources of chlorine are leaks from drain systems and de-icing salts applied to roadways. The chloride ion concentration and activity in the corrosive aqueous

8 soil electrolyte will vary as soil conditions alternate between wet and dry (6,9). On the other hand, sulfates have a less corrosion effect than chlorides and are generally considered to be more benign in their corrosive action toward metallic materials. The presence of sulfates poses a major risk for metallic materials in the sense that sulfates can be converted to highly corrosive sulfides by anaerobic sulfate reduction (6). Table 2 shows the corrosion based on soil resistivity and the respective corrosion level according to the AWWA C-105 Standard. TABLE 2. Point System for Predicting Soil Corrosivity According to the AWWA C-105 Standard Soil Parameter Assigned Points Soil Parameter Assigned Points Resistivity, Ω cm < 700 700–1000 1000–1200 1200–1500 1500–2000 > 2000 10 8 5 2 1 0 pH 0 - 2 2 – 4 4 – 6.5 6.5 – 7.5 7.5 – 8.5 > 8.5 5 3 0 0 0 3 Redox potential, mV > 100 50 – 100 0 – 50 < 0 0 3.5 4 5 Sulfides Positive Trace Negative 3.5 2 0 Moisture Poor drainage, continuously wet Fair drainage, generally moist Good drainage, generally dry 2 1 0