Introduction to the corrosion of oil, gas, and metal pipelines

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Introduction to the Corrosion of Oil, Gas, and Metal Pipelines 1. Overview In oil and gas transportation, corrosion of metal pipelines occurs in two forms: internal corrosion and external corrosion. There are various factors that affect corrosion inside and outside the pipe. For example, in the petroleum refining process, corrosion caused by CO2 and H2S; in buried pipelines, factors such as soil type, permeability, electrical conductivity, soluble salt content, and pH value are all causes of corrosion. 2. Corrosion mechanisms of metal pipelines 2.1 Corrosion mechanism of CO2 Dry CO2 has no corrosive effect on steel at room temperature. The oxide formed in carbon steel in dry CO2 is mainly Fe3O4, with a small amount of Fe2O3. At temperatures above 110°F, the FeO present in the Fe3O4 and Fe2O3 oxide layers in contact with the steel surface is thought to be what controls the diffusion of oxygen ions across the oxide film; the growth rate of these oxides is generally considered to be the factor that determines this diffusion. The moisture content in CO2 is about 1000 ppm of H2O; at a humidity of 100°F, this accelerates the corrosion of steel. The mechanism is as follows: carbon deposits on the oxide film and CO is released, ultimately leading to the rupture of the film. 3Fe + 2CO2 = Fe3O4 + 2C (1) 3Fe + 4CO2 = Fe3O4 + 4C (2) CO2 reacts in water to form a weakly dissociating acid: CO2 + H2O ≒ H2CO3 ≒ H+ + HCO3-. As the pH level decreases, the rate of iron corrosion increases. At room temperature, the final product of the corrosion reaction between iron and CO2/H+ is: Fe + H2CO3 = FeCO3 + H2 ↑ (3) Iron carbonate does not constitute an effective protective layer; it rather leads to pitting corrosion. At 40°C and 1 atmosphere pressure with PH=4, at the same pH value, an aqueous CO2 solution is more corrosive than a strong acid solution. Because the pH value reflects the actual hydrogen ion concentration, representing all available acids. The pH value of weakly ionized acids corresponds only to a portion of the maximum acid ionization. Another explanation suggests that undissociated carbonic acid catalyzes reaction (3); the release of hydrogen and the reaction with intermediate products proceed as follows: Fe + H2CO3 = Fe(HCO3)2 + 2H2↑ (4). Experiments show that at temperatures below 60°C, the reaction rate is controlled by either the rate of CO2 hydrolysis to form carbonic acid or the rate of CO2 diffusion to the metal surface. At temperatures above 60°C, iron carbonate scale forms on the metal surface, and the corrosion rate is influenced by the mass transfer process through the scale layer. 2.2 Corrosion caused by H2S and dissolved oxygen: When H2S dissolves in water, it forms a weak acid, and its effect is more severe than that of CO2. H2S can cause spontaneous brittle fracture in high-strength steel, that is, sulfide stress cracking, which is a form of hydrogen embrittlement. When the H2S concentration in water is below 0.1 ppm and the partial pressure is below 0.001 atmospheres, it can cause sulfide stress cracking. As the H2S concentration and stress increase, the damage decreases. Because sulfide stress cracking does not occur when the temperature is above 180°F (82–22°C). Therefore, maintaining a high temperature is an effective way to prevent such cracking. Using low-strength steel where possible can also prevent cracking. Oxygen dissolved in water is the main cause of corrosion in water storage systems. The corrosion reaction of steel is as follows: 2Fe + 2H2O + O2 → 2Fe2+ + 4OH- → 2Fe(OH)2↓ (5) In addition to causing corrosion in the pipelines of the water injection system, it also leads to pipeline blockages. The main method of control is to use vacuum degasers and purifiers to remove oxygen. A degasser is used to reduce the oxygen concentration to 0.3 ppm, and the remaining oxygen is removed using a purifying agent, usually sodium sulfate or the dehydrating agent N2H4. The reaction equations are as follows: O2 + Na2SO3 → Na2SO4 (6) N2H4 + O2 → 2H2O. At room temperature, the reaction rate between Na2SO3 and O2 is slow; however, when it reacts with other chemicals, the reaction rate increases as the temperature rises – in other words, catalysts can accelerate the reaction between oxygen and sulfur ions. Corrosion is exacerbated when both H2S and CO2 are present in the water injection system, as CO2 destroys the existing iron sulfide protective layer. Oxygen can oxidize ferrous ions (Fe2+) to ferric ions (Fe3+). When the pH value is greater than 4, the reaction accelerates due to the formation of iron hydroxide precipitate. If the reaction does not occur on the metal surface, more Fe2+ ions need to enter the solution to maintain balance: under normal conditions, Fe ≒ Fe2+ + 2e (7). If Fe2+ ≒ Fe3+ + 2e (8), then corrosion will continue to increase as per Fe ≒ Fe2+ + 2e. If there is sufficient oxygen in the water, 2Fe(OH)3 precipitates can form on the metal surface to provide protection; however, if an appropriate amount of Cl- (chloride ions) is present in the water, it interferes with the formation of this protective layer, allowing corrosion to continue. When there is a difference in oxygen concentration between two different areas within the same system, corrosion occurs at the location with the lowest oxygen concentration, such as at the interface between water and air or in gaps.