Top Of Line Corrosion With High CO2 And Traces Of H2S

Top of line corrosion has been studied in a flow loop where moist gas is circulated and water is condensed on 1.8 m long carbon steel pipes with external cooling. The condensed water is collected at the end of each test section. This setup enables measurement of top of line corrosion on large surfaces with low water condensation rates. An experiment with high CO_{2 INTRODUCTION: Top of line corrosion can occur when water condenses in the upper part of wet gas pipelines. The condensing water will have a low pH and high corrosivity, but becomes rapidly saturated with corrosion products, leading to increased pH and possible formation of protective corrosion product films. The water chemistry in the thin film of condensed water in the top of the pipeline can be very different from the bulk water phase in the bottom of the line. Laboratory studies of top of line corrosion were performed already around 1990^{1, 2}Water condensing in the top of a wet gas pipeline will form small droplets or a thin film on the steel surface. The condensed water can become rapidly supersaturated with corrosion products, resulting in increased pH and iron carbonate film formation. The top of line corrosion rate then becomes dependent on the water condensation rate and the amount of iron which can be dissolved in the condensing water ^{1, 5}For sweet systems the top of line corrosion (TLC) rate can be estimated from the water condensation rate and the concentration of iron in the condensed water. The condensing water will have a low pH and high corrosivity. However, when the water condensation rate is low. . Several top of line corrosion models have been developed based on this dependence of water condensation rate and iron carbonate supersaturation4, 5, 6. The presence of acetic acid in the gas may increase the top of line corrosion rate, as it increases the amount of iron which can be dissolved in the condensing water. Top of the line corrosion models have also been developed from a more detailed mechanistic viewpoint7, 8. , and several cases of top of line corrosion in gas pipelines were reported a few years later3, 4. Common factors for these cases were excessive cooling of the gas, high water condensation rates and presence of organic acid in the gas, with 300 to 2000 ppm acetic acid in the produced water. The acetic acid is transported in the gas phase and will condense together with the water and increase the solubility of iron and the top of line corrosion.} partial pressure and traces of H₂S in the gas showed that most of the iron dissolved by corrosion precipitated as a porous iron sulfide film which did not offer good corrosion protection. Thin, protective iron carbonate films were formed close to the steel surface. This showed that the presence of even small amounts of H₂S can change the top of line corrosion mechanism considerably compared to pure sweet condition.