Top of line corrosion loop experiments have been performed at 85 °C with 10 bar CO2 and 0.1 – 1 mbar H2S. The acetic acid content was varied between 360 and 3030 mg/kg acetic acid in the condensed water. Under these conditions corrosion products consisting mainly of iron carbonate were formed on the steel surface where condensation occurred. This is in contrast to previous experiments under similar conditions at 25 °C, where the corrosion products contained mostly iron sulfide due to the slow precipitation of iron carbonate at low temperature.

The top of line corrosion rate increased strongly with the amount of acetic acid in the condensed water. Top of line corrosion is limited by the amount of iron that can dissolve in the condensed water, and acetic acid increases the corrosion rate because it increases the solubility of iron in the condensing water.

The experimental conditions were simulated by calculations using a chemical solubility software package. The calculations showed that the solubility of iron carbonate increases with increasing acetic acid content. The calculated values fitted well with the measured values in the experiments. The calculations also showed that the saturation ratio for iron carbonate is significantly higher than for iron sulfide under these conditions. The small amounts of H2S in these experiments did not have a notable effect on the resulting corrosion rate under these conditions at 85 °C, where iron carbonate is the main corrosion product.

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