Combined Effect of CO₂, H₂S and Acetic Acid on Bottom of the Line Corrosion

This research work presents a study of the combined influence of the partial pressure of H₂ S and the concentration of free acetic acid on the general and localized CO₂ corrosion at the bottom of the line. Experiments were carried out during 21 days in three 4" internal diameter flow loops at 70 °C with 2 bars of CO₂. The flow regime was stratified for all of the experiments. It was found that trace amounts of H₂ S (from 0.004 to 0.13 bars) greatly retards the CO₂ corrosion with general corrosion rates usually 10 to 100 times lower than their pure CO₂ equivalent. However, the most protective conditions were observed at the lowest partial pressure of H₂ S as the corrosion increased when more H₂ S was added. The presence of a mackinawite film on the coupon surface seems to be the origin of this protectiveness. When acetic acid was added to the system (the tests were performed with 1000 ppm of free acetic acid), the general corrosion was multiplied by 2 in CO₂ environment and by 10 to 50 in H₂ S/CO₂ mixtures. Once again the lowest corrosivity is found at the lowest partial pressure of H₂ S.

CO₂ corrosion in the presence of acetic acid has been extensively studied in the literature by many different authors1-4. Corrosion mechanisms are now very well defined and are already incorporated in prediction models^5,6. The influence of acetic acid on the FeCO₃ film characteristics and formation may be one of the last areas where a widely accepted theory is not completed agreed upon^7-10. In summary, the different chemical and electrochemical reactions involved in CO₂ corrosion in presence of acetic acid are described below:

Chemical equation for Water dissociation, Carbon dioxide dissolution, Carbon dioxide hydration (slow step), Carbonic acid dissociation, Bi-carbonate ion dissociation, Acetic acid (HAc) dissociation, Proton reduction, Carbonic acid reduction, Undissociated acetic acid reduction, Iron oxidation, Iron carbonate precipitation (if supersaturated) (available in full paper)

In addition, as more and more field conditions involve the presence of large quantities of H₂ S, the prediction of sour corrosion appears today as one of the most pressing matters in the industry¹¹. The understanding of H₂ S corrosion mechanisms lags clearly behind, even if a lot of effort has already been made in this direction¹². Although H₂ S gas is about three times more soluble than CO₂ gas, the acid created by dissociation of H₂S is about three times weaker than carbonic acid. Hence, the effect of H₂ S gas on decreasing the solution pH is approximately the same as CO₂ gas. The different chemical and electrochemical reactions involved in the H₂ S corrosion are described below and added to the list above: Chemical equation for H₂ S dissolution, H₂ S dissociation, HS^- dissociation, H₂ S reduction, FeS formation by precipitation (available in full paper).

Some very valuable experimental work has been done on the effect of small amounts of H₂ S on the CO₂ corrosion of carbon steel^13-16. It was found that the presence of small amounts of H₂ S will lead to a rapid and significant reduction of the CO₂ corrosion.