This research work presents a study of the combined influence of the partial pressure of H2S 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 70ºC with 2
bars of CO2. The flow regime was stratified for all of the experiments. It was found that trace amounts
of H2S (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 H2S as the corrosion increased when more H2S 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
H2S/CO2 mixtures. Once again the lowest corrosivity is found at the lowest partial pressure of H2S.