A large amount of effort has been invested trying to understand the mechanisms through which CO2 and H2S act upon metallic surfaces. These efforts have led to different models, correlations or charts.
However, present knowledge on the exact form under which these gases react on metal surfaces is far from complete. A particular aspect in which the amount of information is rather scarce is the effect of small amounts of H2S on an stream containing a large amount of CO2. The aim of this work is to determine the amount of H2S (critical concentration) that causes a maximum in the corrosion rate. Lab tests were done in order to determine the corrosion rate of A-516 steel in the presence of 5 % NaCI, at temperatures ranging from 50 °C to 150 °C and various H2S concentrations up to 40 ppm, under a total pressure of 3.27 MPa of CO2. Results showed that a concentration of H2S in the order of 10 ppm exist for which the corrosion rate reaches a maximum. Furthermore, this concentration was not dependent upon the temperature of the system within the range of study. Keywords: CO2, H2S, carbon steel, pipeline