Effect of Test Solution Compositions on Corrosion Resistance of 13Cr Materials in a Little Amount of H₂S Environment

The effect of the buffer solution composition on the corrosion resistance of super 13Cr martensitic stainless steels is studied. It is widely recognized that the SSC susceptibility of 13Cr stainless steels depends on the H₂S partial pressure and pH in the solution. To control the pH value, buffered solutions are defined in NACE TM0177-96 (solution B) and EFC publications to simulate the actual field conditions in the laboratory tests under normal pressure. However, with some modifications of the buffer agents, there were some cases that the discoloring of the specimen was observed and the pitting occurred with the larger corrosion rate in some test conditions. Therefore, the corrosion resistance of super 13Cr stainless steels are considered to be influenced by the composition of the buffer solution. That is, the corrosion resistance of the steel depend on the concentration of CH₃COONa/CH₃COOH included in the test solution as the buffer agent, even in the same pH. The super 13Cr steel was not passivated in the buffer solution including 0.4 wt% CH₃COONa adjusted pH 3.5 with CH₃COOH recommended in NACE TM0177. Cr was detected in the solution, and it is considered that the CH₃COO^- ion enhanced the dissolution of the steel since it can form the complex ion with Cr and therefore it prevents the surface of the steels from passivation. It is suggested that the buffer solution with 0.04 wt% CH₃COONa with CH₃COOH is appropriate solution for evaluating the corrosion resistance of super 13Cr steels because it has enough buffer power during the corrosion test, and has no effect on forming of the passivation film.