Passivity of Martensite-Based Stainless Steel Oil Country Tubular Goods in Sour Environment

Abstract

Passivity of the martensite-based stainless steels for oil country tubular goods, 15Cr (15Cr-6Ni-2Mo-1Cu) and 17Cr (17Cr-4Ni-2.5Mo-1Cu-1W), in an H2S environment were investigated. The depassivation pH of the 15Cr was around 3.2 to 3.5, and that of the 17Cr was around 3.0 in a 10% H2S environment. The difference in the depassivation pH between the 15Cr and the 17Cr corresponded to the difference in the applicable pH limits of the 15Cr and the 17Cr for sulfide stress cracking (SSC). It was suggested that depassivation is one of the dominant factors that determine the applicable limit for SSC. Analyses of the passivation film were also conducted to investigate the mechanism of the depassivation. Direct observation and analysis of the passivation film were successfully performed with a high-resolution transmission electron microscope. It was revealed that sulfur, which had originated from H2S in the environment, had penetrated into the passivation film. This led to the assumption that the penetration of sulf...