Molybdenum-containing martensitic stainless steels, particularly grades with 95-110 ksi specified minimum yield strength and nominally 1 to 2 wt% Mo, are some of the most commonly used materials for production tubulars in offshore wells. H2S-related cracking at low or room temperature is often the primary limiting factor in the use of these materials in mildly sour oil and gas production service. Improved understanding of the failure mechanism and the relevant environmental exposures may lead to a more reliable and reproducible test methodology for low-temperature cracking susceptibility along with a significant improvement in the risk assessment for use of these materials.
Low temperature H2S-related cracking in martensitic stainless steels is commonly referred to by the industry as sulfide stress cracking (SSC). This label is questionable since, for molybdenum-containing 13Cr grades at most pH values of interest, the material is passive or exhibits very low general corrosion rates with little to no absorption of atomic hydrogen. Typically, the low-temperature cracking is preceded by localized corrosion with cracks emanating from the bottom of the established pits.
This paper provides an overview of the existing literature with regards to the understanding of cracking in molybdenum-containing 13Cr alloys and provides suggested paths of investigation.
In particular, electrochemical methodologies and statistical analyses focused on the threshold conditions for localized corrosion and the likelihood of localized corrosion progressing to cracking are discussed.
Key words: martensitic stainless steels, molybdenum-containing modified 13Cr, H2S, sour, sulfide stress cracking, stress corrosion cracking, pitting