Susceptibility of low-temperature plasma nitrided 17-4 PH (H1150D) to sulphide stress cracking (SSC) in typical oilfield environment

To attend the growing global demand for energy, oil production—expected to grow in the foreseeable future-relies on the extraction from unconventional resources, such as deep deposits off the coast and ultra-deep water. These environments are contaminated with significant amounts of hydrogen sulphide, a by–product that promotes hydrogen absorp- tion and subsequent failure by sulphide stress cracking (SSC). The 17-4 PH—a stainless steel widely used for oilfield components—is susceptible to SSC, in spite of its favourable combination of properties. In this study, plasma–based treatments, often used to improve wear resistance, were identified as potential methods to increase the resistance to SSC. To evaluate it, the 17-4 PH was surface–modified by conventional and low–temperature plasma nitriding (LTPN) and submitted to standard SSC experiments. LTPN resulted in superior resistance to SSC compared to both unmodified and HTPN conditions, at- tributed to the formation of a nitrogen–rich layer comprising mixed iron nitrides (e–Fe2-3N and γ’–Fe4N) and expanded martensite without precipitation of chromium nitrides. The protection provided by this structure was mainly due to the compressive residual stress induced by nitriding, combined with the superior resistance to localise corrosion and the reduced hydrogen uptake by the substrate.