Effects of hydrostatic pressure and Cr content on the stress corrosion cracking of 42CrMo steels in simulated marine environment

The corrosion and stress corrosion cracking (SCC) behavior of 42CrMo steels containing 0.6–1.5 wt% Cr was investigated in simulated seawater under hydrostatic pressures of 0.1–35 MPa. The SCC response depended strongly on Cr content: the 0.6Cr and 0.9Cr steels showed a threshold-like rise in susceptibility mainly between 15 and 25 MPa, whereas the 1.2Cr and 1.5Cr steels remained highly susceptible throughout the pressure range. Fractography indicated that the hydrogen embrittlement (HE) tendency of the low-Cr steels increased with pressure, while the high-Cr steels exhibited extensive intergranular fracture and localized cracking even at low pressure. To explain this contrast, a working hypothesis is proposed in which pronounced HE develops as retained hydrogen approaches a critical level. The greater density of reversible traps associated with finer and denser Cr-rich carbides may raise the retained-hydrogen baseline, allowing the high-Cr steels to approach this level even at low pressure, whereas the low-Cr steels appear to require higher pressure. Immersion and electrochemical measurements further showed that pressure reduced corrosion product retention and polarization resistance, consistent with intensified anodic dissolution (AD). At high pressure, dispersed local attack in the low-Cr steels gradually coalesced, while the high-Cr steels showed extensive corrosion product loss and substrate exposure. Such AD-related damage may promote crack initiation and local hydrogen generation, thereby strengthening the coupling between AD and HE-assisted cracking. These findings indicate that increasing Cr is not unconditionally beneficial under hydrostatic pressure.