Impact of Hydrogen on SCC of Ni Alloys in Low Hydrogen Environments

Abstract

The growth rate of stress corrosion cracking (SCC) in Ni-based alloys is a function of electrochemical potential (EcP), specifically, the difference in specimen EcP versus the EcP of the Ni/NiO phase boundary (EcP_specimen – EcP_Ni/NiO = ΔEcP). At low temperatures (400 – 500°F or 200 - 260°C), the Ni/NiO phase boundary in pressurized systems can move to dilute levels of dissolved hydrogen (< 5 cc H₂/kg H₂O [cc/kg]). Testing was performed to improve understanding of EcP at dilute levels of hydrogen, and to improve empirical modeling of the SCC growth rate as a function of hydrogen or EcP by including a wide range of hydrogen levels. SCC growth rate testing was performed over a wide range of hydrogen concentrations; Alloy 600 HAZ was tested at 640 and 680°F(340 and 360°C). An Alloy 600 HAZ SCC growth rate model is presented, wherein the peak growth rate as a function of EcP was found to occur at a slightly positive ΔEcP (i.e., 10 mV).