Stress Corrosion Cracking of Low Strength, Low Alloy Nickel Steels in Sulfide Environments

Factors governing stress corrosion cracking (SCC) of low strength, low alloy nickel steels (e.g., ASTM A203 Grade E) in sulfide environments were investigated using a three point loaded bent beam specimen. Cracking occurred in both weld and base metal at low H₂S partial pressures (to 0.001 atmosphere) and stress levels, but could be inhibited by a neutralizing amine (morpholine) and methanol addition as used for control of ice and hydrate formation in cryogenic gas plants. Cathodic polarization experiments showed cracking was retarded under these conditions. This indicates the cracking phenomenon involved is not that of hydrogen embrittlement (i.e., sulfide stress cracking) as has been heretofore assumed, but involves localized anodic attack as does the cracking of other low strength alloys in environments such as nitrate, caustic, CO/CO₂, etc. Sulfide corrosion stress cracking (SCSC) is suggested as a term for this type of localized anodic cracking attack in sulfide environments.