Stress Corrosion Cracking Susceptibility of Nickel-Molybdenum Alloys by Slow Strain Rate and Immersion Testing

Stress corrosion cracking (SCO) characteristics of three commercially available nickel-molybdenum alloys (designated B, B2, and B3 [UNS N10001, N10667, and N10675, respectively]) in a reducing environment were investigated, with the alloys in their mill-annealed (MA) state and after heat treatment (HT) to simulate effects of welding and stress relief in clad vessel fabrication. Slow strain rate tests (SSRT) showed the cracking mode (intergranular [IG] or transgranular [TG]) varied with HT and electrochemical potential of the environment. At anodic potentials (-0.2 V us a silver-silver chloride [Ag-AgCl] electrode), Alloys B and B2 in the MA and HT states exhibited TGSCC, with a morphology that indicated an active path corrosion mechanism. At cathodic potentials (-0.5 V_Ag-AgCl and -0.8 V_Ag-AgCl) with hydrogen (H₂) euolution on the specimen surfaces, all three alloys exhibited IGSCC on fracture surfaces in the HT state but not in the MA state. Alloys B2 and B3 showed greater susceptibility to IGSCC than Alloy B. Plant immersion tests showed generally the same results as SSRT at cathodic potentials, except Alloy B3 was similar to Alloy B in its low susceptibility to IGSCC and less susceptible than Alloy B2. Comparison of the SSRT, plant immersion, acid corrosion (20 wt% hydrochloric acid [HCl] at 423 K for 100 h), and mechanical tensile test results indicated IGSCC susceptibility probably was attributable to phase transformations from α to β and γ during high-temperature HT and possibly to coherence, precipitation, segregation, and other grain-boundary characteristics. Results also indicated the presumable sensitization of the grain boundary by HT had no direct influence on IGSCC.