Stress Corrosion Behavior of Fe-Cr-Ni and Other Alloys in High Temperature Caustic Solutions

The resistance of various commercial alloys to stress corrosion cracking (SCO in 50% NaOH has been evaluated at temperatures in the range 284 to 332 C. Austenitic alloys tested included Type 300 series stainless steels, Monel Alloy 400, Inconel Alloys 600, 625, 690, Incoloy Alloys 800, 804, 825, Hastelloy Alloy C, and Nickel 201. Other materials tested were Croloy 2%Cr-1 Mo steel, ferritic stainless steels Type 446, 26-1S, and EB 26-1, and commercial purity titanium. The effects of alloy composition, heat treatment, and test temperature on the severity of SCC were examined. Increasing the nickel content increased the resistance of SCC in deaerated 50% NaOH. In an aerated 50% NaOH solution, both high chromium and high nickel contents were necessary to increase stress corrosion resistance. In deaerated 50% NaOH, ferritic alloys exhibited high corrosion rates and displayed structure sensitive cracking behavior. Titanium corroded rapidly but did not crack. The greatest overall resistance to SCC in deaerated caustic solutions at elevated temperatures is exhibited by high nickel Ni-Cr-Fe alloys thermally treated to reduce residual stresses. The presence of molybdenum and other alloying elements which improve corrosion resistance in acidic chloride environments do not improve stress corrosion resistance in 50% deaerated NaOH.