Environmental Cracking Susceptibility of AISI 422 Stainless Steel in NH₄HCO₃/NH₄OH Solution at 70°C

The environmentally assisted cracking (EAC) susceptibility of AISI 422 (UNS S42200) stainless steel (SS) turbine blades was examined in an aerated NH₄HCO₃/NH₄OH solution at 70°C. Exposure in this solution is intended to simulate a cleaning procedure used to remove deposited copper from turbine blades in commercial power plants. EAC susceptibility is a strong function of the electrochemical potential; hence, it is necessary to compare the potential of contaminated blades to potential ranges such as the active-passive transition, which might promote EAC susceptibility. The ranking of open-circuit or mixed potentials experienced by the various materials/electrolyte combinations relative to the active-passive transition was polished SS/aerated > copper+SS/aerated > copper/aerated = SS active to passive transition > polished SS/deaerated. Constant extension rate tests were conducted by potentiostatically polarizing AISI 422 specimens to these potentials. For the displacement rate chosen, both tensile test parameters as well as scanning electron fractography revealed little differences between the environmental results and those obtained in air, except at the potential associated with the active-passive transition of the SS. However, this potential is 0.3 V more negative than the potential of the Cu-contaminated AISI 422 SS turbine blade. Hence, the chemical cleaning procedure does not promote EAC of AISI 422 SS unless the mixed potential experienced by the Cu-contaminated blades becomes more negative than expected.