Stress corrosion cracking susceptibility of nitrogen alloyed stainless steels in 50% CaCl2 solution

Stress corrosion cracking (SCC) susceptibility of nitrogen alloyed Type AISI 301 stainless steels in 50% CaCl₂ solution at 373 K (100 °C) under open-circuit potential and cathodic polarization conditions was studied using a constant load test method. The electrical properties of the surface films formed were monitored as a function of time and potential by means of the Contact Electric Resistance (CER) method. Nitrogen alloying was found to enhance clearly the passivation. Based on creep measurements in a non-corrosive environment and studied solution, the influence of environment on the steady state strain rate (ε_ss) obtained during the test showed more than fivefold increase in the creep rate, which is a clear indication of the role of creep in SCC mechanism. This environmentally enhanced creep was found in the presence of high resistance passive film on the surface. A new model is used to explain and describe the observed phenomena of SCC. The rate determining step in cracking is the generation of vacancies by selective dissolution. The effect of electrochemical polarization on creep is due to a change in the vacancy flux through the oxide film to the metal. Cathodic polarization resulted in decreasing strain rate by reducing the corrosion rate and suppressing the generation of vacancies. Vacancies generated by corrosion are first consumed by dislocations which enhances creep and later strain localization, but finally, agglomeration of vacancies leads under applied stress to cleavage-like SCC.