Martensite caused by passive film-induced stress during stress corrosion cracking in type 304 stainless steel

After a pre-crept specimen of type 304 stainless steel underwent stress corrosion cracking (SCC) at constant load in a 42% MgCl2 solution for 2 h, α′ martensite on the surface and over the entire specimen increased in volume fraction by 14 and 1.5%, respectively. If the specimen pre-crept in the silicon oil was immersed in the 42% MgCl2 solution at a cathodic potential of −600 mVSCE, in which no SCC occurs, the α′ martensite volume fraction increased by only 0.6%. The critical hydrogen concentration for the formation of hydrogen-induced α′ martensite was 50.9 wppm, which was larger than that which entered into the sample during SCC. The difference between the flow stress of the 304 steel before unloading and the yield stress of the same specimen extended in air after unloading and immersed in the 42% MgCl2 solution to form a passive film is defined as a passive film-induced stress. The passive film forming at the open-circuit potential generated a large tensile stress, and that at a cathodic potential of −600 mVSCE generated a small compressive stress. Therefore, the observed martensite transformation during SCC can be attributed to a passive film-induced tensile stress.