Relationship Between Hydrogen Pickup and Susceptible Paths in Stress Corrosion Cracking of Type 304 Stainless Steel

Abstract

The susceptible paths for stress corrosion cracking in Type 304 stainless steels were identified by electron microscopic examination of thin sections before and after exposure to boiling 42 percent MgCl2 These paths were found to be generated by the corrosion reaction, particuarly the cathodic part of this reaction. Type 304 stainless steel was shown to react with hydrogen to form hydride phases which were attacked rapidly by the corrosion environment and thus would not be detected in the steel after stress corrosion cracking failure. Differences in susceptibility of Type 304 stainless steel to stress corrosion cracking were attributed to the solubility of hydrogen in the steel and the extent of diffusion under applied stress. In resistant steels the hydride phases precipitate rapidly and prevent diffusion of hydrogen to high-tensile-stress regions, while in susceptible steels the precipitation does not occur immediately. It was shown that hydrogen, when in solid solution, diffuses through the austenite lattice under an applied load and forms a stress-oriented transgranular precipitate phase distribution similar to the distribution of the cracks in stress corrosion cracked specimens.