Direct Observations of Stress Corrosion Crack Tips of Type 304 Stainless Steel in a Boiling 42% MgCl₂ Solution Available to Purchase

Using a high voltage electron microscope, direct observations of stress corrosion crack tips in Type 304 sheet specimens of 0.1 mm thickness were carried out to clarify the propagation mechanism for stress corrosion cracking. The propagation behavior of stress corrosion cracks in these sheet specimens were shown to be the same as those in a normal 'bulk' material. It has been found that transgranular stress corrosion cracks propagate along the slip planes, forming a small scale of plastic zone resulting from a stress concentration due to the crack. At the stress corrosion crack tips, the slip planes containing moving dislocations were preferentially attacked to form a crack, but there was no evidence of martensite transformation or tunneling corrosion. Direct observations of crack tips revealed that the crack proceeded along {111} slip planes, while fracture planes were {100} or {110} rather than {111} planes by the etch pit method. Appearance of these low indexed planes in the etch pit method was attributed to the fact that this method could not identify such fine planes as obtained under transmission electron microscope but revealed macroscopic planes composed of various {111} planes.