Observations and Thoughts on Stress Corrosion Mechanisms ASTM 1970 Gillett Memorial Lecture

Since the phenomena of stress corrosion cracking (of austenitic stainless steels) were first reported about 30 years ago, there has been a snowballing of costly failures in stainless steel equipment as well as in newer alloy systems, for example, titanium alloys, which were originally also thought to be immune to stress corrosion fracture. This has resulted in worldwide research on all aspects of stress corrosion phenomena. The incidence of failures can be controlled by judicious design, construction, operation, and maintenance of equipment, but complete understanding of stress corrosion cracking mechanisms remains to be achieved. It is believed, however, that from the cases of stress corrosion failures in aqueous chloride systems enough phenomenological information has been developed that a reasonably complete theoretical picture can be assembled. The main features of the suggested mechanism of corrosion cracking involve anodic dissolution, whereby nickel or another "noble” metal species is locally enriched, becoming sufficiently cathodic for hydrogen evolution to occur and embrittle the de-alloyed structure. The role of tensile stress in the system is to provide continuity and directionality to the local dissolution surface enrichment, hydrogen embrittlement (and mechanical rupture) process. Most all phenomenological features observed in alloys failed in chloride stress corrosion cracking systems can be worked into this model. The conclusion is made on the basis of a growing body of data being collected in many laboratories that the role of hydrogen as an embrittling agent may provide a possible unifying mechanism in stress corrosion and must be carefully reconsidered.