Progress towards Understanding Mechanisms of Hydrogen Embrittlement and Stress Corrosion Cracking

Abstract This review describes the present state of knowledge and current controversies concerning mechanisms of hydrogen embrittlement (HE), especially in steels, nickel, and titanium alloys. Mechanisms of stress-corrosion cracking (SCC) in aluminium alloys, magnesium, and other materials where hydrogen effects are involved are also discussed. HE involving the formation of brittle hydrides ahead of crack tips is only briefly described, and the main focus of the review is on mechanisms of HE and SCC when hydrides do not form. In these circumstances, there are three principal mechanisms that have significant experimental and theoretical support, viz. mechanisms based on: (i) hydrogen-enhanced localised plasticity (HELP), (ii) hydrogen-enhanced decohesion (HEDE), and (iii) adsorption-induced dislocation emission (AIDE). It is concluded that combinations of these mechanisms are likely in many cases, with the dominant process depending on the fracture mode. For example, AIDE probably predominates for cleavage-like fractures and dimpled intergranular fractures, HEDE possibly occurs for some brittle intergranular fractures, and HELP contributes particularly to slip-band fractures.