Premature failure of engineering alloys in service is often associated with exposure to unintended environmental chemistry. High strength carbon steels and stainless steel will fail prematurely if exposed under tensile stress to absorbed atomic hydrogen (hydrogen embrittlement). Similarly, aluminum alloys when exposed to liquid mercury, crack prematurely if tensile stresses are present (liquid metal embrittlement). Silicate glass cracks in presence of water (static fatigue), while plastics fail prematurely in presence of organic solvents. Service environments that contain soluble chlorides may lead to premature transgranular cracking of austenitic stainless steels, but, interestingly, these environments will not cause such failure in ferritic stainless steels. These phenomena are often collectively described as environmentally-assisted cracking (EAC), and have been known for a very long time. Materials engineers are fully able to make materials selection decisions for engineering systems that see service in virtually any environment, despite not knowing with certainty the underlying EAC mechanisms.

 

Multiple schools of thought exist regarding these phenomena. It is not uncommon to find them described as stress corrosion cracking even though it is clear that corrosion is not a prerequisite. Most proposed mechanisms involve either adsorption of embrittling species with lowering of the fracture surface energy, or localized anodic electrochemical processes such as dissolution or film formation. The presentation will assess the current state of EAC, to identify what is known in a mechanistic sense and what remains to be understood in terms of the path forward toward a more complete understanding.