Effects of dissolution on plastic deformation and cracking of metals

Abstract

The effects of surface dissolution on plastic deformation of metals, and the operating mechanisms, depend on many variables: stress; frequency of stress fluctuations; strain rate; temperature; metallographic constituents and their distribution; surface films; adsorbed species; surface energy; electrochemical potential and current; dissolution morphology; crack tip geometry; dislocation mobility; and the chemistry at the crack tip in the environment as well as in the metal. The manifestation of these variables in different relative proportions in different systems causes a variety of metal/environment interactions, the most extreme of which are stress-corrosion cracking and corrosion fatigue. During simultaneous action of stress and dissolution, stress-corrosion cracking occurs only with specific metal/environment combinations within critical electrochemical-potential ranges, whereas corrosion fatigue develops at dissolution rates greater than a critical value. In addition to these forms of degradation, the dissolution process may inject defects — such as vacancies, divacancies, or dislocations — into the metal lattice, with resulting alteration in dislocation behaviour and plastic-deformation characteristics. Dissolution may also influence the films that normally exist on metal surfaces and, consequently, the dislocation arrangements which depend on these films. This review is a systematic presentation and discussion of those critical — and sometimes controversial — experiments that elucidate the effects at an atomic level of surface dissolution on plastic deformation of metals.