Hydrogen induced plasticity in stress corrosion cracking of engineering systems

This paper presents a review of some recent experimental results and advances in the modelling and simulation of hydrogen induced damage in stress corrosion cracking (SCC). Experimental results are presented for different material/ solution systems. They outline the localised character of corrosion-deformation interactions that lead to SCC failure and the importance of a critical defect for localised hydrogen entry and damage, particularly in f.c.c. metals. Detailed observations of the fracture micro-crystallograph of 316L austenitic stainless steel single crystals in boiling MgCl2 are presented. They support the successive steps of the corrosion enhanced plasticity model, which is based on a local softening in the SCC crack region and on the repeated formation of a dislocation pile-up at some distance ahead of the crack. Critical experiments that highlight the nature of hydrogen-plasticity interactions in SCC also support this model. A simulation method for hydrogen-plasticity interactions is presented. It reproduces the decrease of the long-range interactions between dislocations in the presence of hydrogen. A general expression is given for such a decrease as a function of temperature, hydrogen concentration and material parameters. It is shown that solid solution hydrogen favours planar slip and the formation of dislocation pile-ups. The equilibrium configuration of a dislocation pile-up ahead of a SCC crack is studied and it is shown that diffusing hydrogen promotes stress concentrations against microstructural obstacles and periodic micro-fracture along the slip planes. Finally, the effect of hydrogen on a dislocation source at a crack tip is investigated. Hydrogen is shown to promote brittle fracture for particular orientations of f.c.c. single crystals. (C) 2001 Elsevier Science Ltd. Ah rights reserved.