Early stages of crack development in environmentally assisted cracking

The development of damage due to environmentally assisted cracking is commonly described by the broad stages: precursor development; transition to a crack; small crack growth; long crack growth. There still remains a degree of ignorance with respect to the role of the surface preparation method in the earliest stages of damage development; in particular, the effect of machining and grinding on the formation of surface defects, residual stress, and nanocrystalline and heavily deformed surface layers. To increase awareness of the potential impact of the surface state, examples will be illustrated briefly in relation to pitting of 316L and formation of dealloyed layers on duplex stainless steel.

Corrosion pits are the most commonly observed precursor to cracks in aqueous chloride environments, and the transition from pit to crack perhaps the most studied. A perspective on the loci of sites of crack initiation and the criteria for the pit-to-crack transition will be presented and the implications for the optimum methodology for quantifying the early stages of crack growth discussed. The subsequent application will be highlighted using examples from NPL research on stress corrosion cracking and low frequency corrosion fatigue of 12 Cr steam turbine blade steel. A key feature to emerge from the research is the observation of remarkedly enhanced crack growth rates for small and short cracks, which is rationalised on the basis of the solution conductivity dependent electrochemical crack size effect.