Small and short crack growth and the solution-conductivity dependent electrochemical crack size effect

Corrosion fatigue small crack growth rates have been determined for a 12Cr steam turbine steel in aerated 300 ppb Cl− and 300 ppb SO42− under simulated two-shifting conditions. The remarkably enhanced crack growth rate observed previously for a short crack (relative to that for a long crack) was not evident for small cracks initiated from pits. In contrast, the stress corrosion small crack growth rate determined in aerated 35 ppm Cl− was markedly increased with respect to long crack growth. The concept of the solution-conductivity dependent electrochemical crack size effect is introduced to provide a fundamental framework to rationalise the disparate results. The engineering implications for two-shifting operation are discussed.