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 air and in a simulated condensate solution under conditions representing repetitive turbine start-up and shut-down (known as two-shifting operation). Cracks were grown from a single corrosion pit generated galvanostatically using the droplet technique and the crack extension monitored optically and by direct current potential drop. The remarkably enhanced crack growth rate observed previously for a short crack (relative to that for a long crack) was not evident for the small crack. In contrast, the stress corrosion small crack growth rate determined in aerated 35 ppm Cl-, representing a system excursion, was markedly increased with respect to the long crack growth and somewhat greater than that for the short crack. 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.