Environmentally Assisted Small Crack Growth

Corrosion fatigue small crack growth rates have been determined for a 12Cr steam turbine steel in a simulated condensate solution under conditions representing repetitive turbine start-up and shut-down. Cracks were grown from a single corrosion pit generated electrochemically using the droplet technique and the crack length monitored optically and by direct current potential drop (DCPD). A comparison is made with growth rates for short and long cracks determined using fracture mechanics specimens. The enhanced crack growth rate observed for a short crack (relative to that for a long crack) but not realised for the small crack is rationalized on the basis of an electrochemical crack size effect.