Electrochemical short crack effect in environmentally assisted cracking of a steam turbine blade steel

Measurement has been made of the corrosion fatigue short crack growth rate in a 12Cr steam turbine blade steel subjected to low frequency trapezoidal loading in aerated and deaerated 300 ppb1 Cl− and 300 ppb SO42-, simulating early condensate chemistry. No difference in growth rate compared to that for long cracks was observed in deaerated solution but significantly enhanced growth rate was obtained in aerated solution for a short crack of length less than 250 μm. Complementary stress corrosion cracking tests were conducted but to ensure crack development at modest applied stresses the environment adopted was aerated 35 ppm Cl−, representing a severe system upset. In this case, the growth rate of the short crack was up to 20 times higher than that for a long crack (>6 mm), even though the crack length had reached 1.6 mm. An explanation for both sets of data based on the difference in potential drop between a short and long crack is expounded.

► Novel measurements made of short crack growth rate in steam turbine blade steel.
► Stress corrosion short crack growth rate 20 times greater than long crack.
► Corrosion fatigue short crack rate higher than long crack in aerated solution.
► No difference in CF crack growth rate in deaerated solution.
► Difference in crack-tip polarisation for short and long crack is key factor.