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

摘要

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 ppb~1 Cl~- and 300 ppb SO_4~(2-), 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 aer ated solution for a short crack of length less than 250 urn. 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.