Corrosion resistance of new 12% Cr steels for steam turbine applications

摘要

Resistance to stress corrosion cracking and pitting corrosion has been evaluated for martensitic steels containing 10-12% chromium. The materials investigated include conventional and powder metallurgically produced 12 Cr-1 Mo-0.3 V steels, also PM steels with a reduced Cr content (10.6%) in combination with additions of 1% W or 1% W + 0.002% B. Slow strain rate testing in 6% NaCl + 0.5% acetic acid at ambient temperature was used to evaluate the resistance to stress corrosion cracking. Stress corrosion cracking occurred in all the alloys investigated but no significant effect of alloy composition or production route was observed for tempering to yield strengths in the range 740-820 MPa. An increase in the yield strength to 850 MPa resulted in a marginally significant decrease in stress corrosion cracking resistance while a yield strength of 1130 MPa was associated with severe cracking. Pitting corrosion was investigated by measuring pitting potentials in 0.05% and 22% NaCl. No significant difference between the conventionally produced and PM 12Cr-1 Mo-0.3 V steels, or the PM alloys with additions of W or W + B was noted. Both pitting corrosion and stress corrosion cracking exhibited a slightly more intergranular character for the steels with W or W + B additions, this could be related to the more predominant carbides on the prior austenite grain boundaries for these materials. 15 refs, 27 figs, 11 tabs, 3 appendices