Corrosion Fatigue Crack Growth in High Tensile Strength Steel under Cathodic Overprotection

Abstract

The influence of cathodic overprotection at E=-1.2V vs. SCE (Saturated Calomel Electrode) on the corrosion fatigue crack growth behavior of a high-tensil steel HT80 has been invesitgated at R=0.1-0.8 and f=0.1-0.01Hz in 3.5%NaCl solution. The major acceleration factors dominating corrosion fatigue crack growth behavior under cathodic overprotection have been discussed. At the free corrosion potential (E=E_c), an acidified solution within cracks (pH=2) made the crack wall dissolve and the crack blunting, which causes a decrease in Kop from that in dry air. At E=-1.2V vs. SCE the pH value within cracks was 10, causing an increase in K_op due to the caustic corrosion products (caustic corrosion-products induced wedge-effect). The effect of load ratio on da/dN at E=E_c was negligible in terms of ΔK_eff. At E=-1.2V vs. SCE da/dN at R=0.8 accelerated from those at R<0.8 at high ΔK_eff, which was brought about by static SCC being dominating crack growth behavior. On the other hand, da/dN became greater as the frequency decreased both at E=E_c and E=-1.2V vs. SCE. The major acceleration factor was hydrogen embrittlement at E=E_c, whilst they were hydrogen embrittlement and stress-assisted caustic dissolution at low ΔK_eff, and hydrogen embrittlement at high ΔK_eff at E=-1.2V vs. SCE.