Influence of Potential on Crack Growth Retardation during Corrosion Fatigue of Mild Steel in Sea Water
Yohnosuke Suzuki, Shinichi Motoda, Shigeo Tsujikawa
The influence of potential on surface fatigue crack growth retardation of mild steel in sea water was investigated at different applied potentials ranged from -770mV to -650mV (SCE) in terms of the rate of crack growth and from the fractographic observation of the fractured surface of specimens. Sinusoidal load at stress ratio of 0.32 was applied with frequencies of 0.17Hz at which crack arrest occurred because of crack tip blunting caused by dissolution and of 0.33Hz at which crack grew steadily under the freely corroding condition in sea water. Crack arrest reoccurred at potentials of -760mV and -750mV (SCE), which were near the free corrosion potential of unloaded specimen in sea water, in this case∼-760 mV (SCE), at both the frequencies, but this tendency for the crack arrest was less at 0.33Hz than at 0.17Hz. When testing at more positive potentials ranged from -740mV to -650mV (SCE), the crack grew steadily and the rate of crack growth was accelerated as compared with that in air at both the frequencies. The acceleration factor, (dl/dN)cf/(dl/dN)air, increased with increasing positive polarization, and was 2.4 for 0.33Hz and 1.9 for 0.17Hz at -720mV (SCE) and more positive potentials at a ΔK of 5.5MN/m3/2.