POTENTIAL FLUCTUATION DURING EARLY-STAGE OF STRESS-CORROSION CRACKING OF TYPE-304 STAINLESS-STEEL IN CHLORIDE SOLUTION

Potential fluctuation of type-304 stainless steel during stress corrosion cracking (SCC) was measured in 25 wt.% MgCl2 solution. The power spectrum distribution of the observed fluctuation was computed by fast Fourier transform (FFT). During the early stage of SCC (the period when small cracks propagate from a pit), the frequency distribution of the power spectrum had a clear peak at the range from 8 to 10 mHz. The correlation between the number of microcracks and potential fluctuations was examined: both of the numbers showed a linear relation. From this result, it was speculated that the fluctuation was generated by the rapid formation of a bare metal surface and its repassivation with the connection of the microcracks. To confirm this speculation, the change in the power spectrum when the stress increased at an early stage of SCC was investigated, because the propagation rate of an advancing crack can be varied by the intensity of the tensile stress. While a peak at higher frequency appeared with an increase in the tensile stress, the peak at the range from 8 to 10 mHz remained.