1993 Volume 42 Issue 12 Pages 777-781
Fluctuation of the corrosion potential during immersion of solution treated or sensitized SUS 304 steels in 0.05M H2SO4 was analyzed by FFT method to estimate the extents of sensitization non-perturbatively. Differences were readily noted, between the solution treated and sensitized specimens, in the power spectral density (PSD) vs. frequency curves at frequencies lower than about 0.3Hz. The PSDs below 0.3Hz increased markedly with increase in the sensitization time. At a frequency of 0.05Hz, for instance, the PSD was 4×10-12V2·Hz-1 for the solution treated SUS 304. While, it increased to 4×10-11 and 1×10-10V2·Hz-1 after sensitization for 1 and 10h, respectively. An increase in the slope of the PSD vs. frequency curve was also noted with increase in the sensitization time. Thus, it appears that the FFT analysis of the fluctuation of the corrosion potential in H2SO4 can be utilized as a non-perturbative, diagnostic tool for the determination of the extent of sensitization. Additionally, detailed observations of the waveforms of potential fluctuation and optical microscopy of the corresponding specimens have shown that the fluctuation is mainly due to the local anodic dissolution of iron component of the steel and subsequent formation of chromium oxide and/or hydroxide at these local dissolution sites.