Determination of Corrosion Type by Wavelet-Based Fractal Dimension from Electrochemical Noise

The corrosion process of general corrosion, passivation, intergranular corrosion, pitting corrosion and crack propagation during stress corrosion cracking of 304 austenitic stainless steels were investigated by electrochemical noise technique. The wavelet based fractal dimension D was calculated by using discrete wavelet transform for corrosion-types identification. The results showed that the fractal dimensions of 304 stainless steels under general corrosion and passivation were larger than 2, which implied that corrosion behaviors occurred uniformly over the whole two-dimensional areas, and the D value of general corrosion (D=2.411) was slightly larger than that of passivation (D=2.242). Whereas the fractal dimensions of pitting corrosion and crack propagation were much lower than 2, which indicated that localized corrosion occurred, and the D value of pitting corrosion (D=1.540) was larger than that of crack propagation (D=1.001). Besides, the fractal dimension of intergranular corrosion (D=1.970) was in the vicinity to 2, which illustrated that intergranular corrosion occurred more uniformly over the electrode surface than pitting corrosion, whereas more locally than general corrosion. By correlated with the corrosion process and corroded surface morphology of 304 stainless steels, the fractal dimension D can be selected to distinguish corrosion types and evaluate the localized degree of corrosion.