Analysis of Perforation Corrosion of Cold-Rolled and Galvanized Steel Sheets by Extreme-Value Statistics

The perforation corrosion of steel and galvanized steels due to the cyclic corrosion test (CCT) has been analyzed by the extreme-value statistics. The plots of the maximum values of perforation corrosion depth and the cumulative probability gave straight lines and they were fitted to the Gumbel (double exponential probability) distribution.

The decrease of the slope of these lines with CCT time suggested that uniform corrosion occurred in an early stage of CCT but it changed to uneven corrosion with CCT time. The period of the uniform corrosion corresponds to the incubation time, tj, of perforation corrosion and the t_i of the steel was extended proportionately with zinc coating weight.

The perforation corrosion rate of Zn-Ni alloy electroplated steel after the t; was lower than those of other materials. This phenomenon seems to be attributable to formation of a protective corrosion product of ZnCl₃ ·4Zn(OH)₂ and a good adhesion of the product to the steel.