Analysis of the initial stage of localized corrosion on Zn and Zn alloy coated steels by photon rupture method

摘要

A photon rupture method, film removal by a focused pulse of pulsed Nd-YAG laser beam irradiation, has been developed as it enables oxide film stripping at extremely high rates without contamination from the film removal tools. In the present study, Zn and Zn-5 mass percent Al alloy coated steel specimens covered with protective nitrocellulose film were irradiated with a focused pulse of a pulsed Nd-YAG laser beam at a constant potential in 0.5 kmol m~(-3) H_3BO_3-0.05 kmol m~(-3) Na_2B_4O_7 (pH=7.4) with/without 0.05 kmol m~(-3) of chloride ions to investigate the initial stage of localized corrosion. At low potentials, both samples reformed oxide film after the nitrocellulose films were removed by this method. The oxide film formation kinetics of Zn-55 mass percent Al follow an inverse logarithmic law, in agreement with Cabrera-Mott theory. However, at high potentials, localized corrosion producing corrosion products occured at the area where nitrocellulose film was removed. The dissolution current of the Zn coated steel samples is higher than that of Zn-5 mass percent Al coated samples at the same applied potential.