Initial stage of localized corrosion in artificial pits formed with photon rupture on Zn–5 mass% Al alloy-coated steel

The photon rupture method, by which oxide film and metal are removed by focused pulsed Nd–YAG laser beam irradiation, was applied to form artificial micro-pits in Zn–5 mass% Al alloy-coated steel. The zinc alloy-coated layer was removed by pulsed laser irradiation treatment for about one second in a neutral buffer solution with NaCl. The rest potential transient with the laser treatment was measured. In the early stage of the laser treatment the rest potential of zinc alloy-coated steel changed to the negative direction immediately after every irradiation of a laser pulse and then returned to the previous value. However, after the steel substrate was exposed to the solution, the rest potential moved to the positive direction immediately after every irradiation of a laser pulse and then returned to the previous value. The amplitude and duration of the potential change after the laser irradiation increased with repetition of laser irradiation, related to the pit depth and the exposed area ratio of coated layer/steel substrate. The rest potential fluctuation difference can be explained by galvanic reaction change in the artificial pit formed by laser irradiation on the Zn alloy-coated steel.