Effect of Anions on Initial Stage of Localized Corrosion of Zn-55 mass% Al Alloy Coated Steels by Photon Rupture Method and Anodizing

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Photon rupture with a focused single pulse of pulsed YAG-laser irradiation was used to investigate abrupt destruction and repair of passive oxide films or localized corrosion on anodized Zn-55 mass% Al coated steels. The specimens were irradiated with a focused pulse of a pulsed Nd-YAG laser beam at a constant potential in borate solutions, pH=9.2, with and without Cl-, NO3-, and SO42- ions and the current transients were monitored. The oxide films were reformed in the borate solution with NO3- and SO42- ions after removal of the oxide film. The oxide film formation kinetics follows the inverse logarithmic law, according to Cabrera-Mott theory at the late stage after t=10ms. As the potential becomes nobler, the current increases. These behaviors can be explained by a preferential dissolution of the metal substrate, which is enhanced at nobler potentials and with the anions. In Cl- containing solutions, localized corrosion of the coated layer occurs at high potentials, while film reformation occurs at low potentials.
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