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The purpose of this study is to clarify the effect of dry and wet processes on the mechanisms of edge corrosion of hot dip 55 mass% Al–Zn alloy coated steel sheet. The corrosion tests employed were continuous salt-spray tests and cyclic corrosion tests using NaCl solution or artificial seawater. It was confirmed that the edge corrosion is suppressed during the salt-spray test using artificial seawater. Furthermore, the suppression was not prevented by dry and wet processes. In order to reveal the corrosion mechanisms, the galvanic current and the cathodic polarization curves at exposed steel were measured and the corrosion products were characterized. It is suggested that the artificial seawater forms anti-corrosive corrosion products and that suppress the cathodic reaction on the steel which is exposed on the shear cut edge. The corrosion products are not changed by dry and wet processes. The numerical analysis also revealed the influence of the dry and wet process on edge corrosion.

Numerical analysis models for galvanic corrosion has been developed in recent years, but most of the ordinary models only can calculate current density and potential distribution in an electrolyte solution. In order to make clear corrosion mechanism, more information such as pH, ion and corrosion product distributions are necessary. However, it is difficult to compute these phenomena by mathematical model because various kinds of ions and complex reactions exist in corrosion progress. Calculating ion movement with satisfying charge conservation is especially difficult.We have developed a new numerical analysis model for galvanic corrosion that can calculate ions movement and reactions. In this model, ion density distribution is corrected by solving Poisson's equation to satisfy charge conservation. Equilibrium reactions are computed after calculating ion transportation.Galvanic corrosion of a model Zn/steel couple in a NaCl solution was calculated with this model. The difference of ion and corrosion product distribution in 50 ppm and 50000 ppm NaCl solutions was discussed. Corrosion product distribution obtained by this numerical analysis model was well agreed with OH− distribution on a galvanic corrosion specimen measured by FT-IR method qualitatively.

Hot-dip 55%Al–Zn alloy coated steel sheet has superior corrosion resistance both on flat panel and near shear cut edge in various atmospheric environments. However, the corrosion often occurs near the shear cut edge in continuous wet conditions, for example, NaCl solution spray test (SST, JIS Z2371), and this has been explained by galvanic model. In this paper, it is clarified that the corrosion near shear cut edge is suppressed by artificial sea water and its second ingredient, MgCl2.To make clear the corrosion mechanisms on MgCl2 suppression of the corrosion near shear cut edge, the measurement of cathodic polarization curve on the Fe electrode after the corrosion test using a AZ/Fe/AZ galvanic electrode, the estimation of corrosion products by numerical analysis in consideration of the substances migration and the precipitation reactions, and the analysis of corrosion products by FT-IR spectroscopy were conducted. As a result, in the galvanic condition, it is clarified that Mg(OH)2 precipitates on the Fe exposed cut edge. Moreover, the corrosion near shear cut edge is suppressed by the effect of the cathodic reaction suppression of Mg(OH)2.