2018 Volume 67 Issue 6 Pages 256-260
Steel plate painted by inorganic zinc primer has high performance of corrosion protection by zinc particles. However, details of corrosion mechanism are not clear. Numerical analysis model is valid to make clear that of initial corrosion mechanism. We have been developed a numerical analysis model of corrosion phenomena, which can calculate ions movement and chemical reactions. From calculation result, zinc particles were corroded at first by rich dissolved oxygen and corrosion products such as MgCO3 and CaCO3 were precipitated around the zinc particles at the top of the primer. The calculated distributions of MgCO3 and CaCO3 were agreed with the cross section EPMA analysis results of exposed specimen, but the calculated distribution of zinc corrosion products were different from the cross section EPMA analysis results of exposed specimen. After several studies, it was cleared that cathodic current density of the painted steel after exposure test was decreased to 1/8 from that of before exposure. Numerical reanalysis was conducted with 1/8 decreased cathodic current density of the zinc particle, and then corrosion products of zinc were agreed with the EPMA cross section. It indicates that the corrosion products around the zinc particles disturbed oxygen diffusion and its cathodic current density was decreased.