Galvanized steels are widely used in various industrial fields such as automobiles and construction since they have excellent corrosion protection in atmospheric corrosion environments.  The corrosion protection property of Zn coating has been improved by addition of another elements such as Mg and Al.  However, the effect of Mg on corrosion protection of the galvanized coating is still unclear.  In this study, we investigated polarization behavior of iron under artificial zinc corrosion products formed in an aqueous solution containing Mg²⁺.

A pure iron bar (purity: 99.9%) was annealed at 850 ^oC for 2 h and sliced into disks of 12 mm in diameter and several millimeter in thickness.  The surface of the iron disk was abraded with SiC papers up to JIS #2000 grit and then the disk was etched chemically to get a mirror-like surface finish.  To form artificial zinc corrosion products on an iron disk, it was polarized cathodically at various potential for 1.8 ks in an aqueous solution of 1 M MgCl₂ containing ZnCl₂.  The zinc corrosion products formed on the iron disk was investigated by SEM and other analytical instruments.  Polarization behavior of iron covered with the artificial zinc corrosion products was investigated by the measurements of cathodic and anodic polarization curves in 0.1 M NaCl.

Artificial zinc corrosion products were successfully formed on an iron disk during cathodic polarization in 1 M MgCl₂ containing ZnCl₂.  It was found from surface analysis that the amount of the artificial zinc corrosion products formed on the disk changes depending on polarization potential.  At around -0.85 V (SSE), the whole surface of the iron disk was covered homogeneously with zinc corrosion products.  Cathodic polarization curves for iron covered with artificial zinc corrosion products showed that oxygen reduction reaction (ORR) is slightly inhibited in comparison with ORR on bare iron.  On the other hand, anodic polarization curves for iron covered with artificial zinc corrosion products showed that anodic dissolution of iron under the zinc corrosion products also tends to be suppressed.  The effect of Mg on polarization behavior of iron covered with artificial zinc corrosion products is going to be discussed in detail.