Corrosion of Galvanized Steel with Zn-Mg Alloy Coatings in Aqueous Solutions

Abstract

The galvanized steel with zinc-magnesium (Zn-Mg) alloy coating has a superior corrosion resistance in wet-dry environment containing chloride. However, the galvanized steel with Zn-Mg alloy coating processed by physical vapor deposition shows surface color degradation. When the galvanized steel with Zn-Mg alloy coating is exposed to a humid environment at high temperature, the surface of the Zn-Mg alloy coated steel easily turns to dark within a short period of time. It is suspected that this phenomenon is closely related to the formation of magnesium based corrosion product (dark rust) on the outermost surface. Since it can lead to deterioration in the coating quality, it is very critical problem for the galvanized steel with Zn-Mg alloy coating. Therefore, this research is focused on understanding of the nature of dark rust formed on the coating surface and developing the prevention methods against the formation of dark rust.Contrary to the hot dip galvanized steel (GI) and electrogalvanized steel (EG), the galvanized steel with Zn-Mg alloy coating and pure magnesium coated steel shows a surface darkening phenomenon after they are exposed to the humid environment at high temperature. It has been revealed that this is due to the anodic dissolution of magnesium from the coating layer and precipitation of needle like magnesium-based corrosion product, Mg(OH)2, on the outermost surface after the magnesium ions exceed the solubility limit in aqueous solutions. Accelerated anodic dissolution of magnesium from the galvanized steel with Zn-Mg alloy coating was caused by the galvanic corrosion between zinc and magnesium.Since the suspected nature of the dark rust was magnesium based corrosion product, Mg(OH)2, a carbonate treatment using the 0.1M NaHCO3 solution and a zirconium plating method were employed to form a protective layer which can cover up the magnesium on the top surface. Zinc based thin film, hydrozincite, was formed after the carbonate treatment and it was revealed that the film can act as a protective barrier for the formation of dark rust even though it was exposed to DI water at high temperature of 100oC for 20 minutes. In addition to this treatment, zirconium plating also showed a good property in terms of surface color degradation phenomenon. Although the galvanized steel with Zn-Mg alloy coating was not densely covered by the zirconium particles by the existing oxide on the coating surface, it seems that there is a clear effect on the surface darkening problem. Lastly, stannate conversion coating treatment was evaluated for the suppression of the dark rust formation. From the result of surface film analysis with the aid of FE-SEM, EDS and GDS, magnesium stannate MgSnO3 was formed on the outermost surface. It was found that it inhibits the dissolution of magnesium from the film and therefore it affects the suppression of the dark rust formation. Although some protection method for the dark rust was proposed in this research, there is need to develop more effective and efficient ways for overcoming the surface darkening problem of the galvanized steel with Zn-Mg alloy coating.