Newly designed surface control using Si addition in trace quantity for Zn-2Al-3Mg alloy coated steel sheet with improved corrosion resistance

The addition of Si even in trace quantity (0.04 wt%) refined the spangle structure in the outermost part of a Zn-2Al-3Mg coating by precipitating Mg₂Si and Zn-Al intermetallic phases on the ternary eutectic areas. In contrast to the conventional interface control of the Zn-Al-Mg-Si coating, this surface-controlled technique is a break-through in that it suppresses localized corrosion in the ternary eutectic areas of the outer surface by making minimal use of Si. Moreover, the surface coverage by corrosion products with a bi-layer structure and inhibiting property, following the rapid sacrificial dissolution of intermetallic phases, provided long-term corrosion resistance of steel.