Influence of Alloy Content on Microstructure and Corrosion Resistance of Zn-based Alloy Coated Steel Product

The effects of alloy composition on the coating structure and corrosion resistance of hot-dip Zn-based alloy coated steel products were investigated. Zn-based alloy coating layers with different Al and Mg compositions were fabricated using a batch-type galvanizing simulator. Various intermetallic compounds including Zn, Zn/MgZn2 binary eutectic, Zn/Al binary eutectoid and Zn/Al/MgZn2 ternary eutectic phases were formed in the coating layer. The surface and cut-edge corrosion resistance of the Zn-based alloy coating were superior to those of the Zn coating. Zn-based alloy coating containing 15% Al and 3% Mg showed the best corrosion resistance, with red rust formed on the flat surface after 120 hours in the salt spray test. The corrosion products of the Zn-based alloy coating consisted of Simonkolleite (Zn-5(OH)(8)Cl-2 center dot H2O), Hydrozincite (Zn-5(CO3)(2)(OH) and zinc oxide (ZnO). Al-containing corrosion products, Zn2Al(OH)(6)Cl-2 center dot H2O and Al2O3, were formed when more than 5 wt% Al was added. Al-containing corrosion products improved the corrosion resistance of the flat surface of Zn-based alloy coating, but did not affect corrosion resistance in the cut-edge area.