Corrosion behavior of Zn-Mg-Al alloy coated steel sheets depending on the alloy coating microstructure

The effects of Mg and Al content on the microstructure and corrosion resistance of hot-dip Zn-Mg-Al alloy coated steels were investigated. Pure Zn and Zn-based alloy coatings containing 0 to 5 wt.% of Mg and 0.2 to 15 wt.% of Al compositions were produced by a hot-dip galvanizing. The coating consists of complex microstructures including Zn phase, MgZn₂ rich phases (MgZn₂ intermetallic compounds, Zn/MgZn₂ binary eutectic and Zn/MgZn2/Al ternary eutectic) and Zn-Al intermetallic compounds. MgZn₂ rich phases played the most important role in increasing surface corrosion resistance. The volume fraction of MgZn₂ rich phases (Zn/MgZn₂ binary + Zn/Al/MgZn₂ ternary + MgZn₂ intermetallic compound) rather than total Mg contents in alloy coating is important to determine surface corrosion resistance of Zn-Mg-Al alloy coated steel. Zn-3%Mg-2.5%Al coated steel showed the best cut edge corrosion resistance induced by large volume of MgZn₂ rich phase especially Zn/MgZn₂ binary eutectic phase.