Zn-Al-Mg Coatings: Thermodynamic Analysis and Microstructure Related Properties

Demands for highly corrosion resistant coated steel are growing. As a result, Zn-Al-Mg coatings were developed. The possibilities of these coatings were investigated and the thermodynamics of the Zn-rich corner of the Zn-Al-Mg system were modelled. Different Zn-Al-Mgcoatings were produced and the microstructure was studied. Simulations of the solidification microstructures were carried out. The properties of the different coatings, like corrosion resistance and formability, were investigated. The thermodynamic model fairly accurately predicted the liquidus and transformation temperatures for low amounts of Al (≤4wt%) and Mg (≤3wt%). In the coatings the MgZn2 phase was present instead of the thermodynamically stable Mg2Zn11. The coatings with 3wt%Mg consisted of primary Al-fcc, MgZn2 crystals and ternary Zn-hcp/Alfcc/ MgZn2 eutectic. The addition of small amounts of Mg to a galvanizing bath caused a Znhcp/ MgZn2 eutectic to grow at the grain boundaries. Mg additions to a Zn+5wt%Al bath resulted in coarsening of the Zn-hcp/Al-fcc eutectic when added in small amounts and, when added in larger amounts (>0.2wt%Mg), a ternary Zn-hcp/Al-fcc/MgZn2 eutectic appeared. Cyclic corrosion tests and bending tests showed that the addition of Mg greatly enhanced the corrosion resistance, but decreased the cracking resistance of the coatings.