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.
TASK 2.5.5 NATURAL EXPOSURE TESTING IN CALIFORNIA
