Role of Si Alloying in the Microstructure Evolution and Corrosion Performance of ZnAlMg Coatings

The impact of Si addition on the microstructure and corrosion behavior of ZnAlMg coatings prepared in a laboratory simulator using several thermal application regimes for each coating composition is described. The coatings exposed to a cyclic accelerated corrosion test Volvo ACT II were evaluated in terms of mass loss, kinetics of white and red rust formation, and release of soluble ions. Both Si addition and the thermal regime affected the coating behavior. The best corrosion stability was provided by Zn3.7Al3Mg3Si with the microstructure consisting almost exclusively of very fine binary and ternary eutectics. The electrochemical role of particular phases was studied using a modified ZRA setup. Mg 2 Si was an important electrochemical actor serving as the principal anode in the early stage of the corrosion process. After its capacity as the source of Mg 2+ had been expended, it was replaced by MgZn 2 .