12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Novel Si-alloyed coatings provide further improvement of corrosion resistance compared to commercial ZnAlMg, yet the mechanism is still a matter of research. Latest studies focused either on binary electrochemical behavior of phases present in the coating or on accelerated and outdoor tests. To provide the link between the two and help understanding the corrosion mechanism we combined the two approaches. Using selection of phases stable in the ZnAlMgSi at normal conditions (etha, alpha, MgZn2, Mg2Si, Si) and ZRA method in unorthodox setup (indovidual phases as working electrode, all other phases as counter electrode), we were able to directly show evolution of galvanic behavior of each phase over time. This was done both in diluted chloride solution, simulating contamination of fresh coating as well as in solution saturated with ZnCl2, AlCl3 and MgCl2, simulating behavior in thin electrolyte after saturation with corrosion products an chloride. As a reference, data from ZnAlMg system (without Si and Mg2Si) were used. Consumption of phases in terms of sacrificial-anode capacity can be showed directly using this method. The results also show that while Mg2Si and Si act as a significant porization agent, their effect on galvanic current is low. This is due to high inactivity of Si and quick selective dissolution of Mg from Mg2Si. Yet, the shift in potential of the whole coating is significant. “Real” behavior of the ZnAlMgSi was studied in accelerated corrosion test both as in form of an alloy and steel coating. Frequent inspection of corrosion products combined with SKP measurements provides link between galvanic behavior of phases and behavior of coating/coated steel after contamination. The results show that while microstructure refinement caused by Si addition is significant, the electrochemical effect also plays major role.