Zr-based conversion coating on Zn and Zn-Al-Mg alloy coating: Understanding the accelerating effect of Cu(II) and NO3-

The reactivity of a Zn-Al-Mg galvanized steel substrate was monitored during a two-step conversion coating sequence with an alkaline pretreatment followed by conversion coating with hexafluorozirconic acid (H2ZrF6). The main effect of alkaline pretreatment was to remove initial oxides and to selectively dissolve Al, limiting the dissolution of Al in the zirconate bath. The commercial alkaline cleaner dissolved Mg from the MgZn2 intermetallic phase. The effect of NO3- and Cu(II) on the reactivity of a commercial Zn-Al-Mg alloy coating was investigated in H2ZrF6, simulating a Zr-based conversion coating process. NO3- served as an oxidant which enhanced the production of OH- leading to a more consequent ZrO2 deposition. Cu(II) underwent a displacement reaction with Zn (0) to form Cu(0) which catalyzed the reduction of NO3- and H+. The interplay between activation and passivation was demonstrated by the occurrence of oscillations in the both NO3- and Cu(II) containing electrolyte under certain conditions.