Surface modification of ZnMgAl-coated steel by dielectric-barrier discharge plasma

In this study, ZnMgAl alloy coated steel sheets were exposed to a dielectric-barrier discharge plasma with different gas mixtures (Ar, Ar + 5% O2, Ar + 5% H2O). Five different plasma-induced processes were identified at the surface: (i) etching of aliphatic carbon groups, (ii) conversion of absorbed carboxylates to carbonates, (iii) field-induced migration of Zn and Mg towards the surface, (iv) increased oxide layer thickness, and (v) homogenization of the surface potential of the originally very heterogeneous alloy. The relative contribution of each process depended on the specific gas mixture. Peel-test studies showed that all atmospheric-pressure plasma treatments improved the adhesive properties of the alloy coating for two different adhesives (acrylate and epoxy amine). The highest improvement was associated to the case of Ar + 5% H2O gas mixture, where all described surface processes took place to a high degree.