Growth and modification of passive layers on new Zn alloys using atmospheric-pressure plasmas

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The corrosion and adhesion characteristics of oxide-covered alloys are mainly determined by the properties of the passive surface oxide film. On the one hand, the chemistry, morphology and crystalline structure of oxide films define the electrochemical properties and thereby the corrosion resistance of the substrate. On the other hand, the surface chemistry and the topography play a crucial role in the nature of the chemical and mechanical interactions at interfaces formed with polymeric coatings or adhesives, as well as their stability. Zinc alloy coatings are coated on steel sheets to provide barrier properties, resistance to atmospheric corrosion and cathodic protection of the construction metal. In the last decade, the development of ZnMg and ZnMgAl alloy coatings led to a breakthrough in the continuous coating of steel coils for automotive and home-appliance industries. ZnMg and ZnMgAl alloys have been shown to provide superior corrosion resistance in comparison to ZnAl-alloy coatings, both with and without the application of protective polymeric coatings. Classical surface technologies applied on metals, such as wet chemical conversion processes aiming at an improvement of corrosion resistance and adhesive properties of applied polymer films lead to a significant etching of the alloy surface and thereby to the formation of waste. With the goal of minimization of process waste without compromising the material properties, plasma surface treatments adaptable to continuous steel processing offer an efficient alternative to wet chemical conversion methods. Moreover, plasma technology allows for a precise process control and thus enables a more defined tuning of material properties relevant for corrosion and adhesion, such as semiconductor characteristics or film/surface chemistry. The aim of this project is to understand the fundamental mechanisms of the changes occurring on novel ZnMgAl alloy surfaces during atmospheric-pressure plasma treatment and to correlate the plasma parameters with the bulk/surface chemistry and electrical properties of the modified passive films, which would enable tailoring of these properties for various technical applications. A further goal is to correlate plasma-induced changes in the composition, semiconductor characteristics and surface chemistry of passive films on ZnMgAl alloys and alloy coatings with application-specific properties, such as adhesion to polymers and corrosion resistance. The synergistic influence of both the alloy composition and the plasma process will be considered. From the standpoint of surface technology, the project will provide an accurate description of the adequate passive film chemistries for improving the corrosion and adhesion properties of ZnMgAl alloys and the plasma process parameters necessary for achieving this.