The effect of zirconium based conversion treatment of zinc-, aluminium- and magnesium oxides on the bonding mechanisms and degradation of ester functionalized molecules

摘要

The need for thinner, weight-saving coatings together with the aim of conserving limited natural resources, create a high industrial interest in reducing the amount of zinc used in galvanized steel. Alloying zinc with aluminium and magnesium significantly enhances the corrosion resistance allowing the use of thinner coatings. Other sectors attracted by magnesium and aluminium are the automotive and aerospace industry which benefit from the high stiffness to weight ratio. In order to provide superior corrosion protection a chemical conversion treatment and organic coating can be applied. Group Ⅳ-A fluorides (ZrF_6~(2-) and TiF_6~(2-)) have shown to be a viable eco-friendly alternative for debatable traditional conversion techniques such as phosphating and hexavalent chromium passivation. It is known that the initial metal oxide properties determine the zirconium converted surface chemistry. This work is a comparative study, relating zinc, magnesium and aluminium oxide properties to the adhesion of ester functionalized molecules. Firstly, N,N'dimethyl succinate is used to mimic the functional groups of widely used polyester coatings. Subsequently, the elucidated bonding mechanisms are validated using well defined model polyester coatings. Finally, the durability of the interfacial bonds are studied by ATR-FTIR and EIS in a Kretschmann geometry. Combining these two techniques provides complementary information about the degradation of the metal-polymer system ATR-FTIR provides chemical information related to interfacial adhesion and delamination processes. Supplementary, semi-quantative accumulation of water in the interfacial region is monitored. On the other hand, water penetration through the bulk polymer and formation of corrosion products at the metal-polymer interface are more easily identified by EIS. Additionally, SKP is used to study the susceptibility of the metal oxides to cathodic delamination.