New thin layer pretreatments versus zinc phosphate - a study of anticorrosive performance with automotive substrates and coatings

 
For many decades, the zinc phosphating process in combination with cathodic electrodipping coating (EC) worked excellently as the global standard for superior corrosion protection for cars. Due to more severe environmental regulations and increasing demands to preserve energy and resources in automobile manufacturing, this process has to be replaced by new technologies. In a research project at Fraunhofer IPA new thin layer pretreatments on various automotive substrate materials are compared with zinc phosphate concerning its corrosion protection properties in combination with EC in the multi-metal-substrate-pretreatment process. This study includes various standard steel and aluminium substrates as well as new substrate materials, e.g. HDG steel with zincmagnesium and coil coatings with corrosion protection primer. The mutual influence of substrate, pretreatment and EC is investigated and possible failure mechanisms are related to the layer properties. Some results of these investigations are presented. They include the new VDA/VDEh cyclic corrosion test (VDA 233-102) and its correlation to the anti-corrosion behaviour at intensified outdoor weathering. In addition, other tests, like the constant climate test with temperature gradient, electro-chemical methods, among others EIS and SKP and studies of the reversibility of water uptake of the EC-coated substrate-pretreatment systems are performed to study the behaviour of the different materials and pretreatments in combination with EC. The interfaces of interest are between metal and conversion layer and/or conversion layer and organic coating. Surface and interfacial studies with FIB/SEM/EDX are applied to understand changes after different loadings and mechanisms related to corrosion behaviour. These studies will offer insights into the performance of these systems and will help to identify strengths and weaknesses of specific combinations of the new pretreatment technologies with (new) substrate materials. Up to now, most investigated systems show good performance in electrochemical tests which is in good accordance with the results of VDA 233-102. The high electrochemical homogeneity from SKP results in high barrier properties of the systems. After loading with thermo cyclic stress, most of the systems keep their high barrier function even near the glass transition temperature of the EC. In water uptake reversibility studies, differences between the systems especially for zinc magnesium (ZM) and aluminium (Al) are observed. The ranking of the reversibility at 20°C and 80°C is not the same for the pretreatments. For steel and galvanized steel the performance of water uptake reversibility is in the same range for all investigated pretreatments. For ZM and Al the performance of the pretreatments strongly depends on the temperature and no correlation to barrier properties can be drawn. For some systems continuous condensation results indicate a weakening in adhesion, but the weakened systems still show good barrier properties and good water uptake reversibility. Further studies and interpretation of the results will provide a deeper insight to the performance characteristics of the material combinations and may help to identify the factors which mainly influence the corrosion protection performance of the investigated systems.