Advanced zinc-based hot dip coatings for the automotive application (AUTOCOAT) : final report

The aims of this project were to establish connections between the coating composition, microstructure, pre-treatment and corrosion performance of ZnAlMg 'ZMA' coatings both in accelerated tests and field exposures. Line hot dip materials including four conventional zinc coatings and three ZMA coatings (ZnMg1%Al1%, ZnMg2%Al2%, ZnMg1.5%Al1.5%) were selected. In addition, a galvanising simulator was used to prepare ZMA coatings with different compositions of Mg and Al (up to 4 %). ZMA coatings were suitable for standard phosphatising process and showed good robustness in respect to process variations. ZMA coatings also showed rather comparable application properties to HDG regarding formability, spot welding and adhesive bonding. Significant improvements of ZMA over standard coatings were observed to be more pronounced for the most aggressive tests and especially for early stages of corrosion. ZMA coatings exposed in stationary sites and on-vehicle showed an improvement of 2 compared to HDG when tested unpainted in open configurations, while no differences were observed on painted samples after 2 years. For hem-flange designs, the improvement of ZMA was not obvious whatever the tests. The corrosion performance of ZMA was not linked to the presence of a specific corrosion product (simonkolleite or hydrotalcite). Rather, it is suggested that the effect of the alloying element is in the change of the microstructure and resulting anode/cathode ratio that will profoundly affect the pH of the cathode and explain the improved corrosion behaviour. This also explains why the improvement after long-term field exposure tests is less pronounced, as this effect is mainly active in the initial stages.