12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Along with the design of the body in white, the paint is very important for the unique esthetic impression of a car. Accordingly, car manufacturers put great emphasis on the quality and visual appearance of the paint. Many R&D- programs investigated the influence of roughness parameters (roughness, waviness, peak count) on the paint appearance of hot-dip galvanized steel sheet, but only few publications dealt with the influence of roughness parameters of electro galvanized steel sheet on paint appearance. Electro galvanizing offers the unique possibility to adjust the roughness (Ra) and the peak count (RPc) by varying the plating conditions and/or the electrolyte composition. Thus eight sets of samples with Ra- values ranging from 0.45 – 1.50 µm and peak counts from 40 – 170 cm-1 were prepared in a lab electro galvanizing cell. The steel substrate was a cold rolled, annealed and skin-passed bake- hardening steel with a roughness of 1.1 µm and a peak count of 120 cm-1 typically used for exposed parts. In close collaboration with three OEM´s this set of samples was passing the whole industrial painting process from cleaning to the application of the clear coat. Afterwards the paint appearance of the samples was evaluated at the respective OEM, as well as in our labs by a wave- scan device returning values for waviness (long and short) and distinctness of image (DOI). Neither the measurements at the OEM´s nor our own evaluations showed any significant differences in paint appearance related to the surface structure of the electro galvanized steel substrate. Moreover a visual assessment could complete this picture. Additionally the friction behavior and adhesive bondability were checked and did not reveal any differences between the samples. In conclusion we are able to say, that the electro galvanizing process allows a great variety of surface structure without adversely affecting the appearance of a high quality automotive paint.