Influence of Structure, Composition and Surface Morphology on Coating Adhesion and Painted Corrosion Performance of ZnNi Alloy Coated Sheet Steels

A number of ZnNi coatings were investigated to determine the effect of phase structure, composition and surface morphology on coating adhesion and painted corrosion performance. The differences in the coating characteristics were achieved by controlled variation of selected process parameters in a series of pilot studies and production trials. It was seen that the coating adhesion, as measured in a draw bead simulator test, is influenced by the Ni content and the morphology of the coating, with 13%Ni and a micronodular surface morphology favoring good coating adhesion. Results of scribe/chipping corrosion tests show that paint creepage from the scribe on ZnNi coatings is equal to or less than that on electrozinc coatings of twice the thickness, and this property is influenced by the Ni content but not by the ZnNi structure or coating morphology. The paint loss from chipping, however, is insensitive to substrate differences, probably due to the good anti-chipping characteristics of the current automotive paint systems, one of which was employed in this study. It is concluded from this study that a generally uniform and consistent corrosion performance can be expected from ZnNi coatings over a wide range of composition and microstructures.