Effect of Additives on the Surface Properties of Chromate Conversion Coatings

The performance of the chromate conversion coatings applied on the electro-galvanized steel has been investigated in terms of the effects of phosphate and silica addition to the chromating solutions. It was found that in the presence of both additives, the chromate film exhibits clearer appearance, higher corrosion resistance, and better fingermarking resistance than when either additive is missing, while paint adhesion was not compatible with wet durability of adhesion even when the two additives coexisted. In order to optimize the dry and wet adhesion of the chromate to paint, the variations of the surface chemical properties and the performance of the chromate film with the changes in the amount of phosphate and/or silica addition were analyzed quantitatively. The hydrogen bonding component of surface tension was found to increase with increasing amount of silica addition because silica is polar and it segregates on the surface, and therefore, it resulted in improvement of dry adhesion and deterioration of wet adhesion. While the hydrogen bonding component once decreased with increasing phosphate addition due to the formation of chromium phosphate, but it was recovered when zinc phosphate began to be formed. The dry and wet adhesion was optimized when the surface contents of silica, zinc phosphate, and zinc chromate were balanced.