INTERNAL-STRESSES IN ZINC-CHROMATE COATINGS

Zinc coatings immersed in a chromating solution acquire a mixed potent ial characteristic of zinc dissolution and the deposition of a chromiu m hydroxide layer. Zinc deposits are initially in a compressive stress but a zinc-chromate coating has a tensile stress which increases with chromate thickness. The stress in the chromate films after drying inc reases only slightly compared to a wet chromate him. Scanning electron and scanning probe microscopy reveals microcracks in the chromate coa tings due to the tensile stress. As the internal stresses in the chrom ate film increase, scanning probe studies show an increase in microcra ck width and depth.