Quantitative Test for Zinc Phosphate Coating Quality

Zinc phosphated steels representing a wide range of salt spray performance were analyzed for coating weight, composition, X-ray structure, morphology and porosity. Of these factors, apparent coating porosity was the only characteristic that consistently correlated with salt spray performance. An electrochemical test has been developed which measures relative porosity values accurately, rapidly, and directly on zinc phosphated steels. This technique has been employed to show that the residual carbonaceous materials tightly bound to the surface of commerical steels need to produce porosity levels of only a few percent in order to drastically reduce the effectiveness of zinc phosphate coatings as a paint base.