Black Spots on Zinc Plating: Important Salt Spray Insight

Summary for black spots on zinc plating Discussion

Black spots on zinc plated parts during salt spray test are generally not treated as white rust, according to D.T. Thakur. He says zinc corrosion products are white, while black or gray spots can appear from impurities such as iron in the deposit or in the passivation film. These spots should not automatically be counted as a failure, but they do suggest weaker areas on the coated surface. The thread also notes that the timing of when the spots appear matters, and Syed Kazmi asks whether they were seen after galvanizing or during the salt spray test. The practical takeaway is that black spots are usually a coating-quality concern rather than a standard white-rust or red-rust classification, though the exact acceptance criteria would depend on the applicable specification.