Causes of Black Spots on Bright Galvanized Coatings After Passivation

The appearance of black spots on the galvanized coating of bright hot-dip galvanized steel pipes after passivation treatment is primarily due to impurities such as copper, iron, and arsenic present in the galvanized coating. Upon passivation, these impurities become visible as black spots. Additionally, chromium oxide formed by the reduction of chromic acid appears green, while iron dichromate formed by the reaction between chromic acid and iron appears brown. When these two compounds mix, they turn black (indicating that the passivation solution has aged). If the galvanized coating is not rinsed after passivation, black patches will remain on its surface.