Why do some bright galvanized surfaces develop black spots after passivation?

The occurrence of black spots on the passivated, bright galvanized surface of hot-dip galvanized steel pipes is primarily due to impurities such as copper, iron, and arsenic present in the galvanized layer. Upon passivation, these impurities manifest as black spots. Additionally, chromium oxide, formed when chromic acid is reduced, appears green; while iron dichromate, produced 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 layer is not rinsed after passivation, black patches will remain on the surface.