As mentioned earlier, the normal passivation film on the galvanized surface of galvanized steel pipes is generally yellow. However, after batches of galvanized steel pipes are cooled in water, although the cooling water inside the pipe holes is drained away, there is still considerable moisture adhering to the surface, which is carried into the passivation solution. Subsequently, this dilutes the passivation solution, significantly reducing its concentration. The consequences include a slower passivation rate, a passivation film that does not meet requirements, and abnormal color. Therefore, it is essential to strictly control the concentration within the specified process range. This necessitates frequent testing of the dichromate (or chromate and chromic acid) content and sulfuric acid content in the passivation solution. Only by doing so can a consistent passivation film be ensured.
Conversely, if an excessive amount of dichromate (or chromate and chromic acid) and sulfuric acid is added to a diluted passivation solution, the passivation film may turn reddish-brown or have purple streaks, with a rough surface appearing, and the pure zinc layer may become thinner. Therefore, strict control of the concentration of the passivation solution is extremely important.
