As previously mentioned, the passivation film on the galvanized layer of galvanized steel pipes is generally yellow in normal conditions. However, after batches of galvanized steel pipes are cooled in water, although the cooling water inside the pipe holes is drained, there is still a considerable amount of water 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 coloration. Therefore, it is essential to strictly control the concentration within the specified process range. This requires frequent testing of the content of dichromate (or chromate and chromic acid) and sulfuric acid 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 the diluted passivation solution, the passivation film will turn reddish-brown or interspersed with purple hues, and the surface will appear rough. Additionally, the pure zinc layer will be thinned. Therefore, strictly controlling the concentration of the passivation solution is of utmost importance.
