Why are there several colors of passivation films, and what color is the passivation film on galvanized steel pipes generally?

The color of the passivation film primarily varies with the content of hexavalent chromium and the thickness of the film. When the concentration of hexavalent chromium is high, the film appears red; when the concentration is low, it appears orange or yellow. Therefore, with different concentrations of hexavalent chromium, the passivation film exhibits various colors ranging from red to yellow. Since hexavalent chromium sometimes exhibits a purple hue, purple may also be mixed in with the colors ranging from red to yellow. Thin passivation films exhibit iridescent colors due to light interference. Generally, the color of the passivation film on galvanized steel pipes is most commonly yellow.