Why do galvanized steel pipes sometimes develop visible turtle cracking on their surface after water cooling?

The reason for the appearance of turtle cracking on galvanized steel pipes after water cooling is primarily due to the pipes not undergoing sufficient air cooling, resulting in a relatively high temperature. When the cooling water temperature is quite low, a large temperature difference is created. Once the galvanized steel pipes are immersed in the water, due to the difference in the coefficients of linear expansion between zinc and iron, the galvanized layer on the surface cools and contracts first, while the steel pipe substrate inside cools and contracts later. This leads to the formation of cracks in the galvanized layer. These cracks are not visible to the naked eye when the pipes are cooled in clear water. However, if the cooling water has high turbidity and chroma, and the cooling water that penetrates into the cracks dries out, visible turtle cracking will appear on the surface of the zinc layer.