74. Why is the product quality of dry-process hot-dip galvanizing better than that of wet-process?

In dry hot-dip galvanizing, adding aluminum to the zinc bath improves the structure and thickness of the iron-zinc alloy layer, resulting in a bright pure zinc coating with excellent bending performance. During the process, the zinc bath is applied to the steel pipe surface, where zinc chloride in the bath begins reacting with the steel substrate. As the pipe temperature rises during drying, this reaction accelerates. When the pipe is immersed in the zinc bath, the iron-zinc reaction reaches its peak, allowing sufficient time for complete reaction between the coating and the zinc bath, thereby minimizing the risk of missed coating. In contrast, wet hot-dip galvanizing lacks these favorable conditions. First, aluminum cannot be added to the zinc bath surface. Even if added, aluminum reacts with the zinc chloride in the bath to form aluminum chloride, which then evaporates. Additionally, in wet hot-dip galvanizing, the pipe only comes into contact with the zinc bath's molten coating when it is not yet fully immersed. The molten coating is very thin, resulting in a short coating time and weak chemical cleaning power. This insufficient activation of the pipe surface affects the iron-zinc reaction, making missed coating more likely.