After pickling, rinsing, and optionally immersion in a diluted hydrochloric acid solution, steel pipes must be coated with a layer of concentrated hydrochloric acid, aqueous zinc chloride, aqueous ammonium chloride, or an aqueous complex salt solution of zinc chloride + ammonium chloride before hot-dip galvanizing, followed by drying. Alternatively, the complex salt of zinc chloride + ammonium chloride can be directly placed in a special container on the surface of the molten zinc bath, so that the steel pipes to be galvanized pass through the molten complex salt layer before entering the zinc bath. The process of producing galvanized steel pipes using these two flux application methods is called the flux method. The former is known as the "dry flux method" or simply the "dry method", and the latter is known as the "wet flux method" or simply the "wet method".
The purpose of using a flux is to ensure that when the steel pipe is immersed in the hot zinc bath for galvanizing, the iron substrate on the surface of the steel pipe reacts normally with the zinc bath within a short period of time to form a complete iron-zinc alloy layer. In the "dry method", the layer of flux film on the surface of the steel pipe can also prevent the metal substrate of the steel pipe from being oxidized.
