Classification of Galvanized Steel Pipe

Sep. 17, 2020

Galvanized steel pipes are divided into cold-galvanized steel pipes and hot-dip galvanized steel pipes. Cold-galvanized steel pipes have been banned, and the latter is also recommended by the state to be temporarily used. In the 1960s and 1970s, developed countries in the world began to develop new types of pipes and gradually banned galvanized pipes. Four ministries and commissions including the Ministry of Construction of China also issued a document that banned galvanized pipes as water supply pipes since 2000. Galvanized pipes are rarely used for cold water pipes in newly built communities, and galvanized pipes are used for hot water pipes in some communities. Hot-dip galvanized steel pipes are widely used in fire fighting, electric power, and highways.

Galvanized Steel Pipe

In order to improve the corrosion resistance of steel pipes, general steel pipes are galvanized. Galvanized pipe can be divided into the cold-plated pipe and hot-dipped pipe. Cold-plated tubes have been banned, and hot-dipped tubes are also advocated by the state to be temporarily usable.   

Hot-dip galvanized pipe

It is to make the molten metal react with the iron matrix to produce an alloy layer so that the matrix and the plating layer are combined. Hot-dip galvanizing is to first pickle the steel pipe. In order to remove the iron oxide on the surface of the steel pipe, after the pickling, it is cleaned in a tank of ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride, and then sent to Hot dip plating tank. Hot-dip galvanizing has the advantages of uniform coating, strong adhesion, and long service life.   

Cold galvanized pipe

Cold galvanizing is electro-galvanizing, the amount of galvanizing is very small, only 10-50g/m2, and its corrosion resistance is much worse than that of hot-dip galvanized pipe. Most of the regular galvanized pipe manufacturers do not use electro-galvanized (cold plating) in order to ensure quality. Only those small enterprises with small scale and outdated equipment use electro-galvanization, and of course, their prices are relatively cheaper. At present, the Ministry of Construction has officially announced that cold-galvanized pipes with backward technology should be eliminated, and cold-galvanized pipes are not allowed to be used as water and gas pipes in the future.

Hot-dip galvanized steel

Complicated physical and chemical reactions occur between the steel pipe matrix and the molten plating solution to form a corrosion-resistant zinc-iron alloy layer with a compact structure. The alloy layer is integrated with the pure zinc layer and the steel pipe matrix. Therefore, its corrosion resistance is strong.  

Cold galvanized steel pipe 

The zinc layer is an electroplated layer, and the zinc layer is layered independently from the steel pipe substrate. The zinc layer is thin, and the zinc layer simply adheres to the steel pipe substrate and easily falls off. Therefore, its corrosion resistance is poor. In new houses, it is prohibited to use cold galvanized pipes as water supply pipes.

The above information is provided by the ASTM steel pipe manufacturer.

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