First, what are the types of galvanized steel pipes?
1. Hot-dip galvanized steel pipe: Hot-dip galvanized steel pipe has the advantages of uniform coating, strong adhesion, and long service life. Most processes in the north use the galvanized strip direct pipe zinc supplementation process. Hot-dip galvanized steel pipe makes the molten metal react with the iron matrix to produce an alloy layer so that the matrix and the coating are combined. Hot-dip galvanizing is to pickle the steel pipe first. To remove the iron oxide on the surface of the steel pipe, after pickling, it is cleaned in an ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride and then sent to the hot dip plating tank. The steel pipe matrix will undergo complex physical and chemical reactions with the molten plating solution to form a corrosion-resistant zinc-iron alloy layer with a tight structure. The alloy layer is integrated with the pure zinc layer and the steel pipe matrix, so it has strong corrosion resistance.
2. Cold-dip galvanized steel pipe: The zinc layer of cold-dip galvanized steel pipe is an electroplating layer, and the zinc layer and the steel pipe matrix are independently layered. The zinc layer is thin, and the zinc layer is simply attached to the steel pipe substrate and is easy to fall off, so its corrosion resistance is poor. Therefore, in residential buildings, it is forbidden to use cold-dip galvanized steel pipes as water supply pipes. Cold galvanizing is electro galvanizing, and the amount of galvanizing is very small, only 10-50g/m2, and its corrosion resistance is much worse than that of hot-dip galvanized steel pipes. To ensure quality, most regular galvanized steel pipe manufacturers do not use electro-galvanizing (cold plating). Only small enterprises with small scale and outdated equipment use electro galvanizing, and of course, their prices are relatively cheaper.

Second, what is the role of galvanized steel pipes?
To improve the corrosion resistance of steel pipes, general steel pipes are galvanized. Most of the old houses are made of galvanized steel pipes. The iron pipes used for gas and heating are also galvanized steel pipes. After a few years of use, a large amount of rust will be generated in the galvanized steel pipes as water pipes. The yellow water flowing out not only pollutes the sanitary ware but also contains bacteria that grow on the rough inner wall. The rust causes excessive heavy metal content in the water, which seriously endangers human health.

Third, technical requirements for the production of galvanized steel pipes
1. The grade and chemical composition of the steel used for galvanized steel pipes shall comply with the grade and chemical composition of the steel used for black pipes specified in GB3091.
2. The manufacturing method of black pipes is selected by the manufacturer. Galvanizing adopts the hot-dip galvanizing method.
3. For galvanized steel pipes delivered with threads, the threads should be machined after galvanizing. The threads should comply with the provisions of YB822. Steel pipe joints should comply with the provisions of YB238; forged cast iron pipe joints should comply with the provisions of YB230.
4. The mechanical properties of steel pipes before galvanizing should comply with the provisions of GB3091.
5. The steel pipe sample shall not turn red (copper-plated color) after being immersed in copper sulfate solution for 5 consecutive times.
6. The bending angle is 90° and the bending radius is 8 times the outer diameter. No filler is used during the test, and the weld of the sample should be placed on the outside or upper part of the bending direction. After the test, there should be no cracks or zinc peeling on the sample.
7. Water pressure test The water pressure test should be carried out on the black pipe, and eddy current testing can also be used instead of the water pressure test. The test pressure or eddy current testing comparison sample size should comply with the provisions of GB3092.