Galvanized pipe can be classified into two kinds: cold galvanized steel pipe and hot dip galvanized steel pipe. The former has been forbidden, and the latter has been advocated temporarily in China.
1. Hot Dip Galvanized Steel Pipe
Hot dip galvanized steel pipe is to make the molten metal react with iron substrate so as to produce alloy layer so that the substrate can be combine with coating combination. Hot dip galvanized steel pipe is first taken acid pickling in order to remove iron oxide on the surface of the steel tube. After acid pickling, clean up in the tank of ammonium chloride or zinc chloride or ammonium chloride and zinc chloride mixed aqueous solution, and then sent to the hot dip plating tank. Hot dip galvanized has the advantage of uniform coating, strong adhesion, long service life, etc.
Steel substrate react complex physically and chemically with molten electrolyte solution, and then form into zinc-iron alloy layer with the compact structure of corrosion resistance. Pure zinc alloy layer, alloy layer and steel substrate are integrated with each other. So, its corrosion resistant ability is extremely strong.
2. Cold Galvanized Steel Pipe
Cold galvanizing is electricity galvanizing, but the galvanized quantity is very few about only 10 g to 50 g per square meter. Its corrosion resistance ability is weaker than that of galvanized pipe. Regular galvanized steel pipe manufacturers mostly do not use electric galvanized zinc (cold galvanized) in order to guarantee the quality. Only those small enterprises with small scale and old equipment adopt electric galvanized zinc. Of course, the price is relatively cheaper. At present, the Ministry of Construction has officially indicated to weed out cold galvanized pipe which is now lagging in technology and it is not allowed to use cold galvanized pipe as water and gas pipe in the future.
The zinc layer is electrodeposited coating, and zinc layer and steel substrate layered independently. Because the thin zinc layer attached on the steel substrate is simply easy to fall off, the poor corrosion resistance is not strong. In the newly-built residence, it is prohibited to use the cold galvanized steel tube
as the feed pipe.