Galvanized steel pipes, to enhance the corrosion resistance of steel pipes, are coated with zinc. There are two types of galvanized steel pipes: hot-dip galvanized and electro-galvanized. Hot-dip galvanized pipes have a thicker zinc coating, while electro-galvanized pipes are more cost-effective but have a less smooth surface.
Hot-dip Galvanized Pipe
Hot-dip galvanized pipes are formed by the reaction of molten metal with the iron substrate to produce an alloy layer, thereby combining both the substrate and the coating. The process begins with the acid pickling of steel pipes to remove iron oxide from the surface. Following acid pickling, the pipes are cleaned in a bath of ammonium chloride or zinc chloride solution, or a mixture of both, and then transferred to the hot-dip galvanizing tank. Hot-dip galvanizing offers advantages such as even coating, strong adhesion, and long service life. The steel pipe substrate undergoes complex physical and chemical reactions with the molten coating to form a tightly structured, corrosion-resistant zinc-iron alloy layer. This alloy layer merges seamlessly with the pure zinc layer and the steel pipe substrate, providing excellent corrosion resistance.
Galvanized Layer Uniformity: Steel pipe specimens must not turn red (copper color) after being continuously immersed in copper sulfate solution for 5 times.
2. Surface Quality: Galvanized steel pipes should have a complete galvanized coating, free of any uncoated black spots and bubbles. Small rough surfaces and localized zinc nodules are permissible.
3. Galvanized Layer Weight: In accordance with the buyer's requirements, the weight of the zinc layer on galvanized steel pipes can be measured, with the average weight not less than 500g/square meter, and no sample should be less than 480g/square meter.
Cold galvanized pipe
Cold galvanized, also known as electro-galvanized, has a very thin coating, only 10-50g/m2, and its corrosion resistance is significantly lower than that of hot-dipped zinc pipes. Reputable galvanized pipe manufacturers generally do not use electro-galvanized (cold galvanized) to ensure quality. Only small, outdated facilities of small enterprises employ this method, and naturally, their prices are relatively lower. The Ministry of Construction has officially announced the elimination of outdated cold galvanized pipes, prohibiting their use in water and gas pipelines in the future. The zinc layer is an electro-galvanized layer, which is separate from the steel pipe substrate. The zinc layer is thin, simply adhered to the steel pipe substrate, and prone to falling off, thus exhibiting poor corrosion resistance. In newly constructed residential buildings, the use of cold galvanized steel pipes as water supply pipes is prohibited.
Application
Galvanized iron pipes, commonly used for gas, greenhouses, and heating systems, can accumulate a significant amount of rust and scale inside when used as water pipes. The resulting yellow water not only contaminates fixtures but also carries bacteria from the rough inner walls. Rust corrosion can lead to elevated levels of heavy metals in the water, posing serious health risks. In the 1960s and 1970s, developed countries internationally began to develop new types of piping materials and gradually phased out the use of galvanized pipes. China's Ministry of Construction and four other ministries issued a directive explicitly banning the use of galvanized pipes from the year 2000 onwards. Cold water pipes in newly constructed residential areas have rarely used galvanized pipes, while some areas still use them for hot water pipes.
