I. Overview of Galvanized Pipes
To improve the corrosion resistance of steel pipes, they are subjected to galvanizing processes. Galvanized pipes (galvanized steel pipes) are divided into hot-dip galvanized and cold-galvanized types. Hot-dip galvanized pipes feature thick, uniform coatings with strong adhesion and long service lives. In contrast, cold-galvanized pipes, despite their lower cost, have rough surfaces and significantly inferior corrosion resistance compared to hot-dip galvanized pipes, leading to their gradual phase-out.
In old buildings in China, galvanized pipes were widely used, and they continue to be the choice for gas and heating iron pipes. However, as water pipes, galvanized pipes accumulate rust and scale over time, contaminating fixtures and fostering bacterial growth on their rough inner walls. More alarmingly, corrosion can elevate heavy metal levels in water, posing serious health risks. In the 1960s and 1970s, some developed countries began developing new pipe materials and gradually banned the use of galvanized pipes. Similarly, China's Ministry of Construction and three other ministries explicitly prohibited the use of galvanized pipes from 2000 onwards. Currently, cold water pipes in newly built residential areas rarely use galvanized pipes, while some hot water pipes still do.
Galvanized pipes are classified into hot-dip galvanized steel pipes and cold-galvanized steel pipes, with the latter being phased out.
Hot-dip Galvanized Pipes/Hot-dip Galvanized Steel Pipes
Hot-dip galvanizing involves reacting molten metal with an iron substrate to form an alloy layer, thereby bonding the substrate and coating. The manufacturing process begins with pickling the steel pipe to remove iron oxide from its surface, followed by cleaning in a bath of ammonium chloride or zinc chloride solution (or a mixture of both). The pipe is then immersed in a hot-dip galvanizing bath.
Hot-dip galvanized steel pipes undergo complex physical and chemical reactions between the steel substrate and molten coating, resulting in a corrosion-resistant, tightly bonded zinc-iron alloy layer. This alloy layer is integrated with the pure zinc layer and steel substrate, giving hot-dip galvanized steel pipes advantages such as uniform coating, strong adhesion, and long service life.
Cold-galvanized Pipes/Cold-galvanized Steel Pipes
Cold-galvanizing, also known as electro-galvanizing, produces a minimal zinc coating of only 10-50g/m2, resulting in significantly inferior corrosion resistance compared to hot-dip galvanized pipes. Reputable galvanized pipe manufacturers typically avoid electro-galvanizing (cold-galvanizing) to ensure quality. Only small-scale, outdated facilities opt for this method due to its lower cost. In cold-galvanized steel pipes, the zinc layer is separately layered on the steel substrate and prone to detachment due to its thinness, leading to poor corrosion resistance. Consequently, cold-galvanized steel pipes have been banned from use as water and gas supply pipes in newly built residences.
