I. Overview of Galvanized Steel Pipes
To enhance the corrosion resistance of steel pipes, they are subjected to galvanization. Galvanized steel pipes are classified into two types: hot-dip galvanized and cold-dip galvanized. Hot-dip galvanized pipes feature a thick, uniformly coated layer with strong adhesion and long service life. In contrast, cold-dip galvanized pipes, though cost-effective, have a less smooth surface and significantly inferior corrosion resistance compared to hot-dip galvanized pipes, leading to their gradual phasing out.
Historically, galvanized pipes were widely used in older buildings in China, and they still prevail in gas and heating systems. As water pipes, galvanized pipes accumulate rust and scale over time, contaminating plumbing fixtures and fostering bacteria growth on their rough inner walls. More seriously, corrosion can elevate heavy metal levels in water, posing a severe health risk. In the 1960s and 1970s, 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. Nowadays, galvanized pipes are rarely used for cold water supply in newly constructed residential areas, though they may still be found in some hot water systems.
II. Classification and Characteristics of Galvanized Steel Pipes
Galvanized steel pipes are categorized into hot-dip and cold-dip types, with the latter gradually being phased out.
Hot-Dip Galvanized Steel Pipes
Hot-dip galvanizing involves the reaction between molten metal and the iron substrate to form an alloy layer, binding the substrate and coating. The manufacturing process starts with pickling the steel pipe to remove iron oxide from its surface. After cleaning in an ammonium chloride or zinc chloride solution, or a mixture of both, the pipe is immersed in a hot-dip galvanizing bath.
The hot-dip galvanized steel pipe undergoes complex physical and chemical reactions between its substrate and the molten galvanizing solution, resulting in a corrosion-resistant, tightly structured zinc-iron alloy layer. This alloy layer integrates seamlessly with the pure zinc layer and the steel substrate, endowing hot-dip galvanized steel pipes with a uniform coating, strong adhesion, and long service life.
III. Production Process of Galvanized Steel Pipes
As the production process of galvanized steel pipes is confidential among manufacturers, only a general outline can be provided: a. Round steel preparation; b. Heating; c. Hot-rolling and piercing; d. Head cutting; e. Pickling; f. Grinding; g. Lubrication; h. Cold rolling; i. Degreasing; j. Solution heat treatment; k. Straightening; l. Pipe cutting; m. Pickling (again, if necessary); n. Final inspection.
