What is a galvanized steel pipe?
A galvanized steel pipe is a carbon steel pipe coated with a protective layer of zinc. This zinc layer serves as sacrificial protection, meaning it corrodes before the underlying carbon steel. There are two types of galvanized steel pipe: hot-dipped galvanized steel pipe and cold galvanized steel pipe. The galvanized layer enhances the anti-corrosion properties of the steel pipe.
Why use zinc as the coating?
Zinc is chosen for coating because it is more reactive than steel. Firstly, the zinc oxide layer on the surface of the galvanized steel pipe is dense, effectively preventing oxidation (corrosion). Secondly, in case of electrochemical corrosion, zinc is corroded first, serving as sacrificial protection for the steel. This slows down the corrosion process and extends the lifespan of the steel pipe.
Application of Galvanized Steel Pipes:
Galvanized steel pipes are primarily used for transmitting coal gas and steam. Initially employed as water pipes, they developed rust over time, resulting in yellow-colored water due to iron oxide contamination. This rust not only tainted dishes and sanitary ware but also harbored bacteria due to the rough internal surface. Consequently, the water became heavily laden with harmful heavy metal elements, posing health risks to consumers. As a result, developed nations began phasing out their use as water pipes since the 1960s.
Types of Galvanized Steel Pipes: Galvanized steel pipes come in two varieties:
- Hot-dipped galvanized steel pipes.
- Cold galvanized pipes, also known as electric galvanized pipes.
Sizes and Specifications of Galvanized Steel Pipes: Nominal Diameter (inch): 1/2 to 4 Nominal Wall Thickness (mm): 2.0 to 4.5 Test Pressure Index:
- Diameter: 10.2 to 168.3 mm - 3 MPa
- Diameter: 177.8 to 323.9 mm - 5 MPa
Introduction to Hot-Dipped Galvanized Steel Pipes:
Hot-dipped galvanized steel pipes undergo a manufacturing process wherein rust-free steel pipes are dipped into zinc liquid at temperatures of 500℃. This process coats the steel surface with a layer of zinc, providing anti-corrosive properties.
Hot galvanizing is an effective anti-corrosive treatment used in various steel structural products. Its use dates back to 1836 when France first employed it in industry, marking over 180 years of application. However, with the advancement of cold-rolled steel in recent decades and the increasing demand for higher standards in protection products for high-voltage transmission, transportation, and communications, the demand for hot galvanizing has continued to rise.
Hot-Dip Galvanizing Process: The process involves several steps: after pickling and washing, the steel is plated, dried, hung for plating, cooled, treated with a solution, cleaned, ground, and finally, the hot galvanized finish is applied.
Zinc Thickness for Hot-Dip Galvanized Steel Pipe: The zinc layer thickness for hot-dip galvanized steel pipes ranges from 35μm to a maximum of 650μm, whereas cold zinc layer thickness is only 5-15μm. The zinc layer provides protection against air corrosion and offers electrochemical protection.
Theory of Hot-Dip Galvanized Steel Pipe's Corrosion Resistance: During hot-dip galvanizing, a zinc-steel alloy layer forms first, followed by a pure zinc layer. When the zinc layer corrodes in the presence of air, it generates a protective film of ZnO, Zn(OH)2, and basic zinc carbonate, which effectively resists air corrosion. If this film is damaged, a new one forms. If the zinc layer is heavily damaged, it triggers electrochemical protection, with zinc acting as the sacrificial element. Hot-dip galvanized steel exhibits superior corrosion resistance compared to cold galvanized pipes.
Lifetime of Hot-Dip Galvanized Steel Pipe: The lifetime of galvanized steel pipes varies depending on environmental conditions. Typically, they last 13 years in heavy industrial areas, 50 years in ocean environments, 104 years in suburbs, and 30 years in cities. Galvanizing eliminates the need for additional rust treatment due to its inherent anti-rust properties.
Suitability of Hot-Dip Galvanizing for Corrosive Conditions: Hot-dip galvanizing is particularly suitable for environments with strong acids, alkalis, and fog, providing effective protection. These fasteners are commonly used in outdoor steel tower construction.
Introduction to Cold Galvanized Steel Pipe:
Cold galvanized steel pipe involves applying a zinc layer onto steel pipes through electric chemical processing. Typically, the zinc layer is around 20~30μm thick.
Manufacturing Process of Cold Galvanized Steel Pipe: The manufacturing process includes degreasing, electroplating, passivation, drying, and packaging.
Zinc Thickness for Cold Galvanized Steel Pipe: The zinc thickness for cold galvanized steel pipe ranges from 5-15 μm, creating a dense layer without organic matter inclusions.
Discontinuation of Cold Galvanization Technology for Pipe Products: Due to the relatively thin zinc layer, many countries have phased out the use of cold galvanized steel pipes for water and coal gas transportation. However, the smooth surface appearance resulting from cold galvanization makes it suitable for manufacturing small components requiring precise plating. These include screws, machinery parts, electronics, precision instruments, chemicals, light industry products, transportation equipment, weaponry, aerospace components, and applications in atomic energy, all of which hold significant importance in various sectors of the economy.
