What Is White Rust on Galvanized Steel?
Updated on: 25/09/2022
White rust on galvanized steel is a common problem when it is stored in a humid and poorly ventilated environment. Generally, after long-term transportation, large amounts of white or grey powder will appear on the surface of the zinc layer, which is commonly referred to as white rust.
What Is White Rust on Galvanized Steel?
Zinc is a very active metal. When the surface of the galvanized layer comes into contact with moist air, it will first chemically react with the moist moisture to form a layer of porous and sticky corrosion products. Subsequently, zinc hydroxide will further react with carbon dioxide to form a thin, dense, and adhesive zinc carbonate corrosion product, which can prevent further corrosion of the coating. But electrochemical reactions will occur, resulting in the local white point-like powdery substance seen by the naked eye. That is white rust, a large amount of white or grey powder.
White Rust
White Rust on Galvanized Steel
What Causes White Rust on Galvanized Steel?
1. High humidity;
2. Corrosion by acid gas or liquid.
3. The galvanized steel products are put into storage without drying in time.
Is White Rust Harmful?
Since the volume of metallic zinc increases by 3 to 5 times when it is converted into zinc oxide or zinc hydroxide, the white rust formed is larger in volume. As a result, white rust looks much more severe than it actually is, seriously affecting the appearance of the zinc coating. But in fact, white rust rarely affects the underlying zinc layer. Generally speaking, a small amount of white rust will not affect the anti-corrosion performance and the service life of the coating, and it will gradually disappear as the surface of the galvanized steel dries. However, if the galvanized metal is stored or transported in a humid environment for a long time, the white rust may not only affects the appearance of the product but also reduces its service life.
Prevention of White Rust
1. Place the galvanized steel in a dry and well-ventilated environment; and keep it far away from acid, alkali, salt, cement, dust, etc.
2. Surface treatment of galvanized parts after water cooling, for example, galvanized pipes can be coated with a layer of varnish; galvanized wires, sheets and meshes can be waxed or oiled; structural parts can be passivated.
3. For outdoor storage, do not stack directly on wet soil or rotting vegetation. It should be elevated from the ground and separated by narrow wooden slats. And remember to place them at a slight angle for drainage.
4. When galvanized parts are to be shipped by sea, wooden strips should be used to separate them, because there may be condensation on the surface. Or the galvanized pieces should be thoroughly dried before packaging.
Prevention of White Rust
How to Remove White Rust from Galvanized Metal?
In most cases, white rust does not mean that the zinc coating has been severely damaged, nor does the life of the galvanized steel will be reduced. When white rust appears, lay out the galvanized steel to dry quickly. A small amount of white rust can be wiped off with a dry cloth. After removing the white rust, check the coating thickness of the white rust area to ensure that there is enough zinc to protect the base metal.
When the white and grey corrosion products turn black, it means that the zinc coating has been corroded, and its service life will be reduced. If there are white deposits or red rust, the galvanized steel is heavily damaged and it must be re-plated or repaired locally according to the relevant standards.
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