White rust or white storage stain of Hot-Dip Galvanized processes.
When products are dipped by the Hot-Dipped Galvanizing process, Zinc coated on steel products will prevent rust (Fe2O2H2O) and corrosion. Zinc is formed as an oxide (O2) through the air which composes of carbon dioxide (CO2). Then, the oxide will be more stable in terms of Zn (CO2) 2 but it takes at least 48 hours for the formation.
White rust formation
White rust occurs when hot-dipped galvanized metal is exposed to pure water or droplet. Zinc hydroxide – the substance formed when zinc reacts with water and air has interacted. Its characteristic is white powdery deposit as white oxide.
Hence, It is necessary to apply chromate to coat the surface of steel products during the new galvanizing in order to wait for the completion of zinc oxide with carbon dioxide. However, this is a short-term solution to prevent hydroxide.
White Rust Prevention
- Keeping steel products coated by Hot-Dipped Galvanized in a dry and ventilated place
- Avoiding wet storage after the completion of dipping process
- Coating steel products with chromate to prevent air and humidity conditions
It is required to remove white rust immediately because
- White rust will restrain oxidation from carbonate
- The surface of steel products will not be perfect and customers will not satisfy it.
How to Repair and Classify the level of White Rust
4 levels of White Rust based on the damage as following;
1. Slight rust
It usually occurs with new Hot-Dip Galvanizing coated steel products which are then exposed to rain or droplet. It results steel products in white rust. This can be solved by polishing the workpiece until the rust is gone. Then, keep steel products in a dry and ventilated place.
2. Medium level of white rust
The white rust in this level can be seen apparently. The rust is larger and darker. This impacts directly to the thickness of steel products. Zinc thickness is likely reduced but it is not more than 5%. This problem can be solved by polishing the surface with a wire brush and then wipe steel products with a cloth moistened with aluminum.
3. Severe white rust.
This white rust level is characterized by almost black colour and shows sign of red rust. Zinc thickness should be checked. This rust will be removed by following details:
- Polishing steel by using a wire brush in the rusty areas
- Appling Epoxy zinc rich to thin film at least 100 microns
4. Re-passive method of Hot-Dip Galvanized surface
Re-passive method is a great solution to remove white rust in case of a continuous propagation by polishing the surface containing white rust and wash it with 5% Sodium Dichromate solution, 0.12 Sulfuric acid
White rust normally occurs after galvanizing due to the management and storage processes. It is not caused by the galvanizing process but it happens in the post-plating period until delivery to the customers. So, all involved parties should manage and reduce causes of white rust effectively. Currently, we can prevent white rust by using Hot-dip galvanized coating.