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“White rust” or “white storage stain” is one of the typical problems faced in galvanized coatings. It occurs as a white, bulky, powdery deposit that forms rapidly on the surface of the galvanized coating, under particular environments.
White rust can cause substantial damage to the galvanized coating and is detrimental to its appearance. The surface of the galvanized coatings mostly contains zinc. The outstanding anti-corrosion performance of steel is credited to zinc’s durability; however, zinc is a relatively “reactive” metal.
Zinc’s durability is established by the stable oxides that form on the surface of this element and these oxides form slowly as zinc is exposed to air. Mostly, carbon dioxide contributes to the formation of these stable oxides.
The freshly galvanized steelwork makes zinc’s surface extremely susceptible since it has been exposed to some level of oxidation. Hence, chromate passivation should be used along with galvanizing operations to protect the galvanized coating during its “youth” phase. This passivation coating provides limited protection to zinc, offering time for the stable oxides to develop on the surface.
White Rust Formation
Pure water (H2O) does not contain any dissolved salts or minerals, and will react swiftly with zinc to form zinc hydroxide—a bulky white and moderately unstable oxide of zinc. When newly galvanized steel is exposed to pure water (for example, rain, dew, or condensation) in an oxygen-deficient setting, the water will continue to react with zinc and slowly corrode the coating.
The typical circumstance where white rust is formed is when galvanized products are securely packed, compactly arranged, or when water seeps into the items and stays for extended periods.
Avoiding White Rust Formation
The formation of white rust can be considerably minimized or completely prevented by following these basic steps:
- The packed work must be dry
- The items must be packed to allow air circulation between the surfaces
- The packed items must be piled up to allow water to drain out
- The surface must be treated with barrier or proprietary water-repellent coatings to prevent moisture from touching the galvanized surface
Treating Galvanized Surfaces Affected by White Rust
After the galvanized surface has been compromised and the zinc hydroxide compounds have developed, it is better to remove the oxide products from the surface, as:
- Their presence stops the development of stable carbonate-based oxides
- They are unappealing
The effect on the galvanized coating can differ from extremely trivial to extremely serious, and various levels of remedial treatment are available to handle various problems relating to white rust.
Light White Rusting
This is characterized by the formation of a light film of white powdery residue, and frequently occurs on galvanized products during heavy rain seasons. It is particularly evident in regions that have been buffed or filed during quality assurance operations. In these treatments, the passivated surface is taken off, and un-oxidized zinc is open to attack from rainwater. Furthermore, if the items are well-drained and well-ventilated, white rust barely forms beyond this superficial phase.
If required, it can be brushed off; however, it will usually wash off in normal weather. This level typically does not require any remedial treatment.
Moderate White Rusting
This can be seen as a distinct darkening and etching of the galvanized coating under the damaged region, with the formation of white rust appearing large. It is crucial to analyze the thickness of the galvanized coating to estimate the degree of the attack on the coating.
In the majority of cases, below 5% of the galvanized coating will have been taken off and there is no need for corrective action as long as the appearance of the affected area is satisfactory for the use of the product. Residues of zinc hydroxide are removed by wire brushing. If the appearance is unappealing, the area affected by white rust can be treated as mentioned below:
- Using a cloth pad wet with aluminum paint, the surface should be rubbed to apply a thin film of aluminum paint to the affected area to blend it with the neighboring unaffected galvanized surfaces
- Using a wire brush, all white corrosion products must be eliminated from the problem area
Severe White Rusting
Severe white rusting is characterized by extremely heavy oxide deposits. The products may be fixed together. Regions under the oxidized area may be almost black, and display signs of red rust. The degree to which the galvanized coating has been damaged can be established by checking the thickness of the coating. Remedial treatment to restore the coating should be performed as given below:
- The affected region should be buffed or wire brushed, to remove all oxidation products and rust
- One or two coatings of approved epoxy zinc-rich paint should be applied, to achieve the necessary dry film thickness of at least 100 µm
Re-Passivating the Galvanized Surface
The surface can be re-passivated where white rusting has occurred, and the item can be continuously exposed to spread the same kind of corrosion. Re-passivation of a surface can be achieved by treating the surface with a solution of 0.1% sulfuric acid and 5% sodium dichromate, then brushing with a solid wire brush for 30 seconds, and finally rinsing the surface thoroughly.
White rust is an occurrence that happens following galvanization. The way it is handled, packed, and stored prior to the installation, and use of the galvanized product is crucial for its prevention.
The formation of white rust is not an indication of the performance of the galvanized coating, but rather highlights the responsibility of all those involved in the supply chain. This ensures that the causes of white rust are known and the possibilities of its occurrence are minimized on the newly galvanized steel.