1.What are the necessary conditions for white rust to form on the surface of galvanized coil?
Direct contact with water: such as rain or flooding.
Condensation: This is the most common and hidden cause. When the ambient temperature fluctuates significantly, moisture in the air condenses into water droplets on the cold surface of the galvanized sheet. If the coils are tightly wound, these water droplets are "trapped" between the layers, preventing them from evaporating.
High humidity: Long-term storage in an environment with a relative humidity above 85% can cause a very thin film of water to form on the surface, even in the absence of visible water droplets, which is sufficient to cause corrosion.
2.What are the sufficient conditions for white rust to form on the surface of galvanized coil?
Tightly wound: Galvanized coils are typically tightly wound during production and transportation, resulting in very limited space between layers.
Locked-out air circulation: In such a confined space, air cannot flow freely. Moisture that enters is difficult to dry out, and the initial oxygen consumption by zinc during corrosion cannot be replenished.
Imperfecting protective film formation: In normal atmospheric conditions, zinc reacts with carbon dioxide to form a dense, durable protective film of basic zinc carbonate (typically gray, a normal appearance). However, in a confined, humid environment, carbon dioxide cannot enter, and instead forms a loose, white powder of basic zinc carbonate and zinc hydroxide, which we see as "white rust."
3.What are the factors that cause white rust on the surface of galvanized coils?
Corrosive ions: Chloride ions (such as seawater and salt spray) or sulfate ions (industrial atmosphere) in water or the environment can significantly accelerate the formation and severity of white rust.
Acidic and alkaline environments:
Acidic substances: Zinc is highly soluble in acid. If the surface is contaminated with acidic substances (such as hand sweat, unwashed acid, or industrial acid gases), it will directly damage the zinc layer and accelerate corrosion in humid environments.
Alkaline substances: Strong bases can also corrode zinc, but to a lesser extent than acids.
Dissimilar metal contact: If galvanized coils come into contact with dissimilar metals such as copper and iron while in an electrolyte, a galvanic cell forms, accelerating zinc corrosion.
Surface damage: If the zinc coating is scratched or dented, exposing the steel substrate, a corrosion cell with a small anode (zinc) and a large cathode (steel) forms, rapidly consuming the zinc layer and producing white corrosion products.
4.How to prevent white rust?
Storage Environment:
Store indoors in a dry, well-ventilated warehouse.
Avoid direct contact with damp floors or walls; use wooden skids to raise the material.
Control the temperature and humidity in the warehouse to prevent large temperature fluctuations that could cause condensation.
Packaging and Transportation:
Use rust-proof paper or rust-proof film to separate layers. These materials have vapor-phase rust inhibition properties.
Ensure the packaging is tightly sealed to prevent the ingress of rain and moisture.
During transportation, cover tightly with a waterproof tarpaulin.
Stacking Method:
If outdoor storage is necessary for a short period of time, the rolls should be placed upright (i.e., standing upright) and covered with a tarpaulin. Ensure adequate ventilation at the bottom and top to prevent internal heat buildup and condensation.
Avoid stacking the rolls flat on top of each other, as this can cause condensation to accumulate between layers.
5.What should I do if white rust has already occurred?
Light white rust: This is usually just a white powdery layer on the surface, which can be removed with a soft cloth or soft-bristle brush. It generally does not affect the corrosion resistance of the zinc coating.
Severe white rust: If large deposits of white or black deposits have formed, or even red rust (rust on the steel substrate), the zinc coating has been severely corroded, significantly reducing its corrosion resistance and requiring professional treatment or downgrading.