1.What is white rust?
In humid environments (especially when there is no air circulation), the zinc layer on the surface of galvanized coils will undergo an electrochemical reaction with moisture and oxygen in the air to form a layer of white, porous basic zinc carbonate corrosion products, which is called "white rust".
2.How does white rust form during transportation?
Condensation: This is the primary cause. During transportation, if there are significant diurnal temperature variations or if the shipment crosses different climate zones, the cargo itself experiences slow temperature changes, while the surrounding air temperature fluctuates rapidly. When warm, humid air comes into contact with the cooler surface of the galvanized coil, the moisture in the air condenses into water droplets that adhere to the steel coil surface.
Enclosed Environment: Inside containers or trucks, the lack of air circulation prevents these water droplets from evaporating quickly enough, allowing them to remain in contact with the zinc layer for extended periods, creating ideal conditions for white rust formation.
Rain and Seawater Corrosion: If the packaging is not secure during transportation, rain or sea splashes can allow moisture to directly penetrate and cause corrosion.
3.What are the dangers of white rust?
Aesthetic Impact: White spots or large areas of white patches on the surface are very unsightly.
Coating Damage: White rust is porous, and its formation consumes the underlying zinc layer. Severe white rust reduces the thickness of the zinc coating, weakening its long-term rust resistance and eventually causing the underlying steel plate to rust (red rust).
Customer Rejection: Galvanized coils with white rust are usually considered a quality issue, and customers are likely to refuse delivery or demand price reductions or compensation.
4.What are some common methods of moisture-proof packaging?
Rust-preventive oil/rust inhibitor
After galvanizing, a thin layer of specialized rust-preventive oil is applied to the surface of the coil. This oil film effectively isolates the zinc layer from moisture and air, serving as the first and most important line of defense against white rust.
Rust-preventive paper/vapor phase corrosion inhibitor paper
The steel coil is wrapped with rust-preventive paper containing vapor phase corrosion inhibitors. This paper continuously and slowly releases a gas that adheres to the metal surface in a confined space, forming a protective layer to prevent corrosion. This is particularly effective for products requiring long-term sea transport or storage.
Inner plastic film packaging
The entire steel coil is tightly wrapped with a thick plastic film (usually PE or PVC). Its main function is to physically block moisture, preventing condensation from directly contacting the coil surface.
Outer packaging and support
Waterproof woven fabric/core paper: A layer of sturdy waterproof woven fabric or multiple layers of core paper are wrapped around the plastic film, providing moisture protection, scratch resistance, and reinforcement.
Steel/Wooden Side Panels: Circular steel or wooden side panels are added to the sides of the steel coil to protect the edges from damage during lifting and transportation.
Desiccant (Desiccant): A certain amount of desiccant is placed inside the final packaging to absorb residual moisture.
5.What are the requirements for transportation and storage?
Inside containers or wagons, steel coils should not be placed directly on the ground; they should be elevated using wooden blocks or similar supports to ensure air circulation at the bottom.
If container transport is used, ensure that the container itself is dry and undamaged.