1.Why is packaging so important?
Cold-rolled coils, due to their production process (cold rolling after pickling), have a clean surface free of oxide scale. However, they also lose the rough but somewhat insulating iron oxide scale of hot-rolled coils. Their surface is like a polished piece of iron, making them highly susceptible to rusting in humid air. Therefore, they must undergo rigorous rust-proof packaging before leaving the factory. The core purposes are:
Physical isolation: Preventing moisture, rainwater, sea fog (containing salt), dust, and contaminants from the outside air from directly contacting the steel coil surface.
Creating a dry microenvironment: The packaging typically contains vapor phase corrosion inhibitors (VCI) and desiccants. VCI evaporates in the enclosed space and adheres to the metal surface, forming a protective film, while the desiccant absorbs existing moisture within the packaging, together maintaining a low-humidity internal environment.
2.Does direct moisture penetration affect rust formation?
Holes and tears allow humid air to enter directly.
If this occurs in areas or seasons with large temperature differences between day and night, the infiltrated moisture will condense into water droplets on the cold steel coil surface, forming "dew point corrosion," a very rapid form of rust.
3.What are the effects of the "chimney effect" on large-scale corrosion?
If the packaging is damaged at the ends or sides, outside air will enter from the bottom, heat up inside (e.g., from sunlight), and then escape from the top, creating a continuous airflow.
This airflow constantly brings in new, humid air and can quickly saturate and deactivate the internal desiccant, leading to increased humidity throughout the steel coil. Rust often then spreads inwards from the damaged area.
4.Which types of packaging damage warrant special attention?
Inner packaging damage: Damage to the rust-proof paper or plastic film that directly contacts the steel coil poses the greatest risk.
Outer packaging (steel or wooden pallets): Although not in direct contact, the loss of physical protection from the inner waterproof and rust-proof paper easily leads to subsequent damage to the inner packaging.
Poor end sealing: The ends of cold-rolled coils are exposed and are usually sealed with special end caps and protective rings. If the end caps fall off or the seal is not tight, moisture can easily penetrate from the end face, and given the large end face area, extensive corrosion is likely.
Long-term contact with damp ground at the bottom: Even if the packaging is intact, if the storage area is wet, prolonged immersion at the bottom of the packaging will allow moisture to gradually seep in.
5.What should be done in practice?
Before leaving the factory: Ensure the packaging is airtight and secure, especially the inner packaging and end seals.
During transportation and handling: Operate according to regulations, using specialized lifting tools (such as C-hooks) to avoid damaging the packaging.
Upon arrival and inspection: Carefully inspect the integrity of the packaging. If any damage is found, focus on inspecting the surface of the corresponding steel coil and take photos for traceability.
During storage: Even if the packaging is intact, store it in a dry, well-ventilated warehouse, avoiding direct sunlight and rain. If the packaging is damaged, immediately perform temporary sealing (such as patching holes with waterproof tape) and prioritize its use.