February 27, 2024 | Export Practice
The top cause of export claims on prepainted steel is rarely impact. It is container rain — moisture condensing on the container roof and dripping onto the coil, leaving a sheet of white rust.
Cargo is loaded at 35 °C in the yard; at sea the night temperature drops to 15 °C. Metal cools fast, the air inside cools slowly, and moisture condenses on the coldest surfaces — the roof and the steel itself. Longer voyages, wider day-night swings and humid routes all make it worse.
The important part: the water does not come from outside. It comes from the air inside and from the pallet timber. External wrapping alone does not stop it.
Layer 1: control moisture at the source. Pallet timber must be dry — a wet pallet is a sponge inside the box. Plywood pallets are low-moisture and exempt from fumigation.
Layer 2: VCI paper. Vapour-phase inhibitor paper releases corrosion-inhibiting vapour into the enclosed space. It only works when the pack is reasonably sealed; an open wrap does nothing.
Layer 3: waterproof paper, edge and corner protection. Blocks drips and handling damage, and stops steel strapping from biting into the coil.
Layer 4: desiccant. Size it to container volume and voyage length rather than guessing. Hang it high and distribute it — bags sitting at the bottom of the load do very little.
Solid wood pallets must comply with ISPM 15, either marked with the IPPC logo or accompanied by a treatment certificate. Miss this and the shipment can be held, rejected or force-treated at destination — usually costing more than the cargo. Plywood pallets sidestep the issue entirely.
1. Keep cargo off the walls. The container wall is where condensation concentrates. Leave a ventilation gap.
2. Never load in the rain. Loading wet seals water inside. If you must, shield the door and add extra desiccant.
Ask the customer to ventilate immediately on opening rather than storing the pack sealed. Light white rust wipes off with a neutral cleaner and a soft cloth with no effect on service; pitted rust means the substrate has started to corrode and needs separate assessment.
