The most common cause of damage in steel coil storage is moisture. The white, powdery layer that forms on a galvanised surface (white rust) is produced when zinc reacts with moisture, and it reduces surface quality.
White rust usually causes no structural weakening; but because appearance and paint adhesion are affected, the product loses value.
How white rust forms
Moisture trapped between coils cannot ventilate and stays on the zinc surface for a long time. Under those conditions the zinc oxidises and a white, powdery layer forms.
Temperature change makes it worse: when a cold coil is brought into a warm, humid space, condensation forms on the surface.
Preventive measures
- Storage in a covered, ventilated area
- Breaking contact with the floor: using pallets or chocks
- Keeping packaging intact; repairing torn packaging at once
- Avoiding sudden temperature changes to prevent condensation
- Regular surface checks on consignments held for a long time
Which products are more sensitive
Galvanised and pre-painted sheet are the most sensitive group, because the surface determines the value of the product. Unoiled cold-rolled sheet is also quickly affected by moisture.
On products such as hot-rolled black sheet and billet, light surface rust is usually not a problem; but corrosion that would reduce the cross-section must not be allowed.
If damage occurs
The situation is established in a written report and photographed. If the condition of the packaging was recorded on entry, when the damage occurred does not remain disputable.
That record is the principal document relied on in the insurance process.