Galvanized coils lose value fastest in the warehouse, not in service. If the storage conditions are wrong, the zinc surface develops white rust, a powdery deposit of zinc hydroxide and zinc oxide that darkens the surface and, in severe cases, consumes part of the coating that was bought for corrosion protection. Because white rust forms when moisture condenses on the zinc, every storage rule in this article is about one thing: keeping the coil dry and ventilated. The guidance follows the handling and storage practice recommended for hot-dip galvanized products supplied to EN 10346, ASTM A653 and JIS G 3302.
Why White Rust Forms
White rust is condensation damage. When humid air contacts a zinc surface that is colder than the dew point, a film of water forms. If the film stays long enough, the zinc reacts with oxygen and moisture to form a loose white deposit. Condensation is worst when the warehouse temperature fluctuates sharply between day and night, when coils are stored directly on a cold concrete floor, or when a coil is unwrapped in a humid area. The risk is highest in rainy seasons and in coastal regions, so the primary task of inventory management is to control humidity, stabilize temperature and keep air moving around every coil.
Warehouse Environment Requirements
| Parameter | Recommended condition |
|---|---|
| Warehouse type | Indoor, dry, well ventilated, free of corrosive gases |
| Relative humidity | Below 60% whenever possible |
| Temperature | Stable; avoid large day-night swings that cause condensation |
| Roof and walls | Leak-proof; no water ingress from rain or roof condensation |
| Monitoring | Thermometer and hygrometer; dehumidifier or air conditioner during humid seasons |
In practice this means an indoor warehouse with a leak-proof roof, adequate insulation to damp temperature swings, and continuous humidity monitoring. During the plum-rain season of southern China and in coastal areas, a dehumidifier is not optional equipment; it is the main defence against white rust.
Stacking and Spacing Specifications
| Rule | Requirement |
|---|---|
| Contact with ground | Never; coils must sit on skids |
| Skid height | At least 15-20 cm, wood or polymer, strong enough for the coil weight |
| Spacing between coils | 10-20 cm horizontally and vertically to form air ducts |
| Preferred orientation | Upright (vertical, like a standing tyre) to avoid moisture pooling at the centre |
| Horizontal stacking | Only with skids between layers and staggered arrangement; never overload the lower coils |
Upright storage is strongly preferred because it prevents moisture from collecting in the bore of the coil. If coils must be laid flat, stack them in a staggered formation with skids between layers and leave ventilation gaps so air can circulate.
Inventory Turnover and Identification
Strict first-in-first-out (FIFO) is the golden rule of coil inventory. Every coil must carry an identification card or label showing the material (GI, GA or GL), specification (thickness, width, inner and outer diameter), coil or batch number, date of receipt, and surface condition such as oiled or passivated. With this information, warehouse staff can ship the oldest coils first and keep the average storage time short. A regular inspection system, daily or weekly depending on the season, should check for white rust, damaged packaging, deformed stacks and damp flooring; any problem found is recorded and dealt with immediately.
Outdoor Storage and Treatment of Existing White Rust
Outdoor storage should be avoided, but when it is unavoidable for a short period, three rules apply: raise the coils on wooden blocks and cover them tightly with waterproof canvas, keep the cover from being completely airtight so air can circulate underneath and around the material, and make sure the outdoor-stored coils are shipped first. If white rust is discovered during inspection, the coil is removed from normal stock and prioritized for use; light white rust can be wiped off with a dry cloth or treated with a mild chemical cleaner, while heavily affected areas are inspected for coating damage and the buyer is notified if the coating mass may be affected.
FAQ
What causes white rust on galvanized coils?
White rust is caused by condensation: moisture films on the zinc surface react with it to form zinc hydroxide and zinc oxide. High humidity, temperature swings and direct contact with cold floors accelerate it.
What relative humidity is safe for galvanized coil storage?
Keep relative humidity below 60%. Above this level, and especially above 70%, condensation forms easily when temperatures change, and the white rust risk rises sharply.
How high must the skids lift a galvanized coil?
At least 15-20 cm above the floor, using wood or polymer skids, so ground moisture cannot reach the coil and air can pass underneath.
Can galvanized coils be stored outdoors?
Only short-term and only with protection: raised on blocks, covered with waterproof canvas, ventilated underneath, and scheduled as the first coils to ship. Long-term outdoor storage will damage the coating.
How should white rust already present be handled?
Remove the coil from normal stock and use it first. Light white rust can be wiped or cleaned chemically; after cleaning, check that the remaining zinc coating still meets the required coating mass, otherwise notify the buyer.
How long can galvanized coils be stored without damage?
With proper indoor conditions, stable temperature and humidity below 60%, galvanized coils can be stored for 6 to 12 months or longer without significant coating degradation, provided packaging remains intact and inspection is regular.