Wet Storage Stain is the term commonly given to the white rust which will later turn to a black and finally a red rust when moisture rests on zinc coating. The white or gray deposit is formed by the accelerated corrosion of the zinc coating when it is stored in damp and poorly ventilated conditions.
Zinc is a very reactive material and will form zinc oxide when mixed with oxygen from the air; this is the thin hard layer that forms the protective film associated with galvanized coatings. The coating needs carbon dioxide to create this layer 2ZnC03.3ZN(OH)2. Once formed, this layer is weather-resistant and minimizes further corrosion.
The problem arises when galvanized parts are deprived of air and exposed to moisture. This causes a different set of chemical changes in the zinc. Droplets of water, when flattened between two surfaces such as items mounted to pallet rack beams create problematic environments for zinc, which needs exposure to air. The zinc is attacked by the water and zinc ions constantly leave the coating to be bound by the water; the absence of carbon dioxide accelerates the corrosion. The zinc cannot be converted to zinc hydroxide to form the protective film and corrosion will persist as long as these conditions prevail.
Galvanized Beams Previous Covered and Deprived of Air
|First Stages – Galvanized Corrosion||Later Stages – Corrosion|
First Stage: Dimples in signage surface create pockets for moisture
Pockets of moisture deprive the zinc coating of carbon dioxide which is needed to form its protective “anodized” film. The first reaction is the white rust which is then accelerated to black rust as the zinc continues to be deprived of air.
Later Stages: Red/Brown Rust
The most advanced step is the red/brown rust that forms after the zinc has been continuously deprived of air.
Stand product where all surface areas will have:
- adequate ventilation
- chemical free environment
- clean surface, free of soils and buildup which prevent air flow
- Use magnetic signage only in dry area
- Remove items from beam which have trapped in moisture and let zinc “breathe”
- If there is sufficient zinc coating remaining, scrape existing white rust with a stiff bristle brush (not wire) and expose to air
- If red/brown rust has formed, remove deposits with a stiff bristle brush (not wire) and recoat any surfaces which have no zinc remaining
- See ASTM – A780 (“Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings) for more detailed instructions on repair.