Moisture trapped in the bundle has caused the zinc coating to oxidize, creating what is commonly referred to as “white rust” or “wet storage stain”.
How does this happen? In order for a galvanized coating to do its job of protecting the base metal, a zinc carbonate passive film must form when the zinc first gets wet and dries. Inside a bundle of tightly-nested deck sheets, water is easily trapped and can quickly corrode the zinc away and begin to attack the base metal. What’s worse, it does not have to rain on the bundle for moisture to get inside. Condensation can also trap moisture inside a bundle of type B roof deck.
Can I repair it and still use the deck? It depends. This is what you should consider:
- Is the deck going to be field painted? If so, the white rust will need to be completely removed using a wire brush or steel wool. This is very labor-intensive and you may decide it is more cost-effective to replace the deck.
- If the deck is not going to be field painted, has the base metal been compromised?
- Use a wire brush to remove the heaviest white rust areas and look for signs of red rust.
- Clean the red rust and look for signs of pitting in the steel where the base metal thickness has been reduced. If there are pits or holes in the base metal, the deck should be replaced.
- Remember, this is thin metal to begin with, so any loss of that metal may result in a significant reduction in strength.
- If the deck is not going to be field painted and the base metal has not been compromised (i.e., the deck is structurally sound), what is the interior environment of the building?
- If the deck is not exposed to view and the environment is dry and non-corrosive, remedial action may not be necessary, but the deck should be installed and covered with roofing the materials as soon as possible.
- If the deck is exposed to view and the environment is dry and non-corrosive, check with the owner to determine what remedial action is required.
- If the interior environment is moderately corrosive or has a high-moisture atmosphere, consult with the designer and owner to determine whether the zinc coating lost due to oxidation is acceptable, or if the impacted areas must be cleaned and treated with a zinc-rich primer.
How can I prevent this in the future? Follow the Steel Deck Institute Guidelines for proper jobsite storage:
- Store the deck off the ground.
- Elevate one end of the bundle to provide drainage.
- Cover the bundle with a waterproof covering, but allow ventilation at each end of the bundle to avoid condensation.
Finally, please keep in mind that the best way to limit damage during storage is to limit the amount of time that the deck is stored!