Wet storage stain is a bulky white or gray zinc corrosion product that develops on closely packed galvanized articles deprived of freely moving air, and exposed to standing water or other moisture. Without the free flow of carbon dioxide over the galvanized coating, the surface retains water and forms an oxygen concentration cell where corrosion of the zinc metal can occur. Corrosion of the zinc metal then forms into the bulky white corrosion product. Wet storage stain occurs after the steel reacts with the environment, not during the galvanizing process.

The attack of wet storage stain is accelerated when the retained moisture contains chlorides from sea water, sulfur compounds from industrial environments, or flux residues from the galvanizing process. However, severe damage can be done to the galvanized coating in a relatively short time by water alone when medium or heavy wet storage stain is left untreated.

See also:
Wet Storage Stain Publication
Wet Storage Stain vs. White Rust Dr. Galv Article