White Rust: Causes and Prevention
*Updated July 2026
Many perceive wet storage stain to be the same as white rust and use the two terms interchangeably. The truth is these two phenomena are not synonymous and there are distinct differences. Though both involve a build up of white corrosion products on the surface, white rust is specifically associated with cooling towers. White rust can occur on unpassivated cooling towers in service in water with a pH level above 8.2 or 8.3. If the cooling towers cannot be chromate quenched, the solution is to use a phosphate treatment and operate the tower at a pH no higher than 7.8 for a few months, then return to normal operating levels.
This November 2018 article from Heating Ventilation Air Conditioning and Refrigeration News reviews the differences between white rust and wet storage stain, and provides a list of causes and prevention for white rust on cooling towers.
See also:
Wet Storage Stain Publication
Wet Storage Stain vs. White Rust Dr. Galv Article
© 2026 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.