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