Changes in the galvanizing process and water treatment chemistry have led to an increase in white rust in cooling towers and evaporative condensers. White rust, once it has formed can reduce the lifespan of a cooling tower from approximately 15-20 years to as little as 3 – 5 years if left untreated.
Understanding White Rust When looking at a galvanized piece of steel, have you noticed white deposits on the surface? White rust is a white deposit that forms on wet galvanized steel surfaces. The white deposit is the loss of zinc in the galvanized coating on the steel. As the zinc is lost on the surface, the base metal underneath is left vulnerable to corrosion. Once white rust has formed, it can be challenging to control, which is why protecting equipment is key in preventing irreversible damage to a cooling tower.
Protecting With Passivation Manufacturers of galvanized cooling towers recognize the risks of corrosion and white rust and defend the steel by applying a protective coating containing zinc. The zinc bonds to the steel underneath, forming a sacrificial layer. If the water in the basin has a pH out of range and contacts steel that has not been passivated, the water will begin to strip away the sacrificial layer through oxidation, exposing the base metal, and leaving it open to corrosion.
The passivation process could take up to 12 – 14 weeks where the zinc coating is converting into a more stable form of zinc that can better withstand long-term exposure to various conditions in a cooling tower. The process uses specialized chemical additions and strict pH control to maintain specific water chemistry conditions. Passivation should be completed before a new tower has a heat load added. If the tower is already online it can still be passivated, but can be more difficult to accomplish successfully, and may require specific chemical additions to control the water’s pH and other parameters.
When white rust has formed on a cooling tower, the water treatment plan will pivot from prevention to control. During tower cleanups, the technician can scrape and brush away the rust, and coatings and paint can be applied. However, the inability to reach every piece of metal inside the equipment can make it difficult to successfully repair, and white rust can continue to develop.
What’s At Risk? Galvanized metal in a cooling tower that is exposed to water must be passivated to prevent white rust. Even towers made from stainless steel, fiberglass, and plastic can contain pieces of hardware that are galvanized steel, leaving the tower susceptible to white rust.
Prevention is crucial in preventing white rust, so if you are planning to purchase a new galvanized cooling tower or concerned about an existing cooling tower, visit bradyservices.com/services.
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