Galvanized steel has been used for years in cooling tower manufacturing. The benefits are that galvanized metal is low cost, high strength, easy to fabricate with, and generally has a long life against corrosion. Other metals, such as stainless steel are preferred due to corrosion resistance but are more costly at the time of purchase.
Cooling towers that have galvanized metal require special care during startup. The goal for starting up a cooling tower with galvanized metal is to form a protective passivation layer on the galvanizing that will provide protection from long-term corrosion.
This passivation process develops under neutral pH conditions, where a grey-colored passivation layer of zinc carbonate will form on the metal to protect it from corrosion. Under alkaline conditions, a white-colored layer of zinc hydroxide will form. This layer will be porous and can allow for accelerated under-deposit corrosion. This white-colored deposit is referred to as white rust. Towers that are not treated properly at start-up can develop this white rust over time that can destroy the galvanized coating and the base metal underneath.
During the first 90 days of operation, certain chemistry and operational parameters are controlled to help develop the passivation layer on the metal. While different cooling tower manufacturers have varying recommendations for the passivation process, controlling the pH between 6.5 and 8.0 is the most important factor to control during the passivation phase. In addition, controlling ortho-phosphate levels in the 12-18 ppm range is also recommended for this process. Please refer to your specific tower manufacturer for other specific items such as water chemistry parameters, etc.
Once the passivation layer is established over the startup phase, you can switch to a standard cooling tower treatment program. QualiChem can assist at this early phase to help determine the best precleaning and passivation approach for your specific cooling tower system’s needs.