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White Rust on Galvanized Steel Date: 8/09 ENGINEERING BULLETIN No. #36A Product: White Rust on Galvanized Steel Date: 8/09 White Rust on Galvanized Steel Introduction White Rust Galvanized steel has a long history as an White rust is a rapid, localized corrosion attack effective and economical material of on zinc that usually appears as a voluminous construction for evaporative cooling systems. white deposit. This rapid corrosion can Galvanized steel consists of a thin coating of completely remove zinc in a localized area with zinc fused to a steel substrate. This combination the resultant reduction in equipment life. If the provides material that has the mechanical white-rust corrosion product is kept wet it often properties of steel enhanced with the corrosion feels waxy; if the corrosion product dries it resistance of zinc. usually feels hard and brittle. Beneath the white Evapco uses two types of galvanized steel. The deposit there will be a localized area where the first is G-235 HMG (Heavy Metal Galvanized) zinc has been attacked. In its early stages this sheet steel which is used for casings and basins. area will appear as a shallow pit. See Figure 1. This steel is coated in a continuous operation at White rust is most likely to appear on new the steel mill. The average minimum coating equipment as it is being put into service. The thickness per ASTM A653 is 2.0 mils per initial few weeks when zinc is exposed to water surface. The typical zinc thickness on HMG must be carefully controlled since this is the sheet runs very close to this minimum average. period where the metal is most prone to white The second type, Hot Dip Galvanized (HDG) rust attack. steel, is a batch-processed metal used for coils White rust attack can occur on an older zinc and other non-sheet components. The minimum surface if it is exposed to high pH, very high average coating thickness for tubes per ASTM conductivity water, or if the old-surface has been A123 is 1.8 mils, though Evapco coated tubes scraped off or aggressively cleaned. typically run thicker. Figure 1 – Waxy White Rust Cells Zinc Passivation Like many metals, the good corrosion properties Parameter Passivation Value of zinc are due to a thin film of corrosion pH 7.0 – 8.0 product that forms on the surface of the metal. This tightly-adherent corrosion film is called a Conductivity <2,400 passivation layer. The formation of the Calcium Hardness >50 ppm passivation layer is essential to achieving long- life with a galvanized product. Chlorides <250 ppm Zinc will naturally form a passivation layer Sulfates <250 ppm when exposed to weathering over an extended Alkalinity <300 ppm period of time. Rain combined with air drying will, over the course of a year or more form a Chlorine (biocide) <0.5 ppm passivation layer on all areas of the zinc which do not come in contact with recirculating water. Fresh zinc has a bright shiny appearance; Thus, the exterior of evaporative equipment and passivated zinc acquires a dull gray metallic all interior areas above the drift eliminators will appearance. See Figure 2 and Figure 3. naturally develop a passivation film. Before the cooling equipment is delivered to the For the components of the system that are site, Evapco recommends that a written exposed to or immersed in recirculating water, passivation plan be developed. This plan should the water chemistry must be carefully controlled be based on the site-specific make-up water and until a passivation film forms. Once the film include how the appropriate water chemistry forms, the recirculating water chemistry, though will be maintained, how the system will be run, still important, is less critical for zinc corrosion. and who will be responsible for conducting the If the passivation film is damaged by abrasion or passivation. chemical attack, it must be re-formed by again carefully controlling the recirculating water Summary chemistry. Galvanized steel is an excellent material of It is very difficult to control these water construction for evaporative cooling systems, parameters and form a passivation layer in a which, with proper care, will have a long service system started under load due to the higher life. Before any water is added to a new piece temperature and the continually changing water of galvanized equipment, the equipment chemistry. Therefore, if possible, the equipment operator should have a detailed plan for should be commissioned with water circulation passivation of the galvanized metal and and without a heat load. The critical parameters continued water treatment after passivation. to form the passivation film are described in the See your local water treatment specialist for following table. It generally takes 4 to 12 weeks more information. to passivate zinc with pH control the most critical parameter and with the first few weeks of water exposure being the most critical time. Figure 2 Figure 3 Bright and Shiny Dull Gray Fresh Zinc Passivated Zinc .
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