4608 (RP-988) -- An Investigation of Zinc Oxide Corrosion of Galvanized Steel in Cooling Towers ✓ Most Recent

An investigation was performed on the effect of various chemical treatments on the oxidation of zinc surfaces by high pH, hard water. The combination of zinc surface coupons and hard water was used to simulate the conditions as they typically exist in galvanized cooling towers. Oxidation on zinc surfaces in high pH hard water is characterized by a white powdery film, which has been referred to as white rust. White rust has been reported to consist of zinc carbonate compounds. The chemical treatments chosen were used in a range of experiments to determine if their use could prevent or minimize the oxidation of the zinc. The zinc corrosion in this investigation was identified as zinc oxide and not primarily zinc carbonates.

The experiments included submerging coupons in treated water, spraying coupons with treated water, and subjecting in-line coupons to a relatively high surface velocity so as to simulate a cooling tower environment. From the experimental results, it appears that the treatment chemicals HEDP, molyb date, and ozone are three treatments that at least inhibit the formation of zinc oxide compared to untreated water. The relatively small coupon mass loss when using the molybdate treatment may have been due in part to its relatively large concentration (~200 ppm) compared to the other treatment chemicals (~10 ppm). In the test of the coupons for 200 hours with ozone and 100 hours without ozone, using the same water reservoir, the ozone treatment tended to minimize zinc oxide formation.

It is concluded the addition or application of treatment chemicals has the potential to significantly reduce, if not prevent, the formation of zinc oxide in high pH hard water, in the absence of a detailed passivation treatment program for galvanized cooling towers. The surface analyses performed for determining the compounds represented by the corrosion indicated zinc oxide, possibly hydrated or with carbon-oxygen bonding.

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