Something known as “white rust” can occur on the surface of galvanized steel during storage, fabrication and initial operation of cooling towers. White rust appears as a white, waxy or fluffy adherent deposit. If it goes unchecked, the galvanized steel coating will continue to erode, eventually leading to an early failure of the galvanization in the cooling tower.
The best cure for white rust is an ounce of prevention when beginning operation of the tower. Consult a water treatment specialist and ask about his program for initial system operation and passivation of a galvanized steel cooling tower.
Early operation with near neutral pH water and use of only reasonable levels of film-forming inhibitors is encouraged. Many water treaters suggest corrosion inhibitor treatments of the phosphate/polymer/phosphonate type, which have not promoted white rust in galvanized steel.
Initially operate the cooling tower with water of pH between 6.5 and 8.0, and with hardness and alkalinity values between 100 and 300 ppm for at least eight weeks. That should be enough time for the protective “basic zinc carbonate” layer to form. As an alternative, other acceptable water treatment methods such as inorganic phosphate passivation may be considered.
Source: Marley Cooling Tower Co.
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