of Galvanized Steel Cooling Towers
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Zinc coating (Galvanizing) of steel to reduce corrosion has been used commercially for almost 150 years. In simple terms, the process consists of dipping the steel in molten zinc to coat it with a thin layer of zinc alloy of usually 1 to 2 ounces per square foot. The corrosion protection results from 1) zinc being anodic with respect to the steel, providing galvanic protection and 2) under atmospheric conditions, the zinc forms a barrier due to the formation of insoluble corrosion products on its surface. The primary compound affording corrosion protection is Zinc Carbonate.
WHAT IS WHITE RUST?
Until around 1986, the phenomenon known as "white rust" did not exist. White Rust is the formation of bulky, white corrosion products formed on galvanized steel cooling tower components. This results in rapid and increased rates of corrosion of the galvanized zinc layer, rendering it ineffective as a corrosion inhibitor. Analysis of the White Rust indicates that it is a complex of Zinc Hydroxide, Zinc Oxides, and Zinc Carbonate. Unlike the Zinc Carbonate alone, this complex is not self-limiting, but continues to form at a fairly rapid rate unless treated properly.
White Rust began to appear in cooling towers with galvanized components for two basic reasons. First, changes in the galvanizing process resulted in the surface of the zinc alloy containing slightly different elements. While the cause is not yet completely understood, it appears that the increased aluminum content may be a factor. Secondly, the shift in cooling water treatment chemistry to higher alkalinity and pH programs also appears to contribute to the problem. Since when White Rust first occurs, it forms as Zinc Hydroxide, Zn(OH)2.
WHERE IS THE WHITE RUST FOUND?
White Rust is commonly found to occur in new cooling towers, often within weeks of start-up. It is thought that White Rust begins to form on the new galvanized surfaces that have not had time to form any corrosion resistant compounds or barrier films.
WHAT IS THE CAUSE?
The primary cause appears to be high pH or alkalinity in the cooling water which results in the zinc hydroxide formation. While the reports vary, a pH over 9 or an alkalinity over 800 ppm seems to be common where White Rust exists. Numerous reports indicate that the soft water may also contribute to White Rust problems. It appears that White Rust occurs in galvanized towers made by all of the manufacturers.
Another cause may be the use of alkaline cleaning compounds used in the start-up cleaning process. Typically, cleaners used to remove oils and other contaminants from piping systems are very alkaline and contain caustic soda. This may expose the new, clean surface to high local pH levels, which result in rapid development of White Rust after start-up.
CAN WHITE RUST BE REMOVED?
At this point, there does not seem to be a clear-cut, easy way to remove existing White Rust. Some success had resulted from power washing and reducing the cooling water pH to the 7-8 range. However, success at this point seems to be limited to halting the progress of the White Rust.
DO NOT reduce the system pH below 7, or acid clean the system. This can result in the complete loss of the galvanizing coating.
HOW TO PREVENT WHITE RUST
The key to success with White Rust is to prevent its development in the first place. Three tower manufacturers have offered their recommendations as follows: All relate to the first few weeks of operation.
After passivation, continue to use the ASSURANCE LINE product, as recommended by your M.I.S. of America Representative. Allow cycles to increase so that the system pH does not go above 9. If White Rust begins to develop, reduce pH to 8.5 or less by increasing bleed-off.
EXISTING GALVANIZED TOWERS
Treat as in #3 above. Do not allow the cooling water pH to go above 9. Report any occurrence of White Rust to your Water Treatment Representative at once.
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