Galvanizing

Factory assembled cooling towers are often constructed from galvanized steel. Steel is a great structural material but requires heightened protection when applied to the corrosive environment of cooling towers.

'Galvanizing' consists of adding zinc to the outer surface of the steel. Since zinc corrodes more easily than steel, the ideal situation will have the zinc slowly depleting over time thereby protecting the steel. Once the zinc is gone, the steel will begin to rust.

There are two ways to galvanize steel. Sheet metal is galvanized during its production by immersion in an electrolyte while zinc is deposited on the surface. The thickness of zinc can be regulated. The G90 specification means .9 oz. of zinc is added per square foot (total both sides), G210 means 2.1 oz, G235 means 2.35 oz, etc..

Zinc can also be deposited to raw (or 'black') steel by immersing it in a molten vat of zinc. The steel is first dipped in a strong acid solution to prepare the surface to receive the zinc. This 'hot dip galvanizing' process is typically reserved for parts that are fabricated into welded sub assemblies. The parts can be quite small or as large as an 18' long coil assembly.

The term 'double dipped' would seem to imply the part is dipped two times to achieve extra zinc thickness but it really means the part is too big to be dipped in just one direction and has to be turned over and dipped again for complete coverage.

The cooling tower operator should always be on the lookout for operational procedures that deplete the zinc layer. Strong chlorine solutions are especially bad. Never toss chlorine tablets into the tower basin; they will burn right through the zinc and steel. Steam condensate is especially bad as are strong acids used for scale removal. Proper water treatment will help retain the protective zinc layer and do away with the need for scale removal by drastic, destructive means.

Give us a call once the zinc is substantially gone and corrosion of the steel begins. At that point, cooling tower life can be extended by installing a urethane barrier to accessible areas in order to arrest corrosion.

Don Davis

Expert Cooling Tower Repair, LLC