Cooling Tower White Rust (Problem Identification and Prevention)

The corrosion of galvanized steel cooling towers is commonly referred to as white rust.   White rust corrosion occurs predominantly with newly installed galvanized steel towers and related cooling tower components.  White rust shows up as scattered pits across galvanized surfaces, and is easily identified by a white, gelatinous or waxy deposit. If the conditions are right, left unattended full penetration through the base metal can easily occur ultimately leading to holes in the tower and premature failure of galvanized steel components.  The water conditions known to aggravate white rust are high alkalinity/high pH, and low hardness water.

Water treatment professionals can recommend various methods of prevention, but some of these methods are only successful when alkalinity/pH and hardness levels can be consistently maintained over an extended time frame. Cooling tower manufacturer’s offer guidance about water quality, and that guidance MUST BE CONSIDERED to protect warranty conditions prior to purchase of a new tower.  A functional water analysis of cycled conditions is an essential, simple, but often overlooked consideration by owners. Armed with proper insights, the decision not to use galvanized steel may very well be the best one.  Other materials of construction include stainless steel or even various types of plastic.  Prevention strategies include:

Preliminary pre-cleaning and Passivation

  • Operating within specified water quality guidelines
  • Use of pH Control where appropriate
  • Factory Coatings
  • After-Market Coatings

Caught early enough, it is possible to save new cooling towers from destruction.  First, the white rust deposits would be cleaned (chemically or mechanically), exposing the base metal.  This step can then be followed by re-application of a zinc-rich coating product and subsequent tight control of water chemistry.  This is an substantial, but avoidable problem.  As always, information and knowledge is the key to a successful strategy.

 

By John Caloritis

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