How to prevent “White Rust”? Galvanized (Zinc-coated) Steel Passivation in Evaporative Cooling Condensers

When new evaporative cooling condensers are commissioned, special measures should be taken to ensure that galvanized steel surfaces are properly passivated to provide maximum protection from corrosion. Passivation is the formation of a protective oxide layer on galvanized steel surfaces.

  1. The new system should be pre-cleaned to remove oils and construction dirt. Before system’s start-up, fill it with water until the galvanized surfaces are fully covered.
  2. The additional soft water must be diluted with hard water to make hardness within the limit of 50 -150mg/l CaCO3
  3. Start-up the circulation.
  4. Don’t feed ammonia into the galvanized pipes during a period of passivation!
  5. The pH of circulating water should be within the limit of 7 – 8. Add acid or acid-based chemical (e.g. Akvabon CO285).
  6.  Add scale and corrosion inhibitor (e.g. Akvabon CO283 in a dose of 10 mg/l of water volume in the system once per week).
  7. Check the concentration of zinc in the circulating water and/or the condition of the galvanized (Zn-coated) surfaces. (In the case when Akvabon CO283 is used, Zn concentration should be 1,0-3,0 mg/l.)
  8. Protect the system from biofouling by biocides dosing (e.g. Akvabon BI363/368). Biocides are used to prevent the surface from the biofouling film which may cause insufficient passivation spots.
  9. As a result, dark grey passivated coating is formed during a period of min. 4-8 weeks and can be seen visually on the galvanized steel.
  10. After the passivation, in case white deposits form on galvanized steel surfaces after the pH is returned to normal service levels, it will be necessary to do repeat the passivation process.