“Cold galvanizing”, or Zinc painting is the application (by brush or spray) of Zinc dust mixed with organic or inorganic binders. Commonly containing 92-95% metallic Zinc in dry film, these Zinc-rich paints can be applied to structural steel and piping which are exposed to a wide range of corrosive atmospheric environments like refineries, power plants, bridges, buildings, mining equipment etc.
With a high percentage of Zinc in the dry film, these paints can synergistically combined with the corrosion inhibitive properties of metallic Zinc. The Zinc dust in paint is integrated with organic binders. These binders allow the Zinc particles to remain in contact with each other so the Zinc paint can provide cathodic protection.
While Zinc protects structures from corrosion by hot-dip galvanizing or cold galvanizing, there is high intrinsic value in using both the techniques together, known as a duplex system.
When hot-dip galvanized steel is painted or powder coated by Zinc rich paints, the duplex coating provides a much more sophisticated manner of corrosion protection known as the synergistic effect. The exterior layer of paint or powder coating acts as an additional barrier and slows down the rate at which the Zinc is consumed – greatly extending the life of the galvanized steel. In return, the underlying galvanized coating acts as a noble primer eliminating the pinhole corrosion and peeling effect often seen on bare steel. Once the exterior layer has been weathered down or damaged, the Zinc beneath is still available to provide cathodic and barrier protection.
As a result of this synergistic effect, the substrate steel is afforded corrosion protection for 1.5 to 2.3 times the sum of the expected life of each system alone, the synergy between the two systems provides protection far superior to other systems.
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Article by- Pavan Kaushik, Head – Corporate Communication, Hindustan Zinc