Chemical Conversion Coatings
Conversion coatings are used intrinsically for long-term corrosion resistance of the powder coating system. The conversion chemistry provides a significant benefit to articles that are exposed to the external environmental or high humidity conditions e.g. architectural components.
The chemical conversion process involves the controlled modification of the surface structure and the creation of a new metal salt complex to a depth of 1 -10 micron. There are essentially two types of conversion coatings used for high performance powder coating systems. These are the phosphate or chromate processes.
The phosphate and chromate processes are used for Iron and Aluminium based substrates respectively.
A new environmentally friendly chromate/phosphate free conversion process is also being evaluated by the architectural sector. However, this technology has not yet been approved to AS3175 or Qualicoat standards.
Iron Phosphate conversion has been used for many years with mild and Stainless Steels, Zinc, Tin, Aluminium and Titanium. The advantage of the Iron Phosphate process is that it reduces cathodic emission of oxygen which if unchecked leads to progressive build up of Iron Oxide corrosion on the product. This results in the expansion of the metal surface through the increased volume of these oxide layers and the subsequent delamination and coating failure of the product.
Zinc Phosphate conversion process is a more recent introduction and is used for plain and galvanized Steels, as well as Aluminium. The conversion process provides very good long term corrosion resistance but has less mechanical flexibility than the Iron Phosphate system.
The Chromium Phosphate conversion process is a hybrid operation. It provides both a chromium and phosphate conversion to the surface of Aluminium and also galvanized Steels. Compared with the straight phosphate processes, Chromium Phosphate provides the most superior corrosion resistant system. As a result of its excellent anti-corrosion properties this system has become highly recommended for architectural finishes, domestic appliance applications and also for aircraft and military equipment.
At Ridgewood Powder Coating we use the latest five stage Chromate Phosphate conversion system for all Aluminium and galvanized steel componentry.
So what do we use for other Iron based products ? Due to the floor space required for a seven stage Iron or Zinc Phosphate pretreatment system, we employ the use of abrasive or ‘whip’ blasting in combination with the new Zinc rich powder primer coatings. This combination process provides excellent corrosion resistance for Iron based materials.