Removal of oil, soil, lubrication greases, metal oxides, welding scales etc. is essential prior to the powder coating process. It can be done by a variety of chemical and mechanical methods. The selection of the method depends on the size and the material of the part to be powder coated, the type of soil to be removed and the performance requirement of the finished product.
The main aims in the preparation of a metal surface prior to powder coating are defined as follows:
- Removal of all foreign matter, eg. Grease, oil, soil, scale, etc.
- Conditioning of the metal surface for the coating that is to be applied.
- Increase the corrosion resistance of the coating metal.
As with other methods of organic coatings attention to the pretreatment stage is essential in order to achieve the full potention of the powder coating.
Surface pretreatment may vary depending upon the specific end-use requirements of the finished products; from a single step cleaning operation to a multi stage sophisticated pretreatment which deposits a conversion coating on the surface of the metal.
Application of a coating of electrostatically charged particles to an earthed metal surface can only be achieved if the surface is free of any composition which has a high electrical resistance. The presence of any insulating film on the surface of the workpiece to be coated will limit or in some cases prevent powder being deposited.
Steel, aluminium, zinc alloys and galvanized steel are common metals on which powder is used. The best performance can be obtained on thoroughly cleaned metal surface.
Steel: For steel surface, maximum corrosion and salt spray resistance are given by a zinc phosphate conversion coating. Medium corrosion and salt spray resistance are given by an iron phosphate conversion coating.
Aluminium: For aluminium and its alloys, although the clean surface is easily coated and adhesion is excellent, performance can be increased using a chromate conversion coating.
Zinc Alloys: With all zinc based substrates such as zintec (It is rolled steel that has been coated with a thin layer of Zinc), Zamac (the alloys are composed Zinc, Aluminium, Magnesium and Copper) and galvanized steel a suitable phosphate coating is recommended. Generally electro-deposited zinc coatings present no pretreatment problems but hot dipped galvanized coating can affect adhesion.
Porous castings and blast cleaned surface: These surfaces can give considerable difficulty with degassing of the powder coating due to entrapment of air. The profile of the metal and thickness of coating must therefore be strictly controlled. Preheating for a few minutes sometimes overcomes this defect. Element have special formulation to overcome of this problem.