Powder Coating Pre-Treatment

Metal Pre-treatment prior to powder coating is an essential step to ensure proper coating performance. Metal Pre-treatment prior to powder coating is of two types.

a) Mechanical surface preparation.
b) Chemical surface Preparation.

For powder coating the most commonly used surface preparation method is chemical pre-treatment.

It is essential for an applicator to know the following:

  1. Need for Pre-treatment
  2. Surface Preparation
  3. De-rusting Processors
  4. Activation
  5. Conversion coatings for:
    • Mild steel
    • Galvanizing steel
    • Aluminum
    • Mazak
  6. Trouble Shooting

1. Need for Pre-treatment:

The performance of powder coating on a metallic surface mainly depends on proper pre- treatment of the substrate.

Proper pre-treatment is an essential factor because of the following advantages:

  1. The pre-treatment process provides a clear, uniform, oil-grease free surface.
  2. It provides good adhesion of film.
  3. It forms an inert layer which inhibits the corrosion of the paint film

General Pre-treatment Process sequence:

  • De-greasing Process
  • Water-Rinse
  • De-rusting
  • Water rinse
  • Activation
  • Conversion Coating
  • Water rinse

De-greasing Process:

Chemical de-greasing methods used in any particular application is closely related to the nature of the surface being cleaned and the amount and type of contamination.

The various chemical decreasing processes applicable to Mild steel, Galvanised Steel & aluminum substrates, are as follows:

Solvent Cleaning

a) Solvent Wipe: Solvent cleaning is the cheapest and best method to remove heavy or sticky oil/grease like substances from any surface. Since all solvents are almost neutral they do not attach to the base metal. In this process, the parts to be cleaned are wiped with rag of cotton soaked in a suitable solvent such as Kerosene, Benzene, Naptha etc. The following are the advantages and disadvantages of solvent wipe method.

Advantages:

  • Cheapest cleaning method available in the industry to remove heavy oils, greases or sticky press compounds.
  • Skilled labour and costly plant installation is not required.
  • Suitable for the smallest job coater as well as for the OEM industry.

Disadvantages:

  • The process is labour intensive.
  • Since all the solvents are flammable, a greater fire risk is associated with the solvent cleaning.
  • Frequent change of cloth and solvent is essential or otherwise it can affect the quality of cleaning
  • Solvents going into the drainage system can cause effluent disposal problems.

Conversion Coatings

Phosphating is universal method of metal Pre-treatment. Phosphating consist of the deposition on the metal surface of insoluble metal phosphates which are actually chemically bonded to the substrate, since this is a chemical reaction, it gives good adhesion for paint film. Phosphating can be either crystalline or amorphous.

This process provides the following:

  • A clean, grease/oil free surface
  • A corrosion inhibitive base for powder coating
  • A non-conductive bond between base metal and powder coating
  • A chemically inert surface which prevent the reaction between the base metal and powder/paint ingredients

Phosphating process is divided into 2 types:

  • Zinc Phosphating (sub-divided into Mono, Di, Tri Carbonic Process)
  • Iron Phosphating