Surface preparation
Metal Pre-Treatment

Metal Pre-Treatment prior to powder coating is of two types mechanical and chemical surface preparation. It is an essential to ensure proper coating performance. For powder coating the most commonly used surface preparation method is chemical pre-treatment.

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Essential Knowledge


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


It is necessary for an applicator to know the following:

Need for Pre-treatment

Surface Preparation

De-rusting Processors


Conversion coatings for mild steel, galvanizing steel, aluminium, mazak

Process Sequence

Water Rinse

Water Rinse


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 & aluminium substrates, are as follows:

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.


  • 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.


  • 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.


Corrosion is common phenomenon for metals. When iron or steel is exposed to humid atmosphere the corrosion process is initiated resulting in rust formation.Rust is the oxide of iron which is loosely adhered to the substrate and hence it is very dangerous if overheated by any surface coating. Rust is readily soluble in acids such as hydrochloric acid, sulphuric acid, phosphoric acid and hence it can be removed by acid cleaning.


  • Cheaper process than blast cleaning or flame cleaning.
  • Can be done anywhere, no big plant or sophisticated equipment is required.
  • Remove rust, mills scale.
  • Makes the surface reactive for the next Phosphating stage.


  • If hydrochloric acid or hot sulphuric acid is used for de-rusting it can create a corrosive atmosphere in the plant.
  • Not suitable in case of spray application.
  • Carry over can create problem in next stage (i.e.) in phophating bath

De-rusting can be done with below acids/acid combinations:

Hydrochloric acid is a fuming acid and is mostly used for ”Pickling”, as it readily dissolves the mill scales formed during the hot rolling operations. Hydrochloric acid pickling is the cheapest de-rusting process. The use of Hydrochloric acid is very limited in the industry due to the corrosive nature of the Hydrochloric acid fumes which can create problems in the coating plant and secondly, the carry over of Hydrochloric acid to the next pre-treatment stage (i.e.) either activation or phosphating, can damage the bath permanently.In the case of Pure Hydrochloric acid de-rusting the tank should be of Stainless Steel.

Activation Process

This process provides fine active crystal centres on the surface of the metal which ultimately results into tine phosphate coating Layer in Phosphating stage. This process helps to attain a uniform phosphate coating. There are two types of activation processes:

Acidic Activation

    This is a cheaper process. Here bath testing and control is not required. Bath can be prepared in hard water. The process results into coarser bigger crystalline coating. Not suitable for non ferrous substrates such as Aluminium.

Basic Activation

    This process is based on titanium based compounds which are most suitable for Ferrous & non Ferrous substrate. It gives more compact, uniform fine crystalline coating having better corrosion resistance.


  • The bath is unstable (Titanium forms colloids in aqueous solutions).
  • Bath cannot be prepared in hard water.

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.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

Zinc Phoshating process is widely in the automobile, hardware, home appliance industry. Zinc Phosphating bath solution contain a saturated solution of phosphoric acid along with Zinc Phosphate. On immersion of an article in such a bath the iron gets attacked by the acid component of the bath, by lowering its concentration at the metal surface.After certain time (within 2min. Maximum) the Phosphate crystalites on the metal surface. Since this is a chemical reaction between the metal and the Phosphating solution, it stops after the formation of crystalline Phosphate layer. It is not advisable to keep the parts/components in the Phosphating bath for more than 10 minutes because the bath pH is acidic (between 4.8 to 5.5, depending on the bath concentration) and at this pH the coating formed gets dissolved slowly thereby leaving the bare metal, which is again attacked by the Phosphating solution forming fresh crystalline layer of Phosphating. This process may go on till the end of the metal and it unnecessarily increases the chemical consumption.

Zinc Phosphating is sub-divided into 3 types:


    Cations are positively charges ions. Zinc is the basic cation in all crystalline phosphating processes. Mono-cationic processes are comparatively cheaper than di & tri cationic processes. The bath solution contains only zinc as a cation. These processes find a very limited use in the industry as they have a limited corrosion resistance.


    In this type the bath solution consist of zinc and Nickel as cations. Zinc imparts adhesion and Nickel contributes to the corrosion resistance. Hence coating formed in this type is more durable and withstand drastic corrosive atmosphere than Mono-Cationic.


    In this type, the bath solution consists of zinc, nickel and manganese as cations. Manganese improves wear resistance of the phosphate coating. Hence coatings formed in this type are more superior w.r.t adhesion, corrosion resistance and wear resistance.

Today Tri-Cationic process is more popular in the metal Pre-Treatment industry because:

  • It gives superior quality coating along with better corrosion and wear resistance.
  • Enerates lesser amount of Soft Sludge as compared with Mono or Di processes.

This process is mostly acceptable by automobile industry (most used prior to Electro-deposition process) Home & consumer appliance industry etc. Suitable for Mazak and Aluminum.