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Conversion Coatings

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 Cationic Process)
  • Iron Phosphating

1. Zinc Phosphating

Zinc phosphating 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 crystalises 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 th 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 process is sub-divided into 3 types:

  • Mono-Cationic Process
  • Di-Cationic Process
  • Tri-Cationic Process

a. Mono-Cationic Process

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.

b. Di-Cationic Process

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.​

c. Tri-Cationic Process

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.
  • Generates 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 Aluminium.

2. Iron Phosphating

Iron phosphating is of limited use in the pre-treatment industry although this has got its own segment. Iron phosphating do not contain any zinc like cation. They are based on sodium or dihydrogen phosphate as amajor ingredient along with accelerators such as molybdate. The coating formed is non-crystalline (amorphous) and having dark bluish colour. The major advantage of Iron phospating is (due to its non-crystalline nature) iron phosphated parts can be welded prior to powder coating. Iron phosphated parts when powder coated give outstanding mechanical properties.

​Conversion coating for Aluminium: Aluminium can be pre-treated by the following ways:

  • Phosphating (Tri-Cationic Zinc)
  • Chromatising (Green, Yellow or Colorless)

a. Phosphating

Aluminium parts can be phosphated like mild steel. For Alumnium mild degreasing chemicals should be used. Heavy, strong degreasing chemicals can cause excessive etching to the base metal. De-rusting process should be avoided in phosphating sequence, which should be re-placed by desmuting process (parts to be dipped in 10% Caustic Solution for 1 to 2 minutes ). Phosphating process is not popular in Aluminium pre-treatment because Aluminium ions beyond a limit can cause poisoning to the phosphating bath.

b. Chromatising

This can be further sub-divided into three different types:

  • Green Chromium Phosphate
  • Yellow Chromium Phosphate
  • Colorless Coating

i. Green Chromium Phosphate

This is general process for the pre-treatment of Aluminium. This bath contian Mixtures of phosphoric acids, chromic acids and additives. This process is more popular in the food industry as it is non toxic. This process forms excellent substrate for powder coatings. The coating formed is dark green in colour weighing 0.5-1.5 gm/m². It gives limited corrosion resistance.

ii. Yellow Chromium Phosphate

This process is more popular in the powder coating industry. The bath contain mixture of chromic acid along with accelerators. This process gives dark yellow coloured (process applicable to OEMs) coating which is having high corrosion resistance. Since this process is based on hexavalent croium, it suffers from the disadvantages such as effluent problems etc.

iii. Colorless Coating

This process is a modified process originated from process b) and used for certain application ( lacquer coat/- clear coat applications) where it is desired to retain the original metallic Aluminium appearance. The coating formed is having limited protected value. This process is not popular in the industry.