Pretreatment

PRETREATMENT

Introduction to Pretreatment

Pretreatment is a process. which is required prior to any powder coating system just to ensure- (A) A clean, oil free surface (B) To impart adhesion (C) To reduce the corrosion rate by stopping the rust creep age. (D) To have a neutral layer in between the powder coating and surface It is very important to note that the performance of any painting system is 100% governed by the correct base pretreatment. As of today, Indian finishing industry is using following surface pretreatment processes prior to powder coating.

Chemical pre-treatment

Details of each stage

1.Degreasing – This stage removes oil, grease, dirt and other dust particles to give a clean, surface for next stage in the process.

2.Water rinse – To remove the chemical from the earlier stage and make the surface totally free from any contamination prior to entering into next stage in the process.

3.Derusting – This stage removes rust from the metal and at the same time make the surface little reactive for the next stage in the process.

4.Water rinse – As per point No. (2)

5.Activation – This process provides fine active crystal centres on the surface of the metal which ultimately results into fine phosphate coating layer in the phosphate stage. Process with activation gives uniform, compact crystalline coating with good corrosion resistance.

6.Phosphating – It gives a deposition of insoluble metal phosphate by chemical reaction. Since the deposited coating forms by a chemical reaction it gives good adhesion and good corrosion resistance to the powder coating film.

7.Water rinse – As per point No. (2)

8.Passivation – This stage helps in sealing the cavities in the phosphate coating and thus increases the corrosion       resistance of the coating.

Pre-treatment stages in details

A. Solvent wipe
This is the cheapest and best method to remove heavy and sticky oil/grease like substances from any surface. The parts to be cleaned are wiped with a rag of cotton soaked in a solvent.

Advantages
Cheapest method of industry to remove heavy oils/sticky press compounds, etc. Does not require skilled labour and costly plant installation. Suitable from smallest job coater to large OEM.

Disadvantages
Labour intensive process. Hazard of fire risk due to use of solvent.
Frequent cleaning of cloth/solvent is necessary to maintain the cleaning standard. Disposal of waste solvent can cause effluent problem.

B. Emulsion Cleaning
They are based on mild alkalies and operate at lower temperatures. They are generally based on kerosene – emulsifier or turpentine emulsifier. These cleaners are generally milky in appearance. They have a limited cleaning tendency compared to alkaline cleaners and they leave a very thin film of solvent/emulsifier over the substrate even after water rinsing . They should be followed by alkali cleaning.

Advantages
Energy Saving as bath operates at lower temperature (45-50°C)
Suitable for all (Ferrous/non ferrous) metals.

Disadvantages
Limited cleaning tendency, cannot remove heavy oil/greases, etc.

C. Alkali cleaning

Alkaline cleaners enjoy superior position in the pretreatment world due to following

  • They operate at room temperatures.
  • Bath is more stable compared to other degreasing processes.
  • Bath control is very simple
  • Costly plant installation is not always required
  • Higher capacity to absorb oil.

These cleaners are based on strong alkalies like sodium hydroxide/sodium silicate with surfactants and fillers.The parts to be cleaned are immersed in the bath at specified temperatures and time.

This process used to remove rust and mill scales from the substrate. It has following advantages and disadvantages. Advantages Cheaper process than blasting or flame cleaning to remove rust. Costly plant/installations are not always needed. Also makes the surface reactive for next phosphating stage. Disadvantages Use of HCL/H2SO4 can create corrosive atmosphere in the plant. Not suitable for Spray process Chemical carry over in next stage (phosphating) can cause problem

Hydrochloric acid based

Sulphuric acid based

Phosphoric acid based

Used mainly–to remove
mill scales

Used for heavy rust

Used for mild Rust

Can create corrosive plantatmosphere due to fuming tendency

Can cause corrosive plantatmosphere

No Harm

Carryover in phosphate
stage can poison the bath

Carryover in phosphate stage can poison the bath

No Harm

Hassevere pitting tendency

Hassevere pitting tendency

No pitting tendancy

Cheapestacid for derusting

Cheaperthan phosphoric

Costlybut more used by the industry

 This stage provides fine active centres on the surface of the metal which results into fine phosphate coating in the phosphating stage. Finer the phosphate coating better the corrosion resistance. This stage also controls the phosphate coating weight thereby controlling the phosphate chemical consumption. There are two types of activation processes.

a) Acidic activation
This is a cheaper process in which bath control is very easy. Bath can be
prepared in hard water. This process gives coarse and bigger crystalline coating. Not suitable for non ferrous metals. Bath is more stable.

b) Basic activation
This process is based on Ti compounds and suitable for both ferrous/non ferrous substrates. It gives more compact/microcrystalline, Uniform phosphate coat with better corrosion resistance.
 The major disadvantages are:
 (i) The bath cannot be made in hard water
 (ii) Shelf life of bath is very low (One week maximum)

It is the universal method of pre treatment. Phosphating consist of the deposition on the metal surface of the insoluble metal phosphate which are chemically bonded to the substrate. This process provides the following. a) A clean, oil grease free surface. b) A corrosion inhibitive base for powder coating c) A non conductive bond between base metal & powder coating. d) A chemically inert surface which prevent the reaction between the best metal and powder ingredients. Types of zinc phosphating
Mono Cationic Di Cationic Tri Cationic
1 Contains only zinc as a basiccation Contains zinc and nickel as basiccations Contains zinc nickel and manganeseas basic cations
2 Zinc imparts good adhesion andcrystal formation Nickel contributes to corrosionresistance Manganese improves the wear andtear properties of the coating.
3 Generates hard sludge Generates hard sludge Generates soft sludge
4 Operatesat higher temperatures Operatesat lower or even at room temperatures Operates at lower or even at room temperatures.
5 Superior in terms of adhesion, corrosion resistance and wear and tear properties.