Metal Pre-Treatment
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.
Essential Knowledge
Performance
Advantages
Proper pre-treatment is an essential factor because of the following advantages:
- the pre-treatment process provides a clear, uniform, oil-grease free surface,
- it provides good adhesion of film,
- it forms an inert layer which inhibits the corrosion of the paint film.
It is necessary for an applicator to know the following:
- Need for Pre-treatment
- Surface Preparation
- De-rusting Processors
- Activation
- Conversion coatings for mild steel, galvanizing steel, aluminium, mazak
Process Sequence
- De-Greasing
- Water Rinse
- De-Rusting
- Water Rinse
- Activation
- Conversion Coating
- Water Rinse
De-Greasing
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.
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.
Vapour de-greasing is quite a sophisticated method of cleaning. In this method, a special type of solvent is used which has the following advantages:
- Mineral & vegetable oils removal is faster
- Has lower boiling point than water
- Non-toxic to human beings
This process requires a closed system in which the item to be cleaned is exposed to the condensing vapours of the solvent. This method effectively removes light oils/greases but cannot remove heavy oils, greases, and sticky press compounds.
Advantages:
- Very good method for cleaning light oils, greases
- Less risk of fire hazard, minimum pollution as it is a closed system
- Since the solvent used is a non-toxic there is no danger for the operating worker
Disadvantages:
- Require costly plant installation and skilled labour,
- Frequent removal of oil & grease is required or otherwise they increase the boiling point of solvent
- Frequent change of cloth and solvent is essential or otherwise it can affect the quality of cleaning
- Thin sheets having heavy oil grease or sticky press compounds can not be removed by the process as there is no mechanical action on the parts
- Costly process
Emulsion cleaners are popular as they are based on mild Alkalis and hence they are suitable to all substrates (Iron, Galvanised steel Mazak etc.) and operate at room temperature. Emulsion cleaners are generally based on either kerosene-emulsifier or turpentine-emulsifier. These cleaners are generally milky in appearance. Emulsion cleaners have a limited cleaning tendency and they eave a very thin film of solvent/emulsifier over the substrate even after water rinsing. Hence, Emulsion Cleaning is always followed by Alkali cleaning. In case of spray process the parts to be cleaned are suspended in a funner wherein they are constantly exposed to the spray solution for 2 to 5 minutes at a pressure of 2-3 kgs/cm2 for better results. In the dip process, the parts to be cleaned are simply immersed in a bath of the cleaning solution.
Advantages:
- Operates at a lower temperature (45-50 degrees C) hence consuming less energy.
- Suitable for all the substrates such as Mild steel, Mazak, Aluminium, Galvanised etc.
Disadvantages:
- Limited cleaning ability as it can not remove heavy grease, oil etc.
- Not suitable for higher temp. as the emulsions are based on solvents which get separated above 70% from the emulsion phase.
Aikali cleaners enjoy superior position in the pre-treatment industry due to following advantages:
- They operate at room temperature.
- Simple bath control.
- Costly plant installation is not always required.
- High capacity to absorb oil.
- Bath stability is higher compared to all other de-greasing processes, low foaming tendency.
These cleaners are based on strong alkalies such as Sodium hydroxide, Sodium silicates, other filters and builders such as soda ash etc, surfactants and additives.
The parts to be cleaned are immersed in this bath at a recommended temperature and time. Strong alkaline cleaners are avoided in case of Manganese phosphate process as they can cause inferior quality coating.
Strong alkaline cleaners are also not suitable for non-ferrous substrates such as Zinc, Aluminium, Brass, Copper, Glass, Galvanised Steel, because all of these metals readily get attacked by strong alkalies.
De-Rusting
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.
Advantages:
- 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.
Disadvantages:
- 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.
Sulphuric Acid is a stronger acid and it is most suitable for heavily rusted components. Sulphuric Acid at an elevated temp.(50-60 degrees C) gives outstanding results in a short time. The use of a hot process is limited in the industry due to the corrosive nature of Sulphuric acid fumes. The use of an inhibitor is a must in hot process to avoid the excess attack of acid on the base metal. The carry over of Sulphuric Acid can permanently damage the Phosphating bath. For Sulphuric acid based de-rusting, the tank should be of either stainless steel or Mild steel with lead lining.
Phosphoric acid based de-rusting process is more popular in the finishing inquiry due to its following advantages:
- It gives a uniform & fine de-rusting pattern which ultimately gives less coarser coating in Phosphating.
- It has less pitting tendency than Hydrochloric acid & sulphuric acid based de-rusting.
- It does not emit any corrosive hazardous fumes.
- Carry over of bath solution does not create much problem.
Aikali cleaners enjoy superior position in the pre-treatment industry due to following advantages:
- They operate at room temperature.
- Simple bath control.
- Costly plant installation is not always required.
- High capacity to absorb oil.
- Bath stability is higher compared to all other de-greasing processes, low foaming tendency.
These cleaners are based on strong alkalies such as Sodium hydroxide, Sodium silicates, other filters and builders such as soda ash etc, surfactants and additives.
The parts to be cleaned are immersed in this bath at a recommended temperature and time. Strong alkaline cleaners are avoided in case of Manganese phosphate process as they can cause inferior quality coating.
Strong alkaline cleaners are also not suitable for non-ferrous substrates such as Zinc, Aluminium, Brass, Copper, Glass, Galvanised Steel, because all of these metals readily get attacked by strong alkalies.
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.
Disadvantages:
- 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:
Mono-Cationic
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.
Di-Cationic
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.
Tri-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.
Iron Phosphating is of limited use in the pre-treatment industry although this has got its own segment. Iron prospecting 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 phosphating is (due to its non-crystalline nature) iron phosphates parts can be welded prior to powder coating. Iron phosphates parts when powder coated give outstanding mechanical properties.
Conversion coating for Aluminium – Aluminium can be pre-treated by the following ways:
- Phosphating ( tri-atomic Zinc)
- Phosphating ( Green, Yellow or colourless)
Aluminium parts can be phosphates like mild steel, For Alumnium mild de-greasing chemicals should be used. Heavy, strong de-greasing chemicals can cause excessive etching to the base metal, De-rusting process should be avoided in Phosphating sequence, which should be re-placed by permuting 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.
