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Metal Pre-Treatment

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

  1. Mechanical surface Preparation
  2. Chemical surface Preparation

For powder coating 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
  1. Need for Pre-Treatment:
  2. ​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. Pre-treatment process provides a clear, uniform, oil-grease free surface
    2. Provides good adhesion of film
    3. It forms an inert layer which inhibit the corrosion of the paint film
  3. Surface Preparation:
  4. The general pre-treatment process for surface preparation follows the sequence as below:
    1. De-Greasing Process
    2. Water Rinse
    3. De-Rusting Process ​
    4. Water Rinse
    5. Activation Process
    6. Conversion Coatings
    7. 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 de-greasing processes applicable to Mild Steel, Galvanised Steel & Aluminum Substrates, are as follows:

  1. Solvent Cleaning
  2. Emulsion Cleaning
  3. Alkali Cleaning
     
  1. Solvent Cleaning
  2. There are 2 methods of Solvent Cleaning:
    1. Solvent Wipe
    2. Vapour De-Greasing
    1. Solvent Wipe:
    2. Solvent cleaning is the cheapest and best method to remove heavy or sticky oil/grease like substances form nay surface. Since all solvents are almost neutral they do not attack 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
      1. Cheapest cleaning method available in the industry to remove heavy oils, greases or sticky press compounds.
      2. Skilled labours and costly plant installation is not required.
      3. Suitable for smallest job coater as well as for OEM industry.

      Disadvantages
      1. The process is labour intensive.
      2. Since all the solvents are flammable, a great fire risk is associated with the solvent cleaning.
      3. Frequent change of cloth and solvent is essential or otherwise it can affect the quality of cleaning.
      4. Solvent going into the drainage can cause effluent disposal problem.

    3. Vapour De-Greasing:
    4. Vapour de-greasing is quite a sophisticated method of cleaning.
      In this method, special type of solvent is used which has following advantages:
      1. Mineral & vegetable oils removal is faster.
      2. Has lower boiling point than water.
      3. 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 solvent. This method effectively removes light oils, greases but cannot remove heavy oils, greases, and sticky press compounds.

      This process has the following advantages and disadvantages:

      Advantages
      1. Very good method for cleaning light oils and greases.
      2. Less risk of fire hazard, minimum pollution as it is a closed system.
      3. Since the solvent used is not-toxic, there is no danger for the operating worker.

      Disadvantages
      1. Requires costly plant installation and skilled labour.
      2. Frequent removal of oil & grease is required or otherwise they increase boiling point of the solvent.
      3. 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.
      4. It is a costly process.

  3. Emulsion Cleaning
  4. Emulsion cleaners are popular as they are based on mild Alkalis and hence 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 leave 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 dip process, the parts to be cleaned are simply immersed in a bath of the cleaning solution.

    This process has the following advantages and disadvantages:

    Advantages
    1. Operates at a lower temperature (45-50ºC) hence consumes less energy.
    2. Suitable for all the substrates such as Mild steel, Mazak, Aluminium, Galvanised steel etc.

    Disadvantages
    1. Limited cleaning ability as it can not remove heavy grease, oil etc.
    2. Not suitable for higher temp. As the emulsions are based on solvents which gets separated above 70ºC from the emulsion phase.

  5. Alkali Cleaning
  6. Alkali cleaners enjoy superior position in the Pre-treatment industry due to following advantages:
     
    1. They operate at room temperature.
    2. 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 substrate such as Zinc, Aluminum, Brass, Copper, Glass, Galvanised Steel, because all these metals readily get attacked by strong alkalies.

First Water Rinse

Regular Water Rinse

De-Rusting Process

Corrosion is common phenomenon for metals. Iron or steel when 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 overcoated 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.

De-Rusting Process:

​Advantages:

  1. Cheaper process than blast cleaning or flame cleaning.
  2. Can be done anywhere, no big plant or sophisticated equipment are required.
  3. Remove rust, Mills scale.
  4. Makes the surface reactive for the next phosphating stage.

Disadvantages:

  1. If Hydrochloric acid or hot Sulphuric acid is used for de-rusting it can create a corrosive atmosphere in the plant.
  2. Not suitable in case of spray application.
  3. Carry over can create problem in next stage (i.e.) in phosphating bath.


De-Rusting can be done with the following acids/acid combinations:

  1. Hydrochloric Acid Pickling:
  2. Hydrochloric acid is fuming acid and 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 problem 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 case of Pure Hydrochloric acid de-rusting the tank should be of Stainless Steel. ​
  3. Sulphuric Acid:
  4. Sulphuric Acid is a strongest acid and it is most suitable for the heavily rusted components. Sulphuric Acid at an elevated temp.(50-60ºC) gives outstanding results in a short time. The use of hot process is limited in the industry due to the corrosive nature of Sulphuric acid fumes. The use of inhibitor is must in hot process to avoid the excess arrack of acid on the base metal. Carry over of Sulphuric Acid bath in phosphating bath 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.
  5. Phosphoric Acid:
  6. Phosphoric acid based de-rusting process is more popular in the finishing indusry due to its following advantages:
    1. It gives a uniform & fine de-rusitng pattern which ultimately gives less coarser coating in phosphating.
    2. It has less pitting tendency than Hydrochloric acid & sulphuric acid based de-rusting.
    3. It does not emit any corrosive hazardous fumes.
    4. Carry over of bath solution does not create much problem.
    Phosphoric acid based de-rusting is costly as compared with Hydrochloric acid or Sulphuric acid. For Phosphoric acid based de-rusting, the bath should be of Stainless steel.

Second Water Rinse

Regular Water Rinse

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:

  1. Acidic Activation
  2. Basic Activation
     
  1. Acidic Activation
  2. 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.
  3. Basic Activation
  4. 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 are:
    i. The bath is unstable (Titanium forms colloids in aqueous solutions)
    ii. 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.

This process provides the following:

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

Phosphating process is divided into 2 types:

  1. Zinc Phosphating (sub-divided into Mono, Di, Tri Cationic Process)
  2. Iron Phosphating
     
  1. Zinc Phosphating
  2. 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:
    1. Mono-Cationic Process
    2. Di-Cationic Process
    3. Tri-Cationic Process
       
    1. Mono-Cationic Process:
    2. 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.
       
    3. Di-Cationic Process:
    4. 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.​
       
    5. Tri-Cationic Process:
    6. 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:
      1. It gives superior quality coating along with better corrosion and wear resistance.
      2. 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.
  3. Iron Phosphating
  4. 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:
    1. Phosphating (Tri-Cationic Zinc)
    2. Chromatising (Green, Yellow or Colorless)
       
    1. Phosphating:
    2. 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.
       
    3. Chromatising:
    4. This can be further sub-divided into three different types:
      1. Green Chromium Phosphate
      2. Yellow Chromium Phosphate
      3. Colorless Coating
         
      1. Green Chromium Phosphate:
      2. 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.
         
      3. Yellow Chromium Phosphate:
      4. 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.
         
      5. Colorless Coating:
      6. 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.
         

Final Water Rinse

Regular Water Rinse