It is essential for an applicator to know the following:
- Need for Pre-treatment:
- 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:
- 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.
- Surface Preparation
- De-rusting Processors
- Conversion coatings for:
- Mild steel
- Galvanizing steel
- Trouble Shooting
Metal Pre-Treatment prior to powder coating is of two types and is an essential to ensure proper coating performance:
- Mechanical surface preparation
- Chemical surface preparation
For powder coating the most commonly used surface preparation method is Chemical Pre-Treatment.
|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 & aluminum 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.
|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:
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.
|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.
|Aikali cleaners enjoy superior position in the pre-treatment industry due to following advantages:
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.
|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.
De-rusting can be done with below acids/acid combinations:
HYDROCHLORIC ACID PICKLING
|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:
|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:
|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:
Phosphating process is divided into 2 types:
|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:
Today Tri-Cationic process is more popular in the metal Pre-Treatment industry because:
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:
|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.