Surface Pre Treatment

Pre-Treatment Systems
Before the metal surface can be painted, it needs to be prepared for the paint layer. It is similar to laying the foundation. The purpose of pre-treatment is to

  • Clean the surface for oil, physical contamination and rust
  • Create a surface condition which has superior adhesion to paint
  • Reduce the chances of reaction of paint with the base metal
  • Add to corrosion resistance

Generally the following stages are used for a good pre-treatment line. However, depending upon the quality specifications, the stages may be decreased or increased.

  • Knock off degreasing
  • Degreasing
  • Water rinse
  • De rusting
  • Activation
  • Phosphating
  • Sealer rinse
  • Hot water rinse

Cleaning affects all the post processes and should be considered the most critical stage for a good pre-treatment. To define a clean surface we should be able to confirm that the surface is free of oil, grease and unwanted contaminants, which adhere to the surface because of the oil and the grease. The first two chemical stages and the two water rinses ensure that all of the oil, grease and contaminants are removed. Customization is used as and when necessary. If the quality requirements are very high, a hot water rinse is added before the chemical cleaner stages. If the requirement is less stringent then a single chemical cleaner stage is used instead of two.

This stage is now mostly redundant however is still quite prevalent. The quality of steel used in the industry is of high grade and care is taken to prevent its rusting. Nonetheless there are still cases where the steel does get rusted due to extreme environment conditions or due to storage. The de rusting stage followed by the two rinses removed all traces or rust. Now we have a very active metal surface ready to receive the layer, which will provide the functional properties.

A single phosphate pre-treatment line will eventually handle a fairly large number of metal substrates. Not all surfaces react identically to the phosphating chemicals. The purpose of activation is to prepare the surface for a uniform crystalline structure.

In the phosphating stage, the metal surface is treated with a solution of phosphoric acid with other chemicals. The purpose is to get a layer or coating which is primarily a phosphate. From a simple Iron phosphate, we now have more exotic salts added to the process to improve the etching and the crystalline structure. These have considerably improved the performance of the phosphate coating.

After the phosphate dip the rinse dip removed the excess and unreacted solution. The sealer dip is used, as the name suggests, to seal the porosity of the crystalline phosphate layer. The improvement in performance can vary from 2 to 10 times. The chromic acid based sealer was very popular however with greater concern about our environment, non-chrome based sealers are gaining popularity.

The post sealer rinse removes the excess chemicals and the hot water rinse gives the advantage that the component dries fast as it comes out of the last rinse. Other than the above mentioned stages and their use, circulation and heating is also required in the chemical tanks.

For the SME segment the square transfer type system is the most economical. This is also popularly called the transporter system or the transporter wagon. The products duly loaded on the jig are carried from one tank to the other in a pre-programmed sequence and pre-defined time.

In certain cases, an oven is also required to completely dry the component before it goes for liquid painting or powder coating. The oven can be electric, diesel or gas fired. The diesel or gas fired ovens are with stainless steel heat exchanger. Forced convection is a must for the oven.

To be able to design a system, the following information is required:

  1. Size of the component
  2. Surface area of the component
  3. Weight of the component
  4. Number of components to be coated per hour
  5. Number of working hours per day
    In some cases, simply the tank size, the expected surface area per batch, weight per batch and the number of batches per hour is also enough.