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Now that we have dealt with the most popular metals and the various forms of those metals we can move on to the preparation of the surface or substrate of the parts. The most important first stage to preparing metal prior to powder coating is called Degreasing. This involves removing, grease, oils, waxes and other surface contaminants that may be on the surface.

This can be carried out in three ways;

Vapour Degreasing

This process involves a tank containing in its lower half a reservoir with a quantity of a chlorinated hydrocarbon solution (non-flammable) which is heated usually by gas but the best way is electric to its boiling point of around 80 degrees centigrade. When boiling a hot vapour is produced that slowly rises up the tank. The parts that need degreasing are placed into the tank. The rising hot vapour condenses on the cold work pieces and in the process causes a washing/cleaning action to take place. The washing action ceases when the vapour has heated the parts up to the same temperature as the vapour. This is by far the best way to degrease metal parts. The hot vapour is very efficient at removing oils, greases and waxes from the surface of the metal parts. Because the solution is non aqueous there are no water based residues that can become trapped in welded joints and cause future corrosion. Also as the parts are heated during degreasing they do not need any sort of drying process. Powder Coatings Ltd have been using Vapour Degreasing to remove surface contaminants from metal parts for over 40 years and find that this method is far and away the most efficient way to prepare metal parts for powder coating.

Aqueous Wash Cleaning

Another method used to degrease metal parts prior to Powder Coating them is an Aqueous Wash. This type of machine is in reality similar to a large dishwasher. The parts that need degreasing are loaded into the machine. In the base of the machine is a reservoir containing hot water and usually some sort of strong detergent. The machine is closed and turned on. The parts are then washed by a series of jets that spray the hot detergent solution over the parts for a number of minutes. They are then rinsed and dried and the cycle finishes.

This type of cleaning process has certain drawbacks; firstly the parts inside have to be stacked so that they drain correctly to ensure that there are no areas where liquids can be trapped. Secondly if there are joins in the parts where they have been welded then water can become trapped and the drying cycle may not fully remove them. This is not so important with non ferrous metals as over time the joints will dry out and pockets of moisture can be dried with pieces of rag but with steel parts unless the residues are dried immediately the entrapped liquids very soon turn rusty which defeats the whole object of coating them in the first place.

Immersion Wash Cleaning

This method involves a series of tanks containing a hot cleaning solution and a hot rinse and a cold rinse. Sometimes a heated tank at the end with lids that acts as a type of drying oven.
The parts that need cleaning are usually loaded into baskets and by means of a hoist are loaded into the tanks, usually in the heated cleaning solution first for a number of minutes and then the cold rinse followed by the hot rinse and finally the drying oven. This process can suffer the same drawbacks as Aqueous Wash Cleaning in the case of ferrous parts due to trapping of liquids in recesses of the parts that are not fully dried.

Solvent Wiping

This method is still used in situations where the parts to be coated are too big for the cleaning plant that is available. It involves a solvent, usually thinners applied to a clean rag. The part that requires degreasing is wiped with the wet/damp solvent rag. After this a dry rag is used to remove the damp solvent/dissolved grease or oil from the surface. This method of degreasing is not recommended as it has many drawbacks. The solvent rag needs to be changed regularly otherwise after wiping a few oily parts with the rag it is effectively just wiping dissolved oil over the surface of the parts. Similarly the dry off rag is collecting oil residues and needs to be changed regularly to avoid spreading oil/grease over the surface.
If done properly it can be effective but basic but if not it leaves a film of oily residue on the surface of the parts that affects the adhesion of the powder coated film.

There are two processes that clean metal parts and also Pre-treat them;

Phosphating

Another form of metal cleaning that converts a few microns of the substrate to a crystalline structure providing a barrier to corrosion under the powder coated film and also improving the bond between the substrate and the powder coating. There are two types of Phosphating; Iron Phosphating and Zinc Phosphating. Both of these are technically called Conversion Coatings, i.e. the action of the solutions used ‘converts’ a few microns of the surface of the metal to either an Iron Phosphate or to a Zinc Phosphate. In the writers view Iron Phosphate is not as good as Zinc Phosphate. Phosphating is done either by immersion into various tanks, generally a cleaning solution followed by rinses and then immersion in the Phosphating Solution followed by a by two cold rinses a hot rinse and sometime Demineralised Water Hot Rinse to ensure that all salts and chemicals have been removed from the surface. If not done by the Immersion Method then the Phosphating is done by the Online method. In this way the parts are hung onto a conveyor that moves through a long covered enclosure containing tanks with the various solutions and nozzles that spray the moving metal parts with the rinses and phosphating solutions and finally moving into an oven that dries the parts.

Chromating

This is a similar process to Phosphating that converts a few microns of the metal surface into a chromate coating. It is mainly used on Aluminium and Zinc Diecastings and can be carried out using the Immersion Process or Online Spray Process.

Phosphating & Chromating not only clean the metal parts but they also convert the surface into a crystalline structure that improves the corrosion resistance of the coated parts and the bond of the powder coated film to the substrate.

However as the solutions used are basically aqueous in nature they can suffer from the same downside as the other water based cleaning methods; firstly the parts have to be placed or hung in a way that does not cause liquids to be trapped in recesses and cavities of the parts and secondly the parts must be thoroughly dried at the end of the process or rust can begin almost immediately.