Before a powder coating can be applied, the surface of the substrate must be cleaned and treated to ensure optimal powder adhesion. If a surface is not properly prepared, any residues or deposits could impair powder adhesion, finish durability and aesthetics. Surface preparation varies by substrate, but may include cleaning, rinsing, etching and pressure blasting.
All materials undergo a pretreatment process to enhance properties such as durability and adhesion. The pretreatment process consists of a multi-stage wash.
In the first stage, a phosphate coating is applied in an acidic environment. If left untreated, iron surfaces will react with corrosive compounds such as oxygen and water and form a layer of metal oxide called rust. Rust is a harmful metal oxide that causes the metal to become weaker and flake away.
The phosphate coating in the first stage impedes the spread of corrosion under the coating by reacting with the surface of the metal and creating an inert surface on the metal. The modified surface shields the metal from corrosive components.
The phosphate coating also promotes the bonding of the powder coating to the metallic surface by providing adhesion characteristics. This is because the physical nature of the metal surface changes, allowing the powder to stick to the metal. The acidic environment of the phosphate coating simultaneously cleans the metal surface, removing any dirt or oil residue.
The metals are further cleaned in several stages using both city water and water with a TDS lower than 40 μS. This is achieved using a technique known as reverse osmosis.
Finally, for aluminum metals, a non-chrome conversion coating is used. Similar to iron, aluminum surfaces form metal oxides known as aluminum oxide or white rust. This conversion coating removes any white rust that may be present in the material and enhance properties such as corrosion resistance and adhesion properties.
Oil, grease, and other residues can be removed from a part’s surface in dip tanks or with wash stations. Wash stations spray material with high pressure hot water, steam, detergent, and other pretreatments to not only clean, but also chemically prepare the material for better coating adhesion.
Materials with surface debris, for example rust or an existing finish, will typically need to be mechanically cleaned. Most often, this is done via some form of pressure blasting. Examples include sand blasting, soda blasting and vapour blasting. Pressure blasting uses a high pressure fluid to shoot abrasives, usually sand, grit or shot. The abrasive material removes surface debris and leaves a cleaner surface, ready for further chemical preparation or finishing.