The Importance of Pretreatment
Metal parts in buildings are often coloured using powder coating. While powder coating adds to the aesthetic appeal, it is the pretreatment that is crucial for the longevity of the product. Pretreatment assists excellent adhesion between the powder coating and the aluminium surface. The surface must be entirely free of aluminium oxides before powder coating as any oxides that remain on the surface can retain air or moisture.
The purpose of the pretreatment process is to clean, etch and degrease so that any coating applied after that provides sufficient durability. For the powder coating to remain bonded to the product, proper pretreatment must be carried out. Pretreatment not only ensures that the product retains its appearance so that building looks great, but also ensures that the products provide effective weather protection.
Types of Pretreatment
There are many forms of pretreatment used in the industry, including hexavalent, trivalent and chrome-free options which all come with a set of advantages and disadvantages. Whatever system is used, the key to a quality end product is to ensure that strict processes are followed when carrying out the pretreatment.
The Process of Pretreatment
Two levels of pretreatments may be carried out, i.e. standard (mandatory) or enhanced (optional). Typically in architectural applications, a mulit-bath pretreatment cycle is undertaken to prepare the surface for a coating. The enhanced pretreatment can include additional steps such as degreasing or a combined degreasing/etching step.
Commonly, the standard process involves profiles being submerged or sprayed in acid, alkali or other chemicals and then being rinsed, in a specific sequence. The levels of acidity or alkalinity are essential, as is the temperature of the solution and the environmental conditions.
All pretreatment processes begin with a cleaning stage. The cleaning step is vital as it removes the surface of oil, grime and dirt, all which can hamper the powder coating. The rinsing stage follows the cleaning stage. In this stage, any residues of the chemicals used to clean the surface are removed. After that comes the conditioning stage which prepares the surface to accept the coating. It helps set the surface to the desired pH level and locks in the pH. The next step is a phosphate stage, which lays down a protective barrier that prevents corrosion and oxidisation. Finally, there are the sealer and dry-off oven stages. Sealing helps the paint adhere to the surface, while the final stage dries the finished product, which is extremely important. As the effect of the pretreatment wears off after the process is undertaken, the aluminium can not be stored any longer than 16 hrs before it is coated, and the sooner the better. The best products are coated immediately after pretreatment.
Following pretreatment, the assessment of the product quality is a complex undertaking. It relies on comprehensive and robust quality systems, regular internal audits and rigorous external audits against recognised industry standards, such as Qualicoat, which demands certain minimum requirements regarding the storage of goods, adhesion of the coating and many other aspects of the process.
To find out more about our pretreatment process or to order a sample, contact our team on 03300 94 94 11.