Pretreatment is the most important step when powder coating process is concerned. Pretreatment process usually does the removal of contaminants on metal surface which enhance coating performance. Once the rust, oil and scale are chemically cleaned by the pretreatment system, the base metal is exposed for oxidation. To avoid this, the material has to be coated with some type of conversion coating which prevents further oxidation before powder coating is performed. So every pretreatment system has incorporated conversion coating tank which also gives additional benefits to the coating substrate as follows.
- Provide temporary in process corrosion protection before application of powder
- Promote good adhesion of the powder coating to the substrate.
- Improve the life of the powder coating
The type of the conversion coating to be used also depends upon the base metal of powder coating. According to the base metal of steel & aluminum, most common types of conversion coatings are, Iron phosphate, Zinc phosphate & Chromium phosphate. After a metal surface receives a conversion coating, the surface should water rinsed to remove unreacted conversion coating chemicals and post-treatment with Chromic Acid is required for sealing function. This process is called passivation.
With the technology development, conversion coating chemicals have been developed by the industry expertise which are suitable to treat all kind of surface from steel to Aluminum. This coating enhances the metallic surface corrosion resistance, obtaining results similar or even superior to the ones obtained through above processes. Apart from the convenient application and hazard free environment friendly operation, many advantages can also be listed in this process.
- It produces a ceramic coat which measures in nano level over metallic substrates, increasing the anticorrosion protection.
- Uniform coating
- Free from any hard metal, phosphate and VOC
- Phosphate free so not generating a sludge.
- BOD & COD free, easy to handle waste water treatment operation
- Low energy cost, working at room temperature with treatment times of around 20 – 120 s.
- Does not require a passivation
The nano-ceramic coating are based on Zirconium irons. When it is applied to the metal substrate, these Zirconium irons react to form a thin Zirconium oxide film that is only 20-80 nm thick. That is a much thinner coating than iron phosphates which are typically 250 nm – 1000 nm.
In order to have a proper function of the nano-ceramic coating, additional care should be taken to maintain bath concentration within the acceptance level. Finding concentration of the bath solution may use colorimeter analyze which is an advance method than usual titration.To determine the concentration of working sample, several sample solutions of a known concentration are first prepared and tested. The concentrations are then plotted on a graph against absorbance, thereby generating a calibration curve. The results of the working sample are compared to that of the known sample on the curve to measure the concentration.
Calibration curve of the conversion coating solution at KIK Lanka (Pvt)Ltd is as follows.
http://www.powdergun.com/knowledge_base.htm_ why you need pretreatment before powder coating
W.C. Lakshan Wijerathne
BSc ( Chem) UOK