When a powder coat finish peels or blisters, the powder is rarely to blame. The problem is almost always underneath, in the preparation of the metal. Between degreasing and powder, there is a step you do not see: the conversion coating. At Chromatique, it is zirconium-based.
A layer a few tens of nanometres thick
The treatment deposits a zirconium oxide film on the metal that the powder grips and that slows corrosion under the finish. This film is extremely thin: 20 to 80 nanometres, according to a Powder Coated Tough article that quotes several suppliers, compared with about 250 for an iron phosphate and up to 1,000 for a zinc phosphate. That is why it is also called thin-film pretreatment.
What zirconium does better
The comparison published by Products Finishing notes four advantages. The zirconium stage works at room temperature, whereas an iron phosphate has to be heated. It produces very little sludge. Most formulations contain no phosphates, whose discharge is restricted in several municipalities. Finally, a single bath treats steel, galvanized steel and aluminum, which matters in a shop where the metals change from one order to the next.
Its limits
The process is less forgiving. It requires good rinses between the degreaser and the treatment, good-quality water and regular monitoring of the bath. The chemicals also cost more than an iron phosphate. A useful note: most of the performance data comes from the chemical suppliers, such as Henkel or Chemetall. The first generations did not do as well as zinc phosphate in corrosion resistance; the suppliers say the gap has now been closed.
Our sequence
At the shop, pretreatment is done in four stages: degreasing with an industrial degreaser, rinsing, zirconium treatment, then oven drying. The part then gets its powder. The complete steps are described on the Our process page.
In summary
- The conversion coating, under the powder, determines adhesion and corrosion resistance.
- Zirconium forms a film of 20 to 80 nanometres, at room temperature, with very little sludge.
- A single bath treats steel, galvanized steel and aluminum.
- The process requires careful rinses and a well-controlled bath.
