Rather than depositing paint, a conversion bath converts the top of the metal into an inorganic layer bonded to it: iron or zinc phosphate on steel, chromate or zirconium oxide on aluminum and steel. The layer gives the powder a uniform, receptive surface and slows corrosion from spreading sideways under the film when it is scratched. Coating weight is the main control check.
Aluminum extrusions skipped the conversion coating, so of course the paint lifted at the cut ends.
Also heard as
- conversion coat
Related terms
Pretreatment
The cleaning and chemical conditioning a part gets before powder, which decides most of its adhesion and corrosion life.
Iron phosphate
A thin, amorphous phosphate conversion coating on steel, the economical standard for indoor and light outdoor parts.
Zinc phosphate
A heavy crystalline phosphate conversion coating that gives steel top-tier corrosion resistance under powder.
Thin-film pretreatment
A zirconium- or titanium-based conversion coating only nanometers thick, run in place of phosphate on many lines.