Evaluation of pre-treatment processes for HRS (hot rolled steel) in powder coating

Hot rolled steel (HRS) is used extensively in the automotive, agricultural and appliance industries. The corrosive response of HRS was investigated after it had been exposed to various surface treatments, prior to powder coating. The behaviour of three conversion coatings: zinc phosphate (ZnP), iron phosphate (FeP) and zirconium (Zr)-based nano-scaled, on HRS was studied. HRS is naturally covered with iron oxide scale and this was removed from the surface by mechanical and chemical processes, prior to the application of surface treatment and organic coatings. The following tests on differently treated panels were conducted to evaluate corrosion performance: adhesion tests such as, crosshatch, pull-off, and conical bending, SEM, XPS, salt spray and electro-chemical impedance studies were also performed. Good correlations were recorded showing that zinc phosphate conversion coating gave the best performance, and zirconium-based nano-structured conversion coating, was superior to that of iron phosphate conversion coatings on HRS.