Surface free energy characterization and adhesion performance of mild steel treated based on zirconium conversion coating: A comparative study

The influence of zirconium based conversion coating on the surface free energy and adhesion performance of an epoxy coated mild steel substrate was evaluated before and after post heating. Surface free energy components (γLW, γ+ and γ) of the mild steel were measured before and after surface treatment using van Oss–Good method. The obtained data revealed that after formation of the conversion layer, γ−component of the surface increased leading to increment of work of adhesion. This may be attributed to the formation of zirconium compounds and adsorbed fluoride ion on the steel surface, which are evident in the Field Emission Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-ray spectroscopy (EDX) results, respectively. Pull-off adhesion test was performed for variety of the surface treatments including zirconium conversion coating (ZCC) and traditional phosphate and chromate layers. The results indicate that ZCC increased adhesion strength of mild steel to the organic coating and the zirconium treated mild steel adhesion performance is better than that of the three-cationic phosphate layer.