Effect of zirconium conversion coating: Adhesion and anti-corrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild steel

• Significant role of Zr conversion coating at interface in compare with ZAPP pigment.
• Comprehensive analyses were performed to evaluate anti corrosion properties.
• Evaluation of ZAPP and Zr conversion coating presences on adhesion phenomena.

An epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigments with and without a hexafluorozirconic acid based conversion coating (ZrCC) pretreatment was applied on carbon mild steel (St 37) in order to investigate the corrosion resistance behavior and adhesion of the coating to substrate. Optimization of pigment volume concentration (PVC) and conversion coating application conditions was carried out and also the interaction of organic and conversion coating was studied using electrochemical impedance spectroscopy (EIS), salt spray and pull-off adhesion tests. Appropriate anti-corrosion properties was gained from organic coating samples with PVC = 30% and conversion coating applied from a solution with pH and acid concentration of 4.5 and 0.01 M, respectively. Results indicated that using conversion coating as a pretreatment before applying organic coating, improves adhesion of the coating to the substrate (by 1–2 MPa) and also provides an effective corrosion protection in long periods of immersion time. The highest corrosion resistance was obtained for pigmented organic coating sample with Zr conversion coating pretreatment after 4 weeks of immersion in NaCl 3.5 wt.%. Field emission scanning electron microscope and energy dispersive X-ray spectroscopy (FE-SEM/EDS) also proved the presence of about 5 wt.% of zirconium at the interface of coating and substrate not only at the onset of immersion but also after 4 weeks of immersion time. Results taken from this research, can clarify the key role of surface modification at interface of coating and substrate in adhesion and also the retention of anti-corrosion properties over the course of time even for an anti-corrosion pigment reinforced organic coating sample.