A comparative study of zinc phosphate and zirconium-based conversion coatings on galvannealed steel: effect on corrosion performance of electrocoated systems

Abstract

ABSTRACT The efficacy of an electrocoated system that employs a Zr-based conversion coating was assessed for the protection of Galvannealed (GA) steel. The aim was to determine if a Zr-based system has the potential to replace a system with a traditional Zinc phosphate (ZnP) conversion coating, where there are concerns for the environmental and economic sustainability of the latter. A comparative study between the two conversion coatings, and a control system with no conversion coating, is presented. Data from AC and DC electrochemistry are utilized in combination with surface analysis investigations of pristine specimens where scanning electron microscopy, x-ray photoelectron spectroscopy and x-ray diffraction have been employed to determine the mechanism through which protection, or coating failure, occurs. Further investigations study the surface morphology and stability of specimens during immersion in a simulated electrocoat bath, and their effect on the performance of fully electrocoated systems. The corrosion current density (icorr) for bare, ZnP and Zr-based conversion coated specimens in chloride-containing solution with no electrocoat were found to be approximately 1.90 μA·cm−2, 0.30 μA·cm−2, and <0.01 μA·cm−2, respectively. Rp values of 0.05 kΩ, 4.2 kΩ and 20.7 kΩ were recorded for bare, ZnP and Zr-based specimens, respectively. The corrosion current density (icorr) for bare, ZnP and Zr-based conversion coated specimens under chloride-containing conditions with electrocoat were found to be 3.82 μA·cm−2, 0.53 μA·cm−2, and 0.06 μA·cm−2, respectively. |Z| values at 0.01 Hz (|Z|0.01 Hz) were measured, yielding 45.5 GΩ, 47.4 GΩ and 31.6 GΩ for bare, ZnP and Zr-based pretreatments respectively, demonstrating excellent barrier properties after electrocoating. Hydrogen evolution reaction (HER) rate values of 3.52·10−2 mol·m−2·s-1, 3.12·10−2 mol·m−2·s-1 and 2.94·10−2mol·m−2·s-1 were recorded for the bare, ZnP and Zr-based systems, respectively. This study produced a ranking order of HER rate. This was bare > ZnP > Zr-based. These results suggest that the Zr-based system is less prone to HER, which interferes with the electrocoat deposition process. A performance ranking order for pretreated and electrocoated systems was established where Zr-based > ZnP > control. The level of thickness and roughness provided by the conversion coating is speculated to be the controlling factor for coating performance, where the ZnP conversion coating was found to provide a thicker layer with greater surfce coverage beneficial for a uniform electrocoat film-build.