Conversion Treatment of Carbon Steel Using an Atmospheric Plasma-Based Process

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Abstract
An atmospheric-pressure, microwave-powered plasma process was used for surface cleaning and fabrication of Zr, Si, and Zr/Si-based bilayer conversion coatings on mild steel for corrosion protection. Si-based films were fabricated from a volatile organometallic precursor in the gas phase, and Zr-based films were produced by converting a solution-based precursor film. The fabricated coatings were characterized in terms of their morphology and composition, and their corrosion properties. Based on the results of electrochemical and corrosion weathering experiments, both Zr- and Si-based conversion coatings resulted in a significant decrease in the corrosion rate of the steel substrate. At the same time, the bilayer Zr/Si sample presented little improvement due to the presence of defects. Polarization measurements revealed that anodic inhibition was the main mechanism for corrosion protection in both Zr- and Si-based conversion coatings. The performance of the conversion coatings was further studied in an epoxy-polyamide paint system using the accelerated cyclic electrochemical technique, as well as salt fog corrosion testing. The results implied that both plasma cleaning of the surface (without any conversion film) and the presence of Zr-based conversion coating increased the low-frequency impedance of the system. However, in the presence of defects in the paint, the anodic inhibition by the Zr-based conversion layers played a key role in increasing the protectiveness and performance of the paint. The results of this study indicate that atmospheric-pressure plasmas can be a promising processing technology to fabricate conversion coatings on mild steel, minimizing chemical waste generated by solvent-based methods.

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