Potassium Permanganate–Fluorozirconate Conversion Coating as a Precursor for Flash-PEO Coatings on AZ31B Magnesium Alloy

Flash-Plasma Electrolytic Oxidation (Flash-PEO) is short anodizing surface treatment that has demonstrated outstanding corrosion resistance associated with self-healing mechanisms. However, similarly to conventional PEO, there is a limitation regarding the incorporation of certain species in the composition of the PEO coating. In that direction, precursor-driven functionalization is a novel approach that can be used in place of the classical one, where the final composition only depends on the electrolyte and the metal.

In this work, a commercial permanganate–fluorozirconate conversion coating (ZrCC) has been used as a precursor prior to a silicate–fluoride (SiF) Flash-PEO treatment. Coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), or X-ray photoelectron spectroscopy (XPS). The corrosion performance was evaluated by open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) after immersion in 0.5 wt. % NaCl. Results have revealed an improvement in the corrosion resistance up to 72h due to Zr and Mn incorporation as well as changes in the porosity and compactness of the Flash-PEO layer.

In summary, the use of precursors such as ZrCC (source of Zr and Mn cations) has been confirmed as an efficient strategy to vary the chemical composition of Flash-PEO coatings, improving their corrosion resistance.