Abstract
REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) regulations demand for an expedient discovery of a Cr(VI)-free alternative corrosion protection for light alloys even though the green alternatives might never be as cheap as current harmful technologies. In the present work, flash- plasma electrolytic oxidation coatings (FPEO) with the process duration < 90 s are developed on AZ31B alloy in varied mixtures of silicate-, phosphate-, aluminate-, and fluoride-based alkaline electrolytes implementing current density and voltage limits. The overall evaluation of the coatings’ anticorrosion performance (electrochemical impedance spectroscopy (EIS), neutral salt spray test (NSST), paintability) shows that from nine optimized FPEO recipes, two (based on phosphate, fluoride, and aluminate or silicate mixtures) are found to be an adequate substitute for commercially used Cr(VI)-based conversion coating (CCC). The FPEO coatings with the best corrosion resistance consume a very low amount of energy (~1 kW h m−2 µm−1). It is also found that the lower the energy consumption of the FPEO process, the better the corrosion resistance of the resultant coating. The superb corrosion protection and a solid environmentally friendly outlook of PEO-based corrosion protection technology may facilitate the economic justification for industrial end-users of the current-consuming process as a replacement of the electroless CCC process.
Highlights
IntroductionThe energy input is considerable, plasma electrolytic oxidation (PEO) coatings exhibit great properties, e.g., very resistive, perfectly adhered to substrate, high thickness, very good paintability (due to the porosity of the top layer), fast growth rate, and intensive incorporation of electrolytes species [3,4]
Since the early 19200 s, Cr-based conversion coatings have been almost exclusively used as corrosion protection for metals and metal alloys
Cr-based protection is very efficient, and very well-known since it has been deeply characterized for many years; it is so readily applied for Mg alloys in industry
Summary
The energy input is considerable, PEO coatings exhibit great properties, e.g., very resistive, perfectly adhered to substrate, high thickness, very good paintability (due to the porosity of the top layer), fast growth rate, and intensive incorporation of electrolytes species [3,4]. The latter fact is very important for Mg alloys since Pilling-Bedworth ratio Vox /Vm = 0.81 for pure MgO [5], which means Mg surface cannot be fully covered with magnesium oxide (the main product of anodizing below breakdown potential)
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