PEO of AZ31 Mg Alloy: Effect of Electrolyte Phosphate Content and Current Density

Study on Microstructure and Properties of Black Micro-Arc Oxidation Coating on AZ31 Magnesium Alloy by Orthogonal Experiment

The effects of CuSO4 concentration, voltage and treating time on the hemisphere emissivity and corrosion resistance of AZ31B magnesium-alloy black micro-arc oxidation coatings were studied by orthogonal experiment. The microstructure, phase composition, corrosion resistance and hemisphere emissivity of the coating were investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, electrochemical test and infrared emissivity spectrometer, respectively. The results showed that the influences of each factor on corrosion current density and the hemisphere emissivity are as follows: voltage > treating time > CuSO4 concentration. The black MAO coatings are mainly composed of WO3, MgAl2O4, CuAl2O4, MgO, CuO and MgF2. The CuO and CuAl2O4 phases are the main reasons for blackness of the coatings. The coating exhibits the best corrosion resistance under the conditions of CuSO4 concentration 1.5 g/L, oxidation voltage 500 V and treating time 10 min. Additionally, the variation trends of hemispherical emissivity and roughness of the black MAO coating are the same when the composition of the coatings is similar. When the concentration of CuSO4 is 1.5 g/L, the oxidation voltage is 450 V and the treatment time is 10 min, the coating with the highest hemispherical emissivity of 0.84 can be obtained.

Effect of Plasma Electrolytic Oxidation on the Short-Term Corrosion Behaviour of AZ91 Magnesium Alloy in Aggressive Chloride Environment

In order to increase the corrosion resistance of magnesium alloy AZ91 in corrosion environments containing chlorides, the alloy surface has been modified by plasma electrolytic oxidation (PEO). The chemical composition of electrolyte in the PEO process consisted of 12 g/L Na3PO4·12 H2O and 1 g/L KOH, and a direct current was applied to the sample. The corrosion resistance of PEO coating and as-cast AZ91 (sample without PEO coating) was assessed using two different electrochemical methods: electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation (PDP) in 0.1 M NaCl at laboratory temperature. In addition to the electrochemical methods, the morphology of the oxidic coating was observed in the cross-sectional and top surface view by using the SEM technique. For better determination of the microstructure and PEO coating, chemical composition EDX analysis was used. The results of the experiments show that the formation of the PEO coating on AZ91 alloy has a more positive effect on the corrosion resistance in 0.1 M NaCl based on electrochemical methods than in the case of the formed coating on AZ31 alloy from the previous study. Based on electrochemical measurements in the selected environment, the formation of PEO coating on AZ91 was accompanied by a significant increase in polarisation resistance after short-term exposure compared to the as-cast surface. The EIS results showed a 73 times higher Rp value for PEO coated AZ91 when compared to the as-cast AZ91. Correspondingly, a 27 times lower icorr value was observed for PEO coated AZ91 than in the case of substrate AZ91 in 0.1 M NaCl. At the same time, the typically porous and inhomogeneous structure of the formed PEO coating on the magnesium alloy AZ91 was demonstrated.

Design and Multidimensional Screening of Flash-PEO Coatings for Mg in Comparison to Commercial Chromium(VI) Conversion Coating

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.