Effect of Processing Parameters On Soft Regime Behavior of Plasma Electrolytic Oxidation of Magnesium

Magnesium and its alloys has a wide range of applications in industrial setups. This can be heavily attributed to the great deal of advantages that it offers such as; low density, high strength to weight ratio, good electromagnetic shielding among many more. However, magnesium and its alloys are highly susceptible to corrosion which greatly hinders their further applications, mainly in solutions containing bromide, chloride and sulphate ions. In recent times, it has been observed that coating magnesium with Zirconium oxide generates a combination of unique and desirable properties such as good mechanical strength, superior refractoriness, high ionic conductivity, excellent corrosion resistance and low thermal conductivity.

Many methods are available for the coat deposition such as plasma spraying, laser ablation, metal organic chemical vapor deposition, sputtering and sol-gel techniques. Nonetheless, these techniques have some limitations which include costly equipment, complicated process and relatively low interface bonding strength, which inhibit their development and application. Alternatively, plasma electrolytic oxidation technique offers a chance to solve these problems. Regardless, the   plasma electrolytic oxidation technique coating is not good enough for many industrial applications where dense and consistent layers with worthy mechanical properties are needed.

Researchers led by professor Chen-Chia Chou  at National Taiwan University of Science and Technology developed a novel ceramic coatings on magnesium alloy by soft plasma electrolytic oxidation regime. They aimed at fabricating high quality ceramic coatings that not only had improved corrosion resistance but also enhanced wear resistance of magnesium alloys. Their work is now published in the peer-reviewed journal, Ceramics International.

The research team examined the compositions, structure and morphologies of the oxide coatings formed using different working parameters by energy dispersive spectroscopy, x-ray diffraction, and scanning electron microscopy. They then evaluated corrosion resistance of the oxide coatings in 3.5 wt% sodium chloride solution using potentiodynamic polarization tests. Eventually, they fabricated Zirconia-containing ceramic coatings on pure magnesium by plasma electrolytic oxidation technique using two different anodic to cathodic charge quantity ratio.

The research team were able to observe that the occurrence of soft regime in CR = 0.85 may be constantly established, which strongly reduce the arcing that usually cause detrimental defects in the oxide layer. They also noted that the occurrence of soft regime in CR = 0.85 was being constantly established, which strongly reduced the arcing that was usually seen to cause detrimental defects in the oxide layer. Growth of the oxide layer can be attributed to a competition between two phenomena, sintering and etching, which induce denser coatings. Unfortunately, the corrosion resistance performance is still not enough, probably due to a big mismatch of the thermal expansion coefficients of the substrate and the ceramic layer, which produces cracks at the interface as the coating grow thicker.

Herein, it has been noted that by using a condition of anodic to cathodic charge quantity ratio smaller than 1 not only induces a “soft regime” during a plasma electrolytic oxidation process but also provides a qualitative improvement in the coating morphology and density both in surface and cross section. The negative current plays a huge role in altering the characteristics of the arc and do the etching process within the soft plasma electrolytic oxidation regime. However, the formation mechanism of the soft regime during plasma electrolytic oxidation process need further investigations.