Aluminum (Al) alloys are used widely in the automotive and aerospace industries, but their corrosion resistance is insufficient in long-term service, particularly in a high-salinity environment. The incorporation of molybdenum disulfide (MoS2) particles into plasma electrolytic oxidation (PEO) coatings formed on an aluminum alloy was investigated in this work. After oxidation, the coatings were investigated by a series of structural, morphological and electrochemical corrosion tests to understand the effect of molybdenum disulfide addition to the base electrolyte. The phase compositions and surface morphology of coatings were analyzed through X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), respectively. The XRD results showed that all coatings were primarily composed of α-aluminum oxide (α-Al2O3), γ-aluminum oxide and molybdenum disulfide phases. The FESEM observations indicated that molybdenum disulfide particles were successfully incorporated in the ceramic coating and the apparent porosities decreased with increasing molybdenum disulfide concentration. The electrochemical corrosion tests indicated that the amount of coating porosity decreased and the corrosion current density decreased from 79·6 μA/cm2 in the bare aluminum alloy to 58·1 and 30·1 μA/cm2 in the PEO coatings without and with the molybdenum disulfide additive, respectively. This behavior could be attributed to the beneficial effect of the molybdenum disulfide particles, which might play an important role in blocking micropores and microcracks.

This paper comprehensively reviews recent research carried out in the field of preparation of bioactive coatings on magnesium (Mg)–aluminum (Al)–zinc (Zn) alloys by using the plasma electrolytic oxidation (PEO) method and investigation of factors affecting it. Magnesium–aluminum–zinc alloys (AZ grades) have been developed for use as biodegradation materials because the addition of aluminum and zinc as alloying elements can ameliorate their mechanical properties and increase corrosion resistance over those of pure magnesium alone without decrement in biocompatibility. Among the common methods for improvement of biodegradation, corrosion resistance and biocompatibility of magnesium alloys, surface modification techniques have been more effective. PEO is a relatively novel surface modification technique that provides excellent wear- and corrosion-resistant coatings on lightweight metals, in particular on magnesium and its alloys. The processing parameters of PEO, such as pulse frequencies, applied voltages, oxidation time, electrolyte concentrations and substrate chemical composition, are some of the major factors that determine the quality of the coatings in terms of corrosion resistance and bioactivity in body fluid solutions. Considering the fact that comprehensive review articles on the effect of PEO parameters on the properties of magnesium alloys have not been published in recent years, the aim of the present text is to provide an understanding of all aspects of these process parameters.