Evolution of plasma electrolytic oxidation coatings formed on AM50 Mg alloy utilizing various alkaline electrolytes with and without glycerol additive and its corrosion behavior
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The widespread application for Mg and its alloys are severely restricted mainly due to its relatively low strength and ductility and poor corrosion behaviour, particularly in the aqueous environment. The plasma electrolytic oxidation (PEO) process for improving the corrosion resistance of the Mg-based alloys, has been widely studied/utilized in the research and commercial arena. Until now only a few studies have been carried on the evolution of the PEO coatings over Mg and its alloys and the effect of glycerol as an electrolytic additive on PEO coating evolution has not been investigated till date. Therefore, this study aims to systematically investigate the evolution and electrochemical behavior of the PEO coatings synthesized under similar operating conditions from the silicate-based, phosphate-based, and mixed silicate-phosphate-based alkaline electrolytes (abbreviated as bSi-PEO, bP-PEO, and bm-PEO respectively) as a function of PEO processing time. Additionally, the influence of glycerol as an additive on the evolution of PEO coatings synthesized from the aforementioned electrolytes (abbreviated as gSi-PEO, gP-PEO, and gm-PEO respectively) is also investigated. PEO processing voltage, maximum pore size, and coated layer thickness increase while pore density decreases and corrosion performance improves with the PEO processing time irrespective of electrolytes type (silicate-based, phosphate-based, and mixed silicate-phosphate-based) with and without glycerol additive. It was observed that the dielectric breakdown voltage, VBD, was in the order bSi-PEO coating > bP-PEO coating > bm-PEO coating propounding their higher dielectric properties. On glycerol addition, the VBD for all the three base electrolytes was higher indicating higher dielectric properties. The maximum pore size, in general, for all the PEO processing times, was in the order silicate-based electrolyte < phosphate-based electrolyte < mixed silicate-phosphate-based electrolyte with and without glycerol additive. This indicates that the micro-discharge intensity during the PEO processing increased in the same order for a given PEO processing time. The maximum pore size decreased with glycerol additive for all the three electrolytes at all the PEO processing times suggesting a reduction in the intensities of micro-discharges. It was also observed that the PEO coating thickness increases in the order silicate-based electrolyte < phosphate-based electrolyte < mixed silicate-phosphate-based electrolyte with and without glycerol additive. This can plausibly be attributed to the enhanced reactivity of PO43- anions than SiO32- anions with Mg2+ cations during the PEO processing. In the case of mixed silicate-phosphate-based electrolyte, it seems that the PO43- and SiO32- anions catalyzed the reactivity of each other with Mg2+ cations and hence resulted in relatively more volume of deposits and thereby resulted in thicker PEO coatings. The glycerol addition to all the three base electrolytes lowered the PEO coating growth rates possibly by increasing electrolytes viscosity, which impedes the migration of ions from the electrolyte towards the substrate. The elemental compositional analysis revealed homogeneous PEO coatings for all the three base electrolytes with and without glycerol additive at all the PEO processing times. The glycerol addition to all the three base electrolytes promoted the formation of the MgO phase in the resulting PEO coatings. Electrochemical studies utilizing 0.5 wt.% NaCl solution revealed that the corrosion performance was in the order bSi-PEO coating > bm-PEO coating > bP-PEO coating during the shorter immersion time. While the corrosion performance during longer immersion time was in the order bP-PEO coating > bSi-PEO coating ≃ bm-PEO coating indicating the bP-PEO degraded relatively less with the immersion time. In general, on glycerol addition to all the three base electrolytes, the corrosion performance improved. This could be due to the thicker inner barrier layer and higher MgO phase content in the PEO coatings despite lower coating thickness on glycerol addition. On glycerol addition, it was observed that for the shorter immersion duration the corrosion performance was in the order gSi-PEO coating > gm-PEO coating > gP-PEO coating indicating the gSi-PEO coating, despite having the least thick PEO coating, had better corrosion performance plausibly due to thicker and compact inner barrier layer. For longer immersion duration the corrosion performance was in the order gP-PEO coating > gSi-PEO coating > gm-PEO coating, indicating that the gP-PEO coating offered better corrosion behaviour as its degradation was retarded plausibly due to the availability of more quantity of amorphous Mg3(PO4)2 phase to recrystallize in aqueous solution, which resulted in the formation of its insoluble crystalline phase.