Influence of Electrolyte Chemistry on Morphology and Corrosion Resistance of Micro Arc Oxidation Coatings Deposited on Magnesium

Abstract

In the present work, micro arc oxidation (MAO) coatings were synthesized on magnesium substrate employing 11 different electrolyte compositions containing systematically varied concentrations of sodium silicate (Na 2 SiO 3 ), potassium hydroxide (KOH), and sodium aluminate (NaAlO 2 ). The resultant coatings were subjected to coating thickness measurement, energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), image analysis, and three-dimensional (3-D) optical profilometry. The corrosion performance of the coatings was evaluated by conducting potentiodynamic polarization tests in 3.5 wt pct NaCl solution. The inter-relationships between the electrolyte chemistry and the resulting chemistry and porosity of the coating, on one hand, and with the aqueous corrosion behavior of the coating, on the other, were studied. The changes in pore morphology and pore distribution in the coatings were found to be significantly influenced by the electrolyte composition. The coatings can have either through-thickness pores or pores in the near surface region alone depending on the electrolyte composition. The deleterious role of KOH especially when its concentration is >20 pct of total electrolyte constituents promoting the formation of large and deep pores in the coating was demonstrated. A reasonable correlation indicating the increasing pore volume implying the increased corrosion was noticed.

Relative hardness and corrosion behavior of micro arc oxidation coatings deposited on binary and ternary magnesium alloys

“…MAO coatings formed on Mg alloys has also been observed by earlier investigators. [13,23,54,55] In the similar line, the coatings formed in the present study (Fig.3) exhibit pores and micro-cracks. However, the pore and crack population is relatively higher towards the coating surface than at the substrate-coating interface.…”

“…The elemental analysis of the MAO coatings obtained on 7 substrates determined through EDS analysis is presented in Table 3 [23,28]. Based on the elemental composition of the coatings (Table 3) and simultaneous phase identification through XRD analysis (Fig.4), it is clear that increasing Al content in the Mg substrate is responsible for the increased formation of MgAl 2 O 4 phase.…”

“…The electrolyte having 120 L volume was prepared by adding 6 g/L sodium silicate (Na 2 SiO 3 ), 2 g/L potassium hydroxide (KOH) and 4 g/L sodium aluminate (NaAlO 2 ) to the distilled water. The above electrolyte combination has been selected based on the outcome of our recent study wherein the electrolyte composition was optimized to produce relatively pore free coatings [23]. The electrolyte was continuously circulated through an acrylic reaction chamber having a working volume of 20 L. The electrolyte was also circulated through an external air cooled heat exchanger such that the electrolyte entering the reaction chamber is maintained at 293±3 K (20 o C).…”

“…However, the current research focus has been constantly and simultaneously being shifted on to the MAO coatings deposited on Mg and its alloys [18][19]. In general, the MAO coatings formed on Mg alloys exhibit a heterogeneous morphology comprising of voids, pores and typical network of microcracks [20][21][22][23][24][25][26]. In order to improve the overall microstructural integrity vis-à-vis the functional properties, numerous studies were carried out so far mainly addressing the influence of MAO process parameters such as electrolyte composition and electrical parameters on the coating formation mechanism, microstructure, mechanical, tribological and electrochemical corrosion behavior [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43].…”

Formation and Corrosion Resistance of Micro-Arc Oxidation Coating on Equal-Channel Angular Pressed AZ91D Mg Alloy

“…Among these surface treatment techniques for the protection of Mg alloys [11][12][13][14][15], anodic oxidation is very popular. Macro-arc oxidation (MAO), as a novel technique developed from traditional anodic oxidation, has attracted more and more attention.…”