Interaction effect between different constituents in silicate-containing electrolyte on PEO coatings on Mg alloy

The influence of different constituents in silicate containing alkaline electrolytes on PEO-coated AZ91D magnesium alloy has been systematically studied using response surface methodology (RSM) coupled with electrochemical measurements and microstructure characterization methods. The results indicate that the most dominant factor that determines the coating performance is the interaction between Na2SiO3 and NaOH, where better corrosion resistance could be achieved when their concentrations are tuned in different directions. Infrared spectrum analysis of the electrolytes innovatively indicates that the degree of polymerization of silicate ions (from one to two-and three-dimensional), varied by the concentration of Na2SiO3 and NaOH, influences the kinetic mechanism of coating formation and thermal-driven gel-forming process under sparking. When silicates species exist as a relatively higher polymerized state, which indicates lower mobility of silicates in the electrolyte and better affinity for trapping Al3+ to form gel-like networks, the resulting coating is inhomogeneous and susceptible to corrosive ions; otherwise, the coating is more homogeneous and exhibits improved corrosion resistance. The work herein aims to provide guidelines for designing PEO electrolyte via correlating the intrinsic nature of the electrolyte with coating properties. (C) 2016 Elsevier B.V. All rights reserved.