Effect of Na2SiO3•5H2O concentration on the microstructure and corrosion properties of two-step PEO coatings formed on AZ91 alloy

In this study, effect of Na2SiO3 center dot 5H(2)O (Na2SiO3) concentration on the two-step peo coating was investigated. The specimens were primarily processed in a solution composed of (NaOH + Na2SiF6 + Na2SiO3 center dot 5H(2)O) for 900 s. The pre-coated specimens were then immersed into secondary solutions containing various Na2SiO3 concentrations and processed for a constant time of 900 s. Scanning electronic microscope (SEM), was used to investigate the structural properties of the coatings while XRD and EDS were used to analyze the elemental and phase composition of the coatings. SEM results revealed that the coatings formed for Na2SiO3 similar to 9 g/L have highly compact layers and least surface porosity similar to 1.18%. From the FWHM and high intensity peak ratios of Mg2SiO4 (112) and Mg (101) phases, it was found that coatings formed in Na2SiO3 similar to 9 g/L solution are highly crystalline and rich in Mg2SiO4 which was further supported by the maximum content of Si in the EDS analysis. From the hardness measurement it was found that the outer layer of the coating have the highest values of hardness and follow increasing trend with Na2SiO3 concentration. However, coating prepared for Na2SiO3 similar to 9 g/L was found to have least difference in hardness values with respect to coating thickness. Finally corrosion analysis of coating was carried out using potentiodynamic polarization in 3.5 wt% NaCl solution which revealed increase in the corrosion current for secondary coatings prepared for 6 g/L and 12 g/L of Na2SiO3, however the coating prepared for 9 g/L of Na2SiO3 was found to have the highest corrosion resistance similar to 627.9 k Omega and corrosion potential similar to-0.19 V. (C) 2017 Elsevier B.V. All rights reserved.