Abstract
The structure, kinetics and thermodynamics of formation of unconventional fluoride conversion coating on wrought AZ61 magnesium alloy are described in this work. Fluoride conversion coating was prepared by immersion of the specimens of AZ61 magnesium alloy into Na[BF4] molten salt at 410, 420, 430, 440 and 450 °C for 0.5, 1, 2, 4, 8, 12, 24 and 36 h. Testing methods, such as scanning electron microscopy, focused ion beam cutting and transmission electron microscopy were used for the morphology analysis, chemical composition and microstructure investigation of the fluoride conversion coating. It was found that the coating was composed of the primary MgF2 and secondary Na[MgF3] layer. The growth mechanism of the primary coating layer was described by the empirical kinetic method. The apparent activation energy and pre-exponential factor, estimated from the Arrhenius plot, are 82.87 kJ·mol−1 and 8.22 s−1, respectively. The kinetic analysis of the coating growth indicates that the process is driven by the parabolic rate law of the stationary one-dimensional diffusion at 410 °C. The Jander equation of the stationary three-dimensional diffusion describes the coating growth at higher investigated temperatures.
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