Abstract
Wrought AZ31 magnesium alloy was used as the experimental material for fluoride conversion coating preparation in Na[BF4] molten salt. Two coating temperatures, 430 °C and 450 °C, and three coating times, 0.5, 2, and 8 h, were used for the coating preparation. A scanning electron microscope and energy-dispersive X-ray spectroscopy were used for an investigation of the surface morphology and the cross-sections of the prepared coatings including chemical composition determination. The corrosion resistance of the prepared specimens was investigated in terms of the potentiodynamic tests, electrochemical impedance spectroscopy and immersion tests in the environment of simulated body fluids at 37 ± 2 °C. The increase in the coating temperature and coating time resulted in higher coatings thicknesses and better corrosion resistance. Higher coating temperature was accompanied by smaller defects uniformly distributed on the coating surface. The defects were most probably created due to the reaction of the AlxMny intermetallic phase with Na[BF4] molten salt and/or with the product of its decomposition, BF3 compound, resulting in the creation of soluble Na3[AlF6] and AlF3 compounds, which were removed from the coating during the removal of the secondary Na[MgF3] layer. The negative influence of the AlxMny intermetallic phase was correlated to the particle size and thus the size of created defects.
Highlights
Magnesium alloys have been investigated in recent years in terms of benefits for their usage as biodegradable metal implants [1,2,3,4,5]
The specimens treated for 8 h in Na[BF4] salt melt showed the first evidence of the corrosion attack after 48 h of immersion in simulated body fluids (SBF) solution regardless of the used coating temperature
Fluoride conversion coating was prepared on AZ31 magnesium alloy by dipping the magnesium alloy into Na[BF4] salt melt at temperatures of 430 and 450 ◦C for 0.5, 2, and 8 h
Summary
Magnesium alloys have been investigated in recent years in terms of benefits for their usage as biodegradable metal implants [1,2,3,4,5]. A positive effect of the fluoride conversion coating, the MgF2 layer, preparation by dipping AZ61 magnesium alloy into Na[BF4] molten salt was shown with the authors in [25,28,31]. The presented paper is focused on the preparation and characterization of the fluoride conversion coating in terms of coating morphology and corrosion resistance to determine the influence of the coated alloy microstructure on the coating characteristics. The fluoride conversion coating was prepared on AZ31 magnesium surface by dipping the magnesium alloy into Na[BF4] molten salt at different treatment conditions. Even though the structure and corrosion resistance of the fluoride conversion coating prepared on AZ31 magnesium alloy was studied in [24,26], the authors did not consider the potential toxicity of the secondary Na[MgF3] layer and analyzed properties of the complex system (of both MgF2 and Na[MgF3] layers). Based on the obtained results, the mechanism of the creation of defects present on the coating s surface will be assumed
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