Film characteristics and electrochemical behavior of hydrothermally synthesized hydroxyapatite/polyacrylic acid composite coatings on WE43 magnesium alloy

Abstract Biodegradable magnesium (Mg) alloys are promising for temporary implants in orthopedic applications, but the prospects have been restricted by their rapid corrosion rate that occurs under physiological conditions. This study demonstrates the incorporation of hydroxyapatite/polyacrylic acid (HAp/PAA) biocomposite layers onto a WE43 magnesium alloy through a simple hydrothermal process to improve the surface morphology, enhance the corrosion resistance, and promote self-healing properties. The addition of polyacrylic acid (PAA) substantially improves the quality of the hydroxyapatite (HAp) layer, as it significantly reduces inside cracks and increases the uniformity of the layer. The HAp crystals become denser and more rigid and thereby suppress the number of fine cracks in the coating as a result of the reaction with ions from PAA. The nucleation process of hydroxyapatite crystals is influenced by hydrothermal time. In this research, the deposition durations were varied for 1, 2, and 3 h. A dense and compact HAp/PAA composite coating resulting from a prolonged (i.e., 3 h) hydrothermal process remarkably decreases the corrosion rate. Furthermore, the HAp/PAA biocomposite coating exhibits excellent self-healing properties, as evidenced by the reformation of a new HAp layer during a 48 h scratch-immersion test in Hanks’ solution at 37 °C. This work therefore shows that the multifunctional HAp/PAA biocomposite coating can be effective for controlling the corrosion rate of a magnesium alloy and, at the same time, for promoting recovery of the protection layer.