Corrosion behaviour of biodegradable magnesium alloys with hydroxyapatite coatings

Bioresorbable magnesium alloys are currently being investigated as implant materials to address the problems associated with traditional metallic implants. However, the use of magnesium in vivo has been limited due to its rapid corrosion in the presence of body fluids. This can result in the liberation of large amounts of hydrogen gas and alkalisation of the body fluid which, in turn, can inhibit wound healing and ultimately lead to necrosis of the surrounding tissue. Calcium phosphate coatings such as hydroxyapatite have been shown to reduce the corrosion rate of magnesium in vivo. Due to the low melting point of magnesium (≈ 600 °C) conventional high temperature techniques to deposit hydroxyapatite, such as plasma spray, are not suitable. This paper investigates the deposition of a hydroxyapatite coating onto the surface of three bioresorbable magnesium alloys using an ambient temperature, blast coating technique. The effect of the coating on the corrosion rate of the alloys in vitro is examined. The coated samples were examined using variety of techniques: x-ray diffraction (XRD), microscopy, elemental mapping, surface roughness and coating adhesion. The corrosion of the alloys was examined using an immersion testing, potentiodynamic polarization and electrochemical impedance spectroscopy. The coatings exhibited high bond strengths (≈ 48 MPa) with no detectable change in the coating's crystallinity after deposition. Each of the coated samples displayed lower corrosion rates than the uncoated equivalent sample. The results indicate that the process can be used to produce highly adherent coatings that decrease the corrosion rate of the alloys in vitro.