Apatite coatings on magnesium based alloy AZ91 for improved corrosion resistance

• ¹ Universidade Federal de São Carlos, Materials Engineering, Brazil

Implants are used for bone reconstitution in orthopedic medicine and traumatology. After bone healing, these devices lose their function and should be removed. However, in many cases, due to the involved costs and/or patient suffering, they are not removed. For that application, biodegradable alloys were developed. After bone reconstitution, they are oxidized and absorbed by the human body. Among them, magnesium based alloys are under the spot. Regular magnesium alloys, however, present one main drawback: high corrosion rate, which leads to mechanical properties loss before the bone reconstitution and excessive production of gas hydrogen that leads to the formation of gas pockets, which cause adverse reactions and inhibit bone growth. In this work commercial magnesium based alloy AZ91 was submitted to surface modification in order to enable the development of a material electrochemicaly more suitable for its use as biodegradable implant. The method employed to modify the surface was the coating with apatite following the Biomimetic Method using modified SBF solution. The alloy AZ91 was submitted to chemical pre-treatment in 1M NaOH solution for the 24-h at 60ºC, and then the samples were rinsed in deionized water and dried at room temperature. The samples were submitted to Biomimetic Method by immersion in the Modified SBF solution for the 7-day period at 37ºC. After this period, the samples were rinsed in deionized water and dried at room temperature. The magnesium based alloy AZ91 with modified surface were analyzed by Scanning Electron Microscopy and X-ray diffraction. Potentiodynamic polarization studies were conducted. The results showed that this coating method was effective to obtain apatite phases on the surface of the magnesium commercial alloy AZ91, which can be considered more appropriate substitutes for bone than Hydroxyapatite (HA). Electrochemical tests showed that the presence of the coating was effective for increasing the corrosion resistance of the alloy, vide Fibure 1. From this work it is possible to establish experimental parameters to develop biodegradable magnesium-based alloys with adequate electrochemical properties, more suitable for the application as biodegradable implant.

Figure 1: Magnesium based alloy AZ91 coated with apatite: SEM image and Potentiodynamic polarization curves – (a) alloy AZ91; (b) alloy AZ91 after chemical pre-treatment in NaOH solution; (c) alloy AZ91 coated with apatite by Biomimetic Method (after chemical pre-treatment in NaOH solution).

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