Enhancement of surface wettability and corrosion resistance of AZ91 Mg alloy by nitrogen plasma immersion ion implantation

Introduction: Magnesium (Mg) alloys boasting light weight and high specific strength are widely used cause to Mg alloys have been widely used in the aerospace and biomedical industry. Unfortunately, magnesium alloys tend to have relatively poor corrosion resistance [1-3] because of the formation of a porous, poorly adherent and inhomogeneous native MgO/Mg(OH)2/MgCO₃ film on the surface in air [1-4]. Surface engineering is a common way to improve the corrosion resistance and biomedical properties of Mg alloys. Among the different surface modification techniques, ion implantation can enhance the chemical and mechanical properties of materials by changing the surface composition. In particular, introduction of nitrogen to the surface of materials has been shown to promote cell response and antibacterial properties [2-4]. 
Materials and Methods: The AZ91 Mg alloy samples underwent nitrogen plasma immersion ion implantation (NPIII) at a base pressure of 2 × 10−1 Pa and bias voltage of −35 kV. The phase, structure, chemical composition, and morphology of the surface were evaluated by grazing-incidence X-ray diffraction (GIXRD) and atomic force microscopy (AFM). The surface contact angles were measured on the Ramé-Hart contact angle instrument equipped with a motorized tilting stage at room temperature. The corrosion behavior was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests in SBF at 37 °C. 
Results and Discussion: Nitrogen plasma immersion ion implantation (N-PIII) improves the corrosion resistance of the AZ91 Mg alloy. A uniform, smooth, and crack-free surface structure is produced by N-PIII (Fig. 1). The corrosion resistance and hydrophobic properties are enhanced as well. The decrease in the corrosion current density after N-PIII and immersion in SBF for 24 h at 37 °C arises from the formation of a ceramic layer on the surface, which plays an important role in mitigating the corrosion reactions between the Mg alloy and SBF (Fig. 2). 
Conclusions: N-PIII is demonstrated to improve the corrosion resistance and hydrophobic properties of AZ91 Mg alloy. The uniform, smooth, and crack-free surface layer produced by N-PIII reduces the current density and increases the surface contact angle. The nitride ceramic layer formed on the AZ91 Mg alloy plays the role of a passivation to enhance the corrosion resistance.