Micro Raman and XPS surface analysis to understand the electrochemical behaviour of AZ31 and AZ91 magnesium alloys as temporary implant materials

Degradable metals and alloys have a potential use for surgical orthopedic applications. In this work, surface oxide/hydroxides formed by degradation, are evaluated in two commercial magnesium alloys (AZ31 and AZ91 alloys) by Raman, electrochemical and X-ray spectroscopic techniques. Electrochemical and surface/composition characterization of the AZ31 and AZ91 alloys are developed in contact with Hanks' Buffered Salts Solution (HBSS) at 37 degrees C. AZ31 alloy shows lower corrosion resistance than AZ91 in HBSS at 37 degrees C, both for short (24 h) and long (17 days) periods of immersion. The surface analysis demonstrate that on both alloys there are abundant corrosion products and the presence of compounds related to apatite. These results show that layers of magnesium oxide and hydroxide can be formed on the alloys surfaces, which gives them a certain degree of protection. The simulated body fluids contain different amounts of Ca2+ ions, carbonates / bicarbonates and phosphates, in addition to the Mg2+ ions (and other cations such as Al3+ and Zn2+) coming from the biomaterial corrosion environment. Thus, specific surface conditions can vary, such as the local pH value, and different corrosion products can precipitate which have different substrate protection properties. These corrosion products can slow down the degradation and influence the adsorption of proteins and cellular adhesion to the Mg surface by changing its chemistry and topography. The use of compositional (Raman and X-ray spectroscopies) and electrochemical (EIS and polarization curves) techniques, allows to do generate advances in the surface and degradation knowledge of typical Magnesium alloys like AZ31 and AZ91, for possible use as temporary implants. Even the analysis was carried out under static conditions, the systematic approach done is complete to picture the degradation mechanism of these alloys. Flow laboratory experiments are consider important to include as future work.