Degradation of AZ31 and AZ91 magnesium alloys in different physiological media: Effect of surface layer stability on electrochemical behaviour

This work deals with the degradation of AZ31 and AZ91 magnesium alloys when they are exposed to three types of physiological media for seven days at 37 °C: Ringer's, Hanks', and simulated body fluid (SBF) solutions. A combination of immersions tests and surface characterisation methods were employed to evaluate the attack on the surface, and the stability of the formed corrosion product layers for each alloy/electrolyte system. Measurements of the Mg-ion released into the electrolytes were also carried out in order to be correlated with the degradation of the alloys. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation (PDP) techniques were employed to compare the performance of the alloys in these different aggressive electrolytes. According to the obtained results, the Mg-alloys exposed to Hanks' media were the less affected, which fact was attributed to a higher stability of the corrosion products layer formed in this medium, in comparison of those formed in Ringer's and SBF solutions. In addition, the corrosion damage was lower for AZ91 than for AZ31 alloy in all environments due to its higher Al content. The mass loss rates calculated from both immersion tests and electrochemical methods followed the same trend for comparative purposes between alloys.