The influence of buffer system and biological fluids on the degradation of magnesium

The influence of frequently used buffer system 4-(2-hydroxyethyl) 21-piperazineethanesulfonic acid (HEPES) compared to CO2/HCO-(3) on the corrosion of magnesium is investigated. Samples were immersed in simulated body fluid (mSBF) while monitored by electrochemical impedance spectroscopy (EIS) for up to 30 days. In CO2/HCO-(3) the initial corrosion rate was 0.11 mm yr(-1). An inner protective layer of magnesium oxide was formed within the first 30 min exposure and subsequently covered by an outer layer of apatite within 24 h. The corrosion mechanism thereafter is best described as passive pitting with a porosity of similar to 10%. Using HEPES as buffer agent increased the corrosion rate to 3.37 mm yr(-1). Cross sectional microscopy show a porous outer corrosion layer allowing rapid diffusion of aggressive ions through the film. Here the EIS results are best described by an active pitting model with an inner layer 5 to 10 times less protective compared to the inner layer formed without HEPES. Further the suitability of human whole blood and plasma as in vitro models for Mg degradation was evaluated. Mg corrosion caused coagulation after 24 h in both biological fluids. The corrosion during the first 24 h is similar to the corrosion in m-SBF with HEPES. (C) 2016 Wiley Periodicals, Inc.