New insights into the effect of Tris-HC1 and Tris on corrosion of magnesium alloy in presence of bicarbonate, sulfate, hydrogen phosphate and dihydrogen phosphate ions

In vitro degradation is an important approach to screening appropriate biomedical magnesium （Mg） alloysat low cost. However, corrosion products deposited on Mg alloys exert a critical impact on corrosion resis-tance. There are no acceptable criteria on the evaluation on degradation rate of Mg alloys. Understandingthe degradation behavior of Mg alloys in presence of Tris buffer is necessary. An investigation was madeto compare the influence of Tris-HCl and Tris on the corrosion behavior of Mg alloy AZ31 in the presenceof various anions of simulated body fluids via hydrogen evolution, pH value and electrochemical tests.The results demonstrated that the Tris-HCl buffer resulted in general corrosion due to the inhibition ofthe formation of corrosion products and thus increased the corrosion rate of the AZ31 alloy. Whereas Trisgave rise to pitting corrosion or general corrosion due to the fact that the hydrolysis of the amino-group ofTris led to an increase in solution pH value, and promoted the formation of corrosion products and thus asignificant reduction in corrosion rate. In addition, the corrosion mechanisms in the presence of Tris-HCland Tris were proposed. Tris-HCl as a buffer prevented the formation of precipitates of HCO3-, SO42-,HPO42- and H2PO4- ions during the corrosion of the AZ31 alloy due to its lower buffering pH value （x,x）.Thus, both the hydrogen evolution rate and corrosion current density of the alloy were approximatelyone order of magnitude higher in presence of Tris-HCl than Tris and Tris-free saline solutions.