Influence of the lithium content on the negative difference effect of Mg-Li alloys

Abstract The so-called ‘negative difference effect’ (NDE) was often defined by the increasing rate of hydrogen evolution from magnesium (Mg) surface under anodic polarization. In this work, a series of electrochemical tests and microstructure observations were performed to provide an evidence that the NDE of Mg-Li alloys can be retarded by increasing lithium content. Potentiostatic, galvanostatic and potentiodynamic polarization experiments using Mg-xLi (x = 4, 7.5 and 14 wt%) alloys electrodes indicated that Mg-4Li alloy maintained the enhancing NDE prior to anodic dissolution as that of conventional Mg alloys. However, the emergence of β-Li phase weakened the NDE of duplex Mg-7.5Li alloy at a low anodic current density, but it was still enhanced apparently after a high applied anodic value (more than 2 mA/cm2). The surface observations, including the plane and cross-sectional morphologies, confirmed that the cracked surface film derived from the anodic dissolution resulted in the catalytic activity of NDE for Mg-4Li and Mg-7.5Li alloys. Furthermore, the NDE of Mg-14Li alloy was suppressed obviously after a prior applied anodic polarization, which was attributed to the persistent and integrated surface film which endured a higher level of applied anodic potential and current.