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

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-4 Li 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-7.5 Li alloy at a low anodic current density,but it was still enhanced apparently after a high applied anodic value(more than 2 mA/cm^2).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-4 Li and Mg-7.5 Li alloys.Furthermore,the NDE of Mg-14 Li 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.

Anodic hydrogen evolution on Mg

We measured the anodic hydrogen evolution rates for various applied anodic current densities and estimated the corresponding cathodic hydrogen evolution rates.The estimated cathodic hydrogen evolution rates were less than the measured anodic hydrogen evolution rates,contradicting the enhanced catalytic activity mechanism of Mg corrosion.In addition,this model was contradicted by the measured apparent Mg valence of 1.2±0.1.In contrast,the uni-positive Mg^(+)mechanism of Mg corrosion was supported by(i)the apparent Mg valence of 1.2±0.1,and(ii)the fact that the measured anodic hydrogen evolution rate increased with increasing weight loss rate.

The anodically polarized Mg surface products and accelerated hydrogen evolution

To clarify the anodic dissolution mechanism of Mg,the hydrogen evolution from pure Mg in acidic solutions under galvanostatic conditions were systematically measured.With increasing anodic current density,the cathodic hydrogen evolution rate decreased,and the anodic hydrogen evolution became faster while some surface area on the Mg was becoming dark under anodic polarization.Based on the surface analysis results and the generally accepted basic electrochemical equations,the evolution kinetics of hydrogen from Mg was deduced,and the most possible surface intermediate active species that could facilitate the anodic Mg dissolution and anodic hydrogen evolution were proposed.This paper further develops the model of incomplete film Mg^(+) dissolution,explains many reported experimental phenomena,and clarifies misunderstandings of current mechanism.

Galvanostatic anodic polarisation of WE43

The Mg corrosion mechanism was explored using galvanostatic polarisation curves,hydrogen evolution and weight loss.The data(a)were consistent with the existence of the uni-positive Mg+ion,(b)indicated that some hydrogen dissolved in the WE43 metal,and(c)indicated that self corrosion was more important than the applied current density in causing weight loss.

What activates the Mg surface——A comparison of Mg dissolution mechanisms

Several mechanisms have been proposed to interpret the widely reported phenomenon of Mg corrosion that the hydrogen evolution rate increases with increasing anodic potential or anodic current density. This paper critically analyzed the two main mechanisms,(1) "the incomplete film univalent Mg+ion mechanism" and(2) "the enhanced catalytic activity mechanism", aiming to clarify the current understanding of the Mg corrosion mechanism and to provide a profound insight into the Mg characteristic electrochemical behavior, anodic polarization accelerating both hydrogen evolution and Mg dissolution. It is expected that the deepened fundamental understanding from this comprehensive mechanistic review will provide a basis of practical applications for Mg alloys and open up a new way to the control of corrosion of Mg alloys in practice.