Low apparent valence of Mg during corrosion

Abstract

Our recent data on Mg corrosion has been reanalysed because of the recent criticism that our previous data analysis was inadequate. Re-analysis leads to similar conclusions as previously. The apparent valence of Mg during corrosion was in each case less than 2.0, and in many cases less than 1.0. Moreover, these values were probably over-estimates. The low values were consistent with the evolving hydrogen gas acting as an insulator, so that the corrosion of parts of the specimen could occur isolated from the electrochemical measurement system.

Film Breakdown and Nano-Porous Mg(OH)2Formation from Corrosion of Magnesium Alloys in Salt Solutions

“…SANS could also potentially provide new insight into anodic film formation relative to the current debate regarding the mechanism of the negative difference effect for Mg, whereby H 2 evolution is observed to increase with increasing anodic potential. Knowledge gained by ex-situ or in-situ SANS study of the evolution of surface area/roughness vs. potential and/or time may help clarify the proposed mechanisms, for which filamentous/dendritic corrosion products have been reported 20,35,[61][62][63][64][65][66] …”

The Role of Surface Films and Dissolution Products on the Negative Difference Effect for Magnesium: Comparison of Cl−versus Cl−Free Solutions

“…6 This phenomenon confounds the understanding of Mg dissolution mechanisms, establishment of a Tafel law and other reaction kinetic parameters, as well as other issues including accurate analysis of corrosion rates from electrochemical impedance spectroscopy (EIS). [7][8][9] There are several theories purporting to explain the origins of the NDE among which include noble impurity element enrichment, 10,11 non-faradaic mass loss via metal spalling, 12,13 formation and dissolution of hydrides and partially protective films, [14][15][16][17] and univalent Mg based dissolution followed by further oxidation of Mg + →Mg 2+ in solution (leading to hydrogen evolution from the reduction of water away from the electrode in the electrolyte); 18 however, no consensus has been reached on the physical origins of the NDE.…”

Possible dissolution pathways participating in the Mg corrosion reaction

“…Atrens et al [4,6] and Shi and Atrens [7,8] reviewed the available experimental evidence and concluded that this uni-positive Mg + reaction sequence was consistent with the available evidence regarding the details of Mg corrosion, in particular (i) that Mg corrodes with an apparent valence of less than 2.0 [9,10], and (ii) that the amount of evolved hydrogen increases with increasing anodic polarisation [4,7,8]. The fact that Mg corrodes with an apparent valence of less than 2.0 is explained by the fact that the corrosion is only partly electrochemical and that there is the dissolution reaction given by Eq.…”

“…A direct consequence of these assumptions is that the increased exchange current density reaction sequence predicts that the apparent valence of Mg during corrosion is always equal to 2.0. However, there is compelling evidence that the apparent valence of Mg is less than 2.0 [9,10,[31][32][33].…”

“…The apparent valence for Mg during corrosion has been evaluated [9,10] using V = 2P i P H (6) in which it is assumed that P i is a good measure of the corrosion rate under electrochemical control as measured by Tafel extrapolation of a cathodic polarisation curve, and P H is the corresponding (and measured essentially simultaneously) corrosion rate evaluated from the hydrogen evolution rate, and furthermore that P H is a good measure of the total corrosion rate as measured by weight loss.…”