Cailiao gongcheng (Oct 2020)
Research progress in anodic hydrogen evolution of magnesium electrochemistry corrosion
Abstract
High chemical activity and unique electrochemical behaviour are responsible for the complicated corrosion mechanism of magnesium. Importantly, the anodic hydrogen evolution on Mg electrodes when applied anodic polarization is one of the greatest important characteristics for Mg corrosion. The phenomenon, referred to as negative difference effect (NDE), has always been of an interest in terms of Mg electrochemistry corrosion. In this article, the research progress of hypotheses and theories proposed to interpret this phenomenon was reviewed: assuming that univalent Mg+ existed during anodic polarization of Mg. According to this, increasing oxidation rate of magnesium by applying an anodic polarization increased the rate of hydrogen evolution since the rate of Mg+ entering into electrolyte increased at the same time. However, this theory has raised criticism since Mg+ has never been experimentally detected, despite the requirement of its finite lifetime. Film theory declared that the dark corrosion products containing bilayer of MgO and Mg(OH)2 films form on the corroded surface of magnesium were believed to exhibit enhanced catalytic activity towards hydrogen evolution as well. In addition, this declaration has been proved experimentally: hydrogen evolution rate on the dark corroded regions was faster than that on the uncorroded regions at free corrosion, especially at the boundaries of the corroded/uncorroded regions. However, the latest experiments demonstrated that the increase in cathodic activity of the corroded regions during anodic polarization provided a minor contribution to the increase in hydrogen evolution rate during anodic polarization, and rather the enhanced hydrogen evolution at anodic potentials (NDE) was dominated by the regions where the net anodic reaction occurred. Assuming that enrichment of impurities such as Fe and Mg atoms took place in corrosion products of Mg. It is believed that Fe enrichment was capable of enhancing catalytic activity towards hydrogen evolution. There were indeed evidences (microscope images) showing the enrichment of Fe atoms around the interface of metal/solution. Unfortunately, the accumulation of these atoms on the electrode only contributed to a very small part to hydrogen evolution in NDE in further studies. Further, some researchers asserted the great possibility of the existence of MgH2 which had a similar mechanism of NDE to univalent Mg+. Still, the experimental existence and stability of such intermediate agents were of great concern. Another study proposed a theory that H2O molecules were always of absorption and desorption on Mg electrode, which provided sufficient H atoms for NDE to take place. Though all these assumptions contributed to explaining the NDE phenomenon to some extent, they were still not so reasonable to be generally accepted. At last, it was pointed that the future development direction of the NDE phenomenon by using advanced in-situ electrochemistry techniques and adjusting the electrochemical parameters, to expect a reasonable NDE mechanism, further to develop the corrosion theory of metals.
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