Absence of negative difference effect on pure magnesium: A consequence of solution and surface chemistry

摘要

Magnesium alloys find applications in automotive, aerospace and biomedical sector as structural materials as well as in energy devices. However, high rates of corrosion at anodic potentials limit the widespread application of Mg and its alloys. The rate of hydrogen evolution, which is a cathodic reaction, increases with increase in applied anodic potential (or applied anodic current density) in Mg alloys; which is known as the negative difference effect (NDE). In the present investigation on pure Mg in sodium bicarbonate solution (0.1 M NaHCO_3); it was observed that although the rates of hydrogen evolution are significantly higher than that in 0.1 M NaCl solutions at all potentials, the so-called NDE is absent in the potential range of -1.8 V_(SCE) to -1.2 V_(SCE). This trend has also been confirmed by hydrogen collection galvanostatically. However, in 0.1 M NaHCO_3 there exists a small range of anodic potentials close to OCP where Mg still exhibits NDE. Unlike the cathodic activation reported for Mg in NaCl solutions the cathodic kinetics of Mg samples in 0.1 M NaHCO_3 which were anodically dissolved (at different anodic potentials/current densities) with different film coverage fractions remained close to that of the polished sample upon potentiodynamic polarisation, with no so-called 'cathodic activation' unlike that on Mg in NaCl solution where magnesium hydroxide film forms. This suggests that the surface film is probably not affecting the catalytic activity of the surface in the potential ranges explored in this electrolyte. Scanning electron microscopy. X-ray photoelectron spectroscopy and X-ray diffraction studies have been utilised to characterise this surface film morphology and chemistry. This investigation clearly demonstrates the complexity of NDE. It is evident that further studies are necessary to understand the extent of film formation, its breakdown and catalytic behaviour during anodic dissolution of Mg in different aqueous solution chemistries to develop a unified understanding of NDE.