Anodic process negative difference effect

Magnesium exhibits a very strange electrochemical phenomenon known as the negative-difference effect (NDE). Electrochemistry classifies corrosion reactions as either anodic or cathodic processes. Normally, the anodic reaction rate increases and the cathodic reaction rate decreases with increasing applied potential or current density. Therefore, for most metals like iron, steels, and zinc etc, an anodic increase of the applied potential causes an increase of the anodic dissolution rate and a simultaneous decrease in the cathodic rate of hydrogen evolution. On magnesium, however, the hydrogen evolution behavior is quite different from that on iron and steels. On first examination such behavior seems contrary to the very basics of electrochemical theory. 

The strange anodic polarization behaviors (negative difference effect, lower apparent valence, low anodic dissolution efficiency, low anodic polarization resistance and poor passivity ) are closely associated with the AHE process which is further related to the onset of localized corrosion or pitting . A comprehensive anodic dissolution model can be employed to understand these. 

Anion effects on the electrooxidation of Ru(OOOl) were explored by introducing Cr and Br into the 0.1 M HCIO4 solution. A sharp rise of anodic current occurs near 0.2 V, which is at a more negative potential than the onset of surface oxidation in sulfuric acid. Hence, even though halide ions are strongly adsorbed, they do not better protect the Ru from surface oxidation than do bisulfate ions. It is likely that a different redox process occurs with halide ions because they form compounds with Ru in several different oxidation states. 

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