Corrosion features hydrogen evolution

Undermining of Mg particles that may take place in a very severe corrosion process has been shown not to influence reaction (1.79) or (1.80) (Song et al, 1997b) either. Therefore, the hydrogen evolution phenomenon in an aqueous solution is one of the most fundamental and important corrosion features of Mg and its alloys. 

Williams [73] carried out similar experiments for a 1 cm HP Mg (100 ppm Fe) sample in 5% NaCl, pH 6.5 and SVET maps were obtained at 2, 8.4, 17 and 28.3 h after specimen immersion. This sample took significantly longer than the CP Mg to show the first sign of breakdown - about 2 h as opposed to a few minutes for CP Mg. The point of breakdown was visually identified as a black dot on the surface with some associated hydrogen evolution. The local current density was over an order of magnitude lower than that obtained with CP Mg at initial breakdown. Thereafter the HP Mg showed features akin to filiform corrosion. In situ SVET showed that the... 

Fe-Cr and Fe-Cr-Ni alloys are of high technical importance, the main benefit for ferritic and austenitic stainless steels resulting from the excellent corrosion resistance of Cr203 layers. Figure 5.31 shows the polarization curve of Fe-15 Cr in 0.5 M H2SO4 [92]. Its characteristic features are determined by the electrochemical properties of the pure alloy components. Hydrogen evolution (with cathodic currents) is observed up to E = -0.2 V followed by the potential range of active dissolution of Cr and Fe + up to OV where passivity starts due to... 

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