Anode polarization resistance

Since the low anodic polarization resistance of a Mg alloy originates... 

Apart from the alloying element, environmental solution is another critical factor determining the passivity of a Mg alloy. Figure 1.10 shows that AZ3 IB has a very low anodic polarization resistance in 5% NaCl, but in a solution of 0.8% NaCl+0.1% CaCl2+0.075% NaHCOs it displays a passive region. Usually, the passivity of a Mg alloy increases with increasing pH value of the environmental solution, and a passive Mg alloy can lose its passivity at low buffered pH values (Shaw and Wolfe, 2005). In nature, the passivity is closely associated with the protectiveness of surface film. Since the quality of a surface film determines the AHE process, the passivity can also be reflected by an AHE process. 

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. 

The low anodic polarization resistance of Mg and its alloys can be ascribed to the breakdown of the film. At potentials more negative than Ept, the surface is fully covered by a surface film according to the model, and thus the anodic polarization current density does not dramatically increase with increasing potential. The presence of surface film on the surface also accounts for the fact that no active dissolution peak appears on the anodic polarization curve. After the polarization potential becomes more positive than Ept and there are considerable film-free sites or areas, the Mg" " generation and the subsequent further anodic oxidation, disproportionation and hydration of Mg" become significant. Therefore, the anodic current density dramatically increases. 

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