Wrought AZ91 alloy

In this section, the wrought AZ91 alloy was fabricated as follows the cast ingots were extruded at 230-240 °C with a horizontal water press, the rate was 2-3m/min and the extrusion ratio was 18 1. 

The potentiodynamic anodic polarization plots of wrought AZ91 alloy without and with Ce in 0.01 M NaOH aqueous solution are shown in Fig. 4.21. It can be seen that the anodic polarization plot of wrought AZ91 alloy without Ce shows an active-passive behavior. A decrease in current density at potential above -1.456Vsce is observed. Within the potential range of passivity, a relative small passive current density (about 4 x 10" A/cm ) is observed, which is potential-dependent. 

The polarization plot of wrought AZ91 alloy with Ce differs from that of wrought AZ91 alloy without Ce. For wrought AZ91 alloy with Ce, the... 

The potentiodynamic anodic polarization plots of wrought AZ91 alloy without and with Ce in 0.01 M NaOH. 

Mott-Schottky (M-S) plots of passive films on wrought AZ91 alloys without and with Ce are shown in Fig. 4.26(a). As it can be seen, the positive slopes are obtained on wrought AZ91 alloy without and with Ce, which indicate n-type semiconductors of passive films for two alloys. Namely, Ce in wrought AZ91 alloy does not change the type of semiconductor. 

The slope of 1/C versus V plot is inversely proportional to carrier concentration (donor or accepted). The slope of wrought AZ91 alloy without... 

M-S plots of the passive films for wrought AZ91 alloy without and with Ce (a), sketch presentation of the electronic structure of n-type passive film at anodic potential (b). 

The results of the potentiostatic polarization and EIS have showed that the passive film formation processes for two alloys are diffusion-controlled processes, which means the diffusion stage is slower than the electrochemical stage during the passive film formation process. The metal ionization process at the interface of metal and film was the electrochemical process. The process was more rapid than the diffusion process, which led to metal ions or oxygen vacancies predominated in the passive film. This kind of passive film has an n-type semiconductor character. Therefore, the passive films of wrought AZ91 alloy without and with Ce addition present n-type semiconductors. 

In summary, short rod-like Al4Ce phases were formed in wronght AZ91 alloy with 1.5 mass% Ce, and DRX had been retarded. The passive cnrrent density inereased and the passive film stability decreased after Ce addition into wrought AZ91 alloy. 

The passive film formed on two alloys had donble layers. The onter layer was the metal hydroxides and the inner layer was the metal oxides. No Ce hydroxides were found at outer layer and Ce in the form of Ce02 existed only in the inner layer of the passive film for wrought AZ91 alloy with Ce. 

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