Potentiostatic polarization active-passive metal

Potentiodynamic and potentiostatic anodic polarization curves obtained at the same sweep rate are identical. They identify corrosion properties of passivating metals and alloys and are very useful in predicting the corrosion properties of materials. Figure 4.5 shows potentiostatic polarization curve of an active-passive metal with more than one passivation potential. 

Fig. 4.4 Potentiostatic polarization curve of an active-passive metal obtained with controlled potential.

E4.1. Calctilate and construct, using mixed potential theory, the critical passivation current density (a) potentiostatic and (b) galvanostatic polarization curve for anodic dissolution for active-passive metal that has the following electrochemical parameters Ecorr= 0-5 V vs. SCE, Epp= — 0.4 V vs. SCE, 4orr= 10 A/cm, K = 0.05,. , = 10 A/cm andEtr= + l-0V vs. SCE. 

Imagine what would happen if the structure is now polarized in the opposite direction. It would amount to polarizing the potential of the anode to that of cathode in the positive direction. Theoretically, such a practice should result in creating corrosion rather than protection. But for some metals, positive polarization forms a protective oxide/hydroxide surface film and this phenomenon of passivation for a limited number of metals results in retardation of corrosion. By this method called anodic protection, it is possible to passivate active-passive metals. Metals, such as iron, chromium and nickel are passivated by anodic polarization, which leads to retardation of corrosion. The potential of this must, however, be maintained in the region of passivity by a potentiostat. Anodic protection is widely applied in transport of acids and corrosives in containers and other applications. 

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