Corrosion & Environmental Degradation

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· Metals such as Fe, Cr, Ni, Ti, and their alloys demonstrate an overpotential behavior involving active, passive, and transpassive corrosion behavior. The diagram illustrates the behavior of these materials. · In the absence of an applied potential, the metal half cell is at its electrode potential E(M/M+) and equilibrium exchange current, i0(M/M+). As a more positive overpotential is applied, the corrosion rate increases along the  M -> M+ + e branch associated with a non-passivating metal. · At the condition Epp,IC, the corrosion rate drops several orders of magnitude and due to an insulating surface film the material is in a passive state.

· Film thickness is on the order of 3 nm and passive behavior is retained until the applied potential is large enough to cause film breakdown. The system then enters the trans-passive state and the corrosion rate increases by several orders of magnitude.

From: M.G. Fontana, "Corrosion Engineering,"