Passivity of Metals

The passivation behavior of a metal is typically studied with a basic electrochemical testing setup. When the potential of a metallic component is controlled and shifted in the more anodic (positive) direction, the current required to cause that shift will vary. If the current required for the shift has the general polarization behavior illustrated here, the metal is termed active-passive and can be anodically protected.

Only a few systems exhibit this behavior in an appreciable and usable way. The corrosion rate of an active-passive metal can be significantly reduced by shifting the potential of the metal so that it is at a value in the passive range. The current required to shift the potential in the anodic direction from the corrosion potential E_corr can be several orders of magnitude greater than the current necessary to maintain the potential at a passive value. The current will peak at the passivation potential value shown as E_pp.

In order to produce passivation the critical current density (i_cc) must be exceeded. The anodic potential must then be maintained in the passive region without allowing it to fall back in the active region or getting into the transpassive region, where the protective anodic film can be damaged and even break down completely. It follows that although a high current density may be required to cause passivation (> i_cc), only a small current density is required to maintain it, and that in the passive region the corrosion rate corresponds to the passive current density (i_p).

Passivity can also be readily produced in the absence of an externally applied passivating potential by using oxidants to control the redox potential of the environment. Very few metals will passivate in non-oxidizing acids or environments, when the redox potential is more cathodic than the potential at which hydrogen can be produced. A good example of that behavior is titanium, and some of its alloys, that can be readily passivated by most acids, whereas mild steel requires a strong oxidizing agent, such as fuming nitric acid, for its passivation.

Alloying with a more easily passivated metal normally increases the ease of passivation and lowers the passivation potential, as in the alloying of iron and chromium in 10% sulfuric acid. Small additions of copper in carbon steels have been found to reduce ip in sulfuric acid. Each alloy system has to be evaluated for its own passivating behavior as illustrated by the case Ni-Cr alloys where both the additions of nickel to chromium and chromium to nickel decrease the critical current density in a mixture of sulfuric acid and 0.25 M potassium sulfate.