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Tuesday, March 29, 2011

How to Explain Potentiodynamic Polarization Curve & its Relation to Passivation of Behaviour of Metals

The concept of passivation has been mentioned earlier in this discussion without a more profound discussion of the characteristics of the phenomena. Passivity is a condition found in some metals and alloys that are capable of resisting corrosion due to the formation of a surface film. These films form under strongly oxidizing conditions or high anodic polarizations. This definition excludes metals possessing a simple barrier film with reduced corrosion at active potential and small anodic polarization. Also, it should be pointed out that insoluble compounds formed by dissolution and re-precipitation are less tenacious and less protective than oxide films formed in situ at the metal surface. This is an exceedingly important characteristic found in several structural metals such as iron, chromium, nickel titanium and aluminium, and its respective alloys being the most noticeable example that of stainless steels.

Figure 1 – Schematic polarization diagram displaying transitions from active corrosion to passive behaviour and to the transpassive state

Metals and alloys that exhibit passive behaviour display a very distinct evolution on the Evans diagram, as exemplified in Figure 1 above. As the potential is increased from the corrosion potential, so the current increases according to normal dissolution behaviour until a critical value (icrit). This point also defines the beginning of stability for passive films, which occurs at potentials higher than Epp (primary passive potential). Beyond this point, the current measured can fall several orders of magnitude to a residual current (ipass). At higher potentials (Et) breakdown of the passive film might occur with an increase in anodic activity, as the metal enters the transpassive state. As always, for self-passivation to occur there must be a cathodic reaction with a nobler potential relative to the anodic reaction and, in this case, superior also to Epp.

One of the strategies employed in the protection of metals is the anodization process. By applying a potential in the passive state (between Epp and transpassivation), and choosing the right media and applied current, it is possible to grow very thick oxide layers to protect the metal surface from oxidation. This constitutes one of the most used techniques in the protection of aluminium alloys. There are several models that try to explain the formation and structure these oxide films; however, much is still uncertain. There are two basic theories: the crystalline oxide model and hydrated polymeric oxide model.

In the crystalline oxide model, as the name implies, passivation depends on the formation of an oxide/hydroxide layer. The exact structure of the oxides is very uncertain and seems to vary from crystalline all the way to completely amorphous. The oxides may contain oxygen and/or hydrogen under several different forms (H+, OH- or H2O), and the number of layers may change according to specific systems as well as the stoichiometry of the oxide.

In polymeric model, on the other hand, water molecules have an important role in passivation as they connect chains of polymeric oxide, whose structure varies from partially crystalline to amorphous.

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