Metals that normally fall victim to corrosion will sometimes exhibit a passivity to corrosion. Passivity is the characteristic of a metal exhibited when that metal does not become active in the corrosion reaction. Passivity is caused by the buildup of a stable, tenacious layer of metal oxide on the surface of the metal. This oxide layer is formed by corrosion on a clean metal surface, where the corrosion products are insoluble in the particular environment to which the metal is exposed. Once the layer, or film, is formed, it acts as a barrier separating the metal surface from the environment. For further corrosion to occur, the reactants must diffuse through the oxide film. Such diffusion is very slow or nonexistent, thus corrosion either decreases markedly or stops.

Metals such as zirconium, chromium, aluminum, and the stainless steels form thin, tenacious oxide films when exposed to the atmosphere or to pure water at room temperature. In some cases, the film is extremely thin and may be invisible to the unaided eye, but it is still very effective in giving these metals a marked passivity.

If there is a net conversion of reactants to products in a system, the system will be chemically unstable, and the reaction will continue until a stable state is attained. This stable state is known as equilibrium.

An active electrochemical cell (oxidation-reduction reaction) is an unstable chemical system. The potential associated with a galvanic cell, for example, steadily decreases as current flows and the oxidation-reduction reaction proceeds. Eventually, the potential falls to zero, the cell no longer supplies electrical energy, and no further net reaction takes place. At this point the system is at equilibrium. In electrochemical cells, the decrease in cell potential caused by the operation of the cell (current flow) is called polarization.

This change in cell potential can be determined. Consider the zinc-copper galvanic cell shown in Figure 3. As the reaction takes place, Zn⁺² ions (produced by the oxidation of zinc metal) pass into solution. The Cu⁺² ions in solution are reduced as the copper metal plates out. Thus, the concentration of Zn⁺² in solution increases and the concentration of Cu⁺² decreases according to the following overall reaction.

Figure 3 A Galvanic Cell

As Zn+² increases and Cu+² decreases, the electrical potential decreases. This decrease in cell potential, which results from changes in concentrations, is one form of polarization called concentration polarization.

Now consider a galvanic cell with zinc and platinum electrodes, such as that shown in Figure 4. The half-reactions in the cell are as follows.

Again, as the cell operates, the cell potential drops. The decrease is partially due to the increase in Zn⁺² concentration and the decrease in H₃0+ concentration, but another type of polarization also occurs in this cell. This second type is associated with the reduction half-reaction.

Figure 4 A Galvanic Cell Showing Absorbed Hydrogen Atoms on a Cathode

The hydrogen atoms formed by the reaction of Equation (2-4) absorb on the surface of the metal and remain there until removed by one of two processes: combination of two hydrogen atoms to form molecular hydrogen, which is then released as a gas or reaction with dissolved oxygen to form water. In the absence of oxygen (deaerated solutions), the first process applies.

Combining Equation (2-6) with Equation (2-4), the net reduction half-reaction is obtained.

Until the absorbed hydrogen atoms are removed from the metal surface, they effectively block the sites at which the reaction of Equation (2-4) can occur. At low temperatures the reaction of Equation (2-6) is slow relative to the reaction of Equation (2-4) because, although the reaction is energetically favored, the combination of two hydrogen atoms requires a large activation energy. Equation (2-6) shows the rate-controlling step of the net reduction half-reaction. Because the oxidation half-reaction can occur no faster than the reduction half-reaction, the rate of the overall oxidation-reduction reaction is controlled by the reaction of Equation (2-6).

The layer of absorbed atomic hydrogen is said to polarize the cell. This type of polarization is called activation polarization and is sometimes referred to as hydrogen polarization, or cathodic polarization, because the polarizing reaction occurs at the cathode.

Both concentration and activation polarization decrease the net oxidation-reduction reaction rate. In corrosion processes, activation polarization usually has the greater effect.

Summary

The important information in this chapter is summarized below.

Corrosion Theory Summary

Ionization is the process of adding electrons to or removing electrons from atoms or molecules which creates ions.

Conductivity is a measure of the ability of a substance to allow electron flow.

Corrosion is the deterioration of a material due to interaction with its environment.

Electrolysis is the decomposition by electric current.

General corrosion is the process whereby the surface of a metal undergoes a slow, relatively uniform, removal of material.

Corrosion is electrochemical in nature because the corrosive chemical reactions involve a transfer of charge. The metal ions go into solution causing the metal to become negatively charged with respect to the electrolyte. The difference in the charge causes a potential to develop and produces a voltage between the electrolyte and the metal.

The oxidation step of the oxidation-reduction process is where an atom (in this case a metal atom) releases electron(s) and becomes a positively-charged ion. The areas where oxidation takes place become electrochemical cells made up of two different substances. The oxidation step results in a chemical transformation that is destructive to the metal. The positive metal ions may go into solution, or they may combine with any available negative ions or water to form ionic compounds. An example of the oxidation step is:

The reduction step of the oxidation-reduction process is where a positively-charged ion gains electron(s). The reduction step for most metals, in an aqueous environment, is the reduction of hydronium ions. An example of the reduction step is:

Passivity is the buildup of a stable, tenacious layer of metal oxide on the surface of the metal that acts as a barrier separating the metal surface from the environment. Passivity decreases or stops the corrosion process because of the formation of the layer.

Polarization is the decrease in cell potential caused by the operation of the electrochemical cell. Polarization can be in two forms; concentration or activation. Concentration polarization is associated with the concentration of ions in solution which shields the metal, thereby causing a decrease in the electrical potential of the cell. Activation polarization is the formation of a layer containing absorbed hydrogen atoms that block the metal's surface from the corrosion process.