POLARIZATION

Polarization reduces the driving force of the corrosion reaction and minimizes metal loss by changing the potential of either the anode or the cathode, or both, so that the difference in potential between them is reduced to a minimum.

A basic concept of kinetics states that the rate of an overall reaction can be controlled by the rate of its slowest step; this applies to the corrosion reaction. Cathodic reactions are generally much slower than those occurring at the anode. Therefore, control of the overall corrosion rate can be accomplished by controlling the rate at the cathode and achieving polarization.

Control of the main cathode reactions, hydrogen or oxygen reduction, requires an understanding of their kinetics. Le Chateller's Principle states that a reaction can proceed to completion with an overabundance of reactants and/or removal of products. Applying this to the cathodic equations listed, it becomes clear that equations "a" and "b" will proceed if hydrogen gas is generated at the cathode; similarly, equations "c" and "d" will proceed if a sufficient supply of oxygen is available at the cathode.

Hydrogen will bubble off the cathode only when the cathode reaches a certain potential. The difference in potential between the cathode and a hydrogen electrode at equilibrium in the same solution is defined as hydrogen overvoltage. This overvoltage decreases with increasing temperature and surface roughness. The overvoltage needed for hydrogen evolution for some common metals is found in the table below.

Normally, the available hydrogen will be insufficient to exceed the overvoltage.

Cathode overvoltage of hydrogen on common metals

In waters of low pH, with relatively high concentrations of hydrogen ions, overvoltage is easily overcome. For this reason, equations "a" and "b" are the rate-controlling cathodic reactions. It also explains the rapid corrosion of iron and similar metals in acids

Some metals do not corrode in acid solution; these "noble" metals are more cathodic than hydrogen and will be reduced in preference to hydrogen ions. For example, copper does not corrode in an acid 1f there are no oxidizing agents present.

Because the evolution of hydrogen gas is part of the corrosion cell process, the cell will be polarized when the cathodic potential is reduced by a film of monatomic adsorbed hydrogen. This buildup of hydrogen, in turn, reduces the driving force of the overall corrosion reaction. Conversely, removal of hydrogen from the cathode surface will depolarize the corrosion reaction and lead to increased metal loss. At low pH, on metals less cathodic than hydrogen, the concentration of the latter can build to a point where overvoltage is overcome and hydrogen gas evolves.

In natural waters, where pH levels are far too high to overcome hydrogen overvoltage, the presence of dissolved oxygen usually controls the cathodic reaction rate. Equations "c" and "d" demonstrate the reactions involved. One logical corrosion control method, therefore, involves governing the amount of oxygen available to the cathode surface. Oxygen is brought to the metal by convection through the bulk of the cooling water and then by the diffusion through a thin laminar water film at the metal surface. If the amount of oxygen diffusion to the metal surface can be controlled, the corrosion reaction can be polarized. This is precisely the mechanism of cathodic corrosion inhibitors. They form an impervious film, which prevents the diffusion of oxygen to the cathode site. Another more costly way to remove oxygen involves mechanical deaeration techniques; these are often used in boiler operations but are usually uneconomical for most open cooling water systems. For ferrous-based materials, oxygen depolarization will be the determining factor in almost all cooling water situations, since pH is maintained at levels where hydrogen evolution effects are minimal.

It is relatively easy to protect closed systems. Any dissolved oxygen present is quickly used in the formation of oxide films along metal surfaces. Since the system is closed, no further oxygen is available. The pH of the system is kept fairly alkaline to maximize hydrogen overvoltage. Therefore, the main cathodic reactions are under control. Compare this to an open cooling water system where heat is rejected by evaporation to the atmosphere. In this situation, the water is constantly being resaturated with oxygen, which results in its subsequent availability to depolarize the corrosion cell. Open systems require more sophisticated corrosion inhibitor applications to maintain proper corrosion control.