Concentration Cell Corrosion

Concentration Cell Corrosion.

A difference in environment between sites on a single metal can also result in increased electrochemical activity.

This difference in environment can be due to non-uniform deposits or fouling on the surface, or, more commonly, built in features which create significant difference in environment.

Crevices at joints are the most common cause of these built in environmental differences. This form of concentration cell corrosion called “crevice corrosion” is often the most difficult form of corrosion to avoid in design and also is one of the most common causes of failure of marine equipment.

Definition.

Concentration cell corrosion is corrosion that is accelerated by differences in environment between separated areas on a single metal.

Mechanism.

Any situation that creates a difference in environment between areas on a single metal can cause concentration cell attack.

The basic mechanism is essentially the same as in galvanic corrosion but in the case of concentration cell corrosion the driving force is the difference in potential between a single metal exposed to different environments rather than the difference in potential between two different metals exposed to a single environment.

The rates of attack experienced in concentration cell corrosion are affected by relative anode/cathode areas in the same manner as in galvanic corrosion. In crevice corrosion, the resistance of the electrolyte to the flow of ions can also be a significant factor in limiting attack in deep tight crevices.

Oxygen Concentration Cells.

Dissolved oxygen has a significant effect on the corrosion of many metals. This is particularly true for alloys such as stainless steels where the corrosion resistance of the alloy is dependent upon abundant oxygen for the stability and self-repair of protective films. Oxygen is also an active participant in the most predominant cathodic reaction in many environments.

The oxygen content of the electrolyte inside a crevice is usually low as oxygen is consumed by both corrosion and biological activity and replacement of oxygen inside the crevice is limited.

The crevice can be formed by metal-to-metal contact, by contact of a metal with a nonmetal or under deposits of debris or fouling. For a metal with a passive film, the metal tends to become active within the crevice where the lack of oxygen causes the passive film to be less stable and less easily repaired.

The resulting active/passive cell has substantial driving potential as noted on the galvanic series.

The anodic area within the crevice is normally small with respect to the cathodic area outside the crevice and with this adverse area ratio, the corrosion inside the crevice can be very rapid. Once initiated, crevice corrosion can also be accelerated by the formation of aggressive chemical compounds within the crevice which further accelerates the attack within the crevice.

In the case of stainless steels, the chromium and nickel chlorides which are formed are very acidic and crevice corrosion can be very rapid once initiated.

Oxygen concentration cell corrosion can also occur on metals which do not have passive films. In this case, the difference in oxygen content makes the area with low oxygen content predominantly anodic with respect to more highly oxygenated areas.

The reason for this is due to the effect of the law of mass action on the predominant cathodic reaction in neutral and alkaline environments.

This reaction is:

2 H2O + O2 + 4 e- ----> 4 OH-

The law of mass action indicates that, where the oxygen content is high, the cathodic reaction will occur more readily than when the oxygen content is low.

Thus areas where the oxygen content is low will not be as effective a cathode, anodic reactions will predominate and the area will act as an anode.

Where the oxygen content is high, the cathodic reactions will predominate and the area will act as a cathode.

Thus, the area inside a crevice will be anodic with respect to the area outside and the same process as described above for active/passive oxygen concentration cells will occur.

Oxygen concentration cell crevice corrosion is particularly insidious. First, many material are susceptible to this form of attack that have otherwise excellent performance in marine environments.

Second, the attack often occurs deep inside crevices in sealed areas, in joints, and in fasteners where a very small amount of corrosion can result in a serious failure.

Third, as it occurs deep within the crevice, it is difficult to detect. Crevice corrosion of this type often remains hidden until revealed by failure.

Source : "Corrosion Control" NAVFAC MO-307 September 1992