DIFFERENTIAL AERATION CELLS

Pits are often covered by an outer coating of corrosion products; in ferrous materials these are called tubercules, barnacles or carbuncles.

The underside of the tubercule may be composed of ferrous hydroxide in which case progressive oxidation will cause a brown hydrated ferric oxide covering to form. The area between the pit and the body of the tubercule is oxygen deficient, with increased concentrations of chloride and sulfate ions.

The water above the deposit contains dissolved oxygen, but the area below the deposit is oxygen deficient. A differential aeration cell is therefore formed with the oxygen deficient area becoming the anode. The result is extensive corrosion and an unacceptably high metal loss rate.

A self-perpetuating corrosion cycle is established. The metal lost through corrosion forms additional deposits which in turn create new differential aeration cells, and result in more corrosion.

Frequently, the cycle induces rapid failure, despite the presence of a corrosion inhibitor. The metal surrounding a deposit will be cathodic to areas beneath it because they may be protected by an inhibitor film. A small anode is, therefore, developed within a relatively large cathodic area.

The damage potential of this relative area effect was described in the section on Physical Factors Affecting Corrosion. In this situation, rapid corrosion ensues, with complete metal perforation not uncommon. It is essential to maintain a clean cooling water system to avoid costly downtime of critical process equipment.

Low-flow areas are particularly susceptible to this form of corrosion. Suspended .materials settle in these areas, causing extensive deposition and subsequent attack. This problem is magnified along heat exchange surfaces when deposition causes unequal heat transfer, which results in temperature differentials, and the creation of a corrosion cell, with cooler areas becoming cathodic to warmer ones.

These conditions lead to rapid corrosion. The process is self-limiting in stagnant or low flow areas because the tubercule will eventually become sufficiently dense to prevent diffusion of the metallic ions into the water. However, in a cooling water system, flow rates are often sufficient to remove this potentially passivating layer of corrosion products.

Fresh metal is, therefore, continuously exposed to corrosion.

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