Concentration cell corrosion damage

Figure 14. Band of concentration cell corrosion damage on bronze pump impeller...

The problem with relying solely on anodic area corrosion inhibition is the risk of local film damage, which concentrates the corrosion current flow and permits a highly active anodic cell to be developed and causing accelerated corrosion to take place. This in turn leads to severe metal wastage, often in the form of deep pitting. 

All three types may be present in heat exchangers. Any object such as loose scale or other deposits resting on a metal surface can give rise to a concentration cell and hence to crevice corrosion, which is reported to be the most potentially damaging type of corrosion in a water cooling system. Corrosion products can nucleate crystallization from supersaturated solutions or subcooled melts, anchor... 

Stainless steels are rarely used in soil applications, as their corrosion performance in soil is generally poor and not better than bare steel. Localized corrosion attack is a particularly serious concern. The presence of chloride ions and concentration cells developed on the surface of these alloys tends to induce localized corrosion damage. 

Corrosion resistance of stainless steels results from formation of a passive oxide film which is stable in an oxidising enviromneait. Any physico-chemical instability of this oxide film either as a result of change in the chemistry of the environment or formation of cracks and/or scratches on the metal surface provides conditions for formation of an oxygen concentration cell which can result in localised corrosion. An example of chemical change of the environment leading into oxide film instability mentioned above is the effect of chloride ions. Chloride ions can locally damage the protective film on stainless steels [44]. 

Moving seawater is less corrosive than stagnant seawater. Under stagnant conditions, oxygen concentration cells are setup on the surface of materials and corrosion is accelerated. Salt water must be rinsed without delay from stainless steel equipment before plant shutdown, otherwise heavy damage by pitting may be caused to the equipment necessitating replacement. 

Heterogeneous surface areas consist of anodic regions at corrosion cells (see Section 2.2.4.2) and objects to be protected which have damaged coating. Local concentrations of the current density develop in the area of a defect and can be determined by measurements of field strength. These occur at the anode in a corrosion cell in the case of free corrosion or at a holiday in a coated object in the case of impressed current polarization (e.g., cathodic protection). Such methods are of general interest in ascertaining the corrosion behavior of metallic construction units... 

Water velocity and turbulence can damage protective films and deposits on metal surfaces causing increased corrosion. Soft metals, such as copper, are particularly susceptible to erosion corrosion, but steel and other metals are also susceptible if the water velocity is sufficiently high. The critical velocity for erosion-corrosion of copper in freshwater is about 5 s (1.52 m/s), but this velocity can drop sharply as the chemical corrosivity of the water increases. Suspended solids in the water can increase the erosion characteristics of the water [1]. Deposits can result in accelerated corrosion from the formation of oxygen differential concentration corrosion cells. 

While concentration and oxygen cells are responsible for perhaps 90 percent of the corrosion in soils and natural waters, other cells, such as the temperature cell shown in Fig. 7.28, may be still quite damaging when they get established. In such cells, the two electrodes are of the same metal, but one is maintained at a higher temperature than the other by some external means. In most cases, the electrode at the higher temperature becomes the anode to cause what has been called thermogalvanic corrosion. 

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