Differential oxygen concentration cells

Oxygen concentration is held almost constant by water flow outside the crevice. Thus, a differential oxygen concentration cell is created. The oxygenated water allows Reaction 2.2 to continue outside the crevice. Regions outside the crevice become cathodic, and metal dissolution ceases there. Within the crevice. Reaction 2.1 continues (Fig. 2.3). Metal ions migrating out of the crevice react with the dissolved oxygen and water to form metal hydroxides (in the case of steel, rust is formed) as in Reactions 2.3 and 2.4 ... 

Underdeposit corrosion is not so much a single corrosion mechanism as it is a generic description of wastage beneath deposits. Attack may appear much the same beneath silt, precipitates, metal oxides, and debris. Differential oxygen concentration cell corrosion may appear much the same beneath all kinds of deposits. However, when deposits tend to directly interact with metal surfaces, attack is easier to recognize. 

Two sections of steel condenser tubing experienced considerable metal loss from internal surfaces. An old section contained a perforation the newer section had not failed. A stratified oxide and deposit layer overlaid all internal surfaces (Fig. 5.14). Corrosion was severe along a longitudinal weld seam in the older section (Fig. 5.15). Differential oxygen concentration cells operated beneath the heavy accumulation of corrosion products and deposits. The older tube perforated along a weld seam. 

Microstructural examinations revealed that branched cracks originated at shallow pit sites on the external surface. The pits, which may have formed during idle periods from differential oxygen concentration cells formed beneath deposits, acted as stress concentrators. The transverse (circumferential) crack orientation and the localization of cracks along just one side of the tube revealed that bending of the tube was responsible for the stresses involved. 

Crevice corrosion of copper alloys is similar in principle to that of stainless steels, but a differential metal ion concentration cell (Figure 53.4(b)) is set up in place of the differential oxygen concentration cell. The copper in the crevice is corroded, forming Cu ions. These diffuse out of the crevice, to maintain overall electrical neutrality, and are oxidized to Cu ions. These are strongly oxidizing and constitute the cathodic agent, being reduced to Cu ions at the cathodic site outside the crevice. Acidification of the crevice solution does not occur in this system. 

In addition to the basic corrosion mechanism of attack by acetic acid, it is well established that differential oxygen concentration cells are set up along metals embedded in wood. The gap between a nail and the wood into which it is embedded resembles the ideal crevice or deep, narrow pit. It is expected, therefore, that the cathodic reaction (oxygen reduction) should take place on the exposed head and that metal dissolution should occur on the shank in the wood. 

Although each form of concentration cell may be considered a discrete form of corrosion, in practice, more than one type may occur simultaneously. These forms of corrosion are all characterized by localized differences in concentration of hydrogen, oxygen, chloride, sulfate, and other minerals, but especially oxygen (producing the so-called differential oxygen concentration cell, or differential-aeration cell). The basic mechanisms surrounding each of these specific forms of concentration cell corrosion are discussed next. 

Under-Deposit Corrosion In the same way that oxygen becomes depleted in a crevice, and a differential-oxygen concentration cell is established, leading to localized corrosion of the oxygen-starved anodic area, so the same phenomenon readily occurs in dirty boilers under deposits, sludge, and other foulants. 

These forms of corrosion are all characterized by localized differences in concentration of hydrogen, oxygen, chloride, sulfate, etc., but especially oxygen (producing the so-called differential oxygen concentration cell or differential aeration cell). 

Pseudomonas sp. Facultatively aerobic, very common heavy slime producer. Can also initiate active corrosion by consuming oxygen and initiating differential oxygen concentration cells. 

In addition to heat-transfer problems, scales can exhibit shielding effects, thus increasing the risk of corrosion through differential oxygen concentration cells. A buildup of scale deposits will also tend to directly impede the flow of cooling water, as well as acting as a key for the accumulation of muds, silt, and biomass. Thus the buildup of a deposit acts as an indirect foulant. 

Figure 6.17 Differential oxygen concentration cell formation in slime deposition...

At first sight it might appear that differential aeration could be explained in terms of a reversible oxygen concentration cell, for which... 

Oxygen Concentration Cell see under Differential Aeration. 

Production of differential aeration cell. A scatter of individual barnacles on a stainless steel surface creates oxygen concentration cells. The formation of biofilm generates several critical conditions for corrosion initiation. Uncovered areas will have free access to oxygen and act as cathodes, while the covered zones act as anodes. Underdeposit corrosion (crevice corrosion) or pitting can occur. Depending on the oxidizing capacity of the bacteria and the chloride ion concentration, the corrosion rate can be accelerated. However, the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)5... 

OXYGEN CONCENTRATION CELL - (see differential aeration cell). 

Figure 7.27 Oxygen differential cell resulting from burial under paving producing an oxygen concentration cell.

An oxygen concentration cell an electrolytic cell resulting from differences in dissolved oxygen at two points. See also differential aeration cell. 

Common exanples of stagnation include nondtain-ing stmctures, dead ends, badly located components, and poor assembly or maintenance practices (Fig. I). General problems include localized corrosion associated with differential aeration (oxygen concentration cells), crevice corrosion, and deposit corrosion. 

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