Differential oxygenation

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. 

D. Zhao, C. Constantinescu, E.W. Hahn, R.P. Mason, Differential oxygen dynamics in two diverse Dunning prostate R3327 rat tumor sublines (MAT-Lu and HI) with respect to growth and respiratory challenge, Int. J. Radiat. Oncol. Biol. Phys. 53 (2002) 744-756. 

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. 

Forms of corrosion of metals and alloys. It should be noted that organisms are more likely to cause localized than general corrosion because of the differential oxygen cell. In each case, the localized attack was found beneath macrofouling layers. Corrosion of copper, steel, and aluminum anodes was significantly higher when connected to cathodes on which the biofilm was allowed to grow naturally Unexpectedly rapid localized... 

In applying a differential oxygen mole balance over the increment Ar located somewhere between R, and R, we recognize that O2 does not react in this region, and reacts only when it reaches the solid carbon interface located at r = R. We shall let species A represent O2. 

Another example of the cyclodextrin cavity s role in complexation photochemistry is given by Weber et al. [96] who reported the photosensitized enantio-differentiating oxygenation of racemic a-pinene (31), using a porphyrin tethered to a p-cyclodextrin derivative (32) (Fig. 6). Upon irradiation with visible light (A, > 350 nm) in the presence of atmospheric oxygen for 8 h, several oxidation products were obtained. The ratios of these products were found to differ depending on the solvent employed, but it was noted that the (S) enantiomers were always formed in excess. For the reaction in the presence of 5 equivalents of 2-methylpyridine, excellent ee s of up to 67% for the (S)-1,2-epoxide (33) and 57%... 

Unsupported Au nanoclusters (or those contacting an inert support material such as BN) exhibit strong size-dependent reactivity, with optimal oxidation performance typically reached < 5 nm diameter [59], For example, colloidal gold stabilized by polyvinylpyrrolidone (PVP) shows pronounced size effects in the aerobic oxidation of benzylic alcohols in water under ambient conditions [60]. Figure 2.1 illustrates this phenomenon for p-hydroxybenzyl alcohol oxidation, wherein 1.3 nm Au clusters achieve 80 % conversion, whereas 9.5 nm clusters are catalytically dead. Differential oxygen adsorption onto these gold clusters is believed to play a crucial role in regulating reactivity. 

---