Differential oxygenation corrosion

In soil, stainless steel is fairly resistant to uniform corrosion, which occurs over the entire surface however, it may be subject to pitting corrosion. Stainless steel is most often used in situations where contamination of the material carried in the pipe is the prime concern. However, as pitting of the buried structures might occur, where soil conditions surrounding the pipe vary, it would be prudent to install stainless steel pipe with a uniform, well-installed backfill where differential oxygen corrosion cells will not occur. Coatings and cathodic protection of buried pipehnes in corrosive soils should be considered. In noncorrosive soils, coatings for stainless steel are recommended. 

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. 

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. 

Oxygen corrosion usually takes the form of deep pitting and involves both tuberculation and differential aeration corrosion mechanisms. The BW commonly is brown and murky, and chemical treatment reserves usually are very low or absent. The source of the oxygen is either MU water dissolved oxygen (DO) or air in-leakage. 

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. 

Another example of a differential-aeration corrosion cell is an iron sheet with a drop of moisture on it (Fig. 12.31). The central region of the drop is oxygen starved compared with the peripheral regions, which therefore become electron-source areas, and corrosion is observed at the central electron-sink section. 

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... 

These forms of corrosion are similar to differential aeration corrosion, in that an oxygen-free region becomes acidic by virtue of the net anodic reaction and consequently corrodes rapidly when coupled to a region in aerated solution. However, a key difference is that these forms of corrosion occur on alloys that are initially passive so there is no limit to the area of the passive, cathodic region. Thus, the severity of the attack may be much greater (Fig. 3). Crevice corrosion occurs when there is a narrow gap between two pieces of metal or a piece of metal and an insulator. The oxygen is consumed by the slow passive corrosion in the crevice, causing the crevice to become sufficiently acidic that the passivity breaks down and active corrosion starts. This then... 

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. 

In the above cell, HCl is in two different concentrations. The activity (molality x activity coefficient) ai is greater than activity 02 fli > <12-Several types of concentration cells are encountered in corrosion. For example, a concentration cell is formed if one end of a pipe is exposed to soil and the other end to air. The end of the pipe in air is exposed to a high concentration of oxygen than the end of the pipe in the soil. The formation of a concentration cell leads to differential aeration corrosion in buried structures in the soil. 

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