Corrosion oxygen-concentration cell

Figure 6.20 Two types of crevice corrosion oxygen concentration cell and metal ion concentration cell774...

As rust accumulates, oxygen migration is reduced through the corrosion product layer. Regions below the rust layer become oxygen depleted. An oxygen concentration cell then develops. Corrosion naturally becomes concentrated into small regions beneath the rust, and tubercles are born. 

Figure 4.1 A) Oxygen concentration cell corrosion beneath a deposit (B) Oxy-...

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. 

Wastage was caused by classic long-term underdeposit corrosion. The combined effects of oxygen concentration cells, low flow, and contamination of system water with high chloride- and sulfate-concentration makeup waters caused corrosion. 

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. 

The carbon dioxide produced can contribute to the corrosion of metal. The deposits of ferric hydroxide that precipitate on the metal surface may produce oxygen concentration cells, causing corrosion under the deposits. Gallionalla and Crenothrix are two examples of iron-oxidizing bacteria. 

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. 

As mentioned earlier, there is an inverse relationship between water volumes and oxygen concentration in soil. As soils dry, conditions become more aerobic and oxygen diffusion rates become higher. The wet-dry or anaerobic-aerobic alternation, either temporal or spatial, leads to higher corrosion rates than would be obtained within a constant environment. Oxygen-concentration-cell formation is enhanced. This same fluctuation in water and air relations also leads to greater variation in biological activity within the soil. 

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. 

Table 7.6 Summary notes boiler section corrosion problems involving oxygen, concentration cells, and low pH.

Manganese and iron oxidation are coupled to cell growth and metabolism of organic carbon. Microbially deposited manganese oxide on stainless and mild steel alters electrochemical properties related to the potential for corrosion. Iron-oxidizing bacteria produce tubercles of iron oxides and hydroxides, creating oxygen-concentration cells that initiate a series of events that individually or collectively are very corrosive. 

MIC almost always acts in concert with other corrosion mechanisms and may, at times, appear to be crevice corrosion, underdeposit acid attack, oxygen-concentration cell corrosion, ion-concentration cell corrosion, and CO2... 

A) Oxygen concentration cell corrosion beneath a deposit (B) Oxygen concentration cell corrosion in a beaker containing an aerated piece of steel and an unaerated piece of steel. A potential develops between the aerated and unaerated steel pieces. Steel exposed to the lower dissolved oxygen concentration corrodes. Beneath a deposit (A) the oxygen-poor environment causes wastage. 

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. 

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... 

In discussing environment, we can look at its effect on a macro scale, e.g. in the atmosphere, in the ocean, etc. and also examine effects on a micro scale, i.e. what is happening on the metal surface or over short distances. Due to the great variety of environments in which metals are put to use, the range of corrosion problems are equally numerous. Often, similar types of corrosion occur in many environments and may stem from similar mechanisms these have been given specific names which indicate how the corrosion has occurred. For example, under-deposit corrosion and crevice corrosion are related, both being due to oxygen concentration cells. 

Thus, any surface geometry or covering which leads to poorer access of oxygen to one part of a metal surface over another will lead to the formation of an oxygen concentration cell and resultant localised corrosion. In such cases, the anodic dissolution occurs as close to the cathodic (aerated) area as possible, especially in poorly conducting media. This is due to the current taking the shortest path, i.e. least resistance in both metal and electrolyte. 

Dispersants may also be used in connection with fouling problems other than simple particulate deposition. For instance in corrosion prevention they may be used to prevent solids deposition and subsequent formation of oxygen concentration cells (see Chapter 10). In the contamination of surfaces by microorganisms (see Chapter 12) dispersants (often called biodispersants) are used to prevent or restrict, the approach of micro-organisms towards heat exchanger surfaces. 

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