Differential aeration corrosion

Richard, A. and Nicolaides, G., Differential Aeration Corrosion of Passivating Metal Under a Moist Film of Locally Variable Thickness , J. Electrochem. Soc., 121, 183 (1974)... 

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. 

Localized, concentration-cell corrosion (differential aeration corrosion), occurring as Tuberculation corrosion Crevice corrosion Under-deposit corrosion Pitting corrosion All forms of localized, concentration-cell corrosion are indirect attack type corrosion mechanisms. They result in severe metal wastage and can also induce other corrosion mechanisms, e.g. Stress corrosion Corrosion fatigue... 

One effect is that bio-fouling of the metal surface often promotes localized attack. When a barnacle or mussel attaches itself to a plate of stainless steel in sea water, a differential aeration corrosion cell is formed. Intense local pitting results which may lead to complete perforation in a relatively short time. In one case a 0.75-inch thick stainless steel plate was perforated beneath a barnacle base after 9 months of immersion in sea water. 

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. 

Apart from corrosion due to differential aeration, corrosion of underground metal structures and pipelines may also arise from stray currents. How this comes about can be seen in the accompanying diagram (Fig. 12.32). The presence of a current-carrying cable in conducting soil results in stray currents passing through the soil. These stray currents may set up a potential difference between two portions of a pipeline, which then develops electron-source (cathodic) and -sink (anodic) areas. Thus, pipelines tend to corrode when they pass near electric lines. 

Differential aeration corrosion Mild steel CeCl3 inhibitor 0-200 ppm in tap water [7]... 

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... 

Differential aeration corrosion near the liquid level... 

In an electrochemical sense, SCC is similar to crevice corrosion and differential aeration corrosion, in that anodic and cathodic areas are spatially separated. With SCC, the anodic processes occur at the crack tip while cathodic processes occur on the crack flanks and near the crack mouth. Hinton et al. (1985) have proposed that the inhibition... 

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. 

As discussed above, deposits can cause accelerated localized corrosion by creating differential aeration cells. This same phenomenon occurs with a biofilm. The nonuniform nature of biofilm formation creates an inherent differential, which is enhanced by the oxygen consumption of organisms in the biofilm. 

Differential Aeration-the stimulation of corrosion at a localized area by differences in oxygen concentration in the electrolytic solution that is in contact with the metal surface. 

Figure 4-434. Differential aeration cell illustrated by waterline corrosion. (From Ref. [165].)...

Figure 53.4 Crevice corrosion driven by (a) a differential aeration cell and (b) a differential metal ion concentration cell...

Fig. 1.48 Examples of differential aeration cells (a) and (b) Differential aeration cells formed by the geometry of a drop of NaCl solution on a steel surface (c) differential aeration cells formed by the geometry of a vertical steel plate partly immersed in a NaCl solution. Increasing concentrations of Na2 CO3 decrease the anodic area (d) until at a sufficient concentration attack is confined to the water line (e) (/) shows the membrane of corrosion products formed at water...

At first sight the mechanism of crevice corrosion appears to be simply the formation of a differential aeration cell in which the freely exposed metal outside the crevice is predominantly cathodic whilst the metal within the crevice is predominantly or solely anodic the large cathode current acts on the small anodic area thus resulting in intense attack. However, although differential aeration plays an important role in the mechanism, the situation in reality is far more complex, owing to the formation of acid within the crevice. 

Fig. 1.52 Mechanism of filiform corrosion showing how atmospheric oxygen and watCT enter the active head through the film (lacquer) and how water leaves through the inactive tail. This results in a high concentration of oxygen at the V -shaped interface between the tail and the head, and to a differential aeration cell (after Uhlig )...

Gladysheva, V. P. and Shatalov, A. Ya., Corrosion of Metals During Differential Aeration , Part 2 Work of Differential Aeration Pairs During Varied Oxygen Concentration Proportions at the Cathode and Anode , Izv. Vyssh. Ucheb. Zaved., Khim. i Khim. Tekhnol., 16, 1663 (1973) C.A., 80, 55218J... 

The rate of water flow is also most important. This determines the supply of oxygen to the rusting surface, and may remove corrosion products that would otherwise stifle further rusting. A plentiful oxygen supply to the cathodic areas will stimulate corrosion, but so may smaller supplies at a slow rate of flow, if this leads to the formation of differential aeration cells (see Section 1.6). 

The US Bureau of Mines found the chemical and galvanic corrosion behaviour of both the TZM and Mo-30W alloy to be generally equal or superior to that of unalloyed molybdenum in many aqueous solutions of acids, bases and salts. Notable exceptions occurred in 6-1 % nitric acid where both alloys corroded appreciably faster than molybdenum. In mercuric chloride solutions the TZM alloy was susceptible to a type of crevice corrosion which was not due to differential aeration. The alloys were usually not adversely affected by contact with dissimilar metals in galvanic couple experiments, but the dissimilar metals sometimes corroded galvanically. Both alloys were resistant to synthetic sea water spray at 60°C. 

Resistance to crevice corrosion Titanium is more resistant to crevice corrosion than most conventional metals and alloys, particularly where differential aeration is involved, e.g. it is very resistant to crevice attack in sea water at normal temperatures. This form of corrosion becomes more severe when acidity develops in a crevice and this is more prone to occur under conditions of heat transfer . Under these circumstances, especially in the presence of halide, even titanium may suffer attack, and the metal should not be employed in strong aqueous halides at temperatures in excess of 130°C. This limiting temperature can be raised to 180°C by use of the Ti-0- 15Pd alloy " or by coating with noble metals. (See also Sections 1.4 and 1.6.)... 

Both iron- and copper-based alloys are corroded more easily on either side of the neutral pH band. In low pH conditions e.g. due to carbon dioxide, the acidic environments attack the alloys readily, causing damage both at the points of initial corrosion and perhaps, consequentially, further along the system, by screening the surface with corrosion products and permitting the development of differential aeration cells. 

Soil corrosion does not lend itself readily to direct study in the laboratory. However, indirect methods involving the action of differential aeration cells have yielded valuable information in comparing the probable corrosivities of different soils towards steel. The details of this technique were described by Denison , Ewing , Schwerdtfeger, and by Logan, Ewing and Denison ... 

Differential Aeration Differences in oxygen concentration in the electrolyte solution in contact with a metal. Differential aeration stimulates corrosion of that area where the oxygen concentration is lower, which becomes the anodic site. 

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