Concentration cell differential aeration

As the reactions proceed, the dissolved oxygen in the small volume of stagnated solution inside the crevice is consumed. However, this does not prevent the dissolution reaction inside the crevice because the electrons reach outside the crevice through the metal, where plenty of oxygen is available for reduction. A concentration cell (differential aeration) is set up between the crevice area and the area outside the crevice. 

A difference in potential will be observed if electrically connected specimens of the same metal are immersed in solutions having different concentrations of ions of the metals. This is one common type of concentration cell. Another type is the oxygen concentration or differential aeration cell. 

Steel surfaces can develop biofilms that may form chemical concentration or differential aeration cells resulting in localised corrosion. In addition, if chloride ions are present, the pH of the electrolyte under tubercles (discrete hemispherical mounds [30]) may further decrease, enhancing localised corrosion. In the presence of certain bacteria such as iron-oxidising bacteria (lOB) [32], the chemical conditions under the tubercles formed by the bacteria may become very acidic as d ions combine with the ferric ions produced by lOB to form a very corrosive acidic ferric chloride solution inside the tubercle [30]. 

Concentration cells have two similar electrodes in contact with a solution of differing composition. The two kinds of concentration cells are salt concentration cells and differential aeration cells [186]. 

Differential Aeration Cells. This type of concentration cell is more important in practice than is the salt concentration cell. The cell may be made from two electrodes of the same metal (i.e., iron), immersed completely in dilute sodium chloride solution (Figure 4-433). The electrolyte around one electrode (cathode) is thoroughly aerated by bubbling air. Simultaneously the electrolyte around the other electrode is deaerated by bubbling nitrogen. The difference in oxygen concentration causes a difference in potential. This, in turn, initiates the flow of current. This type of cell exists in several forms. Some of them are as follows [188]. 

Air—Water Interface. This is another good example of a differential aeration cell (Figure 4-436). Here the water at the surface contains more oxygen than the water slightly below the surface. This difference in concentration can cause preferential attack just below the waterline. 

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

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

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...

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 )...

Oxygen Concentration Cell see under Differential Aeration. 

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. 

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... 

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). 

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... 

Differential aeration galvanic cell. Distilled water is an important medium since it is commonly used. Evans differential aeration cell (a galvanic cell created by a difference in oxygen concentration) for pitting has been shown to be important or essential for cracking in distilled water. (Miller)24... 

Two subtle corrosion effects can occur when a single metal is in contact with an electrolyte -differential aeration and crevice corrosion. Differential aeration can cause corrosion when no obvious galvanic cells are in evidence. To illustrate this effect, suppose we have a cell with a copper anode and cathode. If the concentration of the electrolyte and the temperature of each cell compartment is the same, no potential is generated and no corrosion occurs. However, bubble O2 into the one compartment, which becomes the cathode compartment, and corrosion will occur in the other, which forms the anode compartment. Differential aeration is, in fact, a concentration effect, and can be understood by using the Nernst equation. Electrons will flow from anode to cathode and the anode will corrode. 

DIFFERENTIAL AERATION CELL - An electrolytic cell, the electromagnetic force of which is due to a difference in air (oxygen) concentration at one electrode as compared with that at another electrode of the same material, (see concentration cell)... 

OXYGEN CONCENTRATION CELL - (see differential aeration cell). 

P + Pi brasses of similar composition (Cu—38 wt.% Zn and Cu—41 wt.% Zn, respectively), it has been concluded that this is not simply due to an increased Zn concentration in the p phase, but is likely to reflect the structural differences of hoth phases [67]. Other factors that stimulate the dezincification of both O - and a p brasses are the presence of chlorides, high concentrations of CuCl2 ions, stagnant environments, differential aeration cells and elevated temperatures. In chloride solutions, such conditions are typically encountered at a later stage of corrosion when mass transport restrictions by a deposit, corrosion product, or crevice have heen established and create a local environment, in which the copper in the brass is nearly in equilibrium with CU2O, CuCl, and accumulated CuCl2 anions [68,69]. Experimentally, the necessity... 

A differential aeration galvanic cell consists of differences in oxygen concentration, leading to pitting and cracking in distilled water (96). 

Identical metals in contact with different concentrations In this case, the metal immersed in a dilute solution is dissolved from the electrode and deposited on the electrode immersed in a more concentrated solution. The other type of electrochemical concentration ceU is known as a differential aeration cell. In this case, the electrode potential difference occurs when the electrode is immersed in the same electrolyte with different oxygen partial pressures. Differential aeration initiates crevice corrosion in aluminum or stainless steel when exposed to a chloride environment. 

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