Crevice corrosion oxygen diffusion

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. 

It is appropriate to consider first the crevice corrosion of mild steel in oxygenated neutral sodium chloride, and then to consider systems in which the metal is readily passivated. Initially, the whole surface will be in contact with a solution containing oxygen so that attack, with oxygen reduction providing the cathodic process, occurs on both the freely exposed surface and the surface within the crevice (Fig. 1.50). However, whereas the freely exposed surface will be accessible to dissolved oxygen by convection and diffusion, access of oxygen to the solution within the crevice can occur only... 

In view of this sequence, the crevice geometry parameters of gap width and depth become important. If the gap is sufficiently wide and shallow, oxygen depletion and chloride-ion influx will decrease and metal-ion buildup will be less due to increased diffusion of corrosion products from the crevice. The pH decrease due to hydrolysis of cations will be less, the passive film may be preserved, and if so, crevice corrosion will not occur. These factors are reversed for deep, narrow crevices, and at some critical geometry, crevice corrosion will occur. As with pitting, increased concentration of chloride ions in the environment will increase chloride-ion concentration in the crevice and increase the probability of initiating crevice corrosion. 

CORROSION, OXYGEN DEFICIENCY - A form of crevice corrosion in which galvanic corrosion proceeds because oxygen is prevented from diffusing into the crevice. 

Oxygen is reduced at the actively corroding site or at the filament head. The mechanism is the same as crevice corrosion. A differential aeration cell is established between a deaerated acidified anode at the filament head and an alkaline cathode at the filament tail, saturated with water and oxygen [112]. The metal oxidation is followed by hydrolysis and acidification down to pH 1 [113]. Fe(OH)3, shown in Fig. 7.22, is formed from Fe reacting with the aerated solution in the tail and diffuses through the microcracks in the coating. 

Sedriks et al. [20] and Bakulin et al. [19] found that Ecggn of B/Al MMCs was active to that of their monolithic matrix aUojrs in aerated NaCl solutions. That behavior does not appear to comply with the mixed-potential theory. Bakulin et al. [79], however, found that hot-pressed stacks of aluminum foil processed in the same way as the MMC (but without the BFs) have Ecorr values that are active to the MMCs. The only difference between the monolithic aluminum and the hot-pressed stacks of aluminum foil was crevices in the diffusion bonds between adjacent foils. The crevices, which are sources of additional anodic sites, can polarize the stacks to active potentials. Thus, the B/Al MMCs are actually noble to the matrix material processed in the same way, emd the Ecorr values actually coincide with the mixed-potential theory. Sedriks et al. [20] found that increasing the volume fi ac-tion of BF caused anodic current densities (w.r.t. matrix area) to increase. This implies that BF-matrix interfaces, which increase with BF content, were also sources of anodic sites. Evans and Braddick [89] also reported that BF-matrix interface regions were severely attacked in an oxygenated NaCl solution. These reports indicate that the BF-matrix and foil-foil interfaces are major causes of corrosion. 

Crevice corrosion is another type of localized corrosion relevant to non-alloyed and stainless steel. Narrow gaps- caused by the constructive design or the formation of deposits are a necessary requirement. Fig. 1 -9 c. Crevice corrosion as a result of the different compositions of the electrolyte inside and outside the crevice, i.e. depletion of oxygen in the crevice. In a crevice there is a large metal area/electrolyte volume ratio with a long diffusion path. Solution in the depth of a crevice is depleted of oxygen and therefore has a composition different from that of the bulk solution. As a result of the lack of oxygen anodic metal dissolution occurs at the depth of a crevice. Because diffusion in the crevice is almost impossible, the chemical composition becomes acidic... 

The series of events leading to the formation of a severe crevice can be summarized in the following three stages. Firstly, crevice corrosion is believed to initiate as the result of the differential aeration mechanism mentioned earlier. IDissolved oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal [Fig. 6.21( )]. Secondly, as oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between the... 

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