Crevice corrosion of aluminum

Figure 9.16 Crevice corrosion of aluminum can occur in a saltwater environment...

Crevices are gaps formed on joining two structural members. The formation of a crevice is followed by formation of a differential aeration cell. The oxygen inside the crevice becomes depleted after a length of time and the crevice becomes anodic, Al +- -3H20 3H+-t-Al(0H)3. The precipitation of AI(OH)3 leads to decreased pH and acidification (H" "). Crevice corrosion of aluminum is shovra in Fig. 9.16. If aluminum is coated, a cathodic reduction-reaction would not occur around the crevice mouth and the cathodic reduction reaction would then take place slowly inside the crevice, 2H+-f2e->-H21- Crevices must be eliminated by sealing or welding. 

Actually, the drop of pH is related to more complex reactions and species. Thus, in more sophisticated models, several hydrolysis reactions and metal chloride formation are taken into account but the selection of species and reactions is somewhat different from model to model. Oldfield and Sutton [94] and Watson and Postlethwaite [2] considered only hydroxides as the product of cation hydrolysis. Sharland [96] introduced simple metallic chlorides. The most complete set of species and reactions has been used by Bernhardsson et al. [4], which made available the thermodynamic data of a large number of species, including several iron, nickel, chromium, and molybdenum polycations as well as metal chlorides and hydroxychlorides. Gartland [19] used a more limited set of species (Table 10.3) selected among the Bernhardsson data. According to their experimental results, Hebert and Alkire [95] included Al(OH) " as the hydrolysis product in their model of the crevice corrosion of aluminum alloys. 

Crevices in Waters In most fiesh waters crevice corrosion of aluminum is negligible. In seawater, crevice corrosion takes the form of pitting, and the rate is low. Resistance to crevice corrosion has been found to parallel resistance to pitting corrosion in seawater and is higher for aluminum-magnesium alloys than for alu-minum-magnesium-silicon alloys (Ref 14). 

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