Microtunnelling during localized attack of passive aluminum: The case of salt films vs oxide films

• Published in: Corrosion science 1989, Vol.29 (2), p.363-378
• Main Authors: Baumgärtner, M., Kaesche, H.
• Format: Article
• Language: English
• Subjects: Applied sciences; Corrosion; Corrosion mechanisms; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Metals, semimetals and alloys; Metals. Metallurgy; Physics; Specific materials
• ISSN: 0010-938X; 1879-0496
• DOI: 10.1016/0010-938X(89)90041-3

The present paper is concerned with the stage of visible localized dissolution. The example chosen is aluminum pitted in chloride, bromide and iodide solution, but other cases of pitting show similar features. Pitting proceeds by extremely rapid propagation of microscopic crystallographic tunnels. The rate, averaged over the lifetime of a single tunnelling event, is ca 10 A cm −2, but momentary values reach ca 100 A cm −2. Tunnel side walls (normally {100}-faces) are passive. As the dissolution rate is excessively high, the presence of thin salt films on the active surface can hardly be disputed. The coexistence of salt films and oxide films, across which mass transport processes have enormously different rates, is important. Gross pitting and overall surface roughening are two variants of the result of microtunnelling processes. The situation is further discussed under the general supposition that comparitively shallow crystallographic etch pits and narrow, deep crystallographic tunnels are of the same type of morphology, varied, via intermediate cases, according to solution composition and temperature. The pitting potential is discussed as being essentially the sum of the equilbrium potential of the formation of anhydrous aluminum halide and the overvoltage of nucleation of tunnelling, possibly turning, during the duration of a tunnelling event, into the sum of the equilibrium potential of the formation of hydrated aluminum halide plus the overvoltage of dissolution stabilizing the saturated halide, and ohmic and diffusional contributions.