Local Chemistry and Growth of Single Corrosion Pits in Aluminum

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Abstract
Corrosion of 99.999% Al in at pH 11 was studied at naturally occurring as well as artificial pits. Electrolytic solution extracted from within natural pits was found to have a pH between 3 and 4. Solution extracted from within artificial pits and complexed with ferron exhibited a UV spectrum that indicated that 84% of the dissolved Al was present as monomeric species, and the remainder as polymeric species. The 27Al NMR spectrum of solution extracted from artificial pits was found to be similar to that of synthetic solutions of and . The saturation concentration of the monomeric salts was found to be about 3M, and the pH at 90% of saturation was about 3. Implantation of Fe ions into the surface of pure Al through a 5 μm diam mask was discovered to create a site which, upon immersion at −0.5V SCE, corroded preferentially and thus acted as a nucleation site for pit growth. The current density at such single corrosion pits was determined directly by electrochemical measurement of current and by optical measurement of surface area, and was found to decrease during pit growth with the square root of time. The current density was in quantitative agreement with theoretical predictions based on the hypothesis that the corrosion rate was mass transport controlled by the dissolution of an aluminum oxychloride salt film formed by precipitation of dissolution products.

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