A study of the pitting corrosion of Al byscanning electron microscopy

Abstract

The essentials of a theory are proposed for the pitting of Al, supporting air-formed or barrier-type anodic oxide films, in Cl − solutions. Pits propagate immediately at flaws in the otherwise essentially inert oxide films and proceed to undermine these surface films. Under natural immersion conditions, the morphology and distribution of attack as pitting proceeds depend upon the changing relative cathode/anode area ratio on the electrode, as well as the degree of polarization of the anodic and cathodic processes. During applied polarization, the morphology and distribution of attack are dependent upon the applied potential. Certain concepts developed are applicable to other anodic processes on Al. Crucial information, for testing the theory, is provided by examination of pitted surfaces by scanning electron microscopy. The technique is particularly useful in elucidating the relation between the distribution and morphology of pits and in establishing the role of the surface oxide film throughout the pitting process. The complicated distribution of corrosion products above and around pits on naturally corroded surfaces becomes apparent.