AN ATOMIC-FORCE MICROSCOPY STUDY OF THE CORROSION AND FILMING BEHAVIOR OF ALUMINUM

Corrosion and filming behaviour of aluminium have been investigated by atomic force microscopy (AFM). Preliminary examination of the alumini um surfaces prepared by conventional surface pre-treatment procedures, e.g. electropolishing, mechanical polishing, etc., has shown that mic roscopically smooth aluminium surfaces suitable for AFM study cannot b e obtained readily by existing procedures. Thus, a new and novel techn ique has been developed for the preparation of clean and microscopical ly smooth aluminium surfaces for AFM study. The technique is based on cutting small strips of aluminium specimens, encapsulated in an epoxy resin, by a diamond knife using an ultramicrotome. With this technique , clean aluminium surfaces, which are microscopically smooth to within 1 nm over a scanned area of 1 mu m x 1 mu m or, perhaps, even greater , are readily prepared. Aluminium specimens with such microscopically smooth surfaces were given various treatments and examined by AFM. It was found that AFM, despite its unprecedented high resolution, does no t allow one to one correspondence to be made between local film charac ter and microscopic inhomogeneities of the aluminium surfaces associat ed with fine features such as grain boundaries or cellular boundaries which are known to play a crucial role in the development of chromate chemical conversion coatings or nucleation of pits. Fortunately, howev er, microscopical information can be obtained by transmission electron microscopy of ultramicrotomed sections. Thus, the potential of AFM is realized fully if it is combined with transmission electron microscop y of ultramicrotomed sections. Otherwise and for aluminium in particul ar, AFM is merely an approach to image surfaces in situ or ex situ at resolutions considerably higher than those attained by high resolution scanning electron microscopy. (C) 1997 Elsevier Science Ltd.