An atomic force microscopy study of the corrosion and filming behaviour of aluminium

Corrosion and filming behaviour of aluminium have been investigated by atomic force microscopy (AFM). Preliminary examination of the aluminium surfaces prepared by conventional surface pre-treatment procedures, e.g. electropolishing, mechanical polishing, etc., has shown that microscopically smooth aluminium surfaces suitable for AFM study cannot be obtained readily by existing procedures. Thus, a new and novel technique has been developed for the preparation of clean and microscopically 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 not allow one to one correspondence to be made between local film character and microscopic inhomogeneities of the aluminium surfaces associated 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, however, 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 microscopy of ultramicrotomed sections. Otherwise and for aluminium in particular, 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.