ULTRAMICROTOMY - A ROUTE TOWARDS THE ENHANCED UNDERSTANDING OF THE CORROSION AND FILMING BEHAVIOR OF ALUMINUM AND ITS ALLOYS

Through an ultramicrotomy approach, it is now possible to prepare thin cross-sectional specimens, less than 10 nm thick, of the aluminium su bstrate and its surface film with negligible damage to the film materi al, allowing direct examination of the real film material at atomic-sc ale resolution with simultaneous analysis of a local area of a few nan ometres in diameter. Here, selected examples, some new and some previo usly reported, are collected together and reviewed briefly to show tha t the understanding of the nature of various surface films on aluminiu m and aluminium alloys is increased greatly by cross-sectional transmi ssion electron microscopy. Examples included are: (a) porous anodic ox ide growth over an Al-0.5 wt% Fe alloy containing finely dispersed Al3 Fe particles, (b) porous anodic oxide growth over an Al-1.4 wt% Fe all oy containing finely dispersed Al6Fe particles where the complex inter play between the local film formation over the particles and general f ilm formation over the surrounding matrix regions is now revealed prec isely, (c) chromate conversion coating growth over high purity alumini um where the surface inhomogeneities associated with the grain boundar ies or cellular boundaries of impurity segregation within the metal pl ay a crucial role in the coating development, (d) corrosion of alumini um covered with organic coatings where the highly localized nature of corrosion and its association with impurity segregation within the met al is highlighted, (e) adhesion between aluminium and organic coatings where surface films on the aluminium substrate play a critical role, and (f) crystalline barrier oxide growth over etched aluminium surface s which is of great practical importance in improving the performance of aluminium electrolytic capacitors. Further, it is shown that ultram icrotomy is an extremely powerful technique for the preparation of cle an and microscopically flat surfaces both for high purity aluminium an d aluminium alloys, allowing AFM to be utilized to its full potential in revealing highly localized processes, such as pit initiation, durin g corrosion of aluminium and aluminium alloys. (C) 1998 Elsevier Scien ce Ltd. All rights reserved