3.3. Transmission electron microscopy
Transmission electron micrographs of ultramicrotomed sections of the Al2Cu, Al7Cu2Fe and Al–28 at.% Fe model second phases after galvanostatic anodizing at 5 mA cm−2 in sulphuric acid for 90 s displayed thin anodic films of relatively uniform thickness, with an internal sponge-like appearance; fine pores passing perpendicularly to the alloy surfaces were not readily resolved (Fig. 6). The anodic films appeared to be more compact as the relative content of iron in the model second phase particles increased. Further, disruption of the anodic film and detachment of the film from the Al2Cu substrate were frequently observed during TEM observations. It is thus probable that oxygen gas generation and evolution proceeding during anodizing weakened the oxide through generation of lateral porosity and subsequent film rupture and loss of oxide fragments to the anodizing electrolyte. The anodic films developed on the selected model second phase particles after anodizing in 0.25 M chromic acid at 313 K were similar to those generated in sulphuric acid (Fig. 7). However, the anodic oxides were generally thinner than those formed during sulphuric acid for the same charge passed. The anodic film thicknesses developed on the model Al2Cu, Al7Cu2Fe and Al–28 at.% Fe particles were respectively ∼33, 29 and 60 nm for anodizing in chromic acid and 67, 68 and 75 nm for anodizing in sulphuric acid.