The mechanism of anodic oxidation of alloys

The mechanism of formation of barrier-type anodic films on alloys containing valve and non-valve metals is considered in terms of data yielded by electrochemical methods, a.c. impedance measurements, conventional and scanning electron microscopy and chemical and electron probe microanalysis. The alloying elements enter the film virtually in their alloy proportions. With alloys giving oxides essentially insoluble in the electrolytes, such as Nb-Zr, the metal constituent concentration profiles across the film are determined largely by the cationic and anionic transport numbers and the cationic mobilities. Where one of the alloying elements tends to give soluble compounds, such as vanadium in V-Nb alloys, this situation is complicated by preferential dissolution of that element. The film properties depend on the solution used but the main result, that relatively small additions of valve metals to metals such as vanadium permit thick barrier-type films to be formed, is unaffected. All the as-formed films are initially pore-free, but those produced on alloys rich in non-valve metal are prone to subsequent leaching, with detectable pore formation. Analysis proves that the films on V-Nb alloys are enriched in valve metal, particularly towards the outer surface. This explains why these films can be formed and also why the impedance characteristics and resistance to leaching improve as the film thickens, with the material rich in valve metal developing into an outer, more protective and possibly thicker sheath. The presence of a mass-transfer boundary layer, rich in nonvalve metal dissolution products, also promotes film formation. Films formed in certain nonaqueous electrolytes are richer in the non-valve metal and so are more readily leached in water subsequently, but apparently are also chemically different. Special experiments, involving leaching and reanodizing in various solutions, and the formation of duplex films, provide important data, appearing to indicate substantial anionic movement during film growth.