Oxide films anodized aluminium

Parhutik VP, Makushok IE, Kudriavtsev E, Sokol VA, Khodan AN (1987) An X-ray electronic study of the formation of anodic oxide films on aluminium in nitric acid. Electrochemistry (Elektrokhymia) 23 1538-1544 Kundu M, Khosravi AA, Kulkami SK (1997) Synthesis and study of organically capped ultra small clusters of cadmium sulphide. J Mater Sci 32 245-258... 

Diggle, J.W., Downie, T.C., and Goulding, C.W. 1969. Anodic oxide films on aluminium. Chemical Reviews 69, 365 05. 

Figure 23.3 Bode plots of natural and anodic aluminium oxide films. The aluminium oxide film was formed at 2.5 mA/cm and a formation voltage of 60 V, experimental (points) and fitting (lines).

E. J. W. Verwey [1935] Electrolytic Conduction of a Solid Insulator at High Fields The Formation of the Anodic Oxide Film on Aluminium. Physica 2, 1059-1063. 

Harkness A C. and Young L. (1966), High resistance anodic oxide films on aluminium . 

Nagayama M. and Tamura K. (1967), Dissolution of the anodic oxide film on aluminium in a sulphuric acid solution , E /ectwc/rrw.. 4cto, 12,1097-1107. 

Siejka J. and Ortega C. (1977), An 0 study of field-assisted pore formation in compact anodic oxide films on aluminium , J. Electrochem. Soc., 124, 883-91. 

Sheet aluminium can be given a colour by a similar process. The aluminium is first made the anode in a bath of chromic acid (p. 377) when, instead of oxygen being evolved, the aluminium becomes coated with a very adherent film of aluminium oxide which is very adsorbent. If a dye is added to the bath the oxide film is coloured, this colour being incorporated in a film which also makes the remaining aluminium resistant to corrosion. This process is called anodising aluminium. 

He concluded that for aluminium and titanium certain etching or anodization pretreatment processes produce oxide films on the metal surfaces, which because of their porosity and microscopic roughness, mechanically interlock with the polymer forming much stronger bonds than if the surface were smooth . 

The electrochemical effects of slowly and erratically thickening oxide films on iron cathodes are, of course, eliminated when the film is destroyed by reductive dissolution and the iron is maintained in the film-free condition. Such conditions are obtained when iron is coupled to uncontrolled magnesium anodes in high-conductivity electrolytes and when iron is coupled to aluminium in high-conductivity solutions of pH less than 4-0 or more than 12 0 . In these cases, the primary cathodic reaction (after reduction of the oxide film) is the evolution of hydrogen. 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

Although aluminium is a base metal, it spontaneously forms a highly protective oxide film in most aqueous environments, i.e. it passivates. In consequence, it has a relatively noble corrosion potential and is then unable to act as an anode to steel. Low level mercury, indium or tin additions have been shown to be effective in lowering (i.e. making more negative) the potential of the aluminium they act as activators (depassivators). Each element has been shown to be more effective with the simultaneous addition of zinc . Zinc additions of up to 5% lower the anode operating potential, but above this level no benefit is gained . Below 0 9 7o zinc there is little influence on the performance of aluminium anodes . Table 10.10 lists a number of the more common commercial alloys. 

Table 15.7 shows the effects of thin anodic oxide films on the resistance to industrial and synthetic marine atmospheres (intermittent salt spray) of three grades of pure aluminium. The results are taken from a paper by Champion and Spillett and show how relatively thin films produce a marked improvement in both environments. 

Acid pickles Some of the acid pickles used to clean and etch aluminium alloy surfaces and remove oxide and anodic films, such as the chromic/ sulphuric acid pickle (method O of DEF STAN 03-2) and other chromic-acid bearing pickles (App. Foi DEF-151) probably leave on the surface traces of absorbed or combined chromate which will give at least some protection against mild atmospheres. 

Anodising the formation of oxide films on metals by anodic oxidation of the metal in an electrolyte solution. The term can be used for thin dielectric films but is more particularly applied to thicker films formed on some metals such as aluminium at higher anodic voltages. 

The effect of passivating films on aluminium and magnesium has been the subject of much research. By incorporating chromate/dichromate mixtures and other substances in the electrolyte, a coherent insoluble oxide film is formed which effectively inhibits further corrosion. Sealed cells with aluminium or magnesium anodes may therefore be successfully stored for several years, even at high temperatures. However, once current has been drawn from the cell, the film is broken down and rapid attack on the metal follows due to reactions such as... 

The potential difference developed between aluminium and stainless steel is about the same as that developed between aluminium and copper. The cathodic reaction is easier on copper oxide than that on the highly protective passive oxide of stainless steels. Then, it is not the difference of potential between anode and cathode which counts, but the facility and rate of every reaction. A bare metal is generally a much better cathode than one covered with an oxide. Aluminium is more active than zinc in the electrochemical series. Practically, zinc protects aluminium which becomes covered with an oxide film.20 All more noble metals accelerate corrosion similarly, except when a surface film (e.g., on lead) acts as a barrier to diffusion of oxygen or when the metal is a poor catalyst for reduction of oxygen. 

Moon, S.-M., and Pyun, S.-1.1998. Growth mechanism of anodic oxide films on puie aluminium in aqueous acidic and alkaline solutions. Journal of Solid State Electrochemistry 2.156-161. 

SEM was used for morphological studies of anodic aluminium oxide films, formed at various formation current densities up to a formation voltage of 60 V. The total thickness of the films was determined by cross-section SEM micrographs as shown in Figure 23.1, but it should be emphasised that one can not clearly identify barrier and porous layers of the oxide film by using this technique. 

Figure 23.1 High-resolution cross-sectional SEM micrograph of an anodic aluminium oxide film of 66 nm formed at 0.3 mA/cm and a formation voltage of 60 V (Al thickness remaining 633 nm).

To interpret the impedance data, a model of the anodic aluminium oxide film must be established. The anodic aluminium oxide film is a sandwich film consisting of two layers, a barrier layer and a porous layer (Figure 23.4A [2]). 

Figure 23.4 Two-layer model of the anodic aluminium oxide film (A) and associated equivalent circuit (B)...

The average capacitance and specific resistivity of the barrier aluminium oxide films are determined to be 430. .. 470nF/cm and 1.3. .. 2.4 10 " Qcm, respectively. By using the anodisation factor of 1.2 nm/V for the films formed at low formation voltage, dielectric constants of 5.8. .. 6.4 are calculated from the measured capacitance values. The comparatively low dielectric constant is in agreement with the formation of an amorphous anodic aluminium oxide film as discussed above rather than a crystalline structure for which a higher dielec-... 

The thickness and properties of the barrier aluminium oxide layer were investigated by eleetrochemieal impedanee speetroscopy. The total thickness of the films was determined by seanning electron microscopy of cross-sections. Then, the thiekness of eaeh layer within the aluminium oxide films was calculated. Formation eurrent density, formation voltage, anodization time, and sur-faee roughness of the substrate influenced the electrical and structural properties of the barrier aluminium oxide layer. 

Fig. 14. Growth (in direction G) followed by dissolution (direction D) of anodic aluminium oxide film. The "loop is an ellipsometric signature of a spontaneous substrate roughening-self-smoothing cycle.

Fig. 15. (a) Scanning electron micrograph of sectioned anodic film on aluminium grown ar z.o in 0.1 M NaOH, showing a porous oxide on a smooth substrate, (b) SEM of the anodic filn. dissolved at - 0.5 V for 80s, showing a patchy residual oxide on a roughened substrate. 

Anodization of Al-W alloys with a W concentration in the range of 1-40 at. % has been performed to fabricate a composite nanoporous oxide. An increase of the W content is found to enlarge few times the size of pores. Composite oxide films with the weight ratio WO3/AI2O3 > 50 % have been formed for the first time. Applications of nanostructured aluminium-tungsten oxide composite films are discussed. 

The new nanostructure porous material based on anodic A12O3 and WO3 has been fabricated. The pore sizes can be tuned both by anodic forming regimes and W atom content in initial Al-W alloy films. The composite nanostructures based on anodic aluminium and tungsten oxides opens new opportunities for nanotechnology in electronics and photonics. 

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