Oxide films on aluminium

Aluminium is one of the most thermodynamically reactive metals. Aluminium owes its excellent corrosion resistance in most environments to the protective oxide barrier film formed on and strongly bonded to its surface. This aluminium oxide film is relatively inert and tends to prevent further oxidation of the aluminium. The film can dissolve in the presence of some chemicals and this can lead to dissolution of the metal. When the film is damaged under conditions where normal self-healing does not occur, localized corrosion in the form of pitting or intergranular attack can occur [2.6]. 

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With many organic compounds, aluminium shows high corrosion resistance either in the presence or absence of water. The lower alcohols and phenols are corrosive when they are completely anhydrous —rarely encountered in practice —since repair of breaks in the natural protective oxide film on aluminium cannot take place in the absence of water. Amines generally cause little attack unless very alkaline. 

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. 

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. 

Kimura, A. 2001. Breakaway behavior of snrface oxide film on aluminium-silicon-magnesium alloy powder particles at high temperature in a vaenum. Mater. Trans. 42(7) 1373-1379. 

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

Koimo H., Kobayashi S., Takahashi H. and Nagayama M. (1982), Hydration of barrier oxide films on aluminium and its inhibition by chromate and phosphate ions , Corros. 

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. 

The important thing about the oxide film is that it acts as a barrier which keeps the oxygen and iron atoms apart and cuts down the rate at which these atoms react to form more iron oxide. Aluminium, and most other materials, form oxide barrier layers in just the same sort of way - but the oxide layer on aluminium is a much more effective barrier than the oxide film on iron is. 

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. 

As copper is not an inherently reactive element, it is not surprising that the rate of corrosion, even if unhindered by films of insoluble corrosion products, is usually low. Nevertheless, although the breakdown of a protective oxide film on copper is not likely to lead to such rapid attack as with a more reactive metal such as, say, aluminium, in practice the good behaviour of copper (and more particularly of some of its alloys) often depends to a considerable extent on the maintenance of a protective film of oxide or other insoluble corrosion product. 

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. 

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. 

Erosion/corrosion of the weld bead at the fracture location damaged the aluminum oxide film on the piping, thus allowing the mercury to wet and initiate cracking of the aluminium. 

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 fluxes used when brazing are alkali halide and borate mixtures and compounds, and they have two main functions first, to dissolve the oxide film on the component surface or at least to degrade its adhesion by penetration of naturally occurring flaws and electrolytic action at the oxide-substrate interface, and secondly to prevent formation on the liquid surface of oxide skins which would restrict braze flow. Fluxes can be contained in a bath held at the brazing temperature in which the, usually aluminium, component is placed or else applied as a paste to surfaces of the component or braze. 

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. 

Growth of Aluminium Oxide Films on Silicon Oxide and Films... 

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. 

TYPICAL CONDITIONS FOR THE DEPOSITION OF METAL AND OXIDE FILMS ON GLASS, FUSED SILICA AND ALUMINIUM OXIDE BY SPRAY COATING [75,81] (AA) ACETYLACETONATE. 

Other subjects recently discussed include the photocoabsorption of carbon monoxide and oxygen on ZnO,664 the relative photoabsorption of oxygen and methane over ZnO,665 the photocatalytic properties of ZnO and MgO,66 the effect of irradiation on the dielectric properties of Ti02,667 the photoluminescence of an oxide layer on aluminium,668 the singlet-triplet splitting of the free A-exciton in ZnO,659 the properties of the photosensitive film of copper in iodine solution,580 and the photosensitivity of layers of CuCla and [Fe(ox)8]3-.681... 

Surface layers on metals may have a considerable ohmic resistance. This is, for instance, the case for oxide films on stainless steels, aluminium, chromium etc., even if the film is very thin. When a current I (current density i) is flowing through the film, we will have an ohmic drop, i.e. a resistance polarization expressed by ti = RI = ri, where R (f ) and r (f cm ) are the resistances in the film on the total electrode surface and in 1 cm of it, respectively. Anodic resistance polarization affects the potential on surfaces passivated by oxides or other substances. 

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