S. Tajima

Luminescence, breakdown and colouring of anodic oxide films on aluminium

Abstract El and Pl, breakdown and self-colouring phenomena of anodic oxide films on aluminium, which have been found to be closely related with each other, were reviewed according to newly proposed classifications. Thus the hitherto unsolved problems which had caused confusion in complete understanding of the art are almost solved. Firstly galvanostatic anodic behaviours of Al were classified. Secondly it was shown that el is caused either by (1) selected impurities (activators) (Mn, Eu etc) in Al, (2) excited carboxylate ions in the film formed in aliphatic carboxylic acids, or (3) by, ‘flaws’ in the case of el in inorganic electrolytes, which is not the el in the true sense. Thirdly pl is to be seen in aliphatic carboxylic acid films only and the luminescent centres are the same with those of el in the acid. Aromatic acid films show neither el nor pl since the acid can not enter into the films. Further, reference was made to one type of solid-type led (Al—Al2O3-SnO2) constructed. Fourthly, breakdowns have been classified into: (1) electrochemical breakdown including porous film formation, carboxylate breakdown (pitting) and chloride breakdown, and (2) ‘electric breakdown during anodizing’ or ‘electrolytic breakdown’ which is a complicated mixture of electrochemical and electric breakdown at/or through oxide—electrolyte interface, and which includes ‘burning’ (luminescent ‘flame’ propagation process) and sparking (scintillation and arcing). Finally self-colouring of ‘pure’ Al (except colouring by alloying elements or impurities included) has been classified into: (1) colouring by decomposition or cyclisation of excited carboxylate ions in the film (post-el) higher electric field attained across the film. Thick, coloured films show no more el nor pl. (2) colouring by existence of Al0 particles in the interstitial positions of the film by high electric field assisted oxide growth in the case of the so-called ‘hard anodizing’, locally grown coloured spots in inorganic acid, eg sulphuric acid, and of so-called integral colour anodizing in sulphonated aromatic dibasic acid (or higher aliphatic carboxylic acid) plus a small amount of sulphuric acid, typically K alcolor , D uranodic 300 or P ermalux processes for architectural use. These films show neither el nor pl.

Nature of luminescence during galvanostatic anodizing of high purity aluminium

Abstract Galvanoluminescence of high purity aluminium was found to have two fundamental origins: one was the luminescence associated with scintillation or with some concommitant side reactions, taking place at the flaws; the other was the luminescence associated with anions included in the films during anodization. The former appeared to be the origin of the luminescence in inorganic electrolytes such as ammonium borate, borax, borax plus boric acid etc; and the latter appeared to be responsible for the luminescense in organic acids and their salts, such as oxalic, citric, tartaric and succinic acids and ammonium tartrate.

Localized nature of the luminescence during galvanostatic anodizing of high purity aluminium in inorganic electrolytes

Luminescence during galvanostatic anodizing of high purity aluminium containing Fe, Si and Cu as major impurities in inorganic electrolytes, typically an aqueous solution of ammonium borate, was found to be a local phenomenon which takes place at “flaws” in the growing oxide film. The intensity of the luminescence was found to be closely correlated to the concentration of “flaws” in the film: the intensity of the luminescence being roughly proportional to the concentration of “flaws” in the film. Rough estimate of the concentration of ”flaws“ in the film formed on “as received”, “chemically cleaned” and “electropolished” aluminium plate was 2 × 108.3 × 107 and 105/cm2, respectively. Any mechanism deeming the luminescence to be an electroluminescence in a homogeneous oxide with the collision excitation of the impurities or with the recombination at impurity centres has been rejected.

The development of flaws containing γ′-crystalline alumina regions in barrier anodic films on aluminium

Transmission electron microscopy has been employed to study the formation and development of regions containing γ′-crystalline alumina surrounding flaws in anodic barrier films formed on aluminium in aqueous ammonium borate solutions. The formation of crystalline alumina islands at these localized sites becomes more readily evident when the forming voltage approaches about 100 V and they grow radially with further increase in the forming voltage. The radial growth of the islands and their population density is influenced by increase of bulk temperature of the electrolyte, but is almost unaffected by the concentration of the electrolyte and applied anodizing current density. The population density of the flaws and, hence, γ′-crystalline alumina islands is strongly dependent on the surface condition of the metal and varies from about 2.4 x 108 cm−2 on the as-received specimen to less than 105 cm−2 on the electropolished specimen. The islands of crystalline alumina are thought to form in the vicinity of flaws in the original substrate, by conversion of the barrier film formed by the usual ionic processes, due to local Joule heating effects.

Electron-microscopic and optical studies on the electrocrystallization of alpha-alumina (corundum) films on aluminium☆

Electron-microscopic studies on the electrocrystallization of alpha-alumina films anodically formed in bisulphate melts have been made using Super-Raffinal (99.999 per cent Al), and the mechanism of formation is discussed together with some optical studies of the oxide films. Electron micrographs have been taken on the top surfaces, cross-sections and bottom of the oxide films, and on the surface of the substrate metal, under various conditions of electrolysis; also, comparison was made with the conventional sulphuric acid anodizing films. Oxide films anodically formed in fused salt systems are of all-barrier type. Macro-pores are produced by electric arcing, penetrating the oxide to the substrate metal. Initial pores are small and many but they are gradually filled up with new oxide and healed; the pore size becomes larger with time. Crystals grow directly on aluminium and no boundary layers are observed between them. Refractive index measurement of the stripped films shows that amorphous oxide is first formed, which is immediately transformed to gamma-alumina and finally to alpha-alumina by local heating due to electric arcing. The phenomenon of ‘form double refraction’ is not observed, which indicates the non-existence of micropores. All these facts suggest that the oxidation reaction occurs at or near the melt-oxide interface under the anodic potential, by migration of Al3+ ions through lattice imperfections of the oxide. The view may hold for the oxidation mechanism of the barrier type or duplex (barrier plus porous layer) type oxide anodically formed in aqueous electrolytes with or without solvent action.

Effect of crystal orientation on galvanoluminescence of aluminium

Abstract When high purity aluminium sheets containing large single crystals were anodized galvanostatically in barrier-forming organic acid solutions, such as tartaric, citric acids etc, the distinct difference in the intensity of galvanoluminescence appeared to correspond exactly with the variation of the rate of oxide growth on underlying crystal planes.The intensity of the luminescence was stronger with a crystal plane where the rate of the film growth was higher. Therefrom, the effect of underlying metal orientation on the luminescence was interpreted as a secondary phenomenon due to the difference in the thickness of the film growing on the different crystal planes.

Properties and mechanism of formation of thick anodic oxide films on aluminium from the non-aqueous system boric-acid-formamide☆

Abstract Thick coloured oxide films are produced on aluminium by anodic oxidation in the non-aqueous system of boric-acid-formamide. The films are hard and compact and the colour tone changes from yellow, through brown, to black as the film becomes thicker. The properties and mechanism of formation of the films have been investigated by chemical, electrochemical, physical and optical means and by X-ray analysis. The film may be regarded as of intermediate nature between the barrier and duplex (barrier plus porous) anodic oxide films on aluminium hitherto known. For anodic oxidation, the saturated solution was found preferable. The electric conductivity of the fresh solution is very low, but by ageing at 90–100°C it becomes nearly equal to that of aqueous boric acid solution, which makes it possible for the anodic reaction to proceed. Some other physical and electrochemical properties of the solution were also measured. By electrolysis at 0·5-4 A/dm2 and 20–300 V, at room temperature to 60°, thick coloured oxide films are produced, which may be classified into (i) low current-density films (0·5 A/dm2), (ii) high current-density films (2–4 A/dm2). Both are coloured but the structure is quite different. The former is of organo-metallic nature (Al(HCONH)3), soft and rough, while the latter consists of Al2O3 and B2O3, being hard and compact. Coating ratio is unusually high (2·3-4) due to the presence of B2O3 or organic compounds. With the aid of previous information the mechanism of formation of such films is clarified.

In-depth composition profile of anodic oxide films on aluminium studied by auger electron spectroscopy

In-depth composition profiles of the anodic oxide films grown on Al (111), (110) and (100) single crystal electrodes in ammonium borate were tudied by Auger elecron spectroscopy with Ar ion etching. The oxide film had the composition at Al2O3 as referenced to an authentic α-Al2O3 single crystal. The Auger electron energy and peak shapes of Al and O in the oxide film agreed with those of α-Al2O3 crystal. Regardless of the crystal orientation of the substrate Al and in-depth profile, the oxide film has the chemical composition of Al2O3. The Auger signal peak of boron was approx. 150 of that of oxygen throughout the sputtered distance.

Anodic polishing and passivation of iron in the non-aqueous sulphamic-acid-formamide system☆

Abstract Iron can be anodically electropolished in non-aqueous solution of sulphamic acid (2·9 N HSO3NH2 in HCONH2) at relatively low current density. The galvanostatic polarization curves determined at steady states at 20°C and 40°C show apparently two discontinuous curves, probably caused by passivation. Periodic phenomena can be frequently observed near the transition region. By measurement of electrode potentials in potentiostatic polarization and decay curves, as well as by analytical methods, the mechanism of electropolishing is confirmed to be due to passivation occurring over the anode surface. The Flade potential, and its pH-dependence, which characterizes the transition potential between the active and passive states in aqueous systems, can also be obtained by discharge curves of the passivated iron anode. It is, however, 0·38 V at pH = 0 (the standard state), being rather less noble than 0·56 V which Flade and others have obtained in aqueous solution. By analysis of the anolyte, the basic anode reaction on a Pt surface is considered to be 2HCONH− → HCONH2 + HCNO + 2e; E0 = 0·38 + RT/2F ln aHCNO The standard electrode potential of this reaction is found experimentally to be 0·38 V in 1 N HSO3NH2 solution, coincident with thermodynamic calculation. This fact strongly suggests that generation of oxygen cannot occur since HCONH− ion can more easily discharge above 0·38 V and that the original product produced on the anode is HCNO or related substances. Even by adding a slight amount of water intentionally, no shift of the Flade potential is found, the potential being only somewhat stabilized. To account for the cause of passivation, three assumptions, the formation of an oxide film, the formation of an unknown organic film containing formamide, and the formation of an HCNO gas layer are proposed and it is suggested that either of the latter two is the most probable.

Effect of crystal orientation on anodic oxide film growth of Al

During galvanostatic anodisation of a high purity aluminium sheet of randomly orientated single crystals, the formation of the barrier-type oxide film is accompanied by a bluish white luminescence, whose intensity is different on each crystal grain. The oxide film morphology and film growth kinetics were studied by electron microscopic observation. It was found that the surface of the oxide film with intense luminescent pattern was remarkably rugged, while that with weak luminescent pattern was entirely uniform. The film thickness at each crystal plane was nearly the same (330 nm) at forming voltage (Vf) of 250 V, except that the surface with intense luminescent film had a random fluctuation by ca 20 nm. The oxide film formed on an Al polycrystal grew uniformly and the film thickness increased linearly with the forming voltage up to the final breakdown voltage; then the film grew scarcely, with accompanying surface irregularities. The film breakdown voltage was influenced by the crystal orientation.