Anodising thickness

Alloys are generally of the Al-Mg-Si type with additions of copper and chromium or manganese. Colour varies with the particular alloy and the film thickness. For optimum control of colour, the alloy must be carefully produced with strict attention to composition, homogenisation and heat-treatment, where appropriate, and the anodising conditions must be maintained within narrow limits. It is usual to arrange matters, preferably with automatic control, such that current density is held constant with rising... 

The more or less regular pattern of pores imposes a cellular structure on the film, with the cells approximating in plan to hexagons, each with a central pore, while the bases which form the barrier-layer, are rounded. The metal surface underlying the film, therefore, consists of a close-packed regular array of nearly hemispherical depressions which increase in size with the anodising voltage. The thickness of the individual cell walls is approximately equal to that of the barrier-layer... 

Density Owing to the variable degree of porosity of the anodic film, it is only possible to determine the apparent density, which varies with the anodising conditions and also with the film thickness. 

Fig. 15.2, taken from a paper by Lenz , shows the variation in density with thickness for steam-sealed anodic films produced in sulphuric acid on aluminium of 99-99 7o and 99-5% purity. A mean figure of 2-7 g/cm for sealed, and 2-5 g/cm for unsealed films is accepted by the British Standard for anodised aluminium... 

The hardness of the film is markedly affected by the conditions of anodising. By means of special methods involving dilute electrolytes at low temperatures and relatively high voltages , with or without superimposed alternating current, it is possible to produce compact abrasion-resistant films with thicknesses of 50-75/im and hardnesses of 200-500 VPN, for special applications. 

Flexibility The normal anodic film begins to crack if subjected to an extension exceeding about 0 - 5%. Thinner films up to 5 nm in thickness appear to withstand a greater degree of deformation without obvious failure, and are often used for dyed coatings on continuously anodised strip from which... 

Emissivity Table 15.5 shows the total heat emissivity of various aluminium surfaces, as a percentage of that of a black body. The figures have been recalculated from the data of Hase. The emissivity of anodised aluminium rises rapidly with film thickness up to 3 fim after which the rate of increase diminishes. 

For control or calibration purposes, film thickness can be determined by mounting a sectioned specimen and measuring the oxide film thickness directly on the screen of a projection microscope at a known magnification. Alternatively, the loss in weight of an anodised sample of known area may be found after the film has been stripped in a boiling solution made up as follows ... 

More recently, results of exposure tests for 10 years in a severe industrial environment at Stratford, London, have been reported by the Fulmer Research Institute". A range of pure and alloy specimens, anodised to a maximum film thickness of about 25 m, was exposed at an angle of 45°. 

The hardness and abrasion resistance of anodic coatings have never been easy properties to measure, but the development of a British Standard on hard anodising has made this essential. Film hardness is best measured by making microhardness indents on a cross-section of a film , but a minimum film thickness of 25 tm is required. For abrasion resistance measurements, a test based on a loaded abrasive wheel , which moves backwards and forwards over the film surface, has improved the sensitivity of such measurements. 

Thin alumina membranes (Anodise ) with two different nominal pore diameters (20 nm and 200 nm) were obtained commercially (Whatman International Ltd., Maidstone, Kent, UK). These two types of membranes, designated subsequently as A20 and A200 respectively, had a thickness of 50 pm and an overall diameter of either 25 of 47 mm. X-ray diffraction analysis showed that the alumina in the as received membranes was almost amorphous. Subsequent thermal treatment up to 950 °C produced a crystalline structure (y-alumina), without any significant collapse in the porous structure. Here we describe, in general, the characterisation of the untreated membranes. 

Concerning the two-layer model, the thickness and properties of each layer depend on the nature of the electrolyte and the anodisation conditions. For the application, a permanent control of thickness and electrical properties is necessary. In the present chapter, electrochemical impedance spectroscopy (EIS) was used to study the film properties. The EIS measurements can provide accurate information on the dielectric properties and the thickness of the barrier layer [13-14]. The porous layer cannot be studied by impedance measurements because of the high conductivity of the electrolyte in the pores [15]. The total thickness of the aluminium oxide films was determined by scanning electron microscopy. The thickness of the single layers was then calculated. The information on the film properties was confirmed by electrical characterisation performed on metal/insulator/metal (MIM) structures. 

The morphology of the sputtered aluminium film on a glass substrate (1000 nm thickness) before and after anodisation was studied by AFM microscopy. The surface roughness of both films is approximately 35 nm. The... 

The film is formed on E-200-Al/glass at a current density of 0.5 mA/cm for the anodisation time of 1800 s. The film thickness was controlled by the formation voltage of 10 V. The electrical measurements were carried out on different electrode areas ranging from 0.0025 to 0.04 cm (see Figure 23.12). Below breakdown, low leakage currents of less than 13 nA/cm at 1.67 MV/cm could be observed. The breakdown of the dielectric film occurs when the electrical field strength approaches 8 MV/cm. 

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