Anodising Conditions

In Europe, the QUALANOD label was created for architectural products (directives concerning the quality label for anodic films on wrought aluminium intended to be used for architectural applications). Anodisers that adhere to this label must conform to a strict protocol of operating conditions [21]. Several methods are available for controlling the quality of anodised aluminium for architectural applications [22, 23]. 

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Piggott, A. R., Leckie, H. and Shreir, L. L., Anodic Polarisation of Titanium in HCOOH— 1 Anodic Behaviour of Titanium in Relation to Anodising Conditions , Corros. Sci., S, 165... 

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... 

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. 

Thermal expansion The thermal expansion of the film is only about one-fifth that of aluminium, and cracking or crazing is observed when anodised aluminium is heated above 80°C. The fine hair-cracks produced do not seem to impair the protective properties of the coating if anodising conditions have been correct. 

The protective action of sulphuric films is mainly controlled by the anodising conditions, compact films formed at temperatures below 20° C in 7<7o v/v sulphuric acid being more resistant than the films formed at higher temperatures in more concentrated acid. The wider pores of the latter result in less... 

The formation of defects in porous anodic alumina films fabricated in phosphoric acid solutions has been studied. TEM and SEM examinations revealed that defects appear at the triple point junctions of the cell boundaries, where neighbouring celis meet. The defects represent voids in the anodic alumina and develop into spatial structures that comprise a central void at the triple-point junction extending into tubular branches that pass toward the pore wall. The defects are generated continuously during the anodic film growth under appropriate anodising conditions. Additional post-anodising treatment also results in periodic arrays of circular nanoholes in the pore walls. 

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. 

Zhao, Y. and Xiang, T.Y. (2012) Formation of bioactive titania films under specific anodisation conditions. Surf. Eng., 28 (5), 371-376. 

This review details fabrication methodologies for PAA membranes and examines the ceramic chemistry of their physical and chemical structures formed under different anodising conditions. It describes how these structures change in response to thermal treatment, which is important in the context of their use in high temporature applications. 

Fig. 4. Current-voltage-time relationships during anodisation, demonstrating progression from mild to hard anodisation conditions (left) and mUd-hard-mild conditions (right).

In the short run, within a few months up to a few years. This leads to iridescence [16], chalking, and powdering. These forms of ageing depend more on the anodising conditions (especially sealing) than on the climatic conditions [17]. Some can be eliminated by appropriate cleaning. 

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