Anodisation treatment

Fluoride anodising treatment Caustic soda clean... 

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. 

An innovative technology has been developed by UK anodisers (HMF Ltd. and ASE Ltd.) that allow for the production of anodised surfaces that prevent micro-crack formation and can make possible the production of aluminium tools that have improved wear resistance with cyclic heat resistance up to 500°C and, in addition, very high thermal emissivity. This anodising treatment is referred to as the ALAMO anodising process. 

It is possible to obtain conditions in which the anodic him continues to grow to form a blue or black layer, and this, although not exceptionally protective, has uses in the treatment of baking pans. A typical anodising solution contains 100g/1 Na2HP04 IZHjO and 20 ml/1 phosphoric acid, and is used at 350 A/m at 60-90°C for about 10 min. 

Protective measures range from chemical conversion coatings and anodising to the application of more substantial protective layers, e.g. enamels. For a more detailed treatment of the subject, the reader is advised to consult References 19, 20, 21 and 22. 

The attainment of a clean surface prior to the application of any subsequent treatment or coating is essential, whether this subsequent operation is electroplating, anodising, chemical treatment or organic coating. 

The anodising procedures in general use are shown in Table 15.1, sulphuric acid being the most commonly used electrolyte. Treatment time is IS min to 1 h. 

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... 

A conversion treatment comprising electrolytical anodising of portions of a surface of an HgCdTe body is used in GB-A-1600S99 to form imagers having photo-conductors or photodiodes as detector elements. 

Porous anodising at high current densities provides the formation of selfordering structures without pretexturing treatment [1], Fig. 3 shows SEM images of porous alumina formed at 70 V. Self-ordering occurs with the porous growth (Fig. 3a). 

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. 

Chemical treatments - by immersion in a solution which will etch or dissolve part of the surface, so that the surface becomes chemically active, e.g. acid etching and anodising. 

---