Anodization of aluminium

Reduction of a range of allyl and benzyl chlorides at a stainless steel cathode in dimethylformamide in the presence of carbonyl compounds and using a sacrificial anode of aluminium or zinc, leads to a Reformatsky-type reaction in 40-80 % yields. Allyl halide give products by reaction at both the a- and y-positions. Tetra-chioromethane and bromotrifluoromethane take part in similar reactions provided a... 

Microporous alumina membranes electrochemically grown by anodizing of aluminium can be also produced with an excellent controlling of pore size distribution and porosity [34]. 

The MF membranes are usually made from natural or synthetic polymers such as cellulose acetate (CA), polyvinylidene difiuoride, polyamides, polysulfone, polycarbonate, polypropylene, and polytetrafiuoroethylene (FIFE) (13). Some of the newer MF membranes are ceramic membranes based on alumina, membranes formed during the anodizing of aluminium, and carbon membrane. Glass is being used as a membrane material. Zirconium oxide can also be deposited onto a porous carbon tube. Sintered metal membranes are fabricated from stainless steel, silver, gold, platinum, and nickel, in disks and tubes. The properties of membrane materials are directly reflected in their end applications. Some criteria for their selection are mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and the cost of membrane material as well as manufacturing process. 

On the other hand, a multicolour luminescent image, on the basis of a single phosphor embedded in mesopores or macrocavities of anodic alumina films, can be fabricated using blue PL that is specific for some films. Examples of such blue PL recorded for three types of electrolyte, are presented in Fig. 3. The origin of the blue PL from PAA is considered to be related to generation F-centres during anodizing of aluminium or excitation of residual chemical components absorbed from the electrolytes [8,9]. 

Most commercial anodizing of aluminium is performed in sulphuric acid electrolytes to yield oxide films, which, of the order of tens of micrometres thick, provide good corrosion and abrasion resistance, in addition to a useful dye-absorption capability. Other anodizing processes utilize chromic, oxalic, nitric or phosphoric acids, or mixed electrolytes. 

The replica method has been used for examining the pore structure of films formed in the Anodizing of aluminium. Figure 1 represents a piece of anodized aluminium that had been bent, so that part of the anodic film has spalled off. The deposition and shadowing directions are indicated. Replicas made in this way show the structure of both the top surface of the anodized layer and the fracture surface Fig. 2 shows features of ihe pores described in the article Anodizing. 

Figure 12.17 Structure of the oxide layer formed by the anodization of aluminium.

Naturally occurring oxide films on most metals do not usually provide optimum corrosion protection, and this may be modified or replaced to provide a further means of corrosion control. Common examples are the anodizing of aluminium alloys or the chromating of aluminium, zinc, cadmium or magnesium. With anodizing, the natural oxide film on the aluminium is thickened electrolytieally by up to 5 p.m. Chromating, described in detail later in the chapter, replaces the existing metal oxide film with a mixed chromium/metal oxide film of better corrosion resistance. 

UK Defence Standardization, DEF STAN 03-24 (1997). Chromic Acid Anodizing of Aluminium and Alitminium Alloys, Issue 3 (unsealed process). 

Rare earths have been used in conjunction with anodizing of aluminium in several ways. Firstly, they have been used in fundamental investigations of the anodic oxidation of aluminium alloys where the rare earth element (cerium, hafnium, neodymium and samarium) was employed as an alloying element. These smdies revealed the distribution of the alloying element in the anodic alumina and the outward migration rate of the rare earth cations relative to aluminium ions. It has been shown also that rare earths (particularly cerium) can inhibit ejection of ions from films during anodizing in alkaline electrolytes. 

Fundamental investigations of the effects of alloying elements in anodizing of aluminium alloys have been made using ion implantation (Mackintosh... 

Arrowsmith D.J. and Moth D.A. (1986), Chromic acid anodizing of aluminium . Tram. Inst. Met. Finish., 64, 91-3. 

Habazaki H., Shimizu K., Skeldon R, Thompson G.E., Wood G.C. and Zhou X. (1997a), Effects of alloying elements in anodizing of aluminium alloys , Trans. Inst Met. Finish., 15,18-23. 

Saenz de Meira M., Curioni M., Skeldon P. and Thompson, G. E. (2010), The behaviour of second phase particles during anodizing of aluminium alloys , Corros. Sci., 52, 2489-97. 

Aerts, T., Tourwe, E., Pintelon, R., De Graeve, I. Terryn, H. (2011). Modelling of the porous anodizing of aluminium Generation of experimental input data and optimization of the considered model, Surface Coatings Technology 205(19) 4388-4396. 

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