Phosphating of Aluminium

On exposure to air, metallic aluminium immediately forms a thin film of oxide, AI2O3, while in boiling water or steam the film which is formed consists mainly of boehemite, AI2O3 H2O. In order to act as satisfactory protective films, however, both these compounds have to be formed by 

A buildup of Al + in the zinc phosphating baths will restrict the amount of coating formed unless these ions are ranoved by addition of fluoride or borate. An advantage of zinc phosphating, however, is that it avoids the use of toxic hexavalent chromium compounds. Some simple bath compositions for zinc and manganese phosphating of aluminium are 

When alumininm is treated with a mixture of phosphoric and chromic acids, a pale green mostly amorphous film consisting of alnminium and chromium phosphates is obtained. This process involves the reduction of the chromic acid by the (nascent) hydrogen released by the attack of the phosphoric acid on the alnmininm (12.40, 12.41). 

The chromium phosphate treatment produces largely amorphous films 2-10 mm thick which contain about 50% CrP04 and 20% AIPO4, the remainder being mostly water of hydration. 

Fluoride is included in the bath in order to prevent the precipitation of chromium in the coating, by forming soluble CrF3. Typical compositions are 

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The phosphates of lime are the most valuable natural fertilisers and raw material of the fertiliser and phosphorus industry, which uses also to a less extent phosphates of aluminium and the apatites. Over 80 per cent, of the easily decomposible phosphorites mined are used in the preparation of superphosphate or other fertilisers (1980). 

The solution is filtered from calcium sulphate and more ammonia is added until the phosphates of aluminium and iron settle. On concentrating the filtrate phosphates and sulphates of ammonia may be crystallised.2... 

If a phosphate was present, the precipitate obtained on addition of ammonium chloride and hydroxide may contain phosphates of such metals as calcium, etc., which are normally precipitated in later groups in systematic analysis. The precipitate is therefore dissolved in dilute hydrochloric acid, and the solution nearly neutralised with sodium carbonate. Sodium acetate is now added, and the whole boiled. The precipitate contains the phosphates of aluminium, chromium, and iron, and is treated, as already indicated, with sodium peroxide and water. 

Fig. 3.1.3. Amygdaloid from Malpelo Island, D.F., Colombia. Some of the amygdules and some of the lathlike crystals of the mesostasis are phosphates of aluminium, whereas other amygdules are opal (or mixtures) and some of the lathlike crystals (originally feldspars) are now quartz. Magnification 33x. Reproduced with permission (McConnell, 1943).

Thus in contact with aluminium and ferric phosphates, the aluminium and ferric ion concentrations are reduced and so the phosphate ion concentration is increased to maintain the solubility product at their constant levels. AIPO4 represents various hydrated and hydroxyl phosphates of aluminium, including any adsorbed or precipitated surface layers on oxides and alumino-silicates. FePO similarly, represents various hydrated and hydroxyl phosphates of iron including adsorbed or precipitated surface layers on iron oxide. 

The anodic oxidation of magnesium does not normally produce a film that has sufficient corrosion resistance to withstand exposure without further protection by painting, and the solutions used are complex mixtures containing phosphates, fluorides and chromates. In the case of aluminium, a relatively simple treatment produces a hard, compact, strongly adherent film of oxide, which affords considerably increased protection against corrosive attack . 

Acid treatments The principal acid processes were developed in the USA under the name Alodine, and are marketed in the UK as Alocrom and under other names. The original solutions were based on acid solutions containing phosphate, chromate and fluoride ions. Immersion for up to 5 min in the cold or warm solution leads to the deposition of a greenish film containing the phosphates of chromium and aluminium, and possibly some hexavalent chromate. The more recent Alocrom 1 200 process uses an acid solution containing chromate, fluoride and nitrate. Room-temperature immersion for 15 s to 3 min deposits golden-brown coatings which contain chromate as a major constituent. 

When aluminium is immersed in water, the air-formed oxide film of amorphous 7-alumina initially thickens (at a faster rate than in air) and then an outer layer of crystalline hydrated alumina forms, which eventually tends to stifle the reaction In near-neutral air-saturated solutions, the corrosion of aluminium is generally inhibited by anions which are inhibitive for iron, e.g. chromate, benzoate, phosphate, acetate. Inhibition also occurs in solutions containing sulphate or nitrate ions, which are aggressive towards iron. Aggressive anions for aluminium include the halide ions F ,... 

Discussion. Some of the details of this method have already been given in Section 11.11(C), This procedure separates aluminium from beryllium, the alkaline earths, magnesium, and phosphate. For the gravimetric determination a 2 per cent or 5 per cent solution of oxine in 2M acetic add may be used 1 mL of the latter solution is suffident to predpitate 3 mg of aluminium. For practice in this determination, use about 0.40 g, accurately weighed, of aluminium ammonium sulphate. Dissolve it in 100 mL of water, heat to 70-80 °C, add the appropriate volume of the oxine reagent, and (if a precipitate has not already formed) slowly introduce 2M ammonium acetate solution until a precipitate just appears, heat to boiling, and then add 25 mL of 2M ammonium acetate solution dropwise and with constant stirring (to ensure complete predpitation). 

Phosphate, arsenate, and vanadate interfere. Borate, fluoride, and large amounts of aluminium, calcium, magnesium, and the alkali metals have no effect in the determination, but large amounts of iron (> 5 per cent) appear to produce slightly low results. 

Bigay, J., Deterre, P., Pfister, C., Chabre, M. (1987). Fluoride complexes of aluminium or beryllium act on G-proteins as reversibly bound analogues of the y-phosphate of GTP. EMBO J. 6, 2907-2912. 

Combeau, C. Carlier, M.-F. (1989). Characterization of the aluminium and beryllium fluoride species bound to F-actin and microtubules at the site of the y-phosphate of the nucleotide. J. Biol. Chem. 264,19017-19022. 

Presence of aluminium and calcium. The quantity of aluminium in vaccines containing aluminium hydroxide or aluminium phosphate as an adjuvant is limited to 1.25 mg per dose and it is nsnally estimated compleximetrically. The qnantity of calcium is limited to 1.3 mg per dose and is usually estimated by flame photometry. 

A novel basic support and catalyst have been prepared by activation of aluminium phosphate with ammonia. Fine control of time and temperature allows to adjust the 0/N ratio of these oxynitride solids and thus to tune the acid-base properties. The aluminophosphate oxynitrides are active in Knoevenagel condensation, but a basicity range can not yet determined. Supporting Pt or Pt/Sn on AlPONs allows to prepare catalysts that are highly active and selective in dehydrogenation reactions. 

The most important of these are the refractory cements formed by the heat treatment of aluminium acid phosphate solutions. This subject has been well reviewed by Kingery (1950a), Morris et al. (1977), Cassidy (1977) and O Hara, Duga Sheets (1972). The chemistry of these binders is extremely complex as the action of heat on acid phosphates gives rise to polymeric phosphates, with P-O-P linkages, and these are very complex systems (Ray, 1979). 

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

Ando, Shinada Hiraoka (1974) examined cements formed by the reaction between magnesium oxide and concentrated aqueous solutions of aluminium dihydrogen phosphate. Later, Finch Sharp (1989) made a detailed examination of the cement-forming reaction and reported that the reaction yielded cements of moderate strength. 

Salmon, J. E. Wall, J. G. L. (1958). Aluminium phosphates. Part II. Ion-exchange and pH-titration studies of aluminium phosphate complexes in solution. Journal of the Chemical Society, 1128-34. 

Although commonly formed from endogenous material, the occurrence of synovial crystals formed following environmental exposure to exogenous agents is indicated by the identification of both aluminium phosphate and aluminium silicate particulates (Netter etal., 1983, 1991). It is noteworthy in this context that arthritic symptoms have been reported following the... 

In the beginning of the eighties, the clinical application of DFO expanded to a new type of patient, namely those on maintenance dialysis. As we will see in Chapter 12, some patients suffered from aluminium overload, mostly due to the use of aluminium salts as phosphate binders, while others had obvious transfusional iron overload in the pre-erythropoietin era. DFO was therefore used either to remove aluminium, excess iron or both. Nephrologists established that DFO therapy did not increase the overall incidence of bacterial infections but that it slightly increased the risk of bacteraemia caused by Y. enterocolitica or Y. pseudotuberculosis, as had been previously observed in thalassaemic patients (Boelaert et ah, 1987 Tielemans et ah,... 

The principal method by which aluminium adjuvanted vaccines are prepared entails mixing the antigen in solution with a preformed aluminium phosphate (or hydroxide) precipitate under chemically defined conditions (e.g. of pH). Adsorption of the antigen to the aluminium-based gel ensues, with such preparations being generally termed aluminium-adsorbed vaccines 1 mg of aluminium hydroxide will usually adsorb in the order of 50-200 pg of protein. 

Chemical effects include stable compound formation and ionization, both of which decrease the population of free atoms in the sample vapour and thereby lower the measured absorbance. Examples of compound formation include reactions between alkaline earth metals and oxyanions such as aluminates, silicates and phosphates, as well as the formation of stable oxides of aluminium, vanadium, boron etc. 

By a corresponding reaction with phosphorus oxychloride2 in the presence of aluminium chloride and other catalysts such as ferric chloride, iodine, etc., trichloroethyl phosphate can be prepared in a stepwise fashion. This and related compounds possess the property of reducing the inflammability of organic materials and are recommended for special clothing. 

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