Aluminium and Zinc

The form of Figure 1.43 is common among many metals in solutions of acidic to neutral pH of non-complexing anions. Some metals such as aluminium and zinc, whose oxides are amphoteric, lose their passivity in alkaline solutions, a feature reflected in the potential/pH diagram. This is likely to arise from the rapid rate at which the oxide is attacked by the solution, rather than from direct attack on the metal, although at low potential, active dissolution is predicted thermodynamically The reader is referred to the classical work of Pourbaix for a full treatment of potential/pH diagrams of pure metals in equilibrium with water. 

Fig. 1.67 Effect of mild-steel cathode on the weight loss of aluminium and zinc anodes (100 cm area) and on the number of coulombs flowing in the aluminium-mild steel and zinc-mild steel couples over a 96 h period in 1 -0 n NaCl at 25°C (after Pryor and Keir )...

Fi -1.70. Polarity reversal of the Al-2n couple in 1 C n sbdium chloriUe hi 25 C., Ciirve a, aluminium and zinc electrodes l J mto apart 16 ml of solutioii per square centimetre of clcc-trh( e curve ft aluminium and zinc electrodes 20 mm apart, 10 ml of solution per square centimetre ofelectrot ... 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

C.C.I.T.T. Study Group VI working party on corrosion by alternating currents (C. Cabrillac). A bibliography with over 60 references giving particular emphasis on buried lead, iron, aluminium and zinc structures, I.T.U., Geneva... 

The metals most used for corrosion protection by metal spraying are aluminium and zinc, both of which are anodic to steel in most environments. Physical properties of these coatings are shown in Table 12.6. 

Nagaska el ai., Weathering Test on Zinc, Aluminium and Zinc-Aluminium Alloy Sprayed Coating for 5000 Days, Proc. lOth Int. Thermal Spray. Corf., DVS, Diisseldorf, Poster report (1983)... 

Protection of Iron and Steel by Aluminium and Zinc against Atmospheric Corrosion , Sprayed Metal Coatings, BS 2569 Part 1 (1964)... 

As noted above, however, aluminium and zinc are the most important and are often found in combination in the liquids used for the zinc... 

The liquids used in these early formulations were 50% solutions of orthophosphoric acid, often containing aluminium and zinc. Chemical analyses were published between 1904 and 1972 (Voelker, 1916a Greve, 1913 Watts, 1915 Paffenbarger, Schoonover Souder, 1938 Axelsson, 1964 Wilson et al, 1972). 

Dental silicate cement liquids are concentrated aqueous solutions of orthophosphoric acid generally containing aluminium and zinc (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b Wilson et al., 1972). The optimum orthophosphoric acid concentration is 48 to 55 % by mass (Wilson et al, 1970a), although higher concentrations are encountered. Aluminium is present as phosphate complexes and zinc as a simple ion (see Section 6.1.2). Examples are given in Table 6.6. 

Cement strength and resistance to aqueous attack are also critically dependent on phosphoric add concentration, and there is an optimum concentration for the development of maximum strength and resistance to aqueous attack (Wilson et al., 1970a Wilson, Paddon Crisp, 1979). The effect is particularly critical when the phosphoric add liquid contains aluminium and zinc. 

All commercial examples of phosphoric add solutions used in these cements contain metal ions, whose role has been discussed in Section 6.1.2. In the case of the dental silicate cement, aluminium and zinc are the metals added to liquids of normal commerdal cements and have a significant effect on cement properties (Table 6.8) (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b). Aluminium accelerates setting for it forms phosphate complexes and is the prindpal cation of the phosphatic matrix. Zinc retards setting for it serves to neutralize the addic liquid - it... 

Handy, R.D., F.B. Eddy, and G. Romain. 1989. In vitro evidence for the ionoregulatory role of rainbow trout mucus in acid, acid/aluminium and zinc toxicity. Jour. Fish Biol. 35 737-747. 

Occurrence. Gallium ores are rare. It occurs in aluminium and zinc ores, for instance bauxite and zinc blende. 

If this alloy is put into hot water it readily yields hydrogen the hydrogen yield is proportionate to the aluminium and zinc content. 

Long experience in the storage of blasting caps filled with lead azide has shown that this substance reacts with copper or brass to form cupric azide, which is highly sensitive to friction and impact. For this reason lead azide is compressed only into aluminium and zinc cases. 

Table 10.5 shows the effects of adding a few drops of sodium hydroxide solution to solutions containing various metal ions, and of adding an excess. The colours of the insoluble metal hydroxides can be used to identify the metal cations present in solution. In some cases the precipitate dissolves in excess hydroxide, owing to the amphoteric nature of the metal hydroxide. This amphoteric nature can also be used to help identify metals such as aluminium and zinc. 

Both aluminium and zinc metals will also react readily with moderately concentrated acids and alkalis. 

It is true to say that almost all the reactions by which a metal is extracted from its ore are reduction reactions. Discuss this statement with respect to the extraction of iron, aluminium and zinc. 

Catalysts for epoxide polymerisation of quite different characteristics comprise metalloporphyrins of aluminium and zinc, such as (5,10,15,20-tetraphe-nylporphinato)aluminium chloride [(tpp)AlCl], methoxide [(Mtpp)AlOMe] or 1-propanethiolate [(tpp)AlSPr] and (5,10,15,20-tetraphenyl-21-methylporphi-nato)zinc methoxide [(Mtpp)ZnOMe] [32 35] ... 

If a precipitate forms on neutralizing the solution, the presence of arsenic, antimony and tin sulphides and possibly salts of amphoteric elements (lead, tin, aluminium, and zinc) is indicated. The precipitate should be filtered off and rejected, f CaS04 and Ca(B02)2 may separate from sufficiently concentrated solutions. 

Metals such as iron and copper are generally most soluble in acidic water (i.e. pH < 7), and solubility increases as the pH drops. Other metals, such as aluminium and zinc, are more soluble in alkaline water, especially when the pH is above 10. In mildly acidic water (i.e. pH 45-6.5), metals such as iron and copper have a low solubility under extreme anaerobic and aerobic conditions. This is due to the formation of sulfide minerals that have a low solubility under highly anaerobic conditions, and the formation of low-solubility hydroxide and oxide minerals under highly aerobic conditions. 

Haines A, Iliffe S, Morgan P, Dormandy T, Wood B (1991) Serum aluminium and zinc and other variables in patients with and without cognitive impairment in the community. Clin Chim Acta 198 261-266... 

Add a little, and then an excess, of ammonium sulphide to a solution of chrome alum. Compare the result with that in ( ). Docs chromium form a sulphide Do aluminium and zinc ... 

Figure 10.10 Formation of hydrotaldtc from the precipitation of a solution of aluminium and zinc nitrates (taken from Clays and Clay Minerals. Vol. 37 F. Thcvcnot e/a/., p. 396,1989).

Astringents are weak protein precipitants, e.g. tannins, salts of aluminium and zinc. 

Magnesium, aluminium and zinc react with COFj at red heat to give their respective fluorides and carbon monoxide [1756]. 

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