Potential of Aluminium

Zinc strongly decreases the potential. Alloy 7072 at 1% zinc is, therefore, used as a cladding of 3003 (see Section B.5.6). The alloys of the 7000 series have the most electronegative potentials. 

Copper alloys of the 2000 series have the least electronegative potentials. Alloy 2017A can, therefore, be protected by a cladding of 1050A (see Section B.5.6). 

Dissolution potentials of the most common aluminium alloys are listed in T able B. 1.4 [ 16]. For a given alloy, variations by 50-100 mV from one author to another have been reported. 

Although intermetallic phases may have a dissolution potential rather different from that of the solid solution (Table B.1.5), they have no influence on the dissolution potential. However, they may give rise to intercrystalline corrosion, exfoliation corrosion, or stress corrosion if localised at or close to grain boundaries (see Section B.2.3). 

Alloy Temper Potential (mV ECS) Alloy Temper Potential (mV ECS) 

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The electrode potential of aluminium would lead us to expect attack by water. The inertness to water is due to the formation of an unreactive layer of oxide on the metal surface. In the presence of mercury, aluminium readily forms an amalgam (destroying the original surface) which is. therefore, rapidly attacked by water. Since mercury can be readily displaced from its soluble salts by aluminium, contact with such salts must be avoided if rapid corrosion and weakening of aluminium structures is to be prevented. 

Bohni, H. and Uhlig, H. H. Environmental Factors Affecting the Critical Pitting Potential of Aluminium , J. Electrochem. Soc., 116, 906 (1969)... 

Practice for measurement of corrosion potentials of aluminium alloys Practice for conducting and evaluating galvanic corrosion tests in electrolytes Practice for liquid impingement erosion testing... 

The frost diagram shown in figure above shows much larger negative slope (negative electride potential) of aluminium. The compounds of thallium (Tl) are toxic but they pose no environmental hazard as they are little used. 

Aluminium is a light metal with a density about one third of that of steel. It conducts electricity well and may be alloyed with other metals to improve its strength characteristics. The standard electrode potential of aluminium is -1.67 V and would be a highly reactive metal were it not for the protective oxide formed on its surface. If aluminium is immersed in a solution containing species which prevent the oxide film from reforming, then the metal corrodes rapidly with the evolution of hydrogen gas from the cathodic regions. 

Main J, Ward MK. Potentiation of aluminium absorption by effervescent analgesic tablets in a haemodialysis patient. BMJ I992 304(6843) I686. 

The corrosion processes associated with aluminium clad fuels in storage basins are electrochemical. Hence basin water plays a key role in the flow of electric current and ions in the process. The amount of metal removed by corrosion is directly related to the current flow. By increasing the resistance of the water, the corrosion of aluminium can be reduced. Fontana [10.4] pointed out that the low corrosion rates in high purity water are primarily due to the low conductivity of the water. Aluminium is passive and protected by its oxide film in the pH range 4-8.5. It has been observed that the pitting potential of aluminium in chloride solutions is independent of pH in the range 4-9 [10.7]. 

The armature. This is the main protection for the body shell. Steel is usually preferred, but the weight saving potential of aluminium and SMC has led to many successful designs using these materials. 

The results of electrocopolymerization and redox initiated copdymerization of methacrylic acid with A, iV -methylenebisaciylamide are presented in Fig. The selection of these two monomers for the present study was based upon the fact that these monomers do not undergo significant side reactions, other than the main vinyl polymerization reaction, during electrolysis of their aqueous solutions l Of these two monomers, only methacrylic acid ionizes in aqueous solutions. It was, therefore, anticipated that the methacrylic acid ion will experience a coulombic repulsicm from the aluminium cathode which will affect its incorporation in the copdymer forming on the cathode. Further, by controlling the degree of ionization of methacrylic add and the potential of aluminium cathode, an electrolytic control of the copdymer composition mi t become possil. ... 

Dissolution Potential of Aluminium Electrochemical Equilibrium (Pourbaix) Diagrams The Electrochemical Behaviour of Aluminium Aluminium as a Passive Metal... 

The dissolution potential of aluminium is measured on a surface which is always covered by a natural oxide layer. This oxide layer consists of three very different elements anodic pores (about 0.5% of the total surface), the cathodic barrier layer, and thicker areas which are neutral [9]. All parameters that modify the properties of the natural oxide layer will also modify the potential of aluminium. 

Alloying elements can modify the potential of aluminium in both directions, as can be seen in Figure B.1.8. The dissolution potential of an alloy is determined by the solid solution, the major part of the metal s surface [15]. 

Figure B.1.8. Influence of alloying elements on the dissolution potential of aluminium alloys [17].

Table B.1.4. Dissolution potentials of aluminium alloys (NaCl solution, H2O2, ASTM G69)...

The dissolution potential of aluminium in most aqueous media is in the order of - 500 mV with respect to a hydrogen electrode, while its standard electrode potential with respect to this same electrode amounts to — 1660 mV. Because of this highly electronegative potential, aluminium is one of the easiest metals to oxidise (Table B.1.1). However, aluminium behaves as a very stable metal, especially in oxidising media (air, water, etc.). 

The comparison of the dissolution potentials of aluminium alloys may reach absurdity, for example, leading to a preference for alloys of the 2000 series, which have a dissolution potential far less negative, about — 650 mV, over those of the 5000 series, which have a more electronegative potential, on the order of - 800 mV (Table B.1.3). And yet the latter show excellent corrosion resistance, while alloys of the 2000 series are highly susceptible to pitting corrosion in natural environments. 

Taking into account the dissolution potential of aluminium, anodes can be made either in zinc or in a special aluminium alloy called Hydral , which contains indium (0.015-0.025%) or tin (0.10-0.20%). Magnesium anodes must not be used, because they lower the potential too much and will thus lead to severe cathodic corrosion of aluminium. 

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