The Electrochemical Behaviour of Aluminium

The electrochemical behaviour of aluminium is influenced by the natural oxide layer that governs the corrosion resistance of aluminium. 

The potential measured on aluminium does not correspond to that of the metal, but represents a mixed potential between the oxide layer and the metal. The potential of the metal cannot be measured, because in oxidising media such as water, the oxide layer will form immediately, within 1 ms or even less. 

For predicting the behaviour of aluminium, the pitting potential does not have the same importance as for steel. 

On steel, pitting is controlled by the initiation step (as detected by the pitting potential), and it propagates quickly. On aluminium, there will always be an initiation of a very large number of superflcial and very small pits around intermetallic phases that have a cathodic potential with respect to the matrix. However, the propagation of pits cannot be deduced from the measurement of the potential. 

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

The electrochemical behaviour of aluminium at the water line differs from that of iron, because aluminium is a passive metal. Corrosion develops in the meniscus, where the water film is very thin. It has been shown that in the case of aluminium, it is not the difference in aeration that is important, but the difference in the concentration of chloride in the thinnest part of the water film, where evaporation is fast. The chloride concentration is higher in the thin part of the film. Moreover, the more electronegative the dissolution potentials, the thinner the film is. The upper part of the meniscus is, therefore, the anodic zone, which corrodes preferentially (Figure B.2.20) [37]. 

The electrochemical behaviour of aluminium is strongly influenced by the permanent presence of a natural oxide film on its surface. Therefore, a mixed potential corresponding to the pitting potential is measured on aluminium (see Section B.1.7) this potential represents a threshold below which pitting corrosion can be prevented. 

The electrochemical behaviour of metal carbonyls in aprotic solvents was investigated by Pickett et al (1) and Seurat et al (2). The electrochemical oxidation of several metal carbonyls was also described in a mixture of the high Lewis acid, room temperature molten salt, composed of aluminium chloride and ethylpyridinium bromide (2 1 molar ratio) and benzene (50% v/v) (3). In these studies Cr(CO)6 was found to be reversibly oxidised, at a potential of 1.53 V vs SCE, to the seventeen electron cation Cr(CO)6" which is stable on the time scale of many seconds. The cyclic voltammogram of C r(CO)6 shows a second oxidation peak at a potential of 2.06 V for further oxidation of the cation Cr(CO)5 and the proposed mechanism is (1) ... 

Kassab, A., Kamel, KM., Abdel Hamid, E. (1987). Effed of molybdate ion on the corrosion behaviour of aluminium in NaOH solutions. J. Electrochemical Society of India 36 27-30. 

Laboratory experiments were carried out to determine the current efficiency for aluminium deposition and to study the electrochemical behaviour of dissolved phosphorus containing complexes. 

Haarberg, G.M., Keppert, M., Thisted, E. and Thonstad, J. (2004) The electrochemical behaviour of phosphorus compounds in cryolite-alumina melts and the role of phosphorus during electrowinning of aluminium. 43rd Annual Conference of Metallurgists, Hamilton, Canada. 

The electrochemical properties of zinc also have a large bearing on its corrosion behaviour. Zinc is negative to Eh /h2 and magnesium and aluminium excepted, to most other metals commonly encountered, including those found in the less pure forms of zinc. This means that when zinc is in contact with these metals sacrificial electrochemical action can take place, with zinc forming the anode. Contact with other metals and impurities can... 

Long term behaviour of NiAl in molten carbonate is probably much better than suggested by the short term electrochemical measurements. This is also suggested by the good behaviour of Kanthal-Al (with only 5.8% of aluminium) at long term immersion, which was ascribed also to the formation of an aluminium oxide layer, in that case between the outer Li(Fe,Cr)0 layer and the base alloy. Small amounts of A1 were found to be beneficial also by Uchida [13],... 

C. Brett, I. Gomes, and J. Martins, The electrochemical behaviour and corrosion of aluminium in chloride media, the elfect of inhibitor anions, Corrosion Science, vol. 36, no. 6, pp. 915-923, 1994. 

The shortcomings of the electrochemical series are readily illustrated, e.g. aluminium has a very negative standard potential yet it exhibits particularly good corrosion resistance in many environments. This is because the electrochemical series considers only A1 metal and Al while in practice AI O films determine the properties of the system. Hence, a much more sophisticated thermodynamic treatment is necessary to predict the behaviour of aluminium. 

The matrix a-phase in Mg alloys is typically anodic to the second phases and is preferentially corroded. Song et al. [7,8] suggested that the primary a and eutectic a-phases, which have different aluminium contents, have different electrochemical behaviour. Both the primary and eutectic a can form a galvanic corrosion cell with the P phase, as illustrated in Fig. 3.10. There are therefore two kinds of corrosion morphology ... 

Mg and possessed a superior corrosion resistance than a couple of Mg alloys and another amorphous alloy Mg65Cu2sYio (Fig. 6.6). Interestingly, the Auger electron spectroscopy (AES) analysis showed that there was no trace of Ga compounds in the passive layer and that it was enriched only with aluminium oxide. However, AES depth profiles suggested the deposition of metallic Ga below the corrosion layer, whieh was further confirmed by the XRD results. It appears that the euhaneed eorrosion resistance was only due to the aluminium oxide enrichment at the surfaee of this alloy. These researeh findings opened up avenues for the development of amorphous alloys with higher aluminium eontent that eould provide not only an improved electrochemical behaviour but superior meehanieal properties as well. 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

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