Electrochemical Aspects of Corrosion

Ulrick R. Evans, the British scientist considered to be the father of Corrosion Science has said that corrosion is largely an electrochemical phenomenon, which may be defined as destruction by electrochemical or chemical agencies . The 

Study of corrosion, therefore, cannot be done disregarding completely the electrochemical process at its base. In the electrochemical interpretation of corrosion, the progressive dissolution of the crystal structure in contact with a liquid, that is called electrolyte, is due to a continuous passage into solution of metallic ions, or ions that are hydrated, that feed an anodic current. For instance, if a zinc bar is immersed into water, some ions are hydrated and will leave the metal surface entering the solution as Zn  

Corrosion, then, is the consequence of a couple of redox reactions. In particular, what is actually occurring in water can be expressed as 

Not all metals are equally subjected to corrosion. Better or lesser tendency of metals to hydrate and corrode can be assessed from their oxidation or reduction potential, also called redox potential. 

In general, the higher the oxidation potential the lesser the tendency to corrode. However, some metals corrode less than other metal with higher redox potential. For example, chromium (—0,74 V), zinc (—0,76 V), titanium (—0,89 V), aluminum (—1,71 V) etc. withstand corrosion much better than iron (—0,42 V). This is due to the fact that the surface of these metals coats with an insoluble very thin layer, just a veil, of hard-bitten oxide not reactive at all that, at variance with rust, passivizes the surface blocking the prosecution of corrosion. Table 13.2 provides a synoptic picture of the standard potentials, the so called electrode potential, relative to oxidation reactions of various metals. The standard electrode potential, abbreviated as , is given in volts and is the measure of the potential of any individual metal electrode which is with solute at an elfective concentration of 1 mol/dm at 1 atm of pressure. These potentials are referred to a hydrogen electrode whose reference potential is assumed equal to zero. This is because it is not possible to measure experimentally the value of the dilference of potential Ay between an electrode and its solution as, for example, in the case of zinc reaction (13.16), because any device used for making the measurement must be inserted in the circuit with two electrodes of which one is put in contact with the metal electrode of interest and the other with the solution. Now, this second electrode creates necessarily another interface metal-solution and the potential difference provided by the system is that between the two metals, without any possibility to infer the absolute value of each of them. This is why it is necessary to introduce a reference electrode, which any other potential can be referred to. To 

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The electrochemical aspects of corrosion research have been the subject of some computer-controlled instrumentation [11]. It seems likely, in this case, that electrochemistry alone is not the answer and other techniques, such as spectroscopy and analytical methods, need to supplement the electrochemical measurements. 

Williams, D. F., Electrochemical Aspects of Corrosion in the Physiological Environment, Fundamental Aspects of Biocompatibility, Vol. 1, CRC Press, Boca Raton, M., 1981, pp. 11-42. 

Brignold, G. J., Electrochemical Aspects of Stress Corrosion of Steels in Alkaline Solutions , Corrosion, 28, 307 (1972)... 

Pourbaix, M., Electrochemical Aspects of Stress-corrosion Cracking , CEBELCOR Rapp. Tech., 118(RT 199) (1971) C.A., 76, 12065e... 

Rowlands, J. C., Electrochemical Aspects of Preferential Phase Corrosion in Complex Alloys , Corros. Sci., 2, 89 (1962)... 

The deliberate raising of the electrical potential of titanium, either by the attachment of discrete particles of a noble metal, such as platinum or palladium, at the surface, or by the application of positive direct current to force the formation of a protective film, is dealt with at a later point. The electrochemical aspect of the corrosion of titanium is comprehensively treated in a number of papers ... 

Electrochemical aspects of the stress-corrosion behaviour have been investigated, mainly in neutral solutions. The open-circuit potential of Ti-8Al-lMo-l V is —800mV (v5. S.C.E.). The crack initiation load reaches... 

Vathsala, K. A. Natarajan, 1989. Some electrochemical aspects of grinding media corrosion and sphalerite flotation. Inter. J. Miner. Process, 26(3 - 4) 193 - 203 Walker, G. W., Stout, J. V. Ill, Richardson, P. E., 1984. Electrochemical flotation of sulphides reaction of chalcopyrite in aqueous solution. Inter. J. Miner. Process, 12 55 - 72 Wang Dianzuo, 1983. Structure and reactivty of organic depressant on flotation. Nonferrous Metals, (2) 47-51... 

The present chapter begins with a brief overview of metallic corrosion and mechanisms of corrosion control. Methods of evaluating polymer performance and electrochemical characterization techniques are discussed. Barrier and adhesion aspects of corrosion control are reviewed, and some critical issues needing further study are outlined. 

The electrochemical, physicochemical and adhesional aspects of corrosion protection by organic coatings are shortly discussed. Attention is drawn to some inoonsist-ancies in the interpretation of protective mechanisms and suggestions are given how protective principles may be optimally realized in practical systems. 

Fig. 12.80. Samples of a brass (64% Cu-26% Zn) alloy bar strained in a NaN02 solution at 0.2 V vs. NHE. Typical stress-corrosion cracking fractures (dashes = 100 pm). (Reprinted from J. R. Galvele, Electrochemical Aspects of Stress Corrosion Cracking, in Modern Aspects of Electrochemistry, R. E. White, J. O M. Bockris, and B. E. Conway, eds., No. 27, p. 234, Plenum, 1995.)...

U. Bertocci and E.N. Pugh, Chemical and Electrochemical Aspects of SCC of Alpha-Brass in Aqueous Ammonia, International Congress on Metallic Corrosion, National Research Council of Canada, Toronto, 1984, p 144-152... 

Bundy KJ (1994) Corrosion and other electrochemical aspects of biomaterials. Crit Rev Biomed Eng 22 139-251. 

Electrochemical methods have been used in corrosion testing ever since the electrochemical nature of corrosion processes was discovered. In the present age electrochemical measurements involve the use of sophisticated black boxes which are invariably blamed for any pitfalls which occur. Hence the European Federation of Corrosion Working Party on Physicochemical Methods of Corrosion Testing felt it desirable to remind research workers, students and instrument designers of the more fundamental aspects of the measurements. 

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

Pourbaix M. Electrochemical aspects of stress corrosion. In Scully JC, editor. The Theory of Stress Corrosion Cracking in Alloys. Brussels NATO, 1971. 

A.K. Tiller. Electrochemical aspects of microbial corrosion An overview. In the Proceedings of Microbial Corrosion, The Metals Society, London, UK, 1983. 

Because the corrosion behavior of metals is governed by complex interactions involving many parameters, it manifests itself in numerous often unanticipated forms. The corrosion resistance of a given metal is not an intrinsic property of that metal, but a systems property. The same metal may rapidly corrode in a certain environment while under different conditions it is stable. From a more fundamental point of view, the corrosion resistance of metals is essentially determined by the reactivity of the metal-environment interface. Therefore the chemical and structural characterization of surfaces and interfaces (cf. Chapter 3) and the smdy of their electrochemical behavior in a given environment (cf. Chapter 4) are important aspects of corrosion science. 

Standards and recommended practices covering different aspects of corrosion have been developed by a number of professional organizations as ASTM, NACE International, ISO and many national bodies. Useful information on the corrosion resistance of materials for a given application can often be obtained from suppliers and from the scientific and technical literature. A number of tables and databases published in the open htcrature [1- 4] present the intrinsic corrosion resistance of materials in different environments. We must keep in mind, however, that such data are not sufficient, because they do not include electrochemical interactions that can lead to localized corrosion or effects due to stress or wear. Engineers confronted with materials selection therefore need to have a basic understanding of corrosion mechanisms. Experience gained with similar equipment or installations is also a useful source of information to avoid corrosion problems. As a last resort, one may turn to laboratory testing. 

The relationship between a corrosion product layer and inhibitors has also been discussed, among others, by Lorenz [66], and Lorenz and Mansfeld [67]. These authors point out that in many practical systems, for instance aerated water and carbon steel, an interaction occurs between, in this case, iron oxide and the inhibitor to the point where the inhibitor is not only adsorbed on the oxide surface, but actually incorporated into the three-dimensional oxide layer. Clearly, three-dimensional or interphase inhibition caimot be achieved in short tests or by filming procedures. Furthermore, measuring techniques have to take into consideration the altered chemical and electrochemical conditions across such bulk interphase layers. It is unfortunate that this aspect of corrosion and corrosion inhibition has not received more attention, and it is suggested that this lack of attention has seriously held back all aspects of corrosion inhibitor applications and monitoring of effectiveness. 

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