Corrosion behaviour

As far as corrosion behaviour is concerned, prestressing steel needs to be distinguished from reinforcing steel with regard to hydrogen embrittlement, since it only affects the former this has been illustrated in Chapter 10. 

In non-carbonated and chloride-free concrete, the passivity of low-alloyed steels is not influenced appreciably by their composition, stracture or surface conditions. Therefore, the usual thermal or mechanical treatments or the roughness of the surface of the rebars have negligible influence on their corrosion behaviour. 

Even the presence of magnetite scale that often covers the surface of the bars, which can cause dangerous localized attack on steel in contact with neutral solutions (such as fresh water or seawater), is not dangerous in concrete. In fact, non-carbonated and chloride-free concrete passivates all the surface of the steel. If adherent oxide films are present, they do not create problems. If the oxide layer contains chlorides, because for example it is formed in a marine environment, it must be removed completely because it can hinder passivation. 

Once the steel becomes active due to carbonation of concrete or chloride penetration, the influence of chemical composition, microstructure and surface finish- 

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As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

The corrosion behaviour of different constituents of an alloy is well known, since the etching techniques used in metallography eu e essentially corrosion processes which take advantage of the different corrosion rates of phases as a means of identification, e.g. the grain boundaries are usually etched more rapidly than the rest of the grain owing to the greater reactivity of the disarrayed metal see Sections 1.3 and 20.4). 

The terminology suggested can be illustrated by reference to the corrosion behaviour of iron ... 

The defect 7-structures may be stabilised by the presence of Li or ions (e.g. LiFejOg). Cation diffusion rates in these and other lattices developed on metal surfaces play an important role in governing corrosion behaviour. 

Before considering specific examples it is appropriate to note that there are, in principle, two quite distinct ways in which crystal defects can affect corrosion behaviour. 

Secondly, crystal defects might be expected to affect the corrosion behaviour of metals which owe their corrosion resistance to the presence of thin passive or thick protective films on their surface. The crystal defects and structural features discussed in Section 20.4 might, in principle, be expected to affect the thickness, strength, adhesion, porosity, composition, solubility, etc. of these surface films, and hence, in turn, the corrosion behaviour of the filmed metal surfaces. Clearly, this is the more common situation in practice. 

There is no evidence that any particular crystal structure is more readily corroded than any other. For example, the difference in the corrosion behaviour of austenitic and ferritic stainless steels is, of course, due to compositional rather than structural differences. 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

Pourbaix, M., Klimezak-Mathieu, Martens, C. and Meunier, J., Potentiokinetic and Cor-rosimetric Investigations of the Corrosion Behaviour of Alloy Steels , Corros. Sci., 3, 239 (1963)... 

Wilde, B. E. and Williams, E., The Relevance of Accelerated Electrochemical Pitting Tests to the Long Term Pitting and Crevice Corrosion Behaviour of Stainless Steels in Marine Environments , J. Electrochem. Soc., 118, 1056 (1971)... 

Herbsleb, G. and Schwenk, W., Flow Dependence of the Pitting Corrosion of Cr-Ni Steel in NaCl Solution. 2 Tests with Ultrasonics , Werkst. Korros., 24, 267 (1973) C.A., 79, 56638n El Din Shams, A. M., Bodran, M. M. and Khalil, S. E., Corrosion Behaviour of Manganese-containing Stainless Steel. 3 Their Susceptibility Towards Pitting Corrosion , Werkst. Korros., 24, 290 (1973) C.A., 79, 56642j... 

Danek, G. J., The effect of seawater velocity on the corrosion behaviour of metals . Naval Engineers Journal, No. 763, (1966)... 

Examples of some of these effects and the resulting mass transfer erosion corrosion behaviour are shown in Figure 1.92. 

Briefly the important developments in copper alloys with respect to their erosion corrosion behaviour in seawater have been ... 

The numerous metals and alloys used in practice show such a wide variation in response to various anions in acid and alkaline solutions that common features are difficult to discern and a basis for predicting corrosion behaviour is not very apparent. 

In some metal components it is possible to form oxides and carbides, and in others, especially those with a relatively wide solid solubility range, to partition the impurity between the solid and the liquid metal to provide an equilibrium distribution of impurities around the circuit. Typical examples of how thermodynamic affinities affect corrosion processes are seen in the way oxygen affects the corrosion behaviour of stainless steels in sodium and lithium environments. In sodium systems oxygen has a pronounced effect on corrosion behaviour whereas in liquid lithium it appears to have less of an effect compared with other impurities such as C and Nj. According to Casteels Li can also penetrate the surface of steels, react with interstitials to form low density compounds which then deform the surface by bulging. For further details see non-metal transfer. 

In the case of CaCl2 and NaCl, the order corresponds with the corrosion behaviour expected from cathodic polarisation curves . The order of aggressiveness of chlorides can also be explained on the basis of redox potentials of the melts, calculated on thermodynamic grounds from the free energies of formation of the appropriate oxides and chlorides . The order of aggressiveness of nitrates is complicated by passivity effects , while that of alkalis in contact with air is... 

Despite the introduction of new, improved methods of refining it has been necessary to enhance the performance of lubricants by the use of additives, either to reinforce existing qualities or to confer additional properties. Once additives were regarded with some suspicion —an oil that needed an additive was necessarily an inferior oil today they are an accepted feature of lubricants. Almost all quality lubricants on sale today contain one or more additives. An enormous range of additives are available for use in lubricants " , some produced by the oil companies and others provided by specialist manufacturers. Additives are usually named after their particular function, but many additives are multifunctional. Thus, an anti-wear additive may also protect a surface against corrosion. The main types of additives that can enhance the anti-corrosion behaviour of lubricants are listed in Table 2.22. 

Table 2.22 Additives that can enhance the anti-corrosion behaviour of lubricants...

Hakansson, B., Yontchev, E., Vannberg, N.-G. and Hedegard, B. An Examination of the Surface Corrosion State of Dental Fillings and Constructions. 1. A Laboratory Investigation of the Corrosion Behaviour of Dental Alloys in Natural Saliva and Saline Solutions , Journal of Oral Rehabilitation, 13, 235-246 (1986)... 

Bundy, K. J., Vogelbaum, M. A. and Desai, V. H., The Influence of Static Stress on the Corrosion Behaviour of 316L Stainless Steel in Ringer s Solution , Journal of Biomedical Materials Research, 20, 493-505 (1986)... 

Revie, R. W. and Greene, N. D., Corrosion Behaviour of Surgical Implant Materials 1 Effects of Sterilisation , Corrosion Science, 9, 755-761 (1969)... 

Sury, P., Corrosion Behaviour of Case and Forged Implant Materials for Artificial Joints, Particularly with Respect to Compound Designs. Research and Development Department, Sulzer Brothers Ltd., CH-8401, Winterthur, Switzerland. 

The basic corrosion behaviour of stainless steels is dependent upon the type and quantity of alloying. Chromium is the universally present element but nickel, molybdenum, copper, nitrogen, vanadium, tungsten, titanium and niobium are also used for a variety of reasons. However, all elements can affect metallurgy, and thus mechanical and physical properties, so sometimes desirable corrosion resisting aspects may involve acceptance of less than ideal mechanical properties and vice versa. 

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