Corrosion general theory

Few, if any, failure mechanisms have received as much attention as stress-corrosion cracking (SCC). Yet despite an enormous research effort over many years, an acceptable, generalized theory that satisfactorily explains all elements of the phenomenon has not been produced. SCC is a complex failure mechanism. Nevertheless, its basic characteristics are well known, and a wealth of practical experience permits at least a moderately comfortable working knowledge of the phenomenon. 

This more generalized theory has some important experimental and practical implications. In particular it allows greater use of polarization data for the interpretation of corrosion phenomena and for the determination of the rate of corrosion during the corrosion process. For purposes of discussion we may designate this as the mixed potential technique. 

RB Mears, RH Brown, EH Dix, A Generalized Theory of Corrosion in Alloys, in Symposium on Stress-Corrosion Cracking of Metals, American Society for Testing Materials, West-Conshohocken, Pennsylvania, 1945, 323. 

The mechanisms of stress corrosion have been the subject of numerous studies. They are so complex that it seems to be difficult or even impossible to develop a general theory [20]. 

Meats R.B., Brown R.H., Dix E.H., A generalized theory of stress-corrosion of alloys. Symposium on Stress-Corrosion Cracking of metals ASTM andAIME, 1944, p. 323-344. 

A corrosion inhibitor is an admixture to the concrete used to prevent the corrosion of embedded metal. The mechanism of inhibition is complex, and there is no general theory applicable to all situations. 

A detailed discussion of galvanic corrosion between dissimilar metals in contact in a corrosive environment has been given in Section 1.7, but in the case of coating discontinuities the effect of the anode/cathode area relationship and the nature of any corrosion products formed at small discontinuities may modify any choice made on strict considerations of general galvanic corrosion theory based on the potentials of the coating and substrate in the environment under consideration. 

Rp data are meaningful for general or uniform corrosion but less so for localized corrosion, including MIC. In addition, the use of the Stem-Geary theory where the corrosion rate is inversely proportional to Rp at potentials close to is valid for conditions controlled by electron transfer, but not for the diffusion-controlled systems frequently found in MC. 

These sort of problems make it difficult to obtain reliable high temperature data on the aqueous chemistry of transition metal ions. Unfortunately the necessary timescales for even the simpler experimental studies are frequently too long for a Ph.D. student to make reasonable progress in 3 years from scratch or for industrial researchers to make much reportable progress before the patience of those supporting the work is exhausted. Results can be reported far more rapidly from, for example, corrosion experiments and since corrosion theories are in general of so little predictive value, each relevant alloy/electrolyte combination needs its own study. In such circumstances it is hardly surprising that thermodynamic studies have been (with a few notable exceptions) relatively poorly supported, while corrosion data continue to be amassed without any reliable thermodynamic framework within which to understand them. 

As corrosion products develop, the rate of diffusion of O2 reduces and, in theory, the (general wastage) corrosion rate slows down, but in practice the presence of deposits tends to promote various forms of localized corrosion involving oxygen, such as tuberculation. 

Given sufficient quantitative information about the electrochemical processes occurring, mixed potential theory can be used to predict a corrosion rate. Unfortunately, in the vast majority of cases, there are few data that can be applied with any confidence. In general, experimental measurements must be made that can be interpreted in terms of mixed potential theory. The most common of these measurements in electrochemical corrosion engineering is the polarization curve. 

There is some reason for optimism in spite of the sub-embryonic character of our knowledge at this point. A number of studies have added information of consequence. By way of illustration, equations have been developed for unimpeded anodic reactions, among others, on the basis of electrochemical theory (IQ). Also the study of corrosion of whiskers (11) and of single crystal metals (1,9) has become somewhat more common. Studies on electrodeposition on whiskers (12) ami on dissolution and electrodeposition in copper-copper sulfate solutions (13) are sure to be pertinent. Certainly the recem Faraday Society Discussion on "Crystal Imperfections and Chemical Reactivity" (14) should prove very useful. Although it may nor. be especially pertinent to the metal-solution systems, the general trend of work and the tentative principles evolved should show the way to significant experiments in this field. 

In general, the approach to classification of mixtures as irritant or corrosive to skin when data are available on the components, but not on the mixture as a whole, is based on the theory of additivity, such that each corrosive or irritant component contributes to the overall irritant or corrosive properties of the mixture in proportion to its potency and concentration. A weighting factor of 10 is used for corrosive components when they are present at a concentration below the concentration limit for classification with Category 1, but are at a concentration that will contribute to the classification of the mixture as an irritant. The mixture is classified as corrosive or irritant when the sum of the concentrations of such components exceeds a cut-off value/concentration limit. 

Solid materials, in general, are more or less subject to corrosion in the environments where they stand, and materials corrosion is one of the most troublesome problems we have been frequently confronted with in the current industrialized world. In the past decades, corrosion science has steadily contributed to the understanding of materials corrosion and its prevention. Modem corrosion science of materials is rooted in the local cell model of metallic corrosion proposed by Evans [1] and in the mixed electrode potential concept of metallic corrosion proved by Wagner and Traud [2]. These two magnificent achievements have combined into what we call the electrochemical theory of metallic corrosion. It describes metallic corrosion as a coupled reaction of anodic metal dissolution and cathodic oxidant reduction. The electrochemical theory of corrosion can be applied not only to metals but also to other solid materials. 

Before examining in detail the theories of aqueous corrosion processes and the bases for making quantitative calculations of corrosion rates, it will be useful to develop qualitatively the major phenomena involved. The following sections review several general types of metal/corrosive-environment combinations, the chemical reactions involved, idealized mechanisms for the transfer of metal ions to the environment, and the electrochemical processes occurring at the interface between the metal and the aqueous environment. 

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