Historical Theories on Corrosion

by: Ulick R. Evans, 1948

The chronological sequence of scientific discovery is rarely the logical one. To arrange the facts of metallic corrosion historically would conceal the true interconnection existing between them, and thus deprive them of significance. Nevertheless, in view of the prevailing interest in the History of Science, many readers may welcome a short narrative showing how knowledge of the subject discussed in this book has grown. The note which follows should serve to indicate some names and dates associated with the advance of understanding, but it must be remembered that the credit for any particular discovery cannot be assigned to a single year or to a particular person. If a recent investigator is cited as the discoverer, objection may fairly be raised by the quotation from older papers of passages which seem to contain the germ of the idea ; yet to allot the entire credit to early investigators may be unjust to later ones, who have established as facts what had previously been mere suggestions.

At the Dawn of History, the first metals to be used were those which were either found native, or could easily be reduced to the elementary state ; such metals do not readily pass into the combined state, and their corrosion can have raised no serious problems. But with the introduction of iron, the problem of its corrosion must have presented itself, although it is an undoubted fact that some of the iron produced in Antiquity is today more free from corrosion than much of that manufactured in later years. This may have been due partly to the fact that iron reduced with charcoal contained less sulphur than modern steel, but it may also be connected with the absence of sulphur compounds from the air in the days before coal was adopted as a fuel ; for it is often the conditions of early exposure which determine the life of metal-work. Whatever the cause, ancient iron has in some cases remained in surprisingly good condition for many centuries ; the Delhi Pillar is the example which has excited most interest, but others could be quoted.

For many centuries there seems to have been little curiosity regarding the causes of corrosion, although a few significant observations were made. As early as 1788, Austin noticed that water, originally neutral, tends to become alkaline when it acts on iron. He attributed the alkalinity to the compound now called ammonia ; this was probably an error, since the alkaline reaction produced by most saline waters is due to sodium hydroxide, the cathodic product of the electrochemical corrosion process.

The belief that corrosion is an electrochemical phenomenon was expressed in a paper published in 1819 by an anonymous French writer, thought to be Thenard, and in 1830 his compatriot, de la Rive, attributed the fact that acid attacks impure zinc more rapidly than the relatively pure varieties to an electric effect set up between zinc and the impurities present. Faraday's researches, especially those conducted between 1834 and 1840, afforded evidence of the essential connection between chemical action and the generation of electric currents ; indeed (since his Laws of Electrochemical Action apply as much to anodic as to cathodic processes) he provided a quantitative basis for the observations of later investigators. One of the most interesting chapters of Faraday's work was concerned with the study of passivity-the subject of a famous correspondence in 1836 with Schonbein, then Professor of Chemistry at the Swiss University of Bale (Basle). The experiments described suggested that the strange inactive condition which Schonbein had observed on iron was favoured by anodic action, but was often dispelled by cathodic treatment ; this was all the more remarkable in that normally anodic action favours corrosion whilst cathodic action tends to prevent it.

After Faraday's time, interest in the electrical mechanism of corrosion processes seems to have waned. This may have been due to the fact that electrochemistry was hardly ready to be applied to the detailed elucidation of corrosion, until the conception of Single Electrode Potentials had been made familiar by publications emerging from the schools of Ostwald and Nernst and culminating in an important paper by Wilsmore (1900). But in Great Britain, at least, attention may have been diverted by alternative suggestions ascribing corrosion to the presence or formation of certain substances. Some of these suggestions are now seen to possess a -modicum of truth, but the underlying ideas are themselves consistent with an electrochemical mechanism.

(to be continued...)

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