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...)