Page 19 - Corrosion Engineering Principles and Practice

2 C h a p t e r 1 T h e S t u d y o f C o r r o s i o n 3

extensive use of iron for weaponry and other artifacts that were, of
course, highly subject to rust and corrosion.
Pliny described corrosion phenomena taking place at the surface
of metals, as well as remedies for minimizing the effects of corrosion.
His reference to the use of oil as a means of protecting bronze objects
against corrosion, as well as of allowing the soldering of lead surfaces,
has been unambiguously verified by modern chemical analysis of
Roman artifacts. According to Pliny, surfaces act as bodies that interact
with each other and external agents. Pliny also speculated on the
causes of metal corrosion (air and fire).
Numerous scientific and engineering discoveries have been made
since then and the general understanding of corrosion mechanisms
has progressed with these. Some of the discoveries that have improved
the field of corrosion are listed in App. A (Historical Perspective). By
the turn of the twentieth century the basic processes behind the
corrosion of iron and steel were relatively well understood. One of
the first modern textbooks on corrosion prevention and control was
published by McGraw-Hill in 1910 [3]. The following are some
excerpts that illustrate the state of knowledge when this landmark
text came out.

On the Theory of Corrosion
In order that rust should be formed iron must go into solution and
hydrogen must be given off in the presence of oxygen or certain
oxidizing agents. This presumes electrolytic action, as every iron ion
that appears at a certain spot demands the disappearance of a
hydrogen ion at another, with a consequent formation of gaseous
hydrogen. The gaseous hydrogen is rarely visible in the process of
rusting, owing to the rather high solubility and great diffusive power
of this element. Substances which increase the concentration of
hydrogen ions, such as acids and acid salts, stimulate corrosion, while
substances which increase the concentration of hydroxyl ions inhibit
it. Chromic acid and its salts inhibit corrosion by producing a
polarizing or dampening effect which prevents the solution of iron
and the separation of hydrogen.

Electrolytic Theory of Corrosion of Iron
From the standpoint of the electrolytic theory, the explanation of the
corrosion of iron is not complicated, and so far has been found in
accordance with all the facts. Briefly stated, the explanation is as
follows: Iron has a certain solution tension, even when the iron is
chemically pure and the solvent pure water. The solution tension is
modified by impurities or additional substances contained in the
metal and in the solvent. The effect of the slightest segregation in the
metal, or even unequal stresses and strains in the surface, will throw
the surface out of equilibrium, and the solution tension will be greater
at some points than at others.