Page 20 - Corrosion Engineering Principles and Practice

4 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 5

The points or nodes of maximum solution pressure will be electro-
positive to those of minimum pressure, and a current will flow,
provided the surface points are in contact, through a conducting film.
If the film is water, or is in any way moist, the higher its conductivity
the faster iron will pass into solution in the electro-positive areas, and
the faster corrosion proceeds. Positive hydrogen ions migrate to the
negative areas, negative hydroxyls to the positives.
On the Effects of Cold Work
A considerable body of evidence has been brought forward from time
to time to show that in addition to the segregation of impurities in
steel, the presence of scratches, sand pitholes, and, in fact, all
indentations or wounds on the surface of steel, will stimulate rusting
by becoming centers of corrosion. Such marks or indentations are
almost invariably electropositive to surrounding areas, and the
depolarization which results in the rapid disengagement of hydrogen
at these spots leads to stimulated pitting. This effect can be very
prettily shown by means of the ferroxyl indicator.*

On Puddle Iron and Steel
†
Mr. J. P. Snow, Chief Engineer of the Boston and Maine Railroad, has
called attention to a very significant case of corrosion in connection
with the destruction of some railroad signal bridges erected in 1894,
and removed and scrapped in 1902. These structures were built at the
time that steel was fast displacing puddled iron as bridge material.
The result was that the bridges were built from stock material which
was partly steel and partly wrought iron. The particular point of interest
in this case lies in the fact that while some of the members of the bridge
structures rusted to the point of destruction in eight years, others were
in practically as good condition as on the day they were erected.
Moreover, the tonnage-craze, from which the quality of product in
so many industries is today suffering, is causing to be placed on the
market a great mass of material, only a small proportion of which is
properly inspected, which is not in proper condition to do its work:
rails and axles which fail in service and steel skeletons for high buildings
which may carry in them the germs of destruction and death.

* The ferroxyl indicator is a mixture of two indicators used to reveal the nature
of surface corrosion on steel. Phenolphthalein in the ferroxyl indicator reveals
surface areas that are becoming basic and potassium ferricyanide which turns
blue in the presence of the iron (II) ions produced during corrosion. The use of
ferroxyl indicator will be discussed in more details in Chap. 7.
† Puddle iron is a type of wrought iron produced in a puddling furnace, a process
invented at the end the eighteenth century. The process results in an iron that
contains a slightly increased carbon content and a higher tensile strength
compared to wrought iron. The puddling furnace also allows a better control of
the chemical composition of the iron. The Eiffel Tower and many bridges were
built with puddle iron.