Page 40 - Corrosion Engineering Principles and Practice

22 C h a p t e r 2 C o r r o s i o n B a s i c s 23

scale, for example, was only invented at the end of the nineteenth
century and objects made of this metal were still considered to be a
novelty when the 2.85-kg aluminum cap was set as the last piece of
the Washington Monument in 1884. Aluminum was then considered
to be a precious metal.
The energy difference between metals and their ores can be
expressed in electrical terms that are in turn related to heats of
formation of the compounds. The difficulty of extracting metals from
their ores in terms of the energy required, and the consequent
tendency to release this energy by corrosion, is reflected by the relative
positions of pure metals in a list, which is discussed later as the
electromotive series in Chap. 4.

2.2 Matter Building Blocks
Since metals are the principal materials which suffer corrosive
deterioration, it is important to have an understanding of their atomic
organization in order to fully understand corrosion.
Metals as well as all materials are made up of atoms; metals are
also composed, of course, of those smaller particles which make up
the atoms. These numerous particles arrange themselves so that those
bearing positive charges or those which are neutral cluster together
to form a nucleus around which negatively charged particles or
electrons rotate in orbits or shells.
Chemical shorthand exists to express these atomic states. For
example, Fe is the chemical shorthand for a neutral atom of iron,
whereas Fe denotes an iron atom that has been stripped of two
2+
3+
electrons and is called a ferrous ion or Fe(II). Similarly, Fe denotes
an iron atom stripped of three electrons and is called a ferric ion or
Fe(III). The process of stripping electrons from atoms is referred to by
electrochemists as oxidation. Note that the term oxidation is not
necessarily associated with oxygen.
An opposite process can also occur in which extra electrons are
added to the neutral atom giving it a net negative charge. Any increase
in negative charge (or decrease in positive charge) of an atom or ion
is called reduction.
Many chemical compounds, such as salts, are made up of two or
more ions of opposite charge. When these are dissolved in water, they
can readily split into two or more separate ions which display equal
but opposite charges. This process is also called ionization. It is these
particles that are responsible for the conduction of electric currents in
aqueous solutions.
For a non reacted atom, the negative particles exactly balance the
positive charges present in the atomic nucleus. The electrons occupy
shells in an orderly fashion to balance the positive charge of the nucleus.
The electrons in the outermost shell are called valence electrons.