Page 37 - Corrosion Engineering Principles and Practice

CHAPTER 2

Corrosion Basics

2.1 Why Metals Corrode
The driving force that causes metals to corrode is a natural consequence
of their temporary existence in metallic form. In order to produce
metals starting from naturally occurring minerals and ores, it is
necessary to provide a certain amount of energy. It is therefore only
natural that when these metals are exposed to their environments they
would revert back to the original state in which they were found.
A typical cycle is illustrated by iron. The primary corrosion product of
iron, for example, is Fe(OH) (or more likely FeO·nH O), but the action
2
2
of oxygen and water can yield other products having different colors:
• Fe O ·H O or hydrous ferrous oxide, sometimes written as
2
2
3
Fe(OH) , is the principal component of red-brown rust. It can
3
form a mineral called hematite, the most common iron ore.
• Fe O ·H O or hydrated magnetite, also called ferrous ferrite
4
3
2
(Fe O ·FeO), is most often green but can be deep blue in the
3
2
presence of organic complexants.
• Fe O or magnetite is black.
3 4
The energy required to convert iron ore to metallic iron is returned
when the iron corrodes to form the original compound. Table 2.1
describes the results of x-ray diffraction of products found on
specimens exposed to real environments where it can be seen that the
metals often revert to naturally occurring mineral forms during the
corrosion process [1]. The amount of energy required and stored in a
metal or that is freed by its corrosion varies from metal to metal. It is
relatively high for metals such as magnesium, aluminum, and iron,
and relatively low for metals such as copper, silver, and gold. Table 2.2
lists a few metals in order of diminishing amounts of energy required
to convert them from their oxides to metal.
The high reactivity of magnesium and aluminum expressed as
energy in Table 2.2 is paralleled by the special efforts that were
historically required to transform these metals from their respective
ores. The industrial process to produce aluminum metal on a large
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