Page 106 - Corrosion Engineering Principles and Practice

80 C h a p t e r 4 C o r r o s i o n T h e r m o d y n a m i c s 81

• Passive region: In such region a metal tends to become coated
with an oxide or hydroxide that may form on the metal either
as a compact and adherent film practically preventing all
direct contact between the metal itself and the environment,
or as a porous deposit which only partially prevents contact
between the metal and the environment.
• Corrosion region: Thermodynamic calculations indicate that,
in such region of an E-pH diagram, a metal is stable as an
ionic (soluble) product and therefore susceptible to corrosion
attack. Experience is required to find out the extent and form
of the corrosion attack that may occur in the corrosion
region(s) of a Pourbaix diagram.

These three regions are indicated in Fig. 4.14 where the chemical
reactivity of pure aluminum is noticeable by the small size of
its immunity region. However, there is fortunately a band of stable
oxy-hydroxide product at all potentials above the immunity region at
neutral pH values. As mentioned earlier, the Pourbaix diagram of
zinc is quite similar. Other metals such as beryllium, gallium, indium,
and cadmium also have this column of corrosion product stable
through the highest potential on a Pourbaix diagram.
The presence of a protective oxide layer on aluminum is the main
reason why aluminum alloys are so broadly used with success in
indoor and outdoor environments provided they fall within the
passivation potential/pH boundaries shown in Fig. 4.14. The
aluminum susceptibility to corrode in both acidic and basic
environments is referred to as an amphoteric behavior. While the
aluminum oxide will form naturally on aluminum, it is common
practice to produce this oxide in a controlled process called
anodization. As described in Chap. 5, the quality and properties of
the protective oxide can thus be greatly enhanced, providing various
finishes for a multitude of applications.

Iron E-pH Diagram
Figure 4.15 illustrates the E-pH diagram for iron at 25°C in the
presence of water or humid environments. This diagram was
calculated by considering all possible reactions associated with iron
in wet or aqueous conditions listed in Table 4.13, excluding therefore
drier forms of corrosion products such as magnetite (Fe O ) or iron
3
4
(ferric) oxide (Fe O ). The various stability regions for these drier
3
2
corrosion products are shown in Fig. 4.16 where the predominant
compounds and ions are also indicated.
At potentials more positive than −0.6 and at pH values below
2+
9, ferrous ion (Fe or Fe II) is the stable substance. This indicates
that iron will corrode under these conditions. In other regions of
the iron E-pH diagram, it can be seen that the corrosion of iron