Page 51 - Corrosion Engineering Principles and Practice

32 C h a p t e r 2 C o r r o s i o n B a s i c s 33

Note that each atom of a substance that appears on the left-hand
side of these equations must also appear on the right-hand side.
There are also some rules that denote in what proportion different
atoms combine with each other. As in the preceding reaction, the
sulfate ions that are an integral part of sulfuric acid do not participate
directly to the corrosion attack and therefore one could write both
Eqs. (2.11) and (2.13) in a simpler form:
(
Zn + 2H → Zn 2+ + H g) (2.14)
0
+
2

Many other metals are also corroded by acids, often yielding
soluble salts and hydrogen gas, as shown in Eqs. (2.15) and (2.16) for
iron and aluminum, respectively:
Fe + 2H → Fe 2+ + H g) (2.15)
+
0
(
2

0
2Al 6H → 2Al 3+ + 3H g) (2.16)
+
+
(
2

Note that zinc and iron react with two H ions, whereas aluminum
+
reacts with three. This is due to the fact that both zinc and iron, when
corroding, lose two electrons and display two positive charges in
their ionic form. They are said to have a valence of 2, whereas
aluminum loses three electrons when leaving an anodic surface and
hence displays three positive charges and is said to have a valence
of 3. Some metals have several common valences, others only one.
Figure 2.9 shows some of the oxidation states found in compounds of
the transition-metal elements.
2.4.2 Corrosion in Neutral and Alkaline Solutions
The corrosion of metals can also occur in fresh water, seawater, salt
solutions, and alkaline or basic media. In almost all of these
environments, corrosion occurs importantly only if dissolved oxygen

Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Tc Ru Rh Pd Ag Cd La Hf Ta W Re Os Ir Pt Au Hg
8
7 6
Oxidation State 5 4

2 3
1
0
21 25 30 39 44 48 57 72 75 80
Atomic Number
FIGURE 2.9 Oxidation states found in compounds of the transition-metal
elements. A solid circle represents a common oxidation state, and a ring
represents a less common (less energetically favorable) oxidation state.