Page 310 - Corrosion Engineering Principles and Practice

280 C h a p t e r 8 C o r r o s i o n b y W a t e r 281

pH is controlled by the carbon dioxide system described in the
following Eqs. (8.6) through (8.8).
CO + H O → H CO (8.6)
2
2
2
3
+
H CO → H + HCO − (8.7)
2 3 3
HCO 3 − → H + + CO 3 2− (8.8)
−
If OH is added to the system as a consequence of one of the above
cathodic processes, then the reactions described in the following Eqs.
(8.9) and (8.10) become possible, with Eq. (8.11) describing the
precipitation of a calcareous deposit.
CO + OH − → HCO 3 − (8.9)
2
OH − + HCO − → H O + CO 2− (8.10)
3 2 3
CO 2− + Ca 2+ → CaCO (s) (8.11)
3 3
The reactions represented by Eqs. (8.6) through (8.11) further
−
indicate that as hydroxyl ions (OH ) are produced, then reactions in
Eqs. (8.7) and (8.8) are displaced to the right, resulting in proton
production. This opposition to the rise in pH accounts for the buffering
capacity of seawater. Irrespective of this, however, these reactions
indicate that the buffering action is accompanied by the formation of
calcareous deposits on cathodic surfaces exposed to seawater as
shown in Eq. (8.11). It is the main reason why the behavior of CaCO
3
in seawater has been so extensively studied. Calcium carbonate
sediments are prevalent and widespread in the oceans in two
crystalline forms, that is, calcite and aragonite[12].
Magnesium compounds, Mg(OH) in particular, can also
2
contribute to the protective character of calcareous deposits.
However, calcium carbonate is thermodynamically stable in surface
seawater, where it is supersaturated, while magnesium hydroxide is
unsaturated and less stable. In fact, Mg(OH) would precipitate only
2
if the pH of seawater was to exceed a pH threshold of 9.5.
Since calcite and magnesium carbonate have similar structures,
these compounds tend to form solid solutions, the Ca:Mg ratio of
these compounds depend on the ratio of these ions in seawater.
Theoretical calculations suggest that calcite in equilibrium with
seawater should contain between 2 and 7 mol percent MgCO . But
3
although low magnesium calcite is the most stable carbonate phase in
seawater, its precipitation and crystal growth are strongly inhibited
by dissolved magnesium. Consequently aragonite is the phase that
actually precipitates when seawater is made more basic. The degree
of saturation for aragonite is described in Eq. (8.12):

K sp, aragonite = (Ca )(CO 3 2− ) (8.12)
2+