Page 342 - Corrosion Engineering Principles and Practice

312 C h a p t e r 8 C o r r o s i o n b y W a t e r 313

FIGURE 8.22 Fouled chiller unit. (Courtesy of Defence R&D, Canada-Atlantic)

8.7 Scaling Indices
The saturation level (SL) of water in a mineral phase is a good
indicator of the potential for scaling due to that specific scalant.
Saturation level is a ratio between the ion activity product (IAP) and
the thermodynamic solubility product (K ) of a specific compound in
sp
that water. For example, when calcium carbonate (CaCO ) is the
3
scalant SL is defined as
a ⋅a
2−
SL = Ca 2+ CO 3 (8.25)
K
sp
where a Ca + a ⋅ 2− is the IAP of the two ions involved in the formation
CO 3
of CaCO , that is, Ca and CO .
2+
2−
3
3
K is a measure of ionic concentration when dissolved ions and
sp
undissolved ions are in equilibrium. When a saturated solution of
sparingly or slightly soluble salt is in contact with undissolved salt,
equilibrium is established between the dissolved ions and the
undissolved salt. In theory, this equilibrium condition is based upon
an undisturbed water maintained at constant temperature and
allowed to remain undisturbed for an infinite period of time.
Water is therefore said to be undersaturated (SL value less than 1.0)
if it can dissolve the scalant, that is, calcium carbonate in the present
example. When water is at equilibrium, SL will be 1.0 by definition.
Supersaturated water (SL value greater than 1.0) will precipitate
calcium carbonate from water if allowed to rest. As the saturation