Civil Engineering Reference
In-Depth Information
Langelier's approach for predicting CaCO
3
deposition was straightforward, and re-
ceived wide acceptance. Early experimentation with the index focused on its ability
to predict deposition, not on its precision as a corrosion measure. Its value as an actual
predictor of corrosion rates went essentially untested until the 1950s with the work of
Stumm, who attempted to correlate the predictive qualities of the index with physical
measures of corrosion rates.
13
These studies indicated that calcium carbonate deposi-
tion and metal corrosion were not necessarily related. A similar conclusion was reached
by Kuch in his studies of German water treatment practices and distribution system
corrosion.
14
Arguably, the most thorough study of the relationship between saturation indices
and corrosion was conducted by Singley et al.
15
In a series of tests conducted on several
metal types using both pipe loop and batch studies, they attempted to define just how
accurate a predictor of metal loss rates the Langelier index could be. Their results
showed that, on its own, the saturation index had little or no correlation with corrosion
rates, and hence has little value as a corrosion predictor. Moreover, their results suggest
that, at least for iron and zinc, pH, by itself, is a better predictor of corrosion rates
than the saturation indices; and, since pH is a major determinant of the of the indices,
it may account for the observed minimal association between corrosion rate and the
saturation value.
The fundamental supposition of the saturation index approach to corrosion con-
trol—that calcium carbonate films provide barrier protection for the underlying
metal—overlooks the fact that calcium carbonate scales are both porous and friable,
and unable to provide meaningful barrier protection in a distribution system unless
laid down to a depth that is hydraulically prohibitive. The supposition also disregards
the tendency of the corroding metal to develop its own scale, which will consist of
the oxidized metal, usually in a variety of mineral configurations. Such scales, while
not always providing barrier protection, may nonetheless effectively passivate the sur-
face. Calcium carbonate deposition may be incorporated into these scales, but there is
no evidence to suggest that its inclusion provides for more effective passivation.
Maintenance of a positive saturation value does have utility with regard to estab-
lishing water quality conditions that may minimize corrosion on some surfaces (i.e.,
elevated pH and buffer intensity). Saturation indices also retain their utility as an
operational tool in precipitation softening. However, it is important to stress that they
do not relate directly to corrosion and are not a useful tool in predicting actual cor-
rosion rates.
Direct Measures of Corrosion
Over the past decade a variety of techniques and analytical procedures have been
developed that allow a more direct measure of corrosion and its consequences. These
procedures make it possible to collect a variety of information about distribution sys-
tems and treatment processes that had not been previously available. Specific analytic
objectives often include:
1. Determining corrosion rates on the variety of distribution materials, including
consumer plumbing materials
2. Determining the concentration of corrosion by-products in water served to the
consumer
