Page 343 - Corrosion Engineering Principles and Practice

314 C h a p t e r 8 C o r r o s i o n b y W a t e r 315

level increases beyond 1.0, the driving force for the precipitation of
calcium carbonate increases.
The following sections describe some indices that are used to
indicate the tendency of given waters to deposit scales on metal
substrates and by extension to predict the corrosivity of specific waters.
Generally speaking, scales precipitated onto metal surfaces can provide
protection of the substrate from general corrosion. If on the other hand,
the scales are defective and contain voids and/or cracks, they could
lead to localized corrosion. However, the assumption that water below
saturation with respect to calcium carbonate is corrosive, while
occasionally correct, is not always reliable.

8.7.1 Langelier Saturation Index
The Langelier saturation index (LSI) is probably the most widely used
indicator of a water scale potential. This index indicates the driving force
for scale formation and growth in terms of pH as a master variable. In
order to calculate the LSI, it is necessary to know the alkalinity (mg L as
−1
2+
CaCO or calcite), the calcium hardness (mg L Ca as CaCO ), the total
−1
3
3
−1
dissolved solids (mg L , TDS), the actual pH, and the temperature of the
water (°C). If TDS is unknown, but conductivity is, one can estimate mg
L TDS using a conversion table (Table 8.15). LSI is defined as
−1
LSI = pH − pH s (8.26)
where pH is the measured water pH
pH is the pH at saturation in calcite or calcium carbonate and
s
is defined as:
pH = (9.3 + A + ) B − (C + D ) (8.27)
s
[Log (TDS) − 1]
A = 10 10 (8.28)

o
B = − 13.12 × Log ( C + 273) + 34.55 (8.29)
10
2+
C = Log 10 (Ca as CaCO ) − 0.4 (8.30)
3
D = Log (alkalinity as CaCO ) (8.31)
10 3
As for the SL reasoning described earlier, the LSI indicates three
situations:
• If LSI is negative: No potential to scale, the water will dissolve
CaCO .
3
• If LSI is positive: Scale can form and CaCO precipitation may
3
occur.
• If LSI is close to zero: Borderline scale potential. Water qual-
ity or changes in temperature, or evaporation could change
the index.