Page 345 - Corrosion Engineering Principles and Practice

316 C h a p t e r 8 C o r r o s i o n b y W a t e r 317

where A = [Log (TDS) − 1]/10 = 0.15
10
B = −13.12 × Log (°C + 273) + 34.55
10
= 2.09 at 25°C and 1.09 at 82°C
C = Log (Ca as CaCO ) − 0.4 = 1.78
2+
10 3
D = Log (alkalinity as CaCO ) = 1.53
10 3
Calculation at 25°C:
pH = (9.3 + 0.15 + 2.09) − (1.78 + 1.53) = 8.2
s
LSI = 7.5 − 8.2 = − 0.7
Hence no tendency to scale

Calculation at 82°C:
pH = (9.3 + 0.15 + 1.09) − (1.78 + 1.53) = 7.2
s
LSI = 7.5 − 7.2 = + 0.3

Hence slight tendency to scale

8.7.2 Other Indices
The Ryznar stability index (RSI) uses a correlation established between an
empirical database of scale thickness observed in municipal water systems
and associated water chemistry data. As with the LSI, the RSI has its basis
in the concept of saturation level. The RSI takes the following form:
RSI = 2(pH ) − pH
s
The empirical correlation of the RSI can be summarized as follows:
• RSI < 6 the scale tendency increases as the index decreases
• RSI > 7 the calcium carbonate formation probably does not
lead to a protective corrosion inhibitor film
• RSI > 8 mild steel corrosion becomes an increasing problem

The Puckorius scaling index (PSI) is based on the buffering capacity
of the water, and the maximum quantity of precipitate that may form
in bringing water to equilibrium. Water high in calcium, but low in
alkalinity and buffering capacity can have a high calcite saturation
level that increases the ion activity product. Such water might have a
high tendency to form scale, but the scale itself might be of such a small
quantity as to be unobservable. The water has the driving force but not
the capacity and ability to maintain pH as precipitate matter forms.
The PSI index is calculated in a manner similar to the RSI.
Puckorius uses an equilibrium pH rather than the actual system pH