Page 346 - 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

to account for the buffering effects:

PSI = 2 (pH ) − pH
s eq
where pH is still the pH at saturation in calcite or calcium carbonate
s
pH = 1.465 × log [Alkalinity] + 4.54
10
eq
−
2−
−
[Alkalinity] = [HCO ] + 2 [CO ] + [OH ]
3 3
The Larson-Skold index is based upon evaluation of in situ corrosion
of mild steel lines transporting Great Lakes waters. Extrapolation to
other waters than the Great Lakes, such as those of low alkalinity or
extreme alkalinity, goes beyond the range of the original data. The
2−
index is the ratio of equivalents per million (epm) of sulfate (SO )
4
and chloride (Cl ) to the epm of alkalinity in the form bicarbonate
−
plus carbonate:
Larson-Skold index = (epm Cl − + epm SO 2− )/
4
(epm HCO − + epm CO 2 − )
3 3 (8.32)
The index has proven a useful tool in predicting the aggressiveness
of once-through cooling waters. The Larson-Skold index might be
interpreted by the following guidelines:
• Index < 0.8 chlorides and sulfate probably will not interfere
with natural film formation.
• 0.8 < index < 1.2 chlorides and sulfates may interfere with
natural film formation. Higher than desired corrosion rates
might be anticipated.
• Index > 1.2 the tendency toward high corrosion rates of a
local type should be expected as the index increases.
The Stiff-Davis index attempts to overcome the shortcomings of
the LSI with respect to waters with high total dissolved solids and
the impact of “common ion” effects on the scale formation driving
force. Like the LSI, the Stiff-Davis index has its basis in the concept
of saturation level. The solubility product used to predict the pH at
saturation (pHs) for a water is empirically modified in the Stiff-
Davis index. The Stiff-Davis index will predict that water is less
scale forming than the LSI calculated for the same water chemistry
and conditions. The deviation between the indices increases with
ionic strength. Interpretation of the index is by the same scale as for
the LSI.
The Oddo-Tomson index accounts for the impact of absolute
pressure and partial pressure of carbon dioxide on the pH of water,
and on the solubility of calcium carbonate [20]. This empirical model
also incorporates corrections for the presence of two or three phases
(water, gas, and oil). Interpretation of the index is by the same scale as
for the LSI and Stiff-Davis indices.