POWER July 2011 - 52

COOLING TOWERS
6. Follow-up test data. Corrosion data collected three years after repairs were made on
each set of cooling towers found that the corrosion rates were significantly reduced from the
as-found condition. Source: Restruction Corp.
2005
300
250
200
150
100
50
2B04 2B08 1A02 1A07 1A09 1B05 1B09 1A05
Sample location
2005
-50
-100
-150
-200
-250
-300
Sample location
2005
350
300
250
200
150
100
50
2B04 2B08 1A02 1A07 1A09 1B05 1B09 1A05
Sample location
The corrosion measurement plan was to
measure corrosion current, corrosion potential,
and concrete resistance at two fan stack
locations on Unit 2. These locations would be
considered pre-repair and serve as the baseline.
Corrosion rates and potentials were not
taken during the investigation phase. It was
apparent that corrosion was occurring and
the source of initiation was the high concrete
temperatures and moisture levels. Furthermore,
no future monitoring of the corrosion
was planned during the investigation phase.
Measuring corrosion rate, potential, and
concrete resistance at seven locations on Unit
1 would serve as the immediate post-repair
condition and provide a comparison to prerepair
condition measured at Unit 2.
A Galvapulse corrosion rate meter was
used to record approximately 12 readings
per assembly. The results are illustrated in
Figure 5. An immediate concrete resistance
52
1A04 2A04
2A08
effect was measured. All seven Unit 1 assemblies
measured post-repair had higher
concrete resistance than the two Unit 2 prerepair
assemblies. On average, resistance
increased 135%.
Corrosion rate measured in micro-meters
per year also showed immediate improvement.
Six of seven post-repaired corrosion
rates were below both pre-repaired rates. On
average, corrosion rate was reduced 80%.
Corrosion potential showed immediate
improvement as well. Adjusted for the silver/silver
chloride half-cell, copper sulfate
electrode readings in the pre-repair were
-315mV CSE and -360 mV CSE (CSE or a
copper sulfate electrode is used as the baseline).
The average post-repair potential was
measured at -270 mV CSE. ASTM Standard
indicates that for potentials between -200
mV and -350 mV CSE, corrosion is " uncertain. "
Readings more positive than -200 mV
www.powermag.com
2008
2008
2B04 2B08 1A02 1A07 1A09 1B05 1B09 1A05
1A04 2A04
2A08
1A04 2A04
2A08
2008
indicate with 90% confidence that no corrosion
activity is present.
Three Years Later
The plant engineering team budgeted additional
funds for repair of Unit 2 assemblies to be completed
in spring/summer 2008. Three years had
passed since completion of Unit 1 repairs. Again,
corrosion measurements were taken upon completion.
The same two locations on Unit 2 were
measured, providing a direct comparison of
pre-repair (year 2005) and post-repair readings.
Five of the original seven assemblies at Unit 1
were remeasured. This provided a comparison
of immediate post-repair and repair-plus-threeyears'-time
readings. Two new locations at both
Unit 1 and Unit 2 were also recorded. Results
still indicated that the reinforcing steel was passive,
as shown in Figure 6.
Concrete resistance increased in both Unit
2 post-repair measurements and Unit 1 measurements
recorded in 2008 versus 2005. Average
concrete resistance increased over the
three-year operation period. Unit 1 resistance
increased from 49 kilo-ohms immediate
post-repair to 115 kilo-ohms measured three
years later.
A U.S. standard has not been developed
for corrosion current. However, a proposed
Norwegian standard indicates values of
11.5 to 58.0 micro-meters per year would
be considered low corrosion. Less than 11.5
micro-meters per year indicates negligible
corrosion. The Norwegian standard converts
corrosion rate units of micro-amps per centimeters
squared to cross-section loss of reinforcing
steel using Faraday's Law.
A low corrosion rate (11.5 to 58.0 micrometers
per year) was found in Unit 1 immediate
post-repair measurement in 2005, and
the 2008 measurement showed low to negligible
corrosion activity. The rebar in Unit
1 remains passive due to the implemented
repair scheme. Unit 2 assemblies that were
measured prior to repair in 2005 and then
again post-repair in 2008 recorded improved
corrosion currents in the low to moderate
corrosion activity levels.
The corrosion current levels have dropped
significantly. Corrosion potentials are also
expected to continue improving as the rebar
becomes more passive. These results demonstrate
that the repairs started in 2000 and
completed in 2008 were successful and the
towers' concrete will remain serviceable for
many years to come. ■
-Bruce A. Collins (bruce@restruction
.com) is vice president of Restruction
Corp. Portions of this article are reproduced
with the permission of the Journal
of Protective Coatings & Linings,
published by the Society for Protective
Coatings (www.sspc.org).
POWER | July 2011
Concrete resistance (kOhms)
Corrosion potential (mV)
Corrosion rate (micrometers/yr)

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POWER July 2011

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