POWER April 2013 - 20

according to BAC Series V operating and maintenance instructions.
In other words, the coating may prevent white rust
formation on structural surfaces, but it will not prevent corrosion
of the tube bundle. Therefore, the chemistry guidelines
in Table 3 must still be followed.
Use Chemical Corrosion Inhibition. Until recently, there
Table 3. BAC chemistry guidelines. Source: BAC Series V
and Low Profile V-Cooling Towers, Closed Circuit Cooling Towers, and
Evaporative Condensers Operating and Maintenance Instructions
Parameter
pH
Hardness as CaCO3
Alkalinity as CaCO3
Total dissolved solids
Chloride as ClBaltibond
6.5
to 9.0
30-500 ppm
500 ppm max
1,200 ppm max
250 ppm max
Galvanized steel
7.0 to 9.0
30-500 ppm
500 ppm max
1,000 ppm max
250 ppm max
4. Good results. The initial test results of Nalco 73801WR have
been excellent. This figure shows the results of a trial performed in a
power plant closed-loop system. Source: Nalco Power ITC
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
11/4/2010 12/5/2010 1/5/2011
2/5/2011 3/8/2011
4/8/2011
Table 4. The chemistry of the makeup water and
cooling tower bulk water are compared with the Association
of Water Technologies guidelines summarized
in Table 2. Source: AWT, Nalco Power ITC
Parameter
pH
Hardness as CaCO3
Alkalinity as CaCO3
Chlorides as ClSulfates
as SO4
Conductivity
2Makeup
source Circulating water
8.7
4.6
210
120
110
1,000
9.3
39
1,000
460
390
4,000
Guideline
6.5 to 9.0
>30 ppm
<500 ppm
<250 ppm
<250 ppm
<2,400 micromhos
5. Tube protection. Surface corrosion on galvanized tubes was
prevented during recent testing. Courtesy: Nalco Power ITC
have been no water chemistry offerings to inhibit the formation
of white rust. However, Nalco researchers have developed
a chemical program that has proven to provide good
protection for galvanized steel systems. Nalco 73801WR is
a synergistic blend of passivation agents that can minimize
white rust corrosion. Because NALCO 73801WR is specifically
intended to inhibit white rust formation, it is designed to
supplement other treatment programs such as mild steel corrosion
inhibition and scale inhibition (Figure 4).
The corrosion rates for galvanized systems dropped significantly,
from 0.75 mils per year (mpy) to 0.1 mpy after the initial
feed of Nalco 73801WR on a test cooling tower. Although
the corrosion rate rises and levels off at 0.25 mpy, that still
translates into a 67% reduction in corrosion rate.
Another example involves a plant using a makeup water
source high in conductivity, alkalinity, sulfates, and chloride,
while operating at relatively high cycles of concentration
for a galvanized cooling tower. This tower is also
experiencing promising results using Nalco 73801WR (Table
4). This is a fairly new trial, so corrosion rates are not available
yet. However, photos show excellent protection of the
system (Figure 5).
Determine the Dose Rate. The typical dose rate for Nalco
73801WR is 50 ppm. However, halogen use rates and water
conditions may dictate higher dose rates. Because there is
no Trasar component in this chemistry, the additive dose rate
must be slaved to the other chemicals used in the program.
The efficacy of the program will be, as with any corrosion-inhibition
program, determined by monitoring corrosion
rates. Monitoring for corrosion rates can be achieved via the
following methods:
■ Galvanized steel corrators measure current flow across two
identical electrodes to which a small current is applied and
measured. From the current flow, corrosion rates can be
calculated and displayed online, from which trend lines can
also be produced.
■ Galvanized coupons of the same metallurgy can be used to
produce absolute corrosion rates and the most accurate measurement
of metal loss.
■ Visual inspections may be subjective, but frequent inspections
can prove to be very valuable in identifying problems before
they become severe.
■ Zinc coating testing is critical, as the coating is what you
are protecting. Trend data collection is also important to
help determine if the zinc coating is leaching into the system.
A baseline should be developed for zinc levels in the
system prior to introducing the program. Once the program
is established, zinc levels should decrease if the program is
working effectively.
Increased awareness of the potential pitfalls of using galvanized
components in cooling towers is needed in the power
industry. Though it is relatively inexpensive when compared
to stainless steel, galvanized steel demands a certain diligence
when developing a cooling water treatment program.
Specifically, end users need to be aware of the makeup water
source and the desired cycles of concentration in the cooling
tower. Only then can they make an informed decision about
a comprehensive water treatment regimen, including a corrosion
inhibition plan, prior to startup. ■
-Kevin Boudreaux (kjboudreaux@nalco.com) is a power industry
technical consultant for Nalco Co.'s Power Group.
20
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Galvanized steel probe (mpy)
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POWER April 2013

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