Power Plant Chemistry
Measurement Advancements:
Oxidation Reduction Potential
Abstract
It is important to understand that these methods all measure the
Measurements which show the potential for corrosion of a plant’s
oxidizing or reducing potential of the system with respect to the
metallurgy due to interaction with the system water have gained
water. However, each of the listed names above specify a
in importance within the last two decades. Among the most
method of obtaining the oxidation/reduction potential in a slightly
successful of these measurements are those which measure an
different manner.
actual potential which is based upon the oxidation/reduction
reactions taking place. The measurement of Oxidation Reduction
ORP, also called redox, utilizes a platinum measuring electrode
Potential (ORP) has been used extensively in both nuclear and
with a Ag/AgCl reference electrode to measure the potential.
fossil fuel power plants to show the potential for corrosion in a
Most fossil plants and the secondary side of some pressurized
system. Optimum ORP levels are dependent upon system
water reactor (PWR) nuclear plants make this measurement at
metallurgy and water chemistry treatment. The system reaction,
ambient temperature through a sampling system, similar to the
and thus the ORP response, will vary depending upon a number
measurements of pH, dissolved oxygen, or conductivity. Other
of factors, such as oxygen, hydrogen and oxygen scavenger
nuclear plants, especially those with a boiling water reactor
concentrations. Comparison of the oxidation/reduction potential
(BWR), make this measurement either in-situ (directly in the
at various points in the system is useful for observing total
process) or in a high flow, at-temperature autoclave system
system response to variations in chemical addition and system
which provides a sample which is exactly representative of the
transients which can lead to more corrosive conditions in the
process water.
water.
The ECP measurement typically utilizes a Ag/AgCl reference
Corrosion Potential
electrode and the system metallurgy itself as the measuring
Within the last 15 to 20 years, extensive work has been done in
both the nuclear and fossil fuel power industries to determine the
response of system metallurgies to water chemistry conditions.
Perhaps the most successful type of measurement used for
determining the corrosivity of the water to the system metal has
been the measurement of corrosion potential. This potential has
been observed to be affected by a number of factors, including
the activity of the oxidizing species (oxygen, copper), activity of
electrode. By doing this, the “actual” potential of the system itself
is determined. This measurement is utilized extensively within
the nuclear industry. BWR plants often use the ECP as a control
point for the addition of hydrogen, and as such have found that
this is a more accurate method for ensuring the correct dosage
into the system.
6,11
PWR plants often use the ECP on the
primary side to observe variations in the primary water during
6
startup, steady state, and shutdown conditions.
the reducing species (oxygen scavenger, hydrogen), type of
oxide layers present, temperature, and flow rate.
1-11
The measurement of corrosion potential utilizes a Ag/AgCl
reference electrode and a measurement electrode of the same
This method for obtaining a measurement of the potential for
corrosion utilizes a reference electrode, typically a silver/silver
chloride (Ag/AgCl) half-cell, and a measuring electrode, typically
platinum or some form of metal which is similar or identical to the
system metallurgy (such as a steel alloy). The names given to
this particular measurement vary extensively – corrosion
potential, oxidation/reduction potential (ORP), electrochemical
potential (ECP), or redox (another name for ORP).
metallurgy as that of the system metal (usually some form of
steel alloy). In theory, utilizing a measuring electrode similar to
the system metal, rather than platinum, should produce a
potential more representative of the system metal itself.
However, many plants have found the platinum electrode to
actually be equally or even more responsive and sensitive to
1,2,5,6,8
system changes.
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
In addition, the platinum electrode has been observed to maintain
m
X + pe
its responsiveness to ORP variations longer than the other
electrode types.1 This measurement is typically used most by
n
Y
-
2
⇒ Xm-p (reduction -- gain of electrons)
(1)
⇒ Yn+q + qe- (oxidation -- loss of electrons)
(2)
the nuclear industry, and is usually measured in conjunction with
either the ECP or ORP measurements.
m
m-p
The X represents the species which is reduced to X
by
n
gaining p electrons. The Y represents the species which is
n+q
The focus of this paper will be primarily on the ORP (redox)
oxidized to Y
measurement, although the results obtained from ECP and
electrons is reduced, and acts as an oxidizing agent. The
corrosion potential are mentioned, since these are usually seen
species which loses electrons is oxidized, and acts as a reducing
to be qualitatively, if not quantitatively, very similar. This
agent.
by losing q electrons. The species which gains
discussion will not address the obvious issues of differences in
results due to various sampling schemes, such as an ambient
In order for the whole oxidation-reduction reaction to occur, the
temperature sampling system, an in-situ installation, or a high
two half-reactions must occur simultaneously. If the example
flow, at-temperature autoclave.
above is simplified by assuming that both oxidation and reduction
reactions involve a single electron, Equation 3 can be written.
In addition, the mV potentials in this paper will be reported in two
ways. The first method will be to show the units as milliVolts
m
n
X +Y
⇔ Xm-1 + Yn+1 (standard redox equation)
(3)
(mV) followed by “SHE”, which stands for Standard Hydrogen
Electrode. This refers to a method of reporting the mV potential
If a solution is strongly oxidizing, it has a deficiency of electrons
with respect to a common standard, in this case the SHE. This
available and thus will attempt to acquire electrons. Likewise, if a
electrode is never used in industrial measurements, since it
solution is strongly reducing, it has electrons available and will
requires a fixed partial pressure of hydrogen for its use. By
attempt to give up electrons. The tendency for a solution to
convention, its potential is usually defined as 0.00 V, regardless
donate or accept electrons can be sensed as an electrical
of temperature. Since the Ag/AgCl is almost always used as the
potential on an inert electrode.
reference electrode in the above measurements, a conversion
can be made which shows the actual potential measured (using
If a solution is strongly oxidizing (electron accepting), it will
the Ag/AgCl) with respect to the SHE. The second method of
withdraw electrons from the measuring electrode’s metal surface,
reporting used will not have any designation following the mV
creating a positive potential on the electrode surface. The ORP
value. This refers to a potential measured with respect to the
will thus be more positive in solutions which have a higher activity
Ag/AgCl electrode at sampling temperature, regardless of
of oxidizing reactants than reducing reactants. Likewise, in a
whether the temperature was ambient or similar in temperature to
reducing solution, a more negative potential will exist since the
the process.
solution has electrons available which are donated to the surface
of the measuring electrode. The total potential measured is a
Oxidation-Reduction Potential
result of a multitude of redox reactions (some reversible and
Whether the measurement is ORP, corrosion potential, or ECP,
some not), temperature variations, non-ideal electrode response,
the basic measurement principle is the same. Chemical
and many other factors. It is therefore more useful to look at the
reactions which involve the transfer of electrons between
ORP reading from a comparison standpoint rather than a
reactants are known as oxidation/reduction, or redox, reactions.
theoretically accurate standpoint.
A species with lesser affinity for electrons in solution will lose
electrons, increasing its electrical charge. This species will
ORP in the Power Plant Water/Steam Cycle
become more positive in the oxidation reaction. This half-
ORP can be used at various points within the water/steam cycle
reaction is termed the oxidation, because the initial reactions
to establish a baseline for determining the potential for corrosion
studied during early investigations of oxidation/reduction involved
in the system. Figure 1 shows some of the common points
oxygen as the oxidizing species. A species with greater affinity
where ORP can be measured in a fossil-based plant. Sampling
for electrons in solution will accept electrons, reducing its
points in a nuclear power plant would be similar. ORP testing at
electrical charge. This half-reaction is termed the reduction,
various plants has shown the potential to vary most with changes
since the species becomes less positive. Equations 1 and 2
in dissolved oxygen and oxygen scavenger, since both affect the
summarize the half reactions taking place.
oxidizing or reducing environment of the system.
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
3
Testing at BWR plants employing hydrogen water chemistry have
Testing was done at this plant to observe the relationship
shown a direct relation between hydrogen and the corrosion
between higher oxygen levels and the ORP value. The ORP
potential as well. It is important to note that the complexity of the
value was relatively low (-300 mV SHE) during normal operation
reactions associated with these parameters is such that a change
of oxygen concentrations of less than 5 ppb. As the oxygen level
in any of the concentrations can potentially cause a variation in
was increased, the ORP level quickly increased to almost +75
the concentration of another.
mV SHE by the time 25 ppb oxygen was present in the system.
At 100 ppb of oxygen, the ORP value had leveled off at around
Dissolved Oxygen
+100 mV SHE. Further increases above 100 ppb showed little
Dissolved oxygen plays an important part of the corrosion
increase in the ORP.
process on metal surfaces in power plants. Oxygen is an
oxidizing substance, so it will directly affect the ORP levels in the
From the previous two examples it can be observed that oxygen
system water. Control points of the oxygen concentration vary
directly affects the ORP of a system, as would be expected since
among different water chemistry treatments, but can typically be
oxygen by definition will increase the oxidizing activity of the
summed up in two methods. The first method attempts to
system. ORP will therefore become more positive at higher
remove as much oxygen as possible (typically down to 5 ppb or
levels of oxygen. Figure 2 shows a theoretical example of what
lower) through some form of mechanical and/or chemical means.
might be expected from ORP readings in the condensate when
The second method maintains a higher level of oxygen (typically
oxygen is introduced into the condenser. Notice the direct
50 to 200 ppb). The type of method chosen will determine the
relationship between oxygen and ORP. Notice also that as
expected potentials.
oxygen levels exceed 100 ppb, the ORP does not increase much
further. This graph does not represent actual data at a plant;
At one PWR nuclear plant, oxygen concentration was maintained
6
below 10 ppb in the main condensate. Shutdown of two
condenser halves was done for cleaning purposes. This resulted
in an increase in condensate temperature which in turn caused
however, it does represent results of the oxygen vs. ORP
relationship that have been observed at numerous plants.
Oxygen Scavenger
an increase in the oxygen content of the condensate. The initial
Oxygen scavengers, such as hydrazine, serve a primary purpose
increase in oxygen was from about 10 ppb to as high as 50 ppb.
of chemically removing oxygen from the system. However, they
ORP measurements in the downcomer tube bundle were seen to
can also be useful for corrosion prevention since the presence of
increase from -200 mV SHE to -100 mV SHE. ORP
most oxygen scavengers will create a more reducing
measurements at the high pressure feed water heater were seen
environment in the system. The results of testing performed at a
to increase from around -50 mV SHE to as high as +100 mV
number of power plants, both nuclear and fossil based, show the
SHE. When the condenser was returned to normal operation,
response of ORP to changing oxygen scavenger concentrations.
the condensate oxygen concentration dropped back down to
about 20 ppb. The downcomer ORP dropped down to around -
At one nuclear PWR plant, hydrazine transients were observed
150 mV SHE, and the high pressure feedwater heater ORP
when flow was lost from the injection pumps.
dropped down to about +10 mV SHE. However, the measured
injection took place in the feedwater system, immediately after
oxygen at the feedwater remained constant at 2 to 3 ppb, and the
the point where the condensate demineralizers can be inserted
measured feedwater hydrazine concentration was seen to remain
into the lineup. The highest levels of hydrazine were observed in
constant at 130 ppb. The ORP was thus much more sensitive to
the feedwater system before there could be much reaction of
system changes at the feedwater and downcomer area than the
hydrazine with oxygen or other oxidizing contaminants, and
oxygen or hydrazine measurements.
before there could be any appreciable thermal breakdown of
1,10
Hydrazine
hydrazine. During this time, the ORP of the feedwater varied
The relationship between ORP and oxygen concentration was
2
only by 10 mV, while a large change was seen in the downcomer
observed at one fossil fuel plant. Oxygen excursions occurred
portion of the steam generators (see Table 1). The effect that
due to an air ingress, with the oxygen level spiking from 10 ppb
hydrazine had on the ORP of the downcomer water was directly
to around 1 ppm, and then leveling off to 150 ppb for about 4
related to the length of the excursion and the change in
hours. The feedwater ORP levels were seen to rise sharply two
hydrazine level. The relationship between more positive ORP
and a half hours later. The ORP increased 200 mV within about
readings and lower hydrazine concentrations was obvious. Note
half an hour, and then slowly decreased over the next six hours.
that the hydrazine levels are given for feedwater hydrazine.
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
4
A previous check of hydrazine levels had given downcomer
It should be noted that an increase in ORP with increasing
hydrazine levels of 54 and 79 ppb where feedwater was 100 ppb.
hydrazine dosage is not normal, and could possibly indicate
something other than hydrazine affecting the ORP reading.
It was noted at one fossil plant that sample locations with higher
hydrazine residuals experienced more reducing (more negative)
potentials.
4,12
The hydrazine concentration was initially 40 ppb
It is important to understand that when no oxygen scavenger is
present, the ORP value of the water is largely dependent upon
and was reduced over a period of time to zero. The ORP was
the oxygen concentration in the water. However, when the
seen to have a direct correlation with the level of hydrazine.
oxygen concentration drops much below 5 ppb due to oxygen
While the level of hydrazine was at 40 ppb, the ORP reading was
scavenger addition, the ORP value becomes more of a function
approximately -340 mV. When the hydrazine feed was stopped
of the oxygen scavenger concentration. Figure 3 shows a
completely, the ORP was seen to rise to about +80 mV, where it
theoretical example of what might be expected from ORP
leveled off. Large step changes of the hydrazine concentration
readings in the final feedwater when hydrazine concentration is
correlated to large step changes in the ORP. At another fossil
varied in the feedwater. Notice the inverse relationship between
plant, the ORP levels at the economizer inlet were recorded
hydrazine and the ORP value. Notice also that as hydrazine
before and after hydrazine feed was cycled on and off. An
levels exceed 100 ppb, the ORP does not decrease much
excellent correlation of ORP vs. hydrazine was observed, with
further. This graph does not represent actual data at a plant;
ORP readings of around -80 mV with hydrazine and +60 mV
however, it does represent results of the hydrazine vs. ORP
without hydrazine.
4
relationship that have been observed at numerous plants.
An interesting effect of the oxygen scavenger and ORP
Hydrogen Water Chemistry
relationship was noted at a few nuclear PWR plants. (Note:
In BWR nuclear plants, hydrogen is often added to the feedwater
Although reported as ECP, both plants tested ORP as well and
to avoid stress corrosion cracking of stainless steel piping. It has
reported qualitatively similar results. As such, the actual
been shown that the intergranular stress corrosion cracking
potentials were somewhat different in magnitude, but the
(IGSCC) of sensitized stainless steel can be minimized by
response was basically the same.) In the secondary side of one
maintaining the ECP below the critical potential, typically -230
plant, the ECP values were observed in the final feedwater for
mV SHE in high purity water.
8
6,11,13
Hydrogen addition to the
various concentrations of hydrazine residual. The ECP was
feedwater decreases the amount of oxidizing species in the
approximately -100 mV SHE when there was no hydrazine
reactor water by recombination in the downcomer, and thus
present in the water. As the hydrazine residual increased, the
reduces the production of oxidizing species from the radiolysis of
ECP value dropped exponentially until it leveled off at
water in the core.
approximately -500 mV SHE at 100 ppb of hydrazine. Increasing
creates a more reducing potential (more negative) due to
the hydrazine concentration beyond 100 ppb did not serve to
decrease of oxidizing species which results from the increased
reduce the ECP any further. A similar test was performed in the
hydrogen concentration. The potential of BWR water with
5
secondary side of another plant.
At two of the sample points
11,13
The addition of hydrogen to the feedwater
“normal” hydrogen water chemistry has been observed to be
(condensate pump discharge and low pressure feedwater pump
typically in the range of +50 to +200 mV SHE.
discharge), a similar effect to that seen at the other plant was
increased dosages of hydrogen will the potential be seen to drop
observed. The condensate sample point’s ECP did not vary from
down to well below the -230 mV range.
around +200 mV SHE until the hydrazine concentration was
observed that the potential can only be accurately measured in-
increased to above 15 ppb. When the hydrazine concentration
situ for proper hydrogen dosage control.
6,11
6,11,14
Only at
It has been
11
was increased above 15 ppb, the ECP began dropping
exponentially until it leveled off around -50 mV at 100 ppb of
hydrazine. At the feedwater sample point, the ECP dropped
linearly from 0 mV SHE at 0 ppb hydrazine down to -350 mV
SHE at 15 ppb hydrazine. When the concentration was
increased above 15 ppb, the ECP actually was observed to
increase slightly, leveling off at about -300 mV SHE at
approximately 100 ppb hydrazine.
Location
Typical power plant water has different properties, such as pH,
oxygen / oxygen scavenger concentrations, or temperature,
depending upon the location in the system. Because of the
varying characteristics of the water, as well as different
metallurgy used throughout the system, the ORP response can
change significantly from one point to another.
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
5
A change in chemistry in one portion of the system may cause no
The second transient was characterized by an inleakage of
change in the ORP at one point and drastically alter the readings
oxygen from systems connected to the feedwater line after the
at another point. This has been observed at many plants.
condensate pumps (and after the first point of ORP monitoring).
Although the majority of the ORP measurements have been
A decrease in hydrazine concentration was also noticed during
made in the feedwater or economizer, other points in the process
this type of transient. Each measuring point had a significant
are also important to monitor for better characterization of the
increase in potential during the time of the transient, with the
overall system response to varying changes, such as load
second point showing the most noticeable increase. The
increases, leaks, or excursions.
increase at points 1 and 3 were about the same. The same
5
8
effect was noticed at another plant. In both plants, the dissolved
At one nuclear plant utilizing a Pressurized Water Reactor
oxygen monitor for the feedwater showed no appreciable
(PWR), readings were taken at various points on two redundant
increases, while the potential measurements were seen to
systems on the secondary water.
1,10
These points included the
increase at multiple points.
Feedwater, Downcomer, Hotleg, Coldleg, and Demineralizer
Influent. An example of varying system responses can be seen
Locational testing during changes of hydrazine dosage was also
by examining Table 1. Testing of ORP response of the system to
performed at this plant. The hydrazine was dosed from after the
hydrazine excursions revealed an interesting system response
condensate pump, at a location immediately after the first
between the feedwater and the downcomer portions. As can be
sampling point. An immediate change was noticed in the second
seen from Table 1, feedwater hydrazine levels dropped
sampling point, as the potential dropped linearly from
considerably during excursions. The ORP response at the
approximately 0 mV SHE down to -350 mV SHE as the hydrazine
downcomer sample point was significant, typically rising by as
concentration was increased from 0 to 15 ppb. As the hydrazine
much as 100 mV during each excursion. However, the effect of
concentration was continually increased to as high as 150 ppb,
loss of hydrazine flow on ORP was minimal in the feedwater
the potential slowly increased from -350 mV SHE to about -300
system. ORP readings differed by no more than about 10 mV
mV SHE. The third sampling point remained at approximately -
each time.
400 mV throughout the range of 0 to 150 ppb hydrazine. The
5
first sampling began at approximately +200 mV SHE and
Another PWR-based nuclear plant measured the ORP response
remained constant until about 15 ppb of hydrazine had been
at the outlet of the condensate pumps (point #1), the outlet of the
introduced. At that dosage point, the potential began to drop in a
feedwater pumps (point #2), and the outlet of the high pressure
slow, exponential fashion to a final potential of around -50 mV
5
heaters (point #3). During steady state conditions, the potential
SHE at 150 ppb of hydrazine.
after the condensate pumps was typically -150 to -100 mV SHE.
The potential after the feedwater pumps was typically -350 to -
Another nuclear plant utilizing a Boiling Water Reactor (BWR)
200 mV SHE. The potential at the outlet of the high pressure
experienced high potentials (typically between 0 to +200 mV
heaters was typically -550 mV SHE.
SHE) during normal hydrogen water chemistry conditions.
11
These potentials did not vary at different points in the system.
However, upon addition of hydrogen at what was considered to
At the same plant, some transient conditions were introduced to
observe the effects on the potential at the three sampling points.
5
be normal rates for hydrogen water chemistry, the potentials first
The first transient was characterized by an increase in the
decreased at sample points distant from the core, while the
condensate oxygen concentration (typically 6 ppb) and a
potentials near to the core remained relatively high. The
decrease in the feedwater hydrazine concentration (typically 70
corrosion potential was thus seen to be considerably different at
to 100 ppb). This effect was caused by leakage of oxygen into
various points. As hydrogen dosage was increased to much
the condenser or the steam itself. The redox potential at point #1
higher than the normal rate, the entire system eventually dropped
was observed to increase by about 20-100 mV, with the larger
to a very low potential (typically less than -300 mV SHE), with
increases seen with larger increases in oxygen concentration and
little variance among sample points, although some higher
larger decreases in hydrazine concentration. Point #2 was
corrosion potentials were still seen in the core.
observed to increase in potential by about 10-50 mV,
characterized by the same situation as point #1. The third point
An interesting phenomenon was observed in measuring different
had no measurable increase in potential for any of the above
sample points at one fossil fuel-based plant that converted from
transient conditions.
All-Volatile Treatment to Oxygenated Treatment (OT).
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
6
During AVT treatment, ORP readings were taken at the Hotwell (-
The primary oxidation reaction on steel surfaces is the oxidation
60 mV), Final Feedwater (-100 mV), Boiler Water (-80 mV) and
and dissolution of iron:
Main Steam (-80 mV). Readings differed by more than 100 mV
at various sample points in the plant, and it was observed that
Fe ⇒ Fe + + 2e
2
-
(5)
the sample locations of more negative potentials had higher
4
hydrazine residuals. Upon switching to OT, the readings were
about +100 to +120 mV at each point.
12
The resulting common
readings were expected, since the water chemistry would be
The two primary reduction reactions on steel surfaces are the
reduction of hydrogen (Equation 6) and the reduction of oxygen
(Equation 7):
similar throughout the system.
+
2H + 2e
-
⇒ H2
It is obvious from the results seen in the previous examples that
the multiple location measurements can play an important part in
-
O2 + 2H2O + 4e
(6)
⇒ 4OH-
(7)
determining the overall cause and effect of various transients and
variations to water chemistry in the system. It is expected that
The reduction of hydrogen is seen to be favored at more
load variations on the system could also generate results that
reducing potentials while the reduction of oxygen is favored at
vary from point to point. Regardless of the cause for the
higher (more oxidizing) potentials.
variation, multiple points are useful for sensing overall system
point appears to be somewhat dependent upon the pH of the
response to process changes and upsets.
plant water.
2,7
15
The optimal ORP control
It has been shown that providing a less reducing
environment in all-ferrous plants at the recommended pH levels
ORP And Corrosion
(typically 8-10 pH depending upon the specific water chemistry
ORP has a strong relation to corrosion within a system, since the
treatment) will minimize corrosion product generation.
reactions associated with corrosion are oxidation/reduction
can be done by lowering oxygen scavenger levels or eliminating
based. Any oxidants or reductants present in the system are
the oxygen scavenger levels and adding oxygen. This process
directly linked to corrosion production in a system. Optimal
converts the magnetite (Fe3O4) to ferric oxide hydrate (FeOOH),
concentration limits of oxidants and reductants vary depending
which has a much lower solubility.
3,4,7,9
This
9
upon the type of metallurgy and the type of water chemistry
treatment being done at a plant. Figure 4 shows an example of
At one plant, the ORP increased from -340 mV to +100 mV when
the optimal control ranges for ORP at the feedwater sampling
the level of hydrazine was reduced from 40 ppb to zero. The
point in both nuclear and fossil plants for various water chemistry
total iron decreased from 14 ppb to about 5 ppb. At another
treatments.
plant, which has an all-ferrous system, hydrazine feed was
4
discontinued to reduce iron transport. The ORP was observed to
The trend in past years at most plants has been to remove as
increase from -125 mV to -50 mV when the hydrazine dosage
much of the dissolved oxygen as possible, which almost always
was dropped from 20 ppb to 0 ppb. The feedwater pH of 9.2 to
requires addition of an oxygen scavenger such as hydrazine.
9.6 remained the same during the test period. Iron transport
This can have a detrimental effect in all-ferrous plants, since
through the feedwater cycle did decrease from an average of
removing oxygen by oxygen scavenger will lead to a strongly
about 3 ppb to less than 1 ppb within an 8 month time period
reducing environment (-300 mV or lower), which has actually
following the change in treatment.
10
been seen to increase the erosion/corrosion of iron-based
materials as well as increasing the transported feedwater
3,4,9
corrosion products.
It has been observed that high levels of
Some plants with all-ferrous metallurgy have begun switching to
an Oxygenated Treatment (OT) in which a small concentration of
flow-accelerated corrosion (FAC) occur in all-ferrous plants when
dissolved oxygen, typically 30-150 ppb, is maintained in order to
the ORP is less than -300 mV due to either an oxygen level of
minimize corrosion. Conversion to OT will correspond to an
less than 1 ppb or oxygen scavenger level of greater than 20
oxidizing environment typically on the order of +100 mV or
ppb, or both.
9
more.
3,4
This has been shown to eliminate flow accelerated
corrosion by forming a protective oxide layer on the material
surface.
9
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
7
One plant experienced an increase of 500 mV during the
Conclusions
transition from reducing to oxidizing operating conditions, during
ORP has been seen to be useful in determining the response of
which the corrosion rates dropped to two to three times lower
the system metallurgy to the water chemistry. The ORP has
than those experienced during reducing conditions.2 This highly
been observed to be affected most by the activity of oxidizing
oxidizing environment is expected since no oxygen scavenger is
(oxygen) and reducing (oxygen scavenger, hydrogen) species in
present and higher concentrations of dissolved oxygen are
the water. The ORP of a system has been observed to be
present, both of which raise the ORP. All-ferrous plants
directly related to various types of corrosion, such as flow
switching to OT and thus more positive ORP levels have
accelerated corrosion (FAC) in all-ferrous plants, intergranular
experienced significant drops in corrosion products.
stress corrosion cracking (IGSCC) in BWR nuclear plants, or
cupric oxide formation in mixed metallurgy plants. The optimum
Systems with mixed metallurgy have been found to have minimal
ORP control point has been seen to vary somewhat from plant to
corrosion occurring when a more reducing (more negative)
plant, largely depending upon type of water treatment,
potential is present.
3,16
While understanding of copper alloy
corrosion is not yet adequate, a relationship with ORP has been
concentration of oxidizing and reducing species, type of
metallurgy, and location of the measurement.
established. Cuprous oxides (Cu2O) and cupric oxides (CuO)
can form on copper base alloys. Formation of cuprous oxide
ORP has been observed to be equally or more sensitive to
provides a protective barrier adjacent to the metal surface.
system transients as traditional measurements of hydrazine and
Cupric oxide can form by oxidation of the cuprous ions. Cuprous
oxygen concentration. It is expected that ORP will become a
oxide formation is thus preferred at lower reducing potentials,
standard measurement at multiple points throughout a plant’s
while a more oxidizing environment will support the growth of the
water/steam cycle, and will be used along with measurements of
cupric oxide. Reducing regimes are thus preferred in mixed
pH, oxygen and oxygen scavenger concentration in order to
metallurgy environments. Even when dissolved oxygen levels
optimize the water chemistry.
are kept below the 7 ppb limits typical of mechanical deaeration,
an oxygen scavenger such as hydrazine, which lowers the
reducing potential, should be maintained, or serious copper
deposition problems can occur.
3
This effect was observed at one plant which has a copper nickel
condenser and all-ferrous feedwater heaters.
10
Hydrazine
injection had been terminated in an attempt to reduce the
potential for erosion/corrosion and iron transport through the
boiler cycle. A slow loss in turbine efficiency was observed after
the change. It was theorized that oxygen in the boiler water
converted copper in boiler deposits allowing it to volatize and
deposit on the first stages on the HP turbine. Hydrazine
injection was then re-established with a goal on maintaining a
more reducing environment in the feedwater and boiler water.
It has been observed at many plants that optimization of the
hydrazine level in copper-based systems can be done with the
ORP measurement in order to prevent copper attack by
excessive hydrazine levels.6 ORP monitoring has been used
while chemically cleaning boilers to ensure the proper
environment to dissolve iron oxide, insure passivation of clean
surfaces, and prevent precipitation of copper. Monitoring has
also been used in ammoniated EDTA cleanings to prevent
11
corrosion following the iron removal stage.
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
8
Figure 1: ORP Sampling Points in a Fossil-based Power Plant
Saturated
Steam
H-P Turbine
Chemical
Feed
I-P Turbine
ORP
Condenser
Drum
Cooling Water
ORP
Hotwell
Super
heater
Economizer
Blowdown
Reheater
Boiler
Deaerator
ORP
ORP
H-P Heaters
Condensate
Polishers
L-P Heaters
ORP
ORP
200
Chemical
Feed
200
180
150
160
100
50
120
100
0
80
-50
ORP, mV SHE
Oxygen, ppb
140
Oxygen, ppb
ORP, Condensate
60
-100
40
-150
20
0
-200
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Time, hours
Figure 2: ORP vs. Oxygen Comparison for Condenser Oxygen Ingress*
* Note: This graph does not contain actual data from plant results. It represents an example of typical results that have been observed in a number of
plants
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
160
9
100
50
140
0
120
100
ORP, mV SHE
Hydrazine, ppb
-50
-100
-150
80
-200
60
Hydrazine, ppb
ORP, mV SHE
-250
40
-300
20
-350
0
-400
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Time, hours
Figure 3: ORP vs. Hydrazine Comparison for Changes in Hydrazine Dosage *
* Note: This graph does not contain actual data from plant results. It represents an example of typical results that have been observed
in a number of plants.
Range 3
Range 2
Range 5
Range 4
Range 1
-800 mV
-600 mV
-400 mV
-200 mV
Range 6
0 mV
100 mV
Figure 4: Typical Oxidation/Reduction Potentials
Range 1: Pressurized Water Reactor Nuclear Plant, Primary Side (mV, SHE)
Range 2: Boiling Water Reactor Nuclear Plant, High Hydrogen Addition (mV, SHE)
Range 3: Boiling Water Reactor Nuclear Plant, Normal Water Chemistry (mV, SHE)
200 mV
Power Plant Chemistry Measurement Advancements: Oxidation Reduction Potential
10
Range 4: (a) Pressurize Water Reactor Nuclear Plant, Secondary Circuit (mV, SHE)
(b) Mixed Metallurgy Fossil Fuel Plant, Oxygen Scavenger Addition (mV, Ag/AgCl)
Range 5: All-Ferrous Fossil Fuel Plant, No Oxygen Scavenger Addition (mV, Ag/AgCl)
Range 6: All-Ferrous Fossil Fuel Plant, Oxygen Addition (mV, Ag/AgCl)
Feedwater Hydrazine, ppb
Downcomer ORP, mV
Date
Time
Before
During
Duration
Before
During
10/2/94
5:00
110
20
2 Hours
-100
+10
11/3/94
15:30
115
30
20 Minutes
-60
+22
11/4/94
3:00
110
20
60 Minutes
-58
+86
11/9/94
13:30
140
40
40 Minutes
-58
+64
11/13/94
21:00
110
50
20 Minutes
-80
+10
Table 1: Downcomer ORP Excursions During Hydrazine Transients
More Information
For more information on ORP, visit
www.honeywellprocess.com, or contact your
Honeywell account manager.
Honeywell Process Solutions
Honeywell
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www.honeywellprocess.com
SO-13-18-ENG
January 2013
© 2013 Honeywell International Inc.