Oxidation Reduction Potential

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.


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.


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.


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).


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


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.


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


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


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 1250 West Sam Houston Parkway South Houston, TX 77042 Honeywell House, Arlington Business Park Bracknell, Berkshire, England RG12 1EB UK Shanghai City Centre, 100 Junyi Road Shanghai, China 20051 www.honeywellprocess.com

SO-13-18-ENG January 2013 © 2013 Honeywell International Inc.