Developing a
feedwater chemistry program that will minimize corrosion across a variety of
metallurgies doesn’t have to be difficult. This article reviews the
requirements for three common metallurgies in condensate and feedwater piping
and the chemistry options that operators have to minimize corrosion in this
critical area of the plant.
Alloys found in
the condensate and feedwater systems of power plants include carbon steel for
piping, pumps, and in some cases heat exchangers. Many systems still have some
copper-based alloys from admiralty brass, and copper-nickel (Cu-Ni) alloys all
the way to 400 Series Monel, primarily as feedwater heater tubes.
The major
corrosion mechanisms affect the carbon steel and copper alloys. These include
flow accelerated corrosion (FAC) and corrosion fatigue in carbon steel as well
as ammonia-induced stress corrosion cracking, and ammonia grooving in copper
alloys. FAC can have a variety of appearances (Figures 1 and 2).
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1 Typical. Classic flow-accelerated
corrosion (FAC) orange peel texture with no oxide coating. Courtesy:
M&M Engineering Associates Inc.
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2. Atypical. Compare the previous example
with this one showing an unusual pattern of FAC in a deaerator. Courtesy:
M&M Engineering Associates Inc.
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Gradually, as
aging feedwater heaters are replaced, plants often choose to go with a
stainless steel alloy such as 304 or 316 for feedwater tubing. When the last
copper feedwater heater is replaced, a change in feedwater chemistry is in
order.
Stainless Steel
Stainless steel
is protected by a tight adherent chromium oxide layer that forms on the
surface. Stainless steels alloys are resistant to essentially all the corrosion
mechanisms that commonly affect copper and carbon steel alloys in feedwater.
There is the
tendency to think that stainless steel is the perfect alloy to replace
copper-alloy feedwater heaters. However, stainless steel has its own Achilles
heel: Chlorides can cause pitting, and chloride and caustic have, in some
cases, led to stress corrosion cracking (SCC).
Typically,
these chemicals are not present in sufficient concentration to cause corrosion
on the tube side of feedwater heaters. However, there are cases where
contamination of the steam that feeds the shell side of the stainless
steel–tubed heat exchanger has resulted in SCC.
Remember, it is
not the average concentration of the chloride or caustic that is of concern.
Spikes in contamination can collect and concentrate in the desuperheating zone
of the shell side of the feedwater heater and in crevices. These are the areas
that can fail, even if the steam is pure most of the time. Where there is a
potential for chloride or caustic contamination of the steam, stainless steels
may not be the best fit or, at a minimum, alloys should be considered that have
a higher resistance to chloride attack, such as 316 or 904L. In general
however, it may be more productive to work on eliminating the potential for
contamination than to alloy around the problem.
The most
commonly quoted downside to the replacement of copper-alloy feedwater heater
tubes with stainless steel is the difference in thermal conductivity. A quick
look at the reference values will show that a 304 stainless steel has only
one-seventh the thermal conductivity of admiralty brass and about one-third the
conductivity of 90-10 Cu-Ni alloy. Numerous papers have been published
discussing why these “textbook” values are unlikely to be experienced in the
real world. This is certainly an important consideration with condenser tubes,
where the potential for cooling water–side deposits and condenser cleanliness
is likely to have a much more prominent effect on heat transfer than the
textbook thermal conductivity of the tube metal. However, feedwater heater
tubes should have little steam- or water-side fouling. Other factors, such as
tube thickness may offset some of the thermal conductivity loss, and there are
other design factors, such as susceptibility to vibration damage, to consider
in selecting a material.
Carbon Steel
Carbon steel is
passivated by the formation of a dual layer of magnetite (Fe3O4). The layer
closest to the metal is dense but very thin, whereas the layer closest to the
water is more porous and less stable. Hydroxide ions are necessary for the
formation of magnetite. Due to the common utility practice of using feedwater
to control the final temperature of superheat and reheat steam, the source of
hydroxide in feedwater must be volatile, and ammonia or an amine is generally
used for this purpose. A solid alkali such as sodium hydroxide must never be
introduced ahead of where the takeoff to the attemporation is located.
Ammonia is very
volatile, remaining in gaseous state during initial condensation. This may
occur in the deaerator, condenser, or on the shell side of a feedwater heater.
This lowers the effective pH of the first condensate and increases the
solubility of the magnetite layer in that area. This can increase the rate of
FAC in these areas.
For carbon
steel, higher pH values are better for the production and stability of
magnetite. Operating with low pH values in the feedwater and condensate
destabilizes magnetite and increases the rate of FAC on carbon steel in the
feedwater system. It also increases the iron in the feedwater, which generally
winds up on the waterwall tubes. This iron deposition increases the risk of
under-deposit corrosion mechanisms, inhibits heat transfer across the tube, and
increases the frequency of chemical cleaning.
A case can be
made for the use of carbon steel feedwater heater tubes, particularly alloys
such as T-22, which contains 2.25% chromium (Cr) and 1% molybdenum (Mo). It has
better thermal conductivity than stainless steel, is highly resistant to
chloride SCC, and because it contains 2.25% Cr, is generally considered immune
to FAC.
Copper Alloys
Copper alloy
corrosion in the power industry has been studied in depth due to problems with
copper deposits on the high-pressure (HP) turbine that reduced turbine
efficiency and the maximum load that the unit could produce.
Zinc-containing
brass alloys such as admiralty brass are particularly susceptible to attack
from ammonia vapors. This can result in ammonia-induced SCC on the steam side
of the condenser or feedwater heater. The same alloys are susceptible to a
mechanism termed “ammonia grooving,” where steam and ammonia condense on the
tube sheet and support plates of the feedwater heater and run over the tubes,
creating a narrow group of corrosion directly adjacent to the tube sheet or
support plate. Copper alloys containing nickel are far less susceptible to
ammonia-induced SCC.
Admiralty brass
alloys have the additional concern of corrosion of zinc in the alloy due to
low-pH conditions in the feedwater or steam. Over time, the zinc can leach from
the brass matrix, leaving only the copper sponge, which has little structural
strength. This mechanism is called dezincification. Although not as common,
copper-nickel alloys can also suffer from dealloying (Figure 3).
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3. Weakened. Dealloying, dezincification in
brass alloys, or removal of nickel from copper-nickel alloys will destroy the
strength of the material. Courtesy: M&M Engineering Associates Inc.
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There are three
separate rates associated with the rate of corrosion of any copper alloy. These
have been referred to as:
·
Rd—the rate at which corrosion products leave the surface as a
dissolved species in the water (typically copper ammonium complexes).
·
Rf—the rate at which corrosion products (copper oxides in
operating steam and condensate systems) form on the surface of the metal.
·
Rs—the rate at which copper corrosion products (typically
oxides) leave the surface as suspended particles.
These rates are
not necessarily correlated with each other and may not occur under the same
chemical conditions. Copper oxide formation (Rf) can be protective, minimizing
further corrosion of the alloy—as long as it remains intact. When chemical
conditions change, such as moving from an oxidizing to a reducing condition, Rd
and Rs may increase dramatically. Protective copper oxides are aggressively
dissolved by the combination of ammonia, carbon dioxide, and oxygen. The most
common place for all three of these to be present is in a copper-tubed
condenser that has air in-leakage issues.
Once these
corrosion products are dissolved or entrained, they are subject to downstream
chemical conditions, where a change in the at-temperature pH or the oxidation
reduction potential (ORP) in a specific location can cause the copper to “plate
out” as copper metal on suction strainers, pump impellers, or on another
feedwater heater tube surface in the form of a pure copper “snakeskin.” They
may also continue on through the feedwater system and deposit on a boiler or
superheater tube or on the HP turbine. Similar conditions (plating out) can
occur in stainless steel sample lines, making the accurate measurement of
copper corrosion products in a conventional sample line difficult.
Chemical
Control of Feedwater
Proper alloy
selection, either in the initial construction or as equipment is replaced,
should be carefully considered. Once the decision is made, the water chemistry
program must follow to minimize corrosion of the feedwater equipment and
deposits in the boiler and turbine. The more metals there are in the mix, the
more things need to be considered in the chemistry program. Copper alloys, in
particular, force compromises, as the optimum chemistry requirements for copper
and iron cannot be met simultaneously.
Feedwater pH
Control. The pH limits recommended on all ferrous-alloy condensate and
feedwater piping are now a minimum of 9.2 with an upper limit of 9.8 or even
10.0 in systems with an air-cooled condenser. If there are no copper alloys in
the system, the biggest downside to having too much ammonia in the system is
the frequent replacement of cation conductivity columns rather than corrosion
in the carbon steel.
For those
operating heat-recovery steam generators (HRSGs), there can be a significant
drop in pH of the low-pressure (LP) drum water as ammonia (and some amines)
leaves with the LP steam. It is important that the LP drum pH be monitored
continuously and controlled certainly within the range of 9.2–9.8. Some suggest
a minimum pH of 9.4 for water in the LP drum to protect downstream
high-pressure and intermediate-pressure economizers.
The current
recommended pH range for systems that have copper in either the main condenser
or feedwater heaters is 9.0–9.3. (See the sidebar for an explanation of the
necessity of accurate pH measurement.) Laboratory studies have shown that is
actually the minimum range for avoiding copper corrosion in the copper alloys
used in feedwater heaters and condensers. Lower feedwater and condensate pH
values (for example, pH 7.0) have higher copper corrosion rates than pH 9, particularly
under oxidizing conditions.
Measuring pHAccurate pH measurement in
high-purity water is difficult. The very low specific conductivity of the
water combined with the potential for ammonia to be lost and carbon dioxide
to be simultaneously absorbed by the sample while it is being collected and
measured can lead to confusing results. Inaccurate pH monitoring can result
in over- or under-feeding of ammonia or amines.
Continuous online pH monitoring
using pH probes specifically developed for high-purity water can improve the
accuracy and reliability of the measurement.
The pH of high-purity waters
can also be calculated from a combination of the specific conductivity and
cation conductivity results. This can be done manually, or there are
commercially available instruments that display a calculated and measured pH.
Due to these issues with pH,
specific conductivity is often used to control the ammonia feed instead of
controlling directly from a pH meter.
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Ammonia or
Amines. The addition of ammonia to condensate is the simplest and most
direct way to raise the pH of the condensate and feedwater into the desired
range to create and stabilize the magnetite layer. In all-ferrous systems,
there should be a clear case or desired objective for using any other chemical
for pH control. On the other hand, the use of neutralizing amines in the
utility steam cycle has a long, successful history, particularly in units that
have copper alloys in the feedwater heaters.
The decision to
use neutralizing amine for iron corrosion should be based primarily on the need
to provide more alkalinity (a higher pH) in an area of concern than can be
achieved simply by increasing the ammonia levels. This may include areas where
steam is first condensing into water, such as in an air-cooled condenser, or
where water/steam mixtures are being released, such as in the deaerator.
Although amines
are more common when copper alloys are found in the feedwater system or
condenser, their presence does not necessarily require the use of a
neutralizing amine. There are many mixed-metallurgy units that operate using
ammonia and that carefully control air in-leakage with very low copper
corrosion rates.
The choice of
which neutralizing amine to use (and there are many) should be based on where
and how it is to function. It is critical that both the basicity (amount of pH
rise per ppm of amine) and volatility of the amine (the ratio of what goes into
the steam versus what remains in the water) is matched to the application.
The criticism
of the general use of amines in high-pressure utility cycles is centered on two
issues: the degradation of these organic molecules in the steam cycle
(particularly in the superheater and reheater) and the consequence of these
degradation products—namely, an increase in the cation conductivity of the
condensate and feedwater.
It has been
long known that as neutralizing amines pass through the steam cycle, they break
down into ammonia and organic acid byproducts such as acetic acid, formic acid,
and carbon dioxide. The percentage of degradation is certainly specific to the
particular amine and concentration in the steam, but it is also unit specific
and depends, at a minimum, on the size and complexity of the superheater and
reheater piping, where it appears most of the degradation occurs.
Those who
advocate for the sole use of ammonia instead of amines point to the degradation
of these products and see them as “single-use” chemicals—good for only one trip
around the steam cycle. If all the amine degrades with one trip through the
superheater and reheater, it cannot be available to minimize the corrosion of
copper condenser tubes or affect the pH of a steam/water mixture in the
feedwater, and so it would not be worth the trouble.
However, there
are many different factors that affect amine degradation rates and, therefore,
how beneficial an amine might be in the system. These include the operating
pressure of the unit, where the copper alloys are located, and whether the unit
even has a reheater. For example, in the standard triple-drum HRSG, a
significant percentage of the amine may leave with the LP steam, where it recycles
through the condenser and preheater sections of the HRSG and never sees the
high-temperature areas. This would significantly increase its longevity and
usefulness.
All these
factors need be taken into account when considering whether an amine would be beneficial
at a particular plant. It would behoove anyone who is considering trying an
amine to set up to sample and test for the amine and degradation products
around the cycle and also quantify improvements to iron and copper corrosion
rates. That will help them determine, for their particular unit, if the
benefits of amine use outweigh the costs.
The degradation
products of any amine will add to the cation conductivity of the condensate and
feedwater. The longevity and chemical structure of the amine will affect the
cation conductivity “bump” that the plant will experience. Degassed cation
conductivity can remove carbon dioxide but generally not all the other organic
acids produced by amines. So if amines are used, the normal cation conductivity
will need to be adjusted for the presence of these products.
Controlling
Oxidation Reduction Potential
It can be
generalized that the ability of an alloy to withstand corrosion is a function
of the stability and tenacity of the oxide layer that forms on the metal surface.
As discussed above, stainless steel has a very tight and tenacious layer of
chromium oxide that prevents corrosion of the metal from oxygen and from the
common pH ranges found in feedwater.
Establishing
and maintaining a good oxide layer on carbon steel is critical to minimizing
FAC. Copper oxides are also protective—as long as they remain in place.
Particularly in
the case of copper alloys, the oxide layer can be easily disrupted. Research
has shown that one of the most corrosive times for copper alloys is when they
cycle between a reducing and oxidizing condition. Therefore, it is imperative
that mixed-metallurgy feedwater systems contain sufficient reducing agent such
as hydrazine or carbohydrazide to maintain a reducing condition at all times.
A reducing
condition is not the same as the absence of dissolved oxygen. Regardless of how
well the deaerator is functioning, if there are copper feedwater heaters in the
system, the continuous addition of a reducing agent is required to achieve the
negative ORP that is protective of copper alloys.
All volatile
reducing agents used in utility cycles break down at temperatures typically
associated with HP feedwater heaters or the economizer—and certainly by the
time the water reaches the boiler. Therefore, regardless of which reducing
agent is added to the condensate pump discharge, there is no protection for the
copper alloy condenser tubes against the combined effect of dissolved oxygen,
carbon dioxide, and ammonia. This is why it is so critical to minimize air
in-leakage and control feedwater pH.
Many units have
been replacing copper alloy feedwater heaters with carbon steel or stainless
steel tubes over the years. When the last copper feedwater heater is replaced,
the reducing agent can almost always be eliminated, regardless of whether the
condenser contains copper alloys or not.
Carbon steel
corrosion is inhibited by the presence of small amounts of dissolved oxygen.
Research has shown that as little as 5 ppb to 10 ppb of dissolved oxygen
significantly reduces the rate of FAC under feedwater conditions. This occurs
because the dissolved oxygen present in the low-temperature feedwater (from the
condenser to the deaerator) forms iron oxides that fill in the pores of the
outer layer of the magnetite, dramatically improving its stability. Even in the
absence of any measurable dissolved oxygen, after the deaerator, the ORP
remains positive and increases the stability of the magnetite layer through the
HP feedwater heaters and economizer.
The formation
of these more resilient protective oxides is the basis of oxygenated treatment,
which is successfully used on all supercritical plants in North America and
many HP drum units. However, simply discontinuing the use of a reducing agent
should never be confused with oxygenated treatment, where pure oxygen is purposefully
injected, the deaerator vents are closed, and the dissolved oxygen levels in
the feedwater are an order of magnitude higher than in a conventional feedwater
system.
Stable
feedwater chemistry in the absence of a reducing agent continues to strengthen
the passive oxide layer throughout the feedwater piping over time. Therefore,
although dissolved oxygen levels may temporarily spike during a startup, it is
also unnecessary to add a reducing agent during layup or for the subsequent
startup. ■
— David
Daniels is a POWER contributing editor and senior principal scientist
at M&M Engineering Associates Inc.
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