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Is oxygen corrosive?
Yes. Oxygen is a
highly reactive
gaseous element. In
the presence of
steel, the corrosion
rate of oxygen
doubles for each
30°F rise in
temperature. For
example, in a boiler
system operating at
400 psig and 444°F,
the corrosion rate
for oxygen is 256
times more reactive
than at room
temperature.
How does oxygen
attack metal
surfaces?
Oxygen
forms localized
corrosion areas
referred to as
“pits.” This
distinctive
formation is readily
distinguishable from
acid attack, caustic
gouging or chelate
corrosion. Oxygen
pits can rapidly
“drill” through
metal surfaces,
leading to metal
fatigue and failure.
What is the oxygen
corrosion mechanism?
Oxygen corrosion is
an electrochemical
process similar to a
simple battery. Iron
dissolves at the
anode and releases
electrons which are
subsequently
consumed by oxygen
at the cathode. (See
Figure below)
Pitting is the
result of this
localized mechanism.
Anode: Fe0 ® Fe+2 +
2e–
Cathode: 2e– + H2O +
1/2O2 ® 2OH–
Where can oxygen
corrosion occur in a
boiler system?
Because oxygen is a
gas, it will “flash”
into the condensate
system, turbines,
and other steam
operated equipment.
Every metal surface
of the boiler system
is vulnerable to
oxygen attack.
Why is oxygen
corrosion a serious
problem?
As oxygen corrodes
the boiler metal, it
dissolves the iron
surface. This
weakens the metal
site, but more
importantly, sends
dissolved iron into
the boiler. This
dissolved iron can
deposit onto boiler
tubes, causing
overheating and tube
failure.
How can oxygen
corrosion be
minimized?
Oxygen control is
generally both a
mechanical and
chemical process.
The majority of
oxygen in the boiler
feedwater is
typically reduced to
less than 20 parts
per billion (ppb) by
heating the water to
reduce its
solubility and
releasing it out of
the system via
venting
(deaeration).
Since even very low levels of oxygen will cause corrosion, a chemical
scavenger is used to
supplement
mechanical
deaeration to reduce
the level to zero.
Can an oxygen
scavenger be used if
the plant currently
does not deaerate
the feedwater?
Yes. Oxygen
scavengers are
effective at high
oxygen levels, but
mechanical
deaeration is more
efficient and
cost-effective in
the long run. Oxygen
corrosion cannot be
completely inhibited
by chemicals alone.
There is no real
substitute for good
mechanical
deaeration.
Chemicals are most
effective when used
only to polish a
properly operating
system.
Should oxygen
scavengers be fed
with other products?
No. Oxygen
scavengers should
not be mixed with
other chemicals
because this may
result in a complete
loss of product
activity. The
addition of water to
a liquid oxygen
scavenger may also
result in a loss of
product activity.
Dilution water
contains oxygen,
which is consumed by
the scavenger and
reduces the overall
effectiveness of the
product. All liquid
oxygen scavengers
should be fed neat
(undiluted).
What types of sodium
sulfite are
available?
Sodium sulfite is a
highly effective
oxygen scavenger and
is available in
several forms:
Uncatalyzed sulfite
—
The reaction of
sulfite with oxygen
is very slow at
temperatures below
200°F.
Without a catalyst,
it takes up to ten
minutes to reduce
the oxygen content
of water from the
saturation point to
70% with sodium
sulfite at room
temperature. This
type of product is
recommended only
when the application
cannot tolerate the
presence of a cobalt
catalyst (catalyst
beds, food
applications, etc.).
Catalyzed sulfite
— The
catalyst, usually a
cobalt salt, speeds
up the reaction rate
of sulfite at low
temperatures
(<240°F). A fully
saturated oxygen
solution can be
completely
deoxygenated in as
little as 30
seconds. A potential
drawback is that the
catalyst can be
deactivated or
precipitated by
improper feed
application, such as
water dilution or
mixing with caustic.
Decharacterized
sulfite
—
This is a term used
by the USDA. It
refers to specific
chemicals which must
be removed from
sodium sulfite for
the product to be
USDA and FDA
approved. Sulfite
which is not
decharacterized has
the ability to give
meat a red or
“fresh” appearance.
What are the
disadvantages of
using sodium
sulfite?
Several
costly disadvantages
are inherent with
using sodium sulfite
as an oxygen
scavenger. These
problems occur
regardless of
whether the sulfite
is catalyzed or
uncatalyzed.
• Adds solids to the
boiler — Additional
solids to the boiler
contributes to an
increased blowdown
requirement.
Blowdown is
literally “money
down the drain.” It
is a direct loss
of energy and increases water treatment costs.
• Cannot be used for
spray attemperation
— The solids
contribution of
sodium sulfite would
cause superheater
and turbine
deposits. Also,
since chemical feed
must occur
after the attemperation water take-off point, all equipment
upstream of that
point would be
subject to oxygen
corrosion and costly
repairs.
Cannot be used at
boiler pressures
above 900 psig.
Sodium sulfite
begins to decompose
at approximately 600
psig and is complete
at 900 psig. The
decomposition
products are H2S and
SO2, both highly
corrosive gases,
which can cause
catalyst poisoning
and severe corrosion
on steam operating
equipment.
What is metal
passivation?
Metal passivation
has traditionally
been considered to
be the reduction of
hematite to
magnetite in
iron-based boiler
tubes. Actually, it
is a process by
which bare metal
surfaces form a
protective oxide
film. The passive
film is very thin
and dense. It is
distinguishable from
the base metal by
the coloration.
In the case of
carbon steel, this
protective film is
magnetite (Fe3O4)
and is black in
color.
Can oxygen
scavengers enhance
metal passivation?
Yes. Hydrazine has
traditionally shown
passivation
properties and Nalco
has developed
several proprietary
products which
effectively enhance
metal passivation.
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