[DFENGINEER]

Figure 2 — In the basic corrosion cell,
iron dissolves at the anode while oxygen
reacts to form hydroxide ions at the
cathode
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 Nalco chemicals can be
used for oxygen scavenging?
• Sodium sulfite based:
NALCO® 19P NALCO® 2549
NALCO® 1720 NALCO® 7408
NALCO® 1722
• Proprietary formulations:
NALCO® 1700 NALCO® 1721
NALCO® 1710 ELIMIN-OX®
2
Oxygen Corrosion
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.
(See Figure 1.) Oxygen pits
can rapidly “drill” through metal
surfaces, leading to metal fatigue
and failure.
by oxygen at the cathode. (See
Figure 2.) 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.
Oxygen Scavenging
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.
Figure 1 — Oxygen attack occurs as a
distinctive pit formation
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

Yacine Ouharzoune
Tech-sales engineer
MiSWACO a SMITH SCHLUMBERGER COMPANY

[eugenedakin]

Hello Yacine,

Feel free to send me an e-mail on the company intranet, I can answer your questions there. Feel free to look up my work number in outlook and call me at the Houston office.

Sincerely,

Eugene Dakin