Corrosion
Corrosion is the deterioration of metal by
Chemical action. When dissolved oxygen is
present in the boiler water, corrosion begins and
continues until all metal has been transformed into
iron oxide or, commonly stated, rust. When rust
forms in the boiler, it may drop out as sludge or
cling to other metal surfaces. It is not economi-
cally possible to prevent at least some of the iron
in the boiler from going into solution. All iron
not protected by a coating or film of something
that keeps out moisture and air is sooner or later
going to become RUST. The idea is to slow down
the process as much as possible by KEEPING
OXYGEN OUT and by maintaining a proper
causticity residual.
The pH level of boiler water is also a factor
in corrosion. The active agent in the corrosion of
the internal water surface of boilers is oxygen;
however, the combined action of oxygen and the
acid action of the water are required for the
corrosion process. To suppress the acid action of
the water, you can raise the pH value of the water
by adding caustic soda. The lower the pH value,
the stronger the acid concentration. The higher
the pH value, the weaker the concentration.
Economically, acid corrosion cannot be stopped
completely, but it can be suppressed by keeping
oxygen out of the boiler and by maintaining a
proper pH value and causticity range.
Prevention and Treatment
for Oxygen Corrosion
The chemical most commonly used in oxygen
removal is sodium sulfite, and it is quite often
referred to as an oxygen scavenger. It is an
example of a chemical that actually reacts with
the harmful constituent. It reacts with oxygen,
forming a neutral compoundsodium sulfate.
When enough sodium sulfite is fed into a
boiler so that a surplus of the chemical is
maintained, any of the oxygen getting into the
boiler water is taken up by the chemical, and the
boiler water is kept virtually free of oxygen. By
maintaining a suitable residual, little, if any,
corrosion due to oxygen occurs. Common prac-
tice in feeding sodium sulfite is to maintain a
surplus residual of about 20 ppm to 50 ppm in
the boiler water. This is generally enough sodium
sulfite to react with normal amounts of oxygen
that might get into the boiler. Higher concentra-
tions of sodium sulfite are unnecessary.
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Sodium sulfite dissolves readily in water and
must be fed at a point between the feed heater
and the boiler so that it is used to take up only
the oxygen that gets by the deaerator or heater.
If the sodium sulfite is fed through the feed lines
by continuous feeding, it is always present in the
feed lines and takes up oxygen in the feedwater
in addition to maintaining a surplus in the boilers.
Another advantage of using sodium sulfite is
that if, for any reason, a feedwater heater or
deaerator becomes inoperative or efficient opera-
tion is temporarily interrupted, the sodium sulfite
residual present in the boiler water can take up
the larger amounts of the oxygen getting in. At
the same time, the concentration of sodium sulfite
drops. This is shown by test analysis of the boiler
feedwater. This test gives the operator ample
warning of an existing malfunction within the
boiler feedwater supply system. Immediate steps
should be taken to correct this off-standard con-
dition. Feedwater or makeup water tanks should
be heated to a temperature of 180°F to 200°F.
This heat alone helps to dispense of most of the
dissolved oxygen before it can enter the boiler.
It also allows for more economical use of sodium
sulfite.
The prevention of corrosion in the boiler
means regulating the alkalinity of the water,
producing protective films, and removing dis-
solved oxygen. These preventive measures are
accomplished by maintaining the proper chemical
residuals in the boiler water and by proper
deaeration.
CarryoverFoaming and Priming
The word priming is used rather loosely to
express the action of the water and steam in a
boiler when an unusual amount of water is being
carried over with the steam. For a given boiler
installation, a certain amount of water or moisture
in the steam is tolerated. The amount depends
upon the use of the steam, the boiler construc-
tion, and the facilities for removing the water
from the steam. The mechanical causes include
deficiency in boiler design, high water level,
improper method of firing, overloading, and
sudden load changes. A poorly designed boiler
may have insufficient steam disengaging space.
It is fairly obvious that the faster the steam is
produced in a given vessel, such as a boiler, the
more violent is the boiling effect. But when the
steam space above the water level is large enough,
the steam leaving the boiler does not show any
evidence of carryover. The size of the steam