troubles in low pressure heating boilers -- which usually operate
at a steam pressure below 15 psig or water pressures below 30
psig and are often of the horizontal fire tube type -- often occur
During the past 60 years we have had many occasions to examine
boiler tubes to determine the reason for their failure. In very
few cases have any defective qualities in the tubing been the
cause of the corrosion. In the vast majority of instances, the
necessity for replacement has been traced to conditions of environment.
In power boilers, it is a rare occurrence to find corrosion of
the type common to heating boilers. This is because operators
of power boilers realize the importance of proper water and fire
side conditions and take care to avoid such problems.
The users of heating boilers are, first of all, usually not aware
of the possibilities of corrosion. Often they have little idea
what causes it and lack the know-how and experience to combat
it. Fortunately, scale is not a major factor in low pressure boilers,
although a buildup of scale at tube ends has occasionally resulted
in failure by grooving next to the tube sheet.
Let us first consider the various mechanisms which lead to pitting
or water side corrosion since this is the most common type. This
accounts for 75 percent of the tubes examined in our laboratory.
Steel does not corrode appreciably in dry air, but only in the
presence of moisture. Likewise, steel will not corrode in clean,
alkaline, freshly-boiled water, if air is kept away.
This has been proven to our satisfaction by placing samples of
tubes in ordinary tap water in flasks and boiling the water, causing
the steam to condense and run back into the flask. When we allowed
the condensed drops of water to be free to contact the air, corrosion
of the tubes took place. When we took the oxygen out of the air
in the flask and condenser by running the air through pyrogallic
acid (which is an oxygen-absorbing liquid), no corrosion of the
tubes took place.
Oxygen and Velocity Factors
This proves that the presence of oxygen is an important
factor in corrosion problems. It was also found that if the heaters
were shut down at night, the corrosion was much more rapid than
if the apparatus were kept boiling. In effect, some of the oxygen
was excluded from the flask by the steam space over the boiling
water. In low pressure heating boilers, however, the return water
usually enters at the bottom, which does not afford the oxygen
reduction which would be obtained if it would drop through the
Pitting is probably the most destructive form of corrosion that
affects the water side of boiler tubes. Frequently, only a few
pits are present and most of the surface is unattacked. In other
cases, the pits cover most of the surface, and as a further extreme,
the pits all run together and the corrosion takes the form of
uniform attack. The frequency of the pits is determined to a large
extent by the degree of acidity or alkalinity of the water.
Acidity and alkalinity are dependent upon the amount of hydrogen-ion
concentration found in the water. Both would be expressed in terms
of the pH scale. A strong acid solution -- strong muriatic or
sulfuric acid -- is rated as 1; a strong alkaline solution --
concentrated caustic soda -- is rated as 14. A neutral water has
a pH value of 7.
Below a pH of 5, the water is actually sufficiently acid to dissolve
the steel, and under these conditions no pits form. Instead, the
corrosion is relatively uniform and the steel gradually gets thinner
until it is too weak to hold the pressure, or a small hole develops.
Between a pH 5 and 9.4, pitting takes place at a rate depending
on the concentration of oxygen in the water. Therefore, while
operating the boiler, it is necessary that all air or as much
air as possible be excluded from the boiler water.
It has been shown that a strip of steel hung in the middle of
a fast moving stream did not rust, while an identical piece hung
in a stagnant pool along the edge of the same stream pitted badly
when connected to the first by a wire. This proves that velocity
and air content have an effect on the corrosion of steel. In most
cases, the pitting in horizontal fire tube boilers takes place
along the top of the tubes on the outside, and it is our belief
that this may in part be due to the difference in velocity of
the rising water and steam bubbles, creating an eddy effect along
the top of the tube and accelerating the corrosion, much as did
the experiment of the flowing stream. In any event, pitting would
not occur in this type of boiler if no oxygen were present in
Practically all ground surface supplies of water contain dissolved
air in quantities depending on its source, time of exposure and
its temperature. Cold water will retain more air than warm water,
as can be seen by filling a clear bottle with cold water from
a tap and allowing it to stand overnight. Small air bubbles will
form on the sides, demonstrating that as the water warms up the
gas is liberated.
This release of the air in the form of bubbles creates a problem
in a newly filled boiler. In a new boiler, or in one which has
been drained and refilled with cold water, as the water warms
up, air bubbles form on the tubes. In a very short time pits develop
under these bubbles, due to the difference in oxygen concentration
under the bubbles and the oxygen concentration in the water surrounding
the bubbles. Penetration as high as 50 percent of the tube wall
has been known to take place in one stagnant period of two weeks
duration. Once these pits form, they proceed rapidly even under
Why New Tubes Corrode
Sometimes a set of new tubes installed in a boiler has
been found to last less than a year, whereas the former tubes
lasted five to ten years. Obviously, something has changed. Often
the tubes are blamed for the failure, when actually there have
been changes associated with the operation and maintenance of
the boiler. A different method of starting up may have been used.
Circumstances may have been such that the boiler was immediately
fired when the old set of tubes were put in, while the new set
may have been exposed to the fresh water for some time and air
bubble pitting may have started, leading to the eventual failure
of the tubes. The temperature of the fill-up water may have been
different; and, therefore, more air was present in the new installation.
The composition of the fill-up water may have changed; a thin
scale may have been laid down at the beginning of the life of
the old tubes, which served as a protection. Changes in electrical
connections may have induced stray currents, leading to possible
electrolytic corrosion. Small air or steam leaks around pipe joints
and valves may have let air into the new setup. Air vents may
have become plugged due to jarring of the piping. In short, any
number of things may have happened and caused the failure.
A large number of boiler tube failures take place in the fall
when starting up for winter operation. These are due to both the
air bubble pitting previously mentioned, and to oxygen sucked
into the system through packing and other sources.
Remove Air From Water
The bottle test shows that air can be removed by heating
both the fill-up water and the regular feed water. After every
filling, a steam boiler should be heated to bring the water to
a good boil and the steam so produced should be vented off to
carry the released gases out of the boiler. Before this boil out,
water treating chemicals should be added so as to get good mixing.
After the boil-out, the vents should be closed and the boiler
used or cooled down if not needed.
In hot water systems, production of steam is not desirable, so
the water temperature should be raised to 180° to 200°
F for a short time to allow most of the air to be driven off through
In larger boiler installations, air is removed from the feed water
by heating it to the boiling point and venting off the dissolved
gases. In small installations, this is hardly practical.
However, in steam systems requiring large quantities of make-up
water, it may be possible to fit the return condensate tank with
a steam coil to preheat the water to near the boiling point. This
tank would have to be vented to release the gases.
Another method suggested by F. N. Speller, a noted authority on
corrosion, is to pass the feed water through a de-activator, which
is a tank containing steel scrap, such as turnings or wires. The
oxygen in the water attacks the steel in the tank so that corrosion
properties are neutralized. The process is satisfactory if the
tank is big enough to permit complete de-activation and if the
scrap steel is renewed often enough. The practice is not frequently
followed in steel heating boiler installations because other methods
of control are usually more desirable.
In addition to the air carried in by make-up water, substantial
quantities may be pulled into the system during operation by the
vacuum in the condensate line, or by the vacuum formed when the
boiler is shut down or the fire is allowed to die off. In small
heating boilers, warm days during spring and fall and even in
the winter often result in cooling down a boiler and radiators.
Condensing steam creates a vacuum which pulls air into the system
through leaking pipe connections, traps, vents, valves and packing.
Proper maintenance of the entire heating system is a must.
Hot water systems should not suffer from air entering with make-up
water because make-up water should not be required. We say should,
but there are cases when it is required because cleaning people
are drawing off hot water, garage men are washing cars with it,
circulating pumps leak, floats become water-logged or automatic
feed systems stick.
Systems are sometimes designed to be pressurized with compressed
air in such a way that a large area of water is exposed, allowing
dissolving of air to take place. We have seen systems where well
water was pumped into a horizontal cylindrical tank which was
pressurized with air across its whole surface. Another system
had hot water from three boilers pumped to an overhead horizontal
tank of about 5,000 gallons capacity, which was pressurized with
compressed air from a pump in another building. No one had any
idea of how much air was being pumped into this system. Eighty
pounds of sodium sulfite (an oxygen scavenger) added per day to
this system could not keep up with the dissolved oxygen being
pumped into it.
Any pressurizing of this type should be in an offshoot of the
system, not in the main stream. If it must be in the main stream,
nitrogen gas should be used for pressurizing.. Obviously, there
are many ways air can get into boiler water; it's difficult to
keep it out. Fortunately, however, there are methods for rendering
How To Remove Oxygen
One method of removing oxygen from boiler water is through
the use of an oxygen-absorbing chemical such as sodium sulfite.
If only small quantities of oxygen are present, the addition of
this chemical is practical. It is impractical, however, to try
to remove large amounts of oxygen by using this chemical in large
quantities, because constant additions would cause foaming. Control
of alkalinity of the water must be maintained in conjunction with
the use of sodium sulfite. The pH should be 9.5 or higher.
Hydrazine is a chemical frequently used in large utility boilers
to remove dissolved oxygen. However, it is not recommended for
heating boilers because it must be closely controlled. Very seldom
is such chemical control available in these installations.
Inhibitors are a class of chemicals which deposit a coating on
the surface of the steel or react with it in some way to protect
it against attack. These inhibitors, usually composed mainly of
sodium chromate, are available from most water treating companies.
When added to the water in the recommended quantities, they will
protect the boiler surfaces during either operations or standby.
Since they are harmful if taken internally, and may stain other
products, they should not be used wherever the steam is to be
used for process work. These compounds have the advantage of imparting
a yellow color in the water, which the boiler user can see in
the gage glass, and thus readily determine if more is needed.
Some trouble has been experienced from use of these compounds
in hot water systems due to the formation of sodium chromate crystals
in pump seals, resulting in leakage. Concentrations lower than
the 2.2 pounds per 100 gallons recommended for steam boilers have
been suggested for hot water boilers.
The value of this compound, and of another inhibitor containing
sodium nitrite and sodium nitrate, was established in a series
of tests performed at the Babcock & Wilcox Research &
Development Center. These tests proved that both the sodium chromate
and the sodium nitrite-nitrate inhibitor were effective not only
in preventing attack by dissolved oxygen, but also in stopping
further attack after it had started. There are some limitations
on the amount of chlorides or sulfates that can be tolerated,
but these are seldom a factor in waters used in heating boilers.
A few years ago, there was a flurry of "gadget" type
water conditioning cure-alls being offered. One such device, designed
to fit into a supply line, was purchased and tested. It proved
ineffective in either preventing or stopping corrosion of the
Don't Drain Chemicals
Many boiler owners completely drain their boilers once
or twice a season under a mistaken belief that the water in the
boiler is dirty. Actually, this practice, and the practice of
periodically draining small quantities of water from the boiler,
should be discouraged. It causes loss of chemicals and requires
make-up water, which brings in more oxygen. However, if additional
chemicals are added each time to compensate for losses, little
harm will be done. Insurance companies require periodic tests
of the low water cut-off, and at such time protection should be
insured by the addition of such chemicals.
Instead of inhibitors, alkalizers such as caustic soda may be
used. It is recommended that 2 oz. of caustic soda per 100 gal.
of boiler water be added at the time of a fill up. This will insure
a pH of 11 to 11.5, which will greatly reduce the pitting effect
of dissolved oxygen. Some prefer a lower concentration, down as
low as 1.3 oz. per 100 gal.; but, except for the possibility of
foaming, the larger quantities can do little harm, and act as
a safety factor should losses take place by draining. However,
alkalizers will not stop pitting once it has started.
In new boilers, or in old boilers which have been retubed, a boiling
out using cleaning compounds is suggested. This is necessary to
remove oils and other coatings put on the tubes by the manufacturer
prior to shipment or storage. These materials are put on the tubes
to protect them from rusting during storage and transit, and have
no place in a boiler. Since they may shield portions of the tube
from direct contact with the water, pitting may be accelerated.
A good boil-out is recommended, using a cleaning compound such
as one of the newer detergents, or a mixture of 2-1/2 lb. of caustic
soda and 2-1/2 lb. of soda ash per 100 gal. of boiler water.
Fire Side Corrosion
Approximately 15 percent of the tubes we have examined
have failed by fire side attack. Corrosion on the fire side of
boiler tubes is caused by moisture condensing from the atmosphere
during periods of shutdown, or from flue gas condensation during
operation. This type of corrosion is especially troublesome in
boiler installations near bodies of water, or where the atmosphere
is otherwise humid. Fire side corrosion is accelerated by the
use of high sulfur fuels. Sulfur gases may condense on tube surfaces
during operation; depending upon the kind of fuel, its sulfur
content and the methods of firing.
Accumulations of soot on the tubes should be periodically removed.
Soot attracts moisture; and air, moisture and steel together result
in attack of the tubes. Cleaning may be daily, weekly or monthly,
depending on the fuel used and the method of firing.
Some hot water boilers -- for example, those in greenhouses --
may operate at water temperatures of 140oF to 150oF. Under such
conditions, the condensing gases from coal or oil firing form
sulfurous acid which attacks the tubes and results in a more uniform
type of corrosion. If the percentage of sulfur in the fuel is
high, this situation is worse. Even in the absence of sulfur compounds,
corrosion may occur during shutdown periods because of high humidity
in the air. When shutting down the boiler under such conditions,
the fire side tube surfaces should be brushed and flushed to remove
the winter's accumulation of soot and other products of combustion.
This should be followed by blowing air through to dry out these
surfaces. A light coat of oil should be applied for further protection.
Also, in extremely humid locations, the stack should be disconnected,
or at least the damper should be closed, and a tray of unslaked
lime placed in the ash pit to keep the fire side dry. This lime
must be renewed whenever it becomes mushy, so the drying effectiveness
will not be lost.
Many samples of scale removed from fire side surfaces have been
found to be acid when mixed with water. The presence of this acid
may cause the tube metal to eat away to eventual failure.
Often, boiler rooms are in damp cellars, some with water on the
floor constantly. During the summer months, in particular, humid
air tends to build up in basements, causing clothes and leather
to mildew from the dampness. Similarly, humid air may have ready
access to the fire side of boiler tubes in basement installations
if the tubes are not properly protected.
Even with gas firing of hot water boilers, serious fire side attack
can take place. Some installations employ outdoor-indoor thermometers
to control system water temperatures as outdoor temperatures fluctuate.
Low water temperatures can result in condensation of moisture
from the flue gas and lead to serious corrosion of the tubes.
High water temperatures reduce the probability of attack.
Some horizontal tube boilers suffer from a mechanism called "necking"
and "grooving." This shows up as a circumferential groove
around the outside of the tube where it enters the tube sheet.
It usually occurs at the beginning of the first pass, which is
the hottest end of the tubes. In all cases, there is some corrosion
in evidence in other areas, but it concentrates at the ends because
of strains from two sources. When tubes are rolled in, some unavoidable
expansion takes place back of the tube sheet. Secondly, when a
boiler heats up, the metal in the tubes expands and lengthens.
Consequently, strains are set up at the ends, which are fixed
in the tube sheets. Sometimes these expansions are so severe that
the tubes loosen in the sheets. Scale forming at the tube ends
tends to flake off, exposing fresh steel to further attack. This
problem can be reduced by more gradual firing, more gradual changes
in temperature, and maintaining the boiler water free of oxygen
and under proper control.
Following the precautions and controls described in this article
should result in many years of trouble-free, economical operation.
Follow These Rules
- Boil out the boiler with an alkaline
cleaner after installing new tubes to remove oil or other
coatings from the tube surfaces. These protective coatings
are commonly applied to new tubes to prevent rusting during
storage and transit, and will cause corrosion if left
on the tubes during operation of the boiler.
- Bring a steam boiler to a good steam
output as soon as it is filled to deaerate the water.
Heat the water in a hot water boiler to 180oF for the
same reason. A temperature of 180oF will not remove all
the air, but the majority will be driven off.
- Add sodium chromate or sodium nitrite
- nitrate inhibitors to the water in the quantities recommended.
- In greenhouses or in damp locations,
put a tray of unslaked lime in the ash pit to absorb moisture,
and close the boiler. Inspect this lime occasionally and
renew when it becomes mushy.
- Keep all boiler and system fittings
- Add sodium chromate or sodium nitrite
- nitrate inhibitors to the water in the quantities recommended.
- Preferably, use a fuel with low
sulfur content to avoid the corrosive action of sulfur
- Brush, flush and dry out the insides
of fire tubes as often as possible to remove soot and
other products of combustion, and to prevent the accumulation
of moisture and condensed sulfur gases.
- Use sodium sulfite regularly in
the boiler feed water to remove dissolved oxygen.
- Use suitable feed water heater or
deaerator to reduce the oxygen content of the boiler feed
- Prevent water leakage and avoid
draining water from the system. Addition of make-up water
results in loss and dilution of the treatment, and introduces
air into the system.
- Don't pressurize a hot water system
with compressed air over large areas of water.