corrosion in steam condensate systems?
Carbon dioxide and oxygen cause most condensate system corrosion. Carbon
dioxide, dissolved in condensed steam, forms corrosive carbonic acid. If oxygen
is present with carbon dioxide, the corrosion rate is much higher and is likely
to produce localized pitting. Ammonia, in combination with oxygen, attacks
How is steam condensate corrosion prevented?
The general approach involves removing oxygen from the feedwater mechanically
and chemically and providing pretreatment of the make-up water to minimize
potential carbon dioxide formation in the boiler. Chemical treatment reduces
corrosion potential further. Volatile amines neutralize carbonic acid formed
when carbon dioxide dissolves in condensate. Volatile filming inhibitors form a
barrier between the metal and the corrosive condensate.
How do chemical oxygen scavengers help control condensate system corrosion?
Deaerators (and feedwater heaters) remove oxygen mechanically. The best-designed
and operated deaerators can reduce oxygen to as low as 0.007 PPM. Since very
small amounts of oxygen can cause corrosion, complete oxygen removal often
requires chemical treatment.
What is the basis for choosing between neutralizing and filming inhibitors?
The proper choice depends on the boiler system, plant layout, operating
conditions and feedwater composition. In general, volatile amines are best
suited to systems with low make-up, low feedwater alkalinity and good oxygen
control. Filming inhibitors usually give more economical protection in systems
with high make-up, air in-leakage, high feedwater alkalinity or intermittent
operation. In most cases, a combination of these treatments may be the best to
combat condensate corrosion.
What characteristics should a good condensate corrosion inhibitor have?
A good volatile neutralizing amine should have a favorable distribution ratio in
steam and condensate so that it protects the entire steam-condensate system. It
should have no insoluble reaction products and should be stable at high
temperatures and pressures. A good filming inhibitor should be easy to disperse
in water. It should be stable under usage conditions and form a thin, protective
film without causing deposits in either the boiler or steam-condensate system
Condensate Corrosion--Factors and Control
Breakdown of carbonate and bicarbonate that enter the boiler is the main source
of carbon dioxide. Left unchecked, this can result in low pH condensate. This
has been observed as grooving in sections of condensate lines that are not
completely filled with water.
Oxygen can enter a condensate system by many sources even if the deaerating
heater is functioning properly. The oxygen, at its worse, can result in deep
pitting of condensate lines. The combination of oxygen and carbon dioxide
corrosion can be particularly troublesome in causing corrosion products to be
produced and transported to the boiler.
Although often not controllable, high flow rates within the condensate system
can produce extremely severe corrosion conditions. This flow-assisted corrosion
is accelerated at low pH and can be minimized by keeping the pH above 9.0.
Other gases that can be corrosive and present in the condensate system include
ammonia, hydrogen sulfide, and sulfur dioxide. The most common of these is
ammonia. Copper corrosion can be as serious as iron corrosion and is made even
more serious in the presence of copper complexing agents such as ammonia. Again,
oxygen in combination with these gases increases copper corrosion.
Neutralizing amines, when fed to the boiler, volatile with the steam and enter
the condensate system. These amines are weak bases and will therefore neutralize
any carbon dioxide present and will raise the pH of the condensate. If oxygen
levels are very low, these neutralizing amines can, by themselves, effectively
control condensate corrosion. However, knowing which ones to feed, how much, and
how to control can be a difficult and confusing decision for the typical user.
There are more than a dozen amines in common use. Each amine is unique in
certain characteristics, each of which affect how the amine functions at a given
point in a given condensate system. Those amine characteristics include
basically value, molecular weight, distribution ratio, and hydrolytic thermal
stability. Since it is necessary to prevent corrosion from the point of initial
steam condensation to the far ends and back of condensate systems, a blend of
neutralizing amines is normally fed.
Volatile Passivating Agents
If oxygen is present, the neutralizing amines alone will not control corrosion.
Fortunately, chemical treatments have been developed which will transport with
the steam and will, in addition to possibly reacting with oxygen, function as
Passivating agents to prevent corrosion. But as is often the case, such new
alternatives bring with them a whole new set of potential problems and set of
rules. Which product to use where, when, and how are questions that are best
answered by an expert in water treatment chemistry who is also highly
knowledgeable about your system. Control and testing are the main problems with
these Passivating agents. That is why corrosion monitoring, always important,
becomes even more so when treatment results depend on a passivation chemical.
Corrosion monitoring is a major task for any water system. Relative pure waters,
like condensate, make the task even more difficult. Some of the methods used and
recommended by Thermidaire Consultants include the following:
Steel and copper corrosion coupons have been used in condensate systems for many
years. While there are concerns as to how accurately they reflect the actual
corrosion rate within a given system, the do provide a good relative measure of
trends in long term corrosion. A consistent, continuing program using coupons at
the same locations and for the same duration of time is therefore most
meaningful. The locations and plumbing for the coupons must be carefully
engineered to avoid meaningless results.
While not a direct measurement of corrosion, continuous measurement of
condensate pH can be very helpful in systems that depend on neutralizing amines
for pH control. Other monitors, such as for conductivity, are also helpful to
guard against condensate contamination. Location of sample points and method of
sampling are critical and should be established by a qualified water treatment
Iron and Copper Testing
Another old but proven effective standby is iron and copper testing. The sample
points, method of collection, and analytical procedures are more critical in
trace metal testing than in any other analysis. Incorrect results are much worse
than no results at all. The sample program should be established to collect
samples at a pre-determined interval. The final feedwater represents the
corrosion products load actually entering the boiler and can be a good indicator
of the expected cleanliness of boiler surfaces over a period of time. Composite
as well as spot samples should be taken. However, since the slightest change in
flows can make sample results worthless, any composite samples collected should
be collected from a continuously flowing sample and with a proven condensate
Corrosion Test Monitors
Electronic corrosion test monitors have now been developed that can be
effectively used in condensate systems. Older corrosion test meters were not
capable of this because of the need for the water being tested to exhibit a
minimum conductivity. As with any of the monitoring methods, results from these
should be evaluated over a long period of time and compared with prior results
versus actual inspections.
Monitoring of your condensate is essential toward protection of, not just the
condensate system itself, but more importantly, to the continued reliability of
the system boilers.