Lay-up of Cooling Towers and Boilers
with Volatile Corrosion Inhibitors
4119 White Bear Parkway
Saint Paul, Minnesota 55110
Volatile corrosion inhibitors (VCIs) were originally
developed by Shell for the military during the last world war. Their
effectiveness and ease of application attracted early users. Over
the years, the field of usage has increased to cover water treatment,
electronics, packaging, process industries, seasonal lay-up, coatings,
reinforced concrete, and metalworking fluids.
Technologically advanced treatment programs were
designed for boilers, cooling towers, heat exchangers, and deaerator
A new class of VCIs has been developed in harmony
with the concern for the environment. These new chemicals are classified
as ambiodic type of inhibitors and inhibit cathodic and anodic electrochemical
reactions. They offer excellent protection to metals and at the
same time they have a very low impact on the environment and are
safe to use. These newly developed treatment programs provide three-phase
- in the water phase
- in interphase between water/air
- in the air/vapor phase
The direct cost of atmospheric corrosion of metals
and alloys costs billions of dollars to businesses around the world
annually. One method that has recently gained favor in the chemical
processing industries for combating such damage is the use of volatile
corrosion inhibitor (VCI).
VCIs condition air or other gaseous environments
with trace amounts of inhibitive material to achieve the protective
effect. Classic methods of protection involve changing the composition
of an alloy, changing the environment, or using contact inhibitors.
In some instances, these methods may prove impractical due to cost,
limited accessibility, risk of contamination, or plain inability
to provide good protection. The VCIs are compatible and safe to
use with boiler water and cooling water treatment chemicals. These
new VCIs are used in operational, standby, and laid up cooling towers,
Vapor phase inhibitors are surface absorption type
inhibitors with passivating properties. Such corrosion inhibitors
are the products of the reaction of organic acids and amines.
While discussing the mechanism of vapor phase transport
it should be noted that amine salts such as dicyclohexylammonium
nitrate and diisobutylammonium sulfate are not extensively dissociated
by water and do not give significant vapor phase inhibition. The
same thing applies with slightly hydrolyzed alkali salts, such as
sodium nitrite and sodium benzoate that are excellent contact corrosion/rust
inhibitors when the solutions are in direct contact with metal surfaces.
Volatile vapor phase inhibitors such as amine carboxylates
undergo substantial hydrolysis in water solution in the presence
of moisture. These inhibitors form stable bonds with metal surfaces.
Amine carboxylates have a high vapor pressure at ambient temperatures
allowing these materials to volatilize and migrate to distant metallic
surfaces. As the virtue of this property direct contact with the
metal surfaces to be protected is not required when using vapor
phase corrosion inhibitors. Volatile inhibitors need only to be
placed in the vicinity of the metals to afford protection. The inhibitor
will migrate through the vapor phase and will be absorbed on the
metal surface. The protective vapors disseminate within an enclosed
space until equilibrium is reached. Equilibrium is set by the compound
particle vapor pressure.
In recent years, various techniques were used for
corrosion protection during wet or dry shutdowns. The traditional
approach for dry lay-up involved the following approaches:
- Nitrogen gas blanketing, and
- The use of desiccants, which must be removed prior to boiler
The traditional approach for wet lay-up involved the use of the
- Oxygen scavengers
- Alkaline chemicals to maintain pH>10
- Use of dicyclohexylammonium nitrate and diisobutylammonium sulfate.
These chemicals are the source of hydroxide ions
and are used as neutralizing inhibitors that neutralize hydrogen
ions in the environment. They become volatile only with steam at
use levels. Examples of neutralizing inhibitors are ammonia (has
high volatility gas at room temperature), and morpholine.
Since these compounds at use levels volatilize only with steam,
they are not considered to be volatile vapor phase inhibitors.
The disadvantages of using silica gel or other
desiccants is that once they are saturated with moisture (H2O)
they will start releasing the moisture back in the environment and
creating a corrosive environment. They do not protect against corrosion
directly but rely on eliminating moisture and thus provide only
Oxygen scavengers are not recommended for long-term
protection and they are unable to protect against oxygen ingression.
Likewise this method does not protect surfaces that are not in contact
with solution. Moreover this solution must be replenished with time
and involve manpower to check concentration levels periodically.
The new treatment for laying up boilers utilizes
a unique blend of vapor phase corrosion compounds and contact corrosion
compounds in convenient water-soluble polyvinyl alcohol (PVA) bags.
The application is completed in following simple steps:
1. After the boiler is cooled down and safe to enter, the
PVA bags are slit open and placed inside the boiler. One bag
protects up to 1000 gallons of void (135 cubic feet) including
the surface area of tubes. For extremely large boilers VCIs
are applied by blowing with help of a blower.
2. Close the openings (manholes, etc.).
- Dissolve VCIs in water (after it is below 60o C)
and fill the boiler with VCI solution.
- The boiler does not need to be filled completely to protect
various void areas due to the migrating nature of the VCIs.
The VCIs will reach equilibrium in the void space and protect the
metal in the system. The performance of these products can be evaluated
by the use of corrosion coupons.
COOLING TOWER LAY-UP
The traditional approach to seasonal lay-up of
cooling towers includes circulating a high dosage of a blend of
polymers and phosphates. Oil based lay-up products are also commonly
used prior to shut down.
Conventional seasonal lay-up programs often use
an oil-based product that does not apply evenly. They can cause
significant gunk balls in the equipment and are difficult to dispose.
Another problem is that oil-based products react with rubbers in
the system and with roof tars. Oil-based products are a good source
of nutrients for various kinds of bacteria, including anaerobic
bacteria. This leads to promoting microbiological growth in the
cooling tower system and hence bacterial corrosion.
Use of traditional cooling tower chemicals i.e.
polyphosphate and organophosphorus (cathodic inhibitors) require
constant water recirculation of 3 feet to 5 feet per second in order
to pass through the metal-water interface and form an anti-corrosion
barrier on the metal surface. Without recirculation, these two inhibitors
remain in the bulk water and are unable to passivate metal. Organophosphorus
is also known to strip iron from unpassivated carbon steel in pipes.
The polyphosphate portion of the treatment then binds with this
iron rendering it incapable of providing pasivation even with recirculation.
The major shortcoming of conventional lay-up products
is that they are strictly contact corrosion inhibitors. They can
only protect the parts of the system that they contact. The overhead
spaces, crevices and other hard to reach spaces remain unprotected.
These parts of the system tend to corrode during down due to the
lack protection. Due to all of the above reasons it becomes important
that a thorough cleaning of the system is performed prior to starting
the tower back for normal usage.
The new treatment for the lay-up of cooling towers
involves a blend of vapor phase corrosion inhibitors with complimenting
contact corrosion inhibitors. Together they provide a synergistic
effect ideal for the seasonal lay up situations. Cooling tower lay-up
is conveniently achieved with help of VCIs. The towers can be laid
up in the following fashion with the help of specially formulated
VCIs in water-soluble PVA bags:
- Place bags into cooling water and circulate the water for 6
to 10 hours.
- The treated water forms a protective barrier film over the metal
- Drain the treated water if freezing during the winter is a concern.
Otherwise the tower can remain filled with the treated water for
the duration of the lay-up.
One carton of VCI water-soluble bags treats up
to 1000 gallons of cooling water (or space). VCIs are compatible
with most of non-oxidizing biocides. When using oxidizing biocides
some precautions need to be taken. One of them is keep the free
halogen levels (e.g. chlorine) in check. It is recommended to keep
the free halogen levels less than 1 PPM. Higher concentration of
the VCIs might be needed. The performance of the program can be
monitored using corrosion coupons.
Problem. A Midwestern college had severe
corrosion problems in past with their boilers and deaerators. A
new boiler and deaerator was recently installed and the chief engineer
wanted to protect this new equipment from this type of corrosion.
The chief engineer noted that almost all of the boiler corrosion
occurred during the idle period when the equipment was drained for
their annual inspections and maintenance.
Solution and Application. Water-soluble
bags with VCIs were utilized for a 20,000 pounds per hour water
tube boiler and a 1000 HP fire tube boiler. In the past years off-season
corrosion had occurred in the off line equipment. The VCI containing
water-soluble bags were placed in the boilers and deaerator at a
rate of one bag per 1,000 gallons. The moisture barrier outer bags
were removed and then the water-soluble bags were slit down the
center and placed strategically in the boiler. The bags were placed
on each end of the boiler both in the steam and mud drum in the
water tube boilers. In the fire tube boiler the bags were placed
on the tubes and belly of the boiler. Corrosion coupons were installed
to evaluate the results and pictures were taken at the end of the
Result. The boilers were opened prior
to being put back on line. Inspection indicated that the boilers
were in the same condition as prior to the shutdown months earlier.
Figure 1 and 2 are the pictures taken of the tubes after the lay-up
period was over. A black protective magnetite film still coated
the internals of the boiler and no rusting or other corrosion was
evident. The iron level of the boiler water was 20 times lower than
during past boiler start-ups.
VCIs provide a unique solution to the challenging
problem of corrosion protection of boilers and cooling towers during
the lay-up period. With their peerless ability to protect in three
phases i.e. the vapor phase, liquid phase, and the vapor-liquid
interphase proves to be ideal for such applications.
Boilers, heat exchangers, deaerators, tanks, cooling
towers, etc. can be protected against corrosion in wet or dry lay-up
using volatile vapor phase inhibitors which are compatible with
water treatment chemicals.
Author would like to acknowledge Al Bly, Randy
Meyer, and Mike Bertrang of US Water Services, St. Paul, MN for
providing the field study data.
- Miksic, B. A., Volatile Corrosion Inhibitors Find a New Home,
Material Engineering Forum, 1997.
- Miksic, B. A. Use & Nature of VCIs, April 83, Anaheim. NACE.
Corrosion 83, National Association of Corrosion Engineers, Use
of Vapor Phase Inhibitors for Corrosion Protection of Metal Products.
- Nathan, C.C., "Corrosion Inhibitors." NACE, p.8, 1973.
- Gerberich, W.W., Communication to Northern Instruments, Institute
of Technology, University of Minnesota, Aug 1975.
- Gandhi, A.G., Effective cooling tower lay-up, AICHE Newsletter,
- Petersen, R. J., Conway, E.J., Midwest Research Institute Project
No. 4165-N, Jan. 1976.
- "Methodology of VCIs for Water Treatment." Published:1997,
NACE, National Association of Corrosion Engineers, Paper #182,