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 systems.
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 corrosion protection:
- 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, and boilers.
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 startup.
The traditional approach for wet lay-up involved the use of the following methods:
- 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 indirect protection.
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:
- 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 surfaces.
- 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 application.
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, 1999.
- 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, New Orleans