Volatile Corrosion Inhibitors Unique Water Treatment Applications
By Ashish Gandhi, Cortec Corporation
The direct cost of atmospheric corrosion in metals and alloys has been estimated to be at $5 billion annually. One method that has recently gained favor in the chemical processing industries for combating such damage is the 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 measures may prove impractical due to cost, limited accessibility, risk of contamination, or plain inability to provide good protection.
Examples of applications where VCIs have been used include air spaces above the liquids, condensers, cooling towers and boilers during shutdown or standby, crude oil pipelines, closed loop cooling systems, open loop cooling systems, brine systems, vessels and other equipment during storage, instrumentation, and control-room equipment. For VCIs to be suited for use in the chemical process industries, they must be inexpensive and easy to apply, durable, nontoxic and nonpolluting. Chief advantages of these inhibitors are procedural and economics. It is unnecessary to prepare the metal surface prior to using VCIs since their vapors can penetrate to remote areas of an enclosure. The nearly undiscriminating nature of some newly developed chemical treatment programs to protect dissimilar metals and to perform in severe environments reduces the need for a rigidly planned maintenance schedule.
HOW THEY WORK
VCIs generally come as solid, for convenience in handling. Volatility is simply a means of transport. Protective vapors disseminate within an enclosed space until equilibrium--determined by the partial vapor pressure--is reached. The inhibiting process starts when the vapors contact the metal surface and condense to form a thin barrier of micro-crystals. In the presence of even minute traces of moisture, the crystals dissolve and develop strong ionic activity.
The result of such activity is adsorption of protective ions onto metal surfaces, with the concurrent formation of a molecular film that fosters breakdown of contact between the metal and an electrolyte. The presence of an invisible monomolecular film does not alter any of the important properties of the metal, even in precise electronic application, where properties such as conductivity, or dimensional tolerances are critical, and where even minute deviations could cause malfunction.
VCIs migrate to distant metallic surfaces. This ability enables VCIs to protect metals without direct contact with metals. VCIs need only to be placed in the vicinity of the metals to provide protection. VCIs will migrate to metallic surfaces through the vapor phase and the inhibitor will be adsorbed on the surface. The protective vapors will distribute within the enclosed space until equilibrium is reached. Equilibrium is set by the compound’s partial vapor pressure.
Too high a vapor pressure will cause the inhibitor to be released to such an extent that a protective concentration cannot be maintained. On the other hand, a low-vapor-pressure inhibitor is not used up as quickly and can thus assure more-durable protection, but more time is needed for a protective vapor concentration. This raises the risk of corrosion during the initial period of saturation, and if the space is not sealed, a protective concentration may never be reached.
BUILT-IN TEMPERATURE ADJUSTMENT
Proper selection of volatile compounds enables controlled and dependable volatilization. The higher the temperature, the stronger the tendency of the metal toward corrosion. The volatilization rate of VCIs has a similar function dependence upon temperature, so that more inhibitive material is evaporated at higher temperatures. VCIs can thus self-adjust to the aggressiveness of the environment, over a wide temperature range.
Volatile corrosion inhibitors were originally developed for protection of ferrous metals in tropical environments, an approach that soon proved limiting because of incompatibility with nonferrous metals. Recent developments are based on the synthesis of compounds that provide satisfactory "general" protection, i.e., they protect most commonly used ferrous and nonferrous metals and alloys.
Investigations of electrochemical behavior show that these compounds belong to family of mixed or "ambiodic" inhibitors capable of slowing both cathodic and anodic corrosion processes (figure 1). Active ingredients in VCIs are usually products of reaction between a volatile amine or amine derivative and organic acid. The product obtained as a result of this reaction, ‘aminocarboxilates’ are the most commonly used VCIs. Cyclohexylamine, dicyclohexylamine, guanidine, aminoalcohols, and other primary, secondary & tertiary amine salts represent the chemical nature of VCIs. VCI compounds, although ionized in water, undergo a substantial hydrolysis that is relatively independent of concentration. This independence contributes to the stability of the film under a variety of conditions.
The absorbed film of the VCI on the metal surface causes a repulsion of water molecules away from the surface. This film also provides a diffusion barrier for oxygen, decreasing the oxygen concentration, and thus reduction of the cathodic reaction. Strong inhibition of the anodic reaction results from the inhibitor’s having two acceptor-donor adsorption centers that form a chemical bond between the metal and the inhibitor. Adsorption of these compound changes the Energy State of metallic surface, leading to rapid passivation that diminishes the tendency of metal to ionize and dissolve. In addition to preventing general attack on ferrous and nonferrous metals, mixed VCIs are found to be effective in preventing galvanic corrosion of coupled metals, pitting and, in some cases hydrogen embrittlement.
VCI performance in industrial and marine environment----------All corrosion rates in mils/yr----------
(1000, 3000, 5000,
|Stainless Steels 410
| 301, 316
| and 321
|1. NI-22790 formulation 2. Dezincification 3. Immune to attack; no pitting or weight loss observed 4. Pitting 5. Pitting reduced 6. Staining 7. No staining 8. Free from pitting and weight loss
WATER TREATMENT APPLICATIONS
Vapor-phase inhibitors have been used for corrosion control in enclosures varying in size from the miniature volume of a hearing aid to the cavities and void spaces in mammoth tankers. There is little or no limitation of usage in relation to the type of atmosphere, and the compounds have found use in local industrial atmospheres containing sulfur dioxide, chlorine or hydrogen sulfide. Enclosures need not be tightly sealed, and the compounds have been used in breathable or periodically opened enclosures.
For the water treatment industry the newly developed treatment programs can be classified as the ambiodic type inhibitor that shows inhibition at cathodic and anodic sites. These treatment programs provide three-phase corrosion protection:
1. In the water phase
2. At the interface between water and air
3. In the air (vapor) phase
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, closed loops systems.
In recent years, various methods were used for corrosion protection during the wet or dry shutdowns.
Boiler Shutdowns (Seasonal/Long term Lay-ups)
The traditional approach for dry lay-up involved two main processes:
1. Nitrogen gas blanketing, and
2. The use of desiccants that must be maintained during the lay-up period and removed prior to boiler startups.
The traditional methods for wet lay-up involved the use of:
1. Use of oxygen scavengers
2. Alkaline chemicals to maintain pH > 10
3. Use of dispersants and/or antiscalants
4. Use of dicyclohexylammonium nitrate and diisobutylammonium sulfate
These chemicals are the source of hydroxide ions and use neutralizing inhibitors that neutralizes hydrogen ion in the environment and become volatile only with steam at use. Examples of neutralizing inhibitors are cyclohexylamine, diethylaminoethanol, morpholine, etc. These compounds are not considered vapor phase inhibitors since at use concentrations they need steam to volatilize. The disadvantages of using silica gel or other desiccants is that once they are saturated with moisture (H20) 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 indirect protection. Oxygen scavengers are not recommended for long-term protection and they are unable to protect against oxygen ingression as well as this method does not protect surfaces not in contact with solution. Moreover these solution must be replenished with time and involves manpower to check levels periodically.
Cooling Tower Shutdowns (Seasonal/Long term Lay-ups)
The traditional approach to seasonal lay-up of cooling towers include two main methods:
1. Wet lay-up (in places where temperature does not drop below freezing)
2. Dry lay-up
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 such product poses a tough challenge for disposal. This practice is environmentally unsound. Another problem with using oil-based products is that they 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.
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 time because they 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 VCI DIFFERENCE
The new method for laying up boilers and cooling towers 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.
2. Close the openings (manholes, etc.).
1. Dissolve VCIs in water (after it is below 60o C) and circulate the water for 4 to 5 hours.
2. 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:
Cooling tower lay-up is also 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.
• Drain the treated water and lay-up the tower.
• Place bags into cooling water and circulate the water for about 6 to 10 hours.
• Lay up the tower with the treated water.
One carton of VCI water-soluble bags treats up to 1000 gallons of cooling water (or space). VCIs are compatible with all non-oxidizing biocides. When using oxidizing biocides, it is necessary to be careful and keep the free chlorine levels in check, under 4 PPM or use a higher concentration of VCIs.
VCIs have the unique ability to control corrosion in the water treatment system. VCIs distinguishes themselves from the conventional contact corrosion inhibitors since they provide:
1. Protection in three phases: vapor phase, liquid phase, and liquid-vapor interface.
2. Biodegradable and environmentally friendly solutions free of nitrites, phosphates, chromates, and heavy metals.
3. Economical solutions for protecting cooling towers, closed loop systems, and boilers during the interim period, seasonal lay-ups or long-term lay-ups.
4. Economical solution for niche applications that requires vapor phase protection.
5. Protection in presence of moisture and other corrosive environment by forming a corrosion inhibiting monomolecular layer of on the metal itself.
6. Savings by eliminating the use of expensive cleaning chemicals.
7. Increased worker safety and efficiency due to the non-toxic, non-hazardous nature of the products packaged in convenient water-soluble bags.
8. Easy start-ups at the end of the lay-up period without any clean up required.
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.
Ashish Gandhi is the Water Treatment and Mothballing Sales Manger at Cortec Corporation. Mr. Gandhi is a chemical engineering graduate from University of Minnesota and a member of AIChE and NACE.
Typical corrosion rates for nonferrous metals exposed to
atmospheres containing VCI.