 |
 |
 |
|
|
|
By Ashish Gandhi, Cortec Corporation
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.
CHEMICAL MAKEUP
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----------
|
|
|
|
| Aluminum (1000, 3000, 5000, 6000 series) |
2.15 | <0.25 |
| Carbon Steel | 21.8 | <0.13 |
| HSLA (high-strength Low-alloy steel) |
1.2 | 0.08 |
| Naval brass | 0.22 | 0.03 |
| Titanium | 0.003 | 0.003 |
| Stainless Steels 410 | 0.014 | 0.015 |
| 304 | <0.16 | 0.017 |
| 301, 316 | ||
| and 321 | 0.008 | 0.008 |
| Copper | 0.226 | 0.017 |
Notes:
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:
Boiler Lay-up:
Dry Lay-up:
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.).
Wet Lay-up:
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.
Flushing Lay-up:
• Place bags into cooling water and circulate the water for 6 to
10 hours.
• Drain the treated water and lay-up the tower.
Wet Lay-up:
• 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.
ADVANTAGES:
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.
REFERENCES
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.
Figure 1.
Typical corrosion rates for nonferrous metals exposed to
atmospheres containing VCI.
[About Cortec] [What's New] [Publications] [Products] [Contact Us] [Related Links] [International]