Volatile Corrosion Inhibitors: Protecting Metal with Vapor

A volatile corrosion inhibitor (VCI) is a chemical that protects metal from rust and other corrosion by slowly releasing a vapor. The vapor spreads through an enclosed space, settles on every exposed metal surface, and forms a very thin protective layer. Because the protection travels through the air, it can reach places that oils, paints and greases cannot easily get to: the inside of a machine, the threads of a bolt, the crevices of a gearbox or the interior of a pipeline.

VCIs are everywhere once you know to look for them. The slightly waxy paper wrapped around new tools and car parts, the tinted plastic bags used to ship machined components, the small foam pads stuck inside electrical cabinets, and the treatments used to mothball pipelines and storage tanks can all be VCI products. They are also known as vapor-phase inhibitors (VPIs), vapor-phase corrosion inhibitors or vapor-transported corrosion inhibitors.

This article explains why corrosion protection matters, how VCIs work at the molecular level, their history, the main chemical types, the ways they are applied, how their performance is tested, their limitations, safety and environmental issues, and some common misconceptions.

Why corrosion protection matters

Corrosion is the gradual destruction of metals by chemical reaction with their environment. For iron and steel, the familiar result is rust. Wikipedia's article on rust explains that iron rusts when it is in contact with both water and oxygen, and that "iron metal is relatively unaffected by pure water or by dry oxygen." Water is the main catalyst. Chloride ions, as in salt water, speed corrosion up, and acidic conditions accelerate it further.

The economic cost is enormous. A study for the US Federal Highway Administration, published in 2002 as "Corrosion Costs and Preventive Strategies in the United States," estimated the total annual direct cost of corrosion in the US at roughly $276 billion in 1998, about 3.2% of US gross domestic product at the time.

A large share of corrosion damage happens not in service but in storage and transport. Metal parts sit in warehouses, travel in shipping containers across oceans, or wait in idle plants and pipelines. Temperature swings cause condensation, humidity creeps in, and polluted or salty air adds to the attack. The rust article notes that silica gel packets are used to control moisture in equipment shipped by sea, for exactly this reason. Traditional answers have been to coat parts in rust-preventive oils or greases, which must later be cleaned off, or to keep them in climate-controlled spaces, which is expensive. VCIs offer a third option.

How a VCI works

The basic sequence

Wikipedia, citing the industry test standard NACE TM0208, describes the VCI process as a sequence of three steps:

1. Volatilization. The inhibitor slowly evaporates (or sublimes) from its source, which may be a coated paper, a plastic film, a powder, a foam emitter or a liquid.
2. Vapor transport. The molecules diffuse through the air in an enclosed space until the space approaches a saturated concentration.
3. Condensation and adsorption. The molecules reach the metal and either attach to the surface or dissolve in the thin film of moisture that is almost always present on metal in humid air, forming a protective molecular layer.

Because the inhibitor keeps evaporating from its source, the protective layer is continuously renewed as long as the space remains enclosed and the source is not exhausted.

What happens on the metal surface

For steel, Wikipedia describes two mechanisms that can operate:

• Adsorption. Inhibitor molecules attach to the metal surface, forming a barrier that blocks corrosive ions and displaces condensed water.
• pH buffering. Inhibitor molecules dissolve into the microscopically thin water layer on the surface and raise its pH. An alkaline surface film favors steel's natural corrosion resistance.

A 2003 article by Christophe Chandler and Boris Miksic of Cortec Corporation, a VCI manufacturer, published in the corrosion engineering magazine Materials Performance, describes the process in more detail. When the vapor condenses, it forms a thin film of microcrystals. In the presence of even minute amounts of moisture, these dissolve and release ions that adsorb onto the metal, forming a molecular film that buffers the surface pH in the range of 5.5 to 8.5.

The same article explains that many modern VCI compounds act as mixed inhibitors, slowing both of the electrochemical reactions involved in corrosion:

• The cathodic reaction, typically oxygen reduction, is slowed because the adsorbed film acts as a diffusion barrier for oxygen.
• The anodic reaction, the dissolution of metal, is slowed because the inhibitor molecules bond chemically to the metal surface and reduce the tendency of metal atoms to ionize.

The authors describe the inhibitor molecule as having a functional group that anchors it firmly to the metal, a central nucleus, and another functional group that controls the thickness and penetrability of the film, forming "a continuous line of defense" against aggressive ions.

Copper is different

Copper and its alloys need different chemistry. Wikipedia notes that for copper, the inhibitor forms a copper benzotriazole complex. Benzotriazole (BTA, chemical formula C6H5N3) is the classic copper inhibitor. According to Wikipedia's benzotriazole article, a copper surface exposed to BTA develops a passive layer consisting of a complex between copper and benzotriazole, a copper(I) coordination polymer that is insoluble in water and many organic solvents. The thicker this layer, the better the protection. BTA is also used in heritage conservation to treat "bronze disease," the progressive chloride-driven corrosion of ancient bronzes.

The vapor-pressure balancing act

The single most important property of a VCI compound is its vapor pressure: how readily it evaporates at a given temperature.

• Too low, and the inhibitor evaporates so slowly that it takes a long time to build up a protective concentration. During that time, the metal is exposed.
• Too high, and the inhibitor evaporates quickly, escapes through any leak, and is soon used up, so protection is short-lived.

Wikipedia gives the most favorable vapor pressure range as about 10⁻³ to 10⁻² pascals at room temperature. For comparison, standard atmospheric pressure is about 101,325 pascals, so these are extremely low vapor pressures. The inhibitor is "volatile" only in a very gentle sense.

The Chandler and Miksic article adds that vapor pressure rises with temperature, and so does the tendency of metals to corrode. A well-chosen VCI therefore releases more inhibitor exactly when the risk of corrosion is highest, which the authors describe as a kind of self-adjustment to the aggressiveness of the environment.

Worked example: converting a historic vapor pressure (calculation)

The first widely used VCI, dicyclohexylammonium nitrite, was described in its 1948 US patent as having a vapor pressure of 0.00016 mm of mercury at 75 °F (about 24 °C). One millimetre of mercury equals 133.322 pascals, so:

0.00016 mmHg × 133.322 Pa/mmHg ≈ 0.021 Pa

That puts it at the upper edge of, and slightly above, the 10⁻³ to 10⁻² Pa range that Wikipedia describes as most favorable, consistent with a compound that builds up protection reasonably quickly.

A history of VCIs

The DICHAN patent

The first wide-scale use of VCIs is traced to a Shell patent for dicyclohexylammonium nitrite, often abbreviated DICHAN. US Patent 2,449,962, "Dicyclohexylammonium nitrite and its preparation," was filed on 26 March 1946 by Aaron Wachter and Nathan Stillman of Berkeley, California, and granted on 21 September 1948. It was assigned to Shell Development Company of San Francisco.

The patent describes the compound as a white solid that melts with decomposition at about 200 to 201 °C, with low water solubility (3.9% at 25 °C) and much better storage stability than related amine nitrites. It lists several uses, starting with use as a vapor-phase corrosion inhibitor, "such as in paper wrapping for corrosion-proof packaging of steel." Other listed uses included inhibiting corrosion in water-containing fluids, as a lubricating-oil additive, and in plastics and protective coatings.

The compound was commercialized as VPI 260. According to Wikipedia, the US military used DICHAN widely in several forms: VCI powder, VCI paper, VCI solution and VCI slushing compound (a temporary protective coating).

Origins in mothballing and the tropics

Chandler and Miksic write that VCIs were originally developed to protect the boilers and piping systems of ships being mothballed, and originally designed to protect ferrous metals in tropical environments. Over the following decades, they were adapted to protect electronics, packaged goods, reinforced concrete, coated metals and metalworking fluids.

Moving beyond DICHAN

Health, safety and performance concerns led to DICHAN being replaced by newer compounds. One relevant issue appears in the original patent itself: during preparation, free acid had to be minimized to avoid forming nitrosoamine (nitrosamine) by-products. Nitrosamines are a family of compounds of significant toxicological concern. Modern VCIs are generally based on other chemistry.

Timeline

The chemistry of modern VCIs

Wikipedia describes modern VCIs as typically salts of moderately strong bases and weak volatile acids. Chandler and Miksic describe the active ingredients as usually the products of a reaction between a weak, volatile base and a weak, volatile acid. Such salts ionize in water but undergo substantial hydrolysis, which helps keep the protective film stable across a range of conditions.

The Chandler and Miksic article includes vapor-pressure data for dicyclohexylammonium nitrate, cyclohexylammonium benzoate and diisopropylammonium nitrate, showing how saturated vapor pressure rises with temperature for each. The specific "VCI A" they tested was based on amine carboxylates, with a melting point starting at 188 °C.

Formulations are tailored to the application (films, oils, coatings, cleaners) and to the metal: ferrous, non-ferrous or multi-metal. A product designed only for steel may be unsuitable or even harmful for copper or zinc, which is why multi-metal formulations combine several inhibitors.

Forms and applications

VCIs are delivered in many forms. Wikipedia and the Chandler and Miksic article describe the following uses.

Packaging paper and film

VCI paper came first, followed by VCI film, in which inhibitors are incorporated into polyethylene. Sodium nitrite can also be impregnated directly into film. Wrapping parts in VCI packaging can eliminate the need to coat them in rust-preventive oil, which means the parts can go straight from the package into assembly without degreasing. Films can be heat-shrunk over large equipment for outdoor storage, and some VCI films are now biodegradable.

Emitters for electrical and electronic enclosures

Capsules, foams or cups containing VCI are placed inside electrical cabinets, junction boxes and control panels. Wikipedia notes that these emitters provide protection against hydrogen sulfide, sulfur dioxide, ammonia and humidity, without altering the electrical, surface or optical properties of the components. Chandler and Miksic note that typical devices protect a volume of 1 to 40 cubic feet (0.03 to 1.13 cubic metres) for two years, and that devices may also include a fungistat to control fungal growth and a volatile buffer to keep surface pH uniform.

Coatings

VCIs have gained popularity in paints and coatings as alternatives to inhibitors that contain heavy metals. Added in small amounts, they adsorb onto the metal under the coating and form a hydrophobic moisture barrier.

Oils

VCI-treated oils protect engines and hydraulic systems that are used intermittently or are being mothballed. The vapor protects surfaces above the oil level that the liquid itself does not touch. VCI oil can also be fogged into voids.

Large enclosed spaces

Tanks, pressure vessels, boilers, piping and heat exchangers can be protected internally by fogging VCI, blowing in VCI powder, or hanging packets inside. Chandler and Miksic note that for larger equipment, the powder is applied by misting and the enclosure is then sealed, with no additional dehumidification or oxygen scavenging needed. Where an enclosure can be sealed airtight, it can be evacuated and then repressurized so that the powder is drawn in.

Pipeline casings

Where a pipeline passes under a road or railway, it is often run inside a larger steel casing pipe. If the carrier pipe and casing touch electrically (a "shorted" casing), the cathodic protection system that normally protects the pipeline can be compromised. Wikipedia describes injecting a mixture of VCI and hydrogel into the annular space between the casing and the carrier pipe, and notes that the US Pipeline and Hazardous Materials Safety Administration (PHMSA) has approved this method for addressing a shorted casing on a cathodically protected pipeline. The relevant industry practice is NACE SP0200, "Steel-Cased Pipeline Practices."

Internal pipeline preservation

New pipelines that have been hydrotested, and pipelines taken out of service, often contain residual water, which can cause corrosion. VCIs are used in several ways:

• Piggable lines: an aqueous VCI solution is pushed through the line between two pigs (cylindrical devices that travel through the pipe) after hydrotesting.
• Unpiggable lines: VCI solution is added at high points so that it flows down into low sections holding residual water.
• Idled sections: VCI solution fills low points to a set depth.

VCIs are also used against top-of-the-line corrosion in wet gas pipelines, where water condenses on the cooler upper wall of the pipe. Liquid corrosion inhibitors carried in the gas stream tend to stay at the bottom of the pipe, while a volatile inhibitor can reach the top.

Chandler and Miksic describe a Norwegian oil and gas operator in the North Sea that replaced an oil-water emulsion with a VCI for hydrotesting and internal preservation. VCI was fogged into hydrotested metering stations at a ratio of 500 g per cubic metre, and the openings were capped. After hydrotesting, aqueous solutions, usually at 1% to 3.5% concentration, were stored for reuse or discharged.

Aboveground storage tank bottoms

The underside of a storage tank floor, which sits on sand, concrete or a liner, is difficult to inspect and protect. Wikipedia describes VCI methods matched to different foundation types:

Tank roofs and headspaces

Dispensers attached to roof ports hold VCI bottles. The VCI saturates the dispenser and diffuses into the tank's vapor space. Wikipedia notes this matters most for tanks storing sour crude oil, whose vapors contain corrosive hydrogen sulfide.

Water treatment and flushing

Aqueous VCI solutions are used to flush and rinse pipelines, pumps, manifolds, pits and heat exchangers before they are stored.

Specialty covers

VCI film covers are made for flanges and valves in chemical plants and on offshore platforms, where equipment sits exposed to marine air.

How much VCI is needed?

Manufacturers provide rules of thumb for dosing. The Chandler and Miksic article gives an empirical formula for VCI powder in large enclosures:

Q = 0.0277 × V × C

where Q is the quantity of VCI powder in ounces, V is the enclosure volume in cubic feet, and C is a confidence factor between 2 and 3. They give a nominal cost of about $0.01 per cubic foot per year (in 2003 terms).

Worked example (hypothetical). A sealed vessel with an internal volume of 1,000 cubic feet (about 28 cubic metres), using a confidence factor of 2.5:

• Q = 0.0277 × 1,000 × 2.5 = 69.25 ounces
• 69.25 oz ÷ 16 = about 4.3 pounds
• 69.25 oz × 28.35 g/oz ≈ 1,960 grams, or just under 2 kg
• Nominal annual cost: 1,000 ft³ × $0.01 = about $10 per year, at the article's 2003 nominal rate

For small enclosures, the same article describes estimating the number of emitter devices as:

N = k_a × k_p × k_s × N₀

where N₀ is the number of devices based on nominal volume, and the k factors adjust for the corrosiveness of the environment (k_a), how often the enclosure is opened or how much it breathes (k_p), and its shape (k_s). This formula captures a practical truth: a cabinet that is opened daily in a coastal chemical plant needs much more inhibitor than a sealed box in a dry warehouse.

These formulas are manufacturer guidance, not universal laws. Actual dosing should follow the specific product's instructions.

How VCIs are tested

The Vapor Inhibiting Ability test

The standard laboratory method for checking whether a VCI works is the Vapor Inhibiting Ability (VIA) test. NACE TM0208 is the industry standard laboratory test for evaluating the vapor-inhibiting ability of VCI materials for temporary protection of ferrous metal surfaces. The Chandler and Miksic article describes a version based on US Federal Test Method Standard 101C, Method 4031, in detail:

1. Specimen. Carbon steel plugs (UNS G1018) about 1.6 cm in diameter and 1.3 cm long are polished with abrasives, cleaned with methanol and dried.
2. Humid atmosphere. A glycerin-water solution is placed in the bottom of a 1-litre jar to create a relative humidity of about 90%.
3. Inhibitor. A small amount of VCI (0.05 g in their test) is placed in a dish in the jar, not touching the steel. A control jar has no VCI.
4. Conditioning. The sealed jar is held at 24 °C for 20 hours so the vapor can build up.
5. Condensation challenge. Cold water, 4 °C below ambient, is added to an aluminum tube attached to the steel plug, chilling it so moisture condenses on its surface. After 3 hours, the water is removed.
6. Evaluation. The plug is inspected. Visible pitting or etching counts as corrosion; staining alone does not.

In their test, the control plug showed heavy corrosion, while the plugs exposed to VCI vapor showed none. Because the VCI never touched the steel, the result demonstrated that protection arrived through the vapor phase.

Field and long-term testing

The same article presents data from a manufacturer's formulation (designated NI-22790) comparing corrosion rates in industrial and marine atmospheres with and without VCI:

A mil is one thousandth of an inch, or 25.4 micrometres. For mild steel, 21.8 mils per year is about 554 micrometres per year, while 0.13 mils per year is about 3.3 micrometres per year. These are manufacturer-reported figures for one formulation, so they show what is possible under the reported conditions rather than what every product achieves.

The condensation problem in storage and shipping

To see why VCIs are so useful, it helps to think about what actually wets metal in storage. Most stored parts are never splashed or rained on. Instead, water arrives as condensation. When warm, humid air meets a cooler metal surface, the air next to the metal cools below its dew point and deposits a thin film of liquid water. A crate that sits in a warm warehouse by day and a cold one at night, or a container that crosses from a humid port into cooler waters, can go through this cycle repeatedly.

That thin film of condensed water is exactly what corrosion needs. As the rust article explains, iron is relatively unaffected by dry oxygen or by pure water alone, but the combination of water and oxygen, especially with dissolved chlorides from sea air, drives rusting. Every condensation event brings fresh water into contact with the metal, and any salt or pollutant dust on the surface dissolves into it.

The VIA test is built around this scenario. Its key step chills the steel plug to 4 °C below the surrounding air in a jar held at around 90% relative humidity, forcing moisture to condense on it. A VCI passes only if the steel stays free of pitting and etching through this challenge. In effect, the test asks whether the inhibitor film already sitting on the metal, delivered entirely through the vapor, is strong enough to protect it when water suddenly arrives. This is also why the pH-buffering mechanism matters: the inhibitor dissolves into the condensed water as it forms, making that water less aggressive.

Top-of-the-line corrosion in gas pipelines

One of the more specialized uses of VCIs illustrates their unique advantage. In pipelines carrying wet natural gas, the gas cools as it travels, and water vapor condenses on the cooler pipe wall. Liquid water collects at the bottom of the pipe, where conventional liquid corrosion inhibitors injected into the line can reach it. But condensation also forms on the upper inside wall, and that water is fresh, often acidified by dissolved carbon dioxide and organic acids such as acetic acid, and out of reach of inhibitors that travel with the liquid at the bottom. Research presented at NACE's 2004 Corrosion conference by Singer, Nesic and Gunaltun examined top-of-the-line corrosion in the presence of acetic acid and carbon dioxide.

A volatile inhibitor can travel with the gas phase and condense along with the water at the top of the pipe. Wikipedia lists azoles, certain acetylene alcohols and a "green" volatile aldehyde among the compounds tested for this purpose, and a 2013 article in Materials Performance by Miksic and colleagues addressed VCIs for top-of-the-line corrosion specifically.

Practical guidance for using VCI packaging

The principles above translate into a few practical rules, drawn from how the sources describe the mechanism and its limits:

1. Identify the metals. Choose a ferrous, non-ferrous or multi-metal formulation to match every metal in the package, including small fittings and plated parts.
2. Start with clean, dry parts. VCIs slow corrosion; they do not remove rust or contamination already present.
3. Enclose the parts. Wrap, bag or box them so the vapor can build up and stay. Close or seal openings, because air exchange carries the inhibitor away.
4. Keep the source close. The inhibitor has to diffuse from its source to every surface. In large or oddly shaped enclosures, more than one source may be needed, which is what the shape factor in the emitter formula accounts for.
5. Allow time. There is an initial period while the vapor concentration builds. Packaging parts well before they are exposed to harsh conditions reduces risk.
6. Minimize openings. Each opening lets the vapor escape and humid air in. Enclosures that are opened often need more inhibitor.
7. Replace on schedule. Note the rated life of emitters and replace them before they are exhausted.
8. Follow the safety data sheet. Handle VCI chemicals as directed, and ventilate large treated enclosures before entering them.

These rules echo the structure of the manufacturer formula N = k_a × k_p × k_s × N₀: the corrosiveness of the environment, how often the enclosure breathes, and its shape all change how much inhibitor is needed.

Advantages of VCIs

• Reach. Vapor can reach recesses, crevices, threads and internal cavities that sprays and dips miss.
• No surface preparation. Chandler and Miksic note that VCIs do not require the metal surface to be prepared before use.
• Clean removal. Parts packaged in VCI paper or film often need no degreasing before use, unlike oil-coated parts.
• Self-renewal. As long as the enclosure stays closed and the source lasts, the film is continually replenished.
• Multi-metal protection. Combined formulations can protect steel, copper, aluminum and other metals in the same package.
• No effect on function. Wikipedia notes that emitters in electrical enclosures do not alter electrical, surface or optical properties of components.
• Low cost per volume. The manufacturer estimate of about $0.01 per cubic foot per year illustrates how little material is needed.

Limitations

They need an enclosure

The whole mechanism depends on the vapor reaching and maintaining a protective concentration. Wikipedia's rust article notes that VCIs lose effectiveness when air circulation disperses them and brings in fresh oxygen and moisture. An open rack in a windy yard is not a suitable environment. Packaging must be closed, cabinets sealed and pipelines capped.

Time to build up

Low vapor pressure gives long-lasting protection but means the vapor takes time to reach a protective level. Chandler and Miksic point out that the metal is exposed to increased risk during this initial saturation period.

Metal compatibility

Formulations must suit the metals present. Some inhibitors that protect steel can attack non-ferrous metals. This is why manufacturers specify whether a product is ferrous, non-ferrous or multi-metal.

Finite life

Every VCI source eventually runs out. Emitters are rated for a period, often around two years for small devices according to the Chandler and Miksic article, and must be replaced.

Temporary protection

VCIs are designed for storage, shipping and idle periods. TM0208 itself refers to "temporary protection." They are not a substitute for paint, galvanizing or cathodic protection on structures in permanent service. For comparison, Wikipedia's rust article notes that a hot-dip galvanized zinc coating 85 micrometres thick wears at about 1 micrometre per year in normal weather, giving roughly 85 years of protection, a very different design goal.

Does not fix existing damage

VCIs slow corrosion; they do not reverse it. Heavily rusted parts need cleaning before VCI packaging makes sense.

Safety and environmental considerations

Toxicity

Early VCI chemistry raised legitimate concerns. The DICHAN patent itself warns about nitrosamine by-products, and DICHAN was eventually replaced partly because of health and safety concerns. Wikipedia's article on dicyclohexylamine, one of the parent amines, lists an oral LD50 in rats of 373 mg/kg (a value the article flags as needing clarification). Benzotriazole has a rat oral LD50 of 500 mg/kg according to Wikipedia, carries hazard statements for being harmful if swallowed and causing serious eye irritation, and is classified as toxic to aquatic life with long-lasting effects. It is not readily degraded and only partly removed in wastewater treatment, so substantial amounts reach surface water.

"Environmentally friendly" VCIs

Manufacturers have developed formulations aimed at lower environmental impact. Chandler and Miksic report tests on their amine carboxylate "VCI A" using OECD and ISO methods:

These are a manufacturer's own results for its own product, published in a trade magazine, and should be read with that in mind. They do illustrate the kind of testing used to assess VCI environmental impact and the direction the industry has moved since the nitrite era.

Practical handling

VCI products are chemicals and should be handled according to their safety data sheets. Because emitters are placed in enclosed spaces, people opening large VCI-treated vessels should follow confined-space procedures and ventilate before entry. Anyone with health concerns about exposure should consult a doctor and the product's safety data sheet.

VCIs compared with other protection methods

In practice, these methods are often combined. VCI packaging may include desiccants, VCIs can be added to coatings and oils, and VCI treatments are used alongside cathodic protection in pipeline casings and tank bottoms.

Common misconceptions

"VCIs coat the metal in a thick film like oil"

The protective layer is molecular, typically invisible and dry to the touch. That is why VCI-protected parts can often be used without cleaning.

"Any plastic bag will do the same thing"

An ordinary bag only separates the part from the outside air; whatever moisture is trapped inside stays there, and temperature swings can still bring it onto the metal as condensation. A VCI bag contains active chemicals that deposit on the metal, which is what the condensation step of the VIA test is designed to check.

"VCIs work in open air"

They need an enclosed space to build and keep a protective vapor concentration. Air movement carries the inhibitor away.

"One VCI protects every metal"

Formulations are designed for ferrous metals, non-ferrous metals, or several metals together. Using the wrong product can leave some metals unprotected.

"VCIs are a permanent solution"

They are temporary protection for storage, transport and idle periods. Sources run out, and enclosures are eventually opened.

Frequently asked questions

How long does VCI protection last?

It depends on the product, the enclosure and the environment. Manufacturer data cited here describe typical small emitters protecting enclosures of 1 to 40 cubic feet for about two years. Opening the enclosure frequently, high temperatures and leaky packaging all shorten protection time.

Do VCI parts need to be cleaned before use?

Usually not. Wikipedia notes that VCI packaging can eliminate the need for rust-preventive oils, and Chandler and Miksic note that VCIs avoid the cleaning and disposal requirements associated with petroleum-based products. Specific applications, such as surfaces to be painted or bonded, should follow the manufacturer's guidance.

Can VCIs protect electronics?

Yes. Emitters are widely used in electrical cabinets and junction boxes, where Wikipedia notes they protect against hydrogen sulfide, sulfur dioxide, ammonia and humidity without altering the electrical, surface or optical properties of components.

What is the difference between a VCI and an ordinary corrosion inhibitor?

Ordinary (contact) corrosion inhibitors are added to a liquid that touches the metal, such as boiler water, coolant, fuel or hydraulic fluid. Wikipedia's corrosion inhibitor article describes examples such as phosphates that form scale in water pipes and zinc dithiophosphates in lubricants. A VCI does the same basic job, forming a protective film, but delivers the inhibitor through the air, so it can protect surfaces that no liquid reaches. Many products combine both, such as VCI oils that protect submerged parts by contact and surfaces above the oil level by vapor.

Can VCIs be used together with desiccants?

Yes. The two attack different parts of the problem: desiccants such as silica gel remove moisture from the enclosed air, while VCIs make the metal surface resistant to whatever moisture remains or condenses. Using both together covers moisture removal and surface protection at the same time.

What standard covers VCI testing?

NACE TM0208 is a laboratory test for evaluating the vapor-inhibiting ability of VCI materials for temporary protection of ferrous metal surfaces. NACE SP0200 covers steel-cased pipeline practices, including VCI fill of casings.

Why was the first VCI replaced?

Dicyclohexylammonium nitrite was replaced due to health, safety and performance concerns, including the risk of nitrosamine by-products that its own patent warned about. Modern VCIs use other amine salts, carboxylates and azoles.

Are VCIs used in museums?

Benzotriazole, one of the key copper inhibitors, is widely used in heritage conservation, particularly to treat bronze disease.

Summary

Volatile corrosion inhibitors protect metal by sending their protection through the air. From a paper wrapper, plastic film, foam emitter, powder or liquid, inhibitor molecules slowly evaporate, fill an enclosed space, and settle on metal surfaces as a thin film that blocks corrosive species, buffers surface moisture, and slows both halves of the corrosion reaction. The idea entered wide use with Shell's dicyclohexylammonium nitrite, patented in 1948 and used extensively by the US military, and has since expanded into amine carboxylates, azoles for copper, and formulations aimed at lower environmental impact.

Today VCIs protect everything from boxed spare parts and electrical cabinets to pipeline casings, idle pipelines and the undersides of storage tanks. Their strengths are reach, cleanliness, multi-metal protection and low cost per volume. Their limits are equally clear: they need an enclosure, they take time to build up, they must match the metals present, and they provide temporary rather than permanent protection. Used within those limits, they are one of the simplest and most effective tools for keeping metal from rusting before it ever goes into service.

Sources

• Volatile corrosion inhibitor (Wikipedia)
• Corrosion inhibitor (Wikipedia)
• Corrosion (Wikipedia)
• Rust (Wikipedia)
• Benzotriazole (Wikipedia)
• Dicyclohexylamine (Wikipedia)
• US Patent 2,449,962: Dicyclohexylammonium nitrite and its preparation (Google Patents)
• Chandler, C. and Miksic, B. A. (2003), Environmentally Friendly VCIs, Materials Performance (Cortec Corporation)
• NACE SP0200-2014, Steel-Cased Pipeline Practices
• Koch et al. (2001), Corrosion Costs and Preventive Strategies in the United States, FHWA/NACE