Inhibitors have become a common measure for dealing with the complex chemical reaction known as corrosion. Inhibitors consist of chemical compounds that decrease the rate of corrosion in metals and metal alloys. The most common industrial corrosion inhibitors come in either spray or liquid form and are applied to components such as tanks, vessels, pipes and other structures operating in harsh environments.
Volatile corrosion inhibitors (VCI), on the other hand, operate a bit differently. They are used to provide corrosion protection to metal components in enclosed areas during long-term storage or shipping. (For more on storage and shipping, read Temporary Corrosion Protection During Storage, Transportation and Handling.) As opposed to traditional corrosion inhibitors that are commonly available in spray or liquid form, VCIs work by emitting gaseous corrosion-inhibiting chemicals (a vapor) from various mediums such as paper, plastic, foam and powder.
The types of VCIs, how they work and their suitable applications will be discussed in detail in this article.
What is a Volatile Corrosion Inhibitor (VCI)?
Electrochemical corrosion is a natural redox reaction that occurs when a metal is exposed to air and moisture. The process involves the flow of electrons from the anode (the metal) to the cathode (oxygen) through an electrolyte (moisture) and leads to the formation of rust. Volatile corrosion inhibitors slow this reaction by passivating the surface and suppressing mechanisms that trigger the oxidation reaction, or by decreasing the corrosiveness of the surrounding environment.
The corrosion inhibiting compounds coated on the VCI packaging material (paper, plastic or foam) are dispersed in vapor form (volatilized) into the surrounding atmosphere. The VCI molecules migrate from the coated media directly to the surface of the metal to form a thin molecular layer of protection. Because VCIs depend on the diffusion of chemicals into the air, they are most effectively used in enclosed environments with limited air circulation.
Types of VCIs and Their Application
VCIs work in different ways and are available in various forms. The choice of VCI is ultimately dependent on several factors, such as:
- The properties of the material to be protected
- The size and shape of the enclosed area
- The environment the metal will be exposed to (humidity, temperature, presence of pollutants, etc.)
- The duration of storage or shipping
Listed below are some of the different types of VCIs and their suitable applications.
VCI Packaging Film
This type of protection consists of polyethylene bags, covers and stretch/shrink film. Films are frequently used to wrap items such as large electrical equipment, irregularly shaped parts and other bulky metal components. The versatility and flexibility of VCI films can also be used to package large machinery during transportation.
VCI packaging films also offer an additional layer of protection by acting as a barrier that prevents moisture from coming into contact with the metal being protected. Additionally, VCI polyethylene bags are transparent, which allow inventory parts to be identified and inspected without removing or opening the bags.
VCI Packaging Paper
Using VCI packaging paper involves covering metal components in paper that has been directly coated with corrosion-inhibiting compounds. The paper can be wrapped or draped over metal components. Alternatively, sheets of VCI paper can be used to layer boxes, containers, crates and barrels containing metal items to be protected.
The properties of the packaging paper increase the rate at which the corrosion-inhibiting chemicals are released into the air and onto the metal. This characteristic makes VCI packaging paper better suited for time-critical applications where corrosion protection is needed immediately to prevent the rapid onset of corrosion. Porous metals such as cast iron, which are susceptible to flash rusting, are perfect examples of metals that are better protected using VCI paper products. (An in-depth discussion of flash rust can be found in the article 7 Things to Know About Flash Rust.)
VCI Packaging Foam
VCI packaging foam is available in various shapes and sizes and is usually placed in enclosed spaces or packages that do not provide corrosion protection. Common applications for VCI packaging foam include inside electrical cabinets/panels, boxes, crates and airtight packages. The foam simply needs to be placed within the enclosed storage area and, unlike paper and plastic VCI packaging, does not need to cover or drape over the entire object or the storage container. This makes it ideal for situations where it is not feasible to keep items wrapped or covered, such as electrical cabinets.
As an additional benefit, VCI foam packaging can provide cushioning for shipping and transporting fragile equipment.
VCI Packaging Emitters, Powders and Tablets
VCI emitters work in a manner similar to VCI packaging paper, plastics and foams by releasing corrosion-inhibiting chemicals into an enclosed area. However, this method uses small (usually palm-sized) packets, breathable pouches, foam or wire strap emitters placed at strategic locations in the enclosed area.
VCI emitters can be used to protect the same enclosed spaces as packaging foam. However, because of their relatively small size, they can be:
- Installed and removed quickly and placed in non-visible locations
- Easily combined with other VCI products such as paper or plastic
Factors that Affect the Efficiency of VCIs
Most VCI products have an in-service life of 2 to 5 years; this means that metal components are expected to be protected during this time frame. However, for VCIs to perform at maximum efficiency, the surface of the item being protected must be clean and free of moisture. Contaminants such as dust, oil, grease or fingerprints affect the VCI's ability to coat and protect the substrate. (Related reading: Anti-Corrosion Coatings for Different Service Exposures.) In addition, a continuous exchange of air within the enclosure, such as by subsequent opening and closing, can disperse the protective layer, therefore increasing the potential for corrosion.
Conclusion
VCIs are an effective method to protect metal items intentionally enclosed for long periods of time.
Several factors should be taken into consideration before deciding which VCI product is most suitable for a given situation. To protect sensitive equipment requiring rapid protection, VCI packaging methods may be combined to enable faster saturation of the corrosion-inhibitor compounds.
It should be noted, however, that VCI products are not effective on contaminated surfaces, non-ventilated areas and metal items with preexisting corrosion.