How VCI Works

In the early 1900's, it was discovered that an organic chemical could protect metal from corrosion by vaporising and depositing on the metals. This compound was developed by Shell after the Second World War to protect military equipment.

Vapour Corrosion Inhibitors or VCIs normally come as solids, for convenience in handling. Volatility is simply a means of transport. Protective vapours disseminate within an enclosed space until equilibrium--determined by the partial vapour pressure--is reached. The inhibiting process begins when the vapours come in contact with the metal surface and condense to form a monomolecular barrier. In the presence of even minute traces of moisture, the molecules 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 or vapour Corrosion inhibitors migrate to distant metallic surfaces. VCIs need only to be placed in the vicinity of the metals to provide protection. VCI molecules have a natural affinity to metals and will migrate to metallic surfaces through the vapour phase. The inhibitor will be adsorbed on the surface and the protective vapours will distribute within the enclosed space until equilibrium is reached. VCI protection to metals is based upon conditioning the entrapped atmosphere within the package with vapour Corrosion inhibitors (VCI). The inhibition materials used are organic in nature and are impregnated into the various elements of the packaging. When VCI molecules come in contact with moisture molecules, they convert the moisture molecules into non-corrosive electrolytes, thus creating a monomolecular barrier layer on the metal surface preventing any direct contact between moisture and metal. They continue to protect even in case of change in pressure due to a breach in the packaging. This is due to the presence of inhibition materials in the vicinity that continue to emit vapours, which replenish the molecules lost to the outside atmosphere.

Proper selection of VCI compounds enables controlled and dependable vaporisation. The higher the temperature, the greater the tendency of the metal toward corrosion. The vaporisation rate of VCIs has a similar function depending on 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.

Vapour 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 overall metal protection, i.e., they protect most commonly used ferrous and nonferrous metals and alloys.

Strong inhibition of the anodic reaction results from the inhibitor's having two acceptor-donor adsorption centres that form a chemical bond between the metal and the inhibitor. Adsorption of these compounds change the energy state of metallic surface, leading to rapid passivation that diminishes the tendency of metal to ionise 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 attack and, in some cases hydrogen embrittlement.