to Metals
to Corrosion and oxidation
Dr. Dmitri Kopeliovich
Volatile corrosion inhibitors (VCI) are substance molecules of which vaporize at a significant pressure and then adsorb on the metal surface forming a thin corrosion protection layer.
VCI are capable to reduce corrosion in enclosed spaces (e.g. package bags).
The protective vapor is emitted by a VCI containing material and then it is distributed throughout the space migrating to the part surface and penetrating into crevices, cracks, small holes and other hard-to-reach regions. The vapors condense on the metal surface in form of microscopic crystals, which dissolve in the moisture film present on the surface.
The ions of the dissolved VCI displace water molecules from the metal surface and form monomolecular invisible protection film reducing the corrosion rate.
The protective VCI film serves as a barrier repelling water and decreasing the diffusion of Oxygen to the metal surface. VCI molecules also controls the PH level in the electrolyte.
Effectiveness of VCI depends on their vapor pressure. Too high vapor pressure allows fast evaporation and distribution of the inhibitor in the surrounding atmosphere. However the resulting protective layer is unstable. Its molecules have a tendency to vaporize from the metal surface.
Too low vapor pressure produces a stable protective layer but the process of its formation is very slow.
First VCI were anodic corrosion inhibitors (sodium nitrite, sodium benzonate). They were developed for protection of ferrous metals. Anodic VCI restrict the anodic reactions by the mechanism of anodic passivation when a thin film of VCI prevents corrosion of the metal.
Anodic VCI are hazardous substances. They also may attack non-ferrous alloys.
Later mixed (ambiodic) Volatile Corrosion Inhibitors (VCI) have been developed. They are capable to combine the effects of both anodic and cathodic inhibitors. A depressing of the cathodic reaction is achieved due to the restriction of the oxygen transportation.
Volatile corrosion inhibitors may be added to various package materials: polymer film (eg. low density polyethylene), paper, foam, powder, powder sachets, oils, tablets. carton chips, etc.).
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Most Volatile Corrosion Inhibitors (VCI) are aminocarboxilates - products of reaction between an organic acid with an amine or amine derivatives.
The following compounds are used as Volatile corrosion inhibitors (VCI):