VCI (Volatile Corrosion Inhibitor) method
Mode of action and use
Inhibitors are substances capable of inhibiting or suppressing chemical reactions. They may be considered the opposite to catalysts, which enable or accelerate certain reactions.
Unlike the protective coating method, the VCI method is an active corrosion protection method, as chemical corrosion processes are actively influenced by inhibitors.
In simple terms, the mode of action (see Figure 1) is as follows: due to its evaporation properties, the VCI substance (applied onto paper, cardboard, film or foam supports or in a powder, spray or oil formulation) passes relatively continuously into the gas phase and is deposited as a film onto the item to be protected (metal surfaces). This change of state proceeds largely independently of ordinary temperatures or humidity levels. Its attraction to metal surfaces is stronger than that of water molecules, resulting in the formation of a continuous protective layer between the metal surface and the surrounding atmosphere which means that the water vapor in the atmosphere is kept away from the metal surface, so preventing any corrosion. VCI molecules are, however, also capable of passing through pre-existing films of water on metal surfaces, so displacing water from the surface. The presence of the VCI inhibits the electrochemical processes which result in corrosion, suppressing either the anodic or cathodic half-reactions. Under certain circumstances, the period of action may extend to two years.
Mode of action and use
Inhibitors are substances capable of inhibiting or suppressing chemical reactions. They may be considered the opposite to catalysts, which enable or accelerate certain reactions.
Unlike the protective coating method, the VCI method is an active corrosion protection method, as chemical corrosion processes are actively influenced by inhibitors.
In simple terms, the mode of action (see Figure 1) is as follows: due to its evaporation properties, the VCI substance (applied onto paper, cardboard, film or foam supports or in a powder, spray or oil formulation) passes relatively continuously into the gas phase and is deposited as a film onto the item to be protected (metal surfaces). This change of state proceeds largely independently of ordinary temperatures or humidity levels. Its attraction to metal surfaces is stronger than that of water molecules, resulting in the formation of a continuous protective layer between the metal surface and the surrounding atmosphere which means that the water vapor in the atmosphere is kept away from the metal surface, so preventing any corrosion. VCI molecules are, however, also capable of passing through pre-existing films of water on metal surfaces, so displacing water from the surface. The presence of the VCI inhibits the electrochemical processes which result in corrosion, suppressing either the anodic or cathodic half-reactions. Under certain circumstances, the period of action may extend to two years.
Figure 1: Mode of action of VCI
The mode of action dictates how VCI materials are used. At item to be protected is, for example, wrapped in VCI paper. The metallic surfaces of the item should be as clean as possible to ensure the effectiveness of the method. The VCI material should be no further than 30 cm away from the item to be protected. Approximately 40 g of active substances should be allowed per 1 m³ of air volume. It is advisable to secure this volume in such a manner that the gas is not continuously removed from the package due to air movement. This can be achieved by ensuring that the container is as well sealed as possible, but airtight heat sealing, as in the desiccant method, is not required.
The VCI method is primarily used for articles made from carbon steel, stainless steel, cast iron, galvanized steel, nickel, chromium, aluminium and copper. The protective action provided and compatibility issues must be checked with the manufacturer.
N.B.: The use of water-miscible, water-mixed and water-immiscible corrosion protection agents, corrosion protection greases and waxes, volatile corrosion inhibitors (VCI) and materials from which volatile corrosion inhibitors may be released (e.g. VCI paper, VCI films, VCI foam, VCI powder, VCI packaging, VCI oils) is governed by the German Technical Regulations for Hazardous Substances, TRGS 615 "Restrictions on the use of corrosion protection agents which may give rise to N-nitrosamines during use".
Comparison of advantages and disadvantages of the VCI method
Advantages
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Since the gas also penetrates
holes and cavities, these areas also receive adequate protection.
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The period of action may extend to
two years.
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The wrapping need not be provided
with an airtight heat seal.
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On completion of transport, the
packaged item need not be cleaned, but is immediately available.
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Disadvantages
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The VCI method is not suitable for
all metals. It may cause considerable damage to nonmetallic articles
(plastics etc.).
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Most VCI active substances may
present a hazard to health, so it is advisable to have their harmlessness
confirmed by the manufacturer and to obtain instructions for use.
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http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm
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