A volatile corrosion inhibitor (VCI) is a material that protects metals from corrosion. Corrosion inhibitors are chemical compounds that can decrease the corrosion rate of a material, typically a metal or an alloy. A volatile corrosion inhibitor (VCI) is a chemical compound that protects metallic surfaces from corrosion by releasing protective vapors. According to NACE International Standard TM0208, these substances operate through a process of volatilization, vapor transport within an enclosed atmosphere, and subsequent condensation onto metal surfaces to form a protective molecular layer, including absorption, dissolution, and hydrophobic effects on metal surfaces, where the rate of corrosion of metal surfaces is thereby inhibited. They are also called vapor-phase inhibitors, vapor-phase corrosion inhibitors, and vapor-transported corrosion inhibitors. VCIs come in various formulations that are dependent on the type of system they will be used in; for example, films, oils, coatings, cleaners, etc. There are also a variety of formulations that provide protection in ferrous, nonferrous, or multi-metal applications. Other variables include the amount of vapor phase compared to the contact phase inhibitors. Because they are volatile at ambient temperature, VCI compounds can reach inaccessible crevices in metallic structures. V.VCI is also called Vacuum VCI, meaning they have special properties of performance in vacuum as well as corrosion protection properties.
History The first wide-scale use of VCIs can be traced to Shell's patent for dicyclohexylammonium nitrite (DICHAN), which was eventually commercialized as VPI 260. DICHAN was used extensively by the US military to protect a wide variety of metallic components from corrosion via various delivery systems, VCI powder, VCI paper, VCI solution, VCI slushing compound, etc. Concerns regarding health, safety, and performance limitations have led to the decline of DICHAN in favor of modern VCI compounds, which are typically salts of moderately strong bases and weak volatile acids. At present, commercial VCI compounds are typically salts of moderately strong bases and weak volatile acids. The typical bases are amines, and the acids are carbonic, nitrous, and carboxylic.
VCI corrosion protection mechanism For steel, the first step will be the volatilization of the inhibitor into the airspace. This may entail simple evolution of the molecule or the chemical may dissociate first and then volatilize. The molecules will then diffuse through the enclosed airspace until some of the molecules reach the metallic surface to be protected. There are two likely paths once the molecules reach the metallic surface. First, the molecule may adsorb onto the metal surface, thereby forming a barrier against corrosive ions and displacing any condensed water. The second path involves the condensed water layer that has been shown to exist on the metallic surface. The VCI molecules will dissolve into the condensed water layer, raising the pH. An alkaline pH has been shown to have a beneficial effect on the corrosion resistance for steel. The mechanism for copper begins the same as for steel, the evolution of the inhibitor. Once at the copper surface, however, the inhibitor will form a copper benzotriazole complex which is protective. Vapor pressure is a critical parameter in VCI effectiveness. The most favorable range of pressure is 10−3 to 10−2 Pa at room temperature. Insufficient pressure leads to the slow establishment of the protective layer; if the pressure is too high, VCI effectiveness is limited to a short time.
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