Volatile corrosion inhibitor

Volatile corrosion inhibitors (VCls), also called vapor phase inhibitors (VPIs), inhibit corrosion by being transported as a vapor from a permeable container to the site susceptible to corrosion. In boilers. 

On contact with the metal surface, the vapor of these salts condenses and reacts with moisture to liberate protective ions, ft is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of these compounds, fast action wanting high volatility, whereas enduring protection requires low volatility. A convenient partial vapor pressure for closed spaces VCIs will lie between 10 to 10 Pa (Table 14.5). 

The first condition for good efficiency of a VCI is its capability to reach the metallic surface to be protected. The second is that the rate of transfer of the corrosion inhibiting molecules should not be too slow to prevent an initial attack of the metal surface by the aggressive environment before the inhibitor can act. These two conditions are related partly to the vapor pressure of the inhibitor, partly to the distance between the source (s) of the inhibitor and the metal surfaces, and partly to the accessibility of the surfaces [14]. 

It is significant that the most effective volatile corrosion inhibitors are the products of the reaction of a weak volatile base with a weak volatile acid. Such substances, although ionized in aqueous solutions, undergo substantial hydrolysis (reaction with water), the extent of 

The nature of the adsorbed film formed at a metal-water interface is an important factor controlling the efficiency of VCIs. Metal surfaces exposed to vapors from VCIs in closed containers give evidence of having been covered by a hydrophobic-adsorbed layer. The contact angle of distilled water on such surfaces increases with time of exposure. Experimental studies on the adsorption of VCIs from the gas phase have confirmed that the VCIs indeed reacted with the metal surface to provide corrosion protection. 

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Other hydrocarbons. They have been used as corrosion inhibitors in glycol heat-exchanger fluids (antifree2es) and as volatile corrosion inhibitors for steel (see Corrosion and corrosion inhibitors). They also stabilize sulfur trioxide. 

Vapor phase inhibitors These are used for the temporary protection of new plant in transit or prior to commissioning. Volatile corrosion inhibitors such as cyclohexylamine derivatives are used. The plant must be sealed or contained to prevent rapid loss of the inhibitor. Sachets of these materials are placed in packing cases. Papers impregnated with them are available for wrapping steel items. These inhibitors are used primarily to protect steel. 

The atmospheric pollution prevailing in special industrial or laboratory locations may induce more severe corrosion, e.g. the vapours from concentrated hydrochloric or acetic acid will etch tin, and moist sulphur dioxide will produce a sulphide tarnish, as will hydrogen sulphide at temperatures above about 100°C. The halogens attack tin readily. The commonly used volatile corrosion inhibitors are without adverse action although the benefit derived from their use is doubtful. 

There are many temporary protectives on the market and it would be impracticable to describe them individually. However, they may be classified according to the type of film formed, i.e. soft film, hard film and oil film the soft film may be further sub-divided into solvent-deposited thin film, hot-dip thick film, smearing and slushing types. All these types are removable with common petroleum solvents. There are also strippable types based on plastics (deposited by hot dipping or from solvents) or rubber latex (deposited from emulsions) these do not adhere to the metal surfaces and are removed by peeling. In addition there are volatile corrosion inhibitors (V.C.I.) consisting of substances, the vapour from which inhibits corrosion of ferrous metals. 

Volatile corrosion inhibitors are particularly useful when oil, grease or other adherent films are unsuitable. They should be used in conjunction with a primary wrap which should form as close an approach to a hermetically-sealed pack as possible. They are widely used to provide protection to precision tools, moulds and dies, and also on a larger scale to car body components. 

The testing of vapour phase inhibitors, usually referred to as volatile corrosion inhibitors, is essentially a matter of placing a test specimen in the vapour space of a closed vessel containing an aggressive atmosphere — frequently water vapour, perhaps with SO2 present—and a quantity of the inhibitor. Variations on the basic technique include provision for circulation of the vapour, the use of paper impregnated with inhibitor, provision for temperature cycling, etc. 

A further longer term wet lay-up alternative is through the use of volatile corrosion inhibitors (VCIs) such as dicyclohexylamine acetate. These are dissolved in the water at a temperature below 60 °C, and the water is circulated for 4 to 5 hours. The boiler does not need to be completely filled because the VCI migrates to all parts of the boiler and reaches equilibrium in each of the void spaces. With traditional lay-up chemicals, the oxygen scavenger may become depleted easily (which is why the reserve usually is so high) and corrosion protection is quickly lost however, with VCI programs, there is always a volatile buffer available that maintains equilibrium and hence corrosion protection. 

Gandhi, Ashish (Cortec Corporation). Volatile Corrosion Inhibitors Unique Water Treatment Applications. The Analyst, Journal of the Association of Water Technologies, USA, Fall 2000. 

When the bottom plates are welded together, the coating is partially destroyed. Research and field work showed that protection can be achieved using volatile corrosion inhibitors under the tank [688]. This works alone or in combination with cathodic protection. Double tank bottoms for leakage monitoring are often specified for new tanks. However, the same problem of coating destruction occurs. Volatile corrosion inhibitors are an excellent solution from both a technical and an economic standpoint. This type of corrosion inhibitor... 

L. Gelner. Protection of storage tank bottoms using volatile corrosion inhibitors (VCI). In Proceedings Volume, volume 1, pages 102-109. 7th NACE Int et al Middle East Corrosion Conf (Manama, Bahrain, 2/26-2/28), 1996. 

Exposed shafts and shaft couplings shall be wrapped with waterproof, moldable waxed cloth or volatile-corrosion inhibitor paper. The seams shall be sealed with oil-proof adhesive tape. 

Coupons should be obtained, suitably prepared, and supplied in a small paper envelope impregnated with a volatile corrosion inhibitor (VCI). Typically they are 3 x x in. thick, with a laser-cut mounting hole. 

In the selection of an appropriate corrosion inhibitor, an important consideration is the problem of adverse competition with other additives designed to adsorb on the liquid-metal interface. Extreme pressure and/or antiwear agents compete for the same sites as the corrosion inhibitor. Eatty amines are good corrosion inhibitors in this type of environment. However, their adverse effect on the performance of ZDDP additives often prohibits their use. Half esters or amides of dodecylsuccinic acid, phosphate esters or thiophosphates are frequently employed. A combination of inhibitors is sometimes used, for example, fatty carboxylic acids or the dimer/trimer analogues of the unsaturated acids, e.g. oleic, used in conjunction with an amine such as an ethanolamine or alkyl amine, or amide such as alkyl imidazoline and sarcosines, Eig. 6.7. Shorter chain carboxylic acid and amines are used as volatile corrosion inhibitors. 

Volatile corrosion inhibitors can be used with objects stored in closed containers, such as tools during shipment. A typical one is a dicyclohexylamine salt that dissociates to produce the amine (13.20). 

The corrosion system presented in Fig. 1.15a, includes metal article 1 and corrosion medium 2. Polymer composite 3 incorporating a volatile corrosion inhibitor of metals is a part of the system protecting against corrosion. 

Engineering goods transported or stored in closed volumes or metal vessels necessitate rust prevention. For this purpose volatile corrosion inhibitors (VCI) are commonly used whose vapors penetrate into narrow gaps and slots of metal structures and become adsorbed on the surfaces, thus inhibiting corrosion propagation. 

Prance Patent 2774995, C 08 K 5/521, 5/10. Compositions containing volatile corrosion inhibitors, articles based on these compositions and their production. 

While attempts have been made to mitigate corrosion of electronics by encapsulating the components in plastics, it is useful to note that polymers are permeable to moisture. Hermetically sealed ceramic packaging is more successful. However, care must be taken to prevent moisture and other contaminants from being sealed in. A useful common approach for mitigating corrosion of circuits housed inside a relatively large-size chassis includes the use of volatile corrosion inhibitors. 

Vapor phase inhibitors/volatile corrosion inhibitors (VCI)... 

L.R.M. Estevao, R.S.V. Nascimento, Modifications in the volatilization rate of volatile corrosion inhibitors by means of host-guest systems, Corros. Sci. 43 (2001) 1133—1153. 

Vapor phase inhibitors (VPIs), also called volatile corrosion inhibitors (VCIs), are compounds that are transported in a closed system to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds such as morpholine or octadecylamine are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide (Boles et al. 2009). Compounds of this type inhibit corrosion by making the environment alkaline. In closed vapor spaces, such as shipping containers, volatile solids such as the nitrite, carbonate, and benzoate salts of dicyclohexylamine, cyclohexylamine, and hexamethyleneimine are used. 

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