Natural products as corrosion inhibitor for metals in corrosive media — A review
Introduction
Pure metals and alloys react chemically/electrochemically with corrosive medium to form a stable compound, in which the loss of metal occurs. The compound so formed is called corrosion product and metal surface becomes corroded. Corrosion involves the movement of metal ions into the solution at active areas (anode), passage of electrons from the metal to an acceptor at less active areas (cathode), an ionic current in the solution and an electronic current in the metal. The cathodic process requires the presence of an electron acceptor such as oxygen or oxidising agents or hydrogen ions [1], [2], [3], [4], [5], [6]. Corrosion can be minimized by suitable strategies which in turn stifle, retard or completely stop the anodic or cathodic reactions or both.
Among the several methods of corrosion control and prevention, the use of corrosion inhibitors [7], [8], [9], [10], [11], [12] is very popular. Corrosion inhibitors are substances which when added in small concentrations to corrosive media decrease or prevent the reaction of the metal with the media. Inhibitors are added to many systems viz., cooling systems, refinery units, chemicals, oil and gas production units, boiler etc. Inhibitors function by adsorption of ions or molecules onto metal surface. They reduce the corrosion rate by,
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Increasing or decreasing the anodic and/or cathodic reaction
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Decreasing the diffusion rate for reactants to the surface of the metal
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Decreasing the electrical resistance of the metal surface
Inhibitors are often easy to apply and offer the advantage of in-situ application without causing any significant disruption to the process. However, there are several considerations when choosing an inhibitor:
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Cost of the inhibitor can be sometimes very high when the material involved is expensive or when the amount needed is huge.
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Toxicity of the inhibitor can cause jeopardizing effects on human beings, and other living species.
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Availability of the inhibitor will determine the selection of it and if the availability is low, the inhibitor becomes often expensive.
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Environment friendliness.
Sanyal [13] in his review has given a vivid account of organic corrosion inhibitors including the classification and mechanism of action. He has attributed the corrosion inhibition potential to the donation of lone pair of electron to metal atoms.
A number of heterocyclic compounds [14], [15], [16], [17] have been reported as corrosion inhibitors and the screening of synthetic heterocyclic compounds is still being continued.
Though many synthetic compounds showed good anticorrosive activity, most of them are highly toxic to both human beings and environment. The safety and environmental issues of corrosion inhibitors arisen in industries has always been a global concern. These inhibitors may cause reversible (temporary) or irreversible (permanent) damage to organ system viz., kidneys or liver, or to disturb a biochemical process or to disturb an enzyme system at some site in the body. The toxicity may manifest either during the synthesis of the compound or during its applications.
These toxic effects have led to the use of natural products as anticorrosion agents which are eco-friendly and harmless. In recent days many alternative eco-friendly corrosion inhibitors have been developed, they range from rare earth elements [18], [19], [20] to organic compounds [21], [22], [23], [24].
In the present review a detailed account of green inhibitors is given.
Section snippets
Natural corrosion inhibitors
In 1930, plant extracts (dried stems, leaves and seeds) of Celandine (Chelidonium majus) and other plants were used in H2SO4 pickling baths. An additive, ZH-1 consisting of finely divided oil cake, a by product formed in the phytin manufacture was developed for the control of corrosion. Animal proteins (by products of meat and milk industries) were also used for retarding acid corrosion. The additives used in acid, included flour, bran, yeast, a mixture of molasses and vegetable oil, starch and
Conclusion
The recent trend of reporting the plant extracts as corrosion inhibitors has one main drawback. Rarely the phytochemical investigation is carried out on the extract and efforts are seldom made to pinpoint the active ingredient present in the plant extract. It is also likely that a mixture of constituents present may potentiate the inhibitive effect of one particular constituent. But it is still amazing to see reports wherein inhibition to the extent of 98% efficiency is achieved. It is certain
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