NATURAL PRODUCTS AS CORROSION INHIBITOR
Rosliza Binti Ramli, TATi University College
Corrosion is a chemical or electrochemical reaction process against certain material, usually metal and its environment which produce the deterioration of the material and its properties. The corrosion reaction produces a less desirable material from the original metal and resulted in the reduced function of a component or system, a significant problem encountered everyday.
Aluminium and its based alloy are widely used in aerospace applications, due to the unique combination of lightweight and high mechanical properties . However, these materials are reactive and prone to corrosion. As such, corrosion of aluminium and its alloy has been a subject of numerous studies due to their importance in the recent years [2, 3].
The most practical method for protection against corrosion is the use of corrosion inhibitor. The development of novel corrosion inhibitors of natural source and non-toxic type has been considered more important and desirable. A good corrosion inhibitor significantly decreases the weight loss, corrosion rates, corrosion currents densities and double layer capacitance, as simultaneously increase the values of polarization resistance.
A corrosion inhibitor is a chemical substance or combination of substances that, when present in the environment, prevents, or reduces corrosion without significant reaction with the components of the environment. Development of effective and environmentally acceptable corrosion inhibitors as alternatives of chromates, which are toxic and carcinogenic, is desirable for inhibition of metal corrosion in corrosive solutions.
It is very important to add a corrosion inhibitor to decrease the rate of aluminium dissolution in such solutions. The need growing for the corrosion inhibitors becomes increasingly necessary to delay or stop the attack of metal in an aggressive solution.
Considerable efforts are made to find suitable natural source to be used as corrosion inhibitor in various corrosive media. Sodium benzoate, natural honey, vanillin and tapioca starch offer interesting possibilities for corrosion inhibition because of their safe use and high solubility in water.
Tobacco, black pepper, acacia gum, hibiscus, anis, black cumin, onion, garlic and natural honey have been reported as good inhibitors for steel . Raja and Sethuraman  reported that rosmarinus officinalis, hisbiscus subdariffa, opuntia, polyaniline and sodium benzoate  give adequate protection against corrosion for aluminium in aggressive media.
Measurements of corrosion parameters
Several methods have been used to measure the corrosion rates of a metal. Among such techniques, the most commonly used are weight loss and electrochemical measurements. Weight loss measurements is the simplest way of measuring the corrosion rate of a metal is to expose the sample to the test medium and measure the weight loss of the material as a function of time. In this technique, samples are completely submersed in chosen corrosive solution. These tests are useful due to its similarity to the actual in-service conditions. However, such procedures can require hundreds of hours or even years to detect corrosion.
Most corrosion phenomena are electrochemical by nature, therefore electrochemical tests methods can be used to characterize corrosion mechanisms and predict their corrosion rates. Two main types of electrochemical techniques that used to measure the corrosion rates are the potentiodynamic polarization and electrochemical impedance spectroscopy.
Potentiodynamic polarization is the characterization of a metal specimen by its current-potential relationship (the current response as a function of the applied potential). Polarization method used to measure corrosion rates has inherent advantages. Usually only a few minutes are required to determine corrosion rate by polarization resistance; whereas conventional weight loss measurement requires several days or more. Polarization measurements are non-destructive and may be repeated numerous times to measure consecutive corrosion rates on the same electrode.
Electrochemical impedance spectroscopy (EIS)
EIS is a non-destructive method that has many advantages over other traditional electrochemical measuring methods. In monitoring the corrosion rate using EIS method, the polarization resistance, the reciprocal of which is proportional to the corrosion rate in same cases, is obtained by subtracting high frequency impedance, which corresponds to the ohmic drop, from low frequency impedance .
EIS has been successfully used to investigate corrosion. This method provides important mechanistic and kinetic information which enables understanding of the characteristics of the system under investigation , and also provides data about processes occurring at corroding metal surfaces . From appropriate electrical equivalent circuits, representative parameters of corrosion process can be extruded.
The recent trend of reporting the natural products as corrosion inhibitors has one main drawback. Ideally, natural products offer interesting possibilities for corrosion inhibition to invent new and better methods of preventing and apply existing methods more intelligently and effectively. It is certain that natural compounds emerge out as effective inhibitors of corrosion in the coming years due to their biodegradability, easy availability and non-toxic nature. Careful perusal of the literature clearly reveals that the era of the green inhibitors has already begun.
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