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Selected oxadiazoles of fatty acids; namely 2-hepta decene-5-mercapto-1-oxa-3,4-diazole (HMOD); 2-undecane-5-mercapto-1-oxa-3,4-diazole (UMOD); and 2-decene-5-mercapto-1-oxa-3,4-diazole (DMOD), were synthesized. Their influence on the inhibition of corrosion of mild steel in 20% formic acid (HCOOH) was investigated by weight loss and potentiodynamic polarization techniques. The inhibition efficiency of the compounds was found to vary with concentration, immersion time and temperature. All the compounds showed good inhibition efficiency (e IE ) in formic acid solution. Adsorption on mild steel obeyed the Langmuir adsorption isotherm. Potentiodynamic polarization revealed that all three inhibitors, HMOD, UMOD, and DMOD are mixed inhibitors. Electrochemical impedance spectroscopy was also used to investigate the inhibition mechanism.

A number of gemini surfactants from the series of 1,2-alkane-bis-(ethyl ammonium bromide) were synthesized and their inhibitive effects on mild steel corrosion in acetic acid were evaluated by weight loss and electrochemical polarization measurements. Their critical micelle concentrations at equilibrium in water at 30 • C were also determined. The results obtained revealed that all the studied gemini surfactants are effective in reducing corrosion of mild steel in acetic acid media. The adsorption of the inhibitors on the mild steel surface obeys Langmuir's adsorption isotherm. The thermodynamic parameters of adsorption revealed a strong interaction between the inhibitors and the corroding mild steel surface. The influence of inhibitor concentration, solution temperature, immersion time and acid concentration on the corrosion of mild steel has also been investigated.

Volatile corrosion inhibitors (VCIs) or vapor phase inhibitors (VPIs) represent a class of compounds that are employed to protect the corrosion or oxidation of ferrous and nonferrous metals and alloys where other surface treatments are impractical. The VCIs release slowly inside the sealed airspace and actively adsorb and prevent corrosion. Several VCIs have been employed as effective inhibitors for several metals like iron, zinc, aluminum, etc. and their alloys. Literature study revealed that VCIs protect metallic corrosion either by forming a surface protective film on the metal surface or neutralizing the corrosive surrounding reagents H 2 O, SO 2 , H 2 S and CO 2 , etc. Because of the high volatility, VCIs easily vaporize and their vapors condense on the metallic surface as well as the volume available in the crevices, pores and cracks thereby gives complete protection where they used. The VCIs can be inserted into coatings, foams, adhesives, powders, sprays and plastics. Several VCIs are known those act as effective inhibitors by themselves and few other VCIs are also known those hydrolyzed products act as corrosion inhibitors. The inhibition ability as well as quantity of the adsorbed VCIs on metallic surfaces can be determined by several commonly employed techniques such as contact angle, radiotracer, polarographic and electrocapilarity methods. Present review article describes the assortment of previous works published on VCIs as corrosion inhibitors for ferrous and non-ferrous metals and alloys. The present report also deals with salient features of VCIs and mechanism of their action.