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2,5-Bis(4-dimethylaminophenyl)-1,3,4-oxadiazole (DAPO) and 2,5-bis(4-dimethylaminophenyl)-1,3,4-thiadiazole (DAPT) have been synthesised and their inhibiting action on the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 at 30 °C has been investigated by various corrosion monitoring techniques. At constant acid concentration, the inhibitor efficiency of both the compounds is found to increase with inhibitor concentration. DAPT is slightly more efficient in 0.5 M H2SO4 than in 1 M HCl whereas DAPO is more efficient in 1 M HCl. Of the two, DAPT appears to be a better inhibitor. Potentiostatic polarisation studies show that both are mixed-type inhibitors in 1 M HCl but cathodic-type in 0.5 M H2SO4. The inhibitors function through adsorption following Langmuir isotherm in both the acids. The electronic properties of DAPO and DAPT, obtained using the AM1 semi-empirical quantum chemical approach, have been correlated with their experimental inhibition efficiencies using the linear resistance model (LR). These inhibitors are considered as a non-cytotoxic substances. [Copyright &y& Elsevier]

A quantum chemical study of the corrosion inhibition efficiency of triazole and oxadiazole derivatives, at the mild steel electrode, in molar hydrochloric acid (1 M HCl) was carried out. The correlation between the molecular structure and inhibition efficiency of these heterocyclic compounds was investigated using a linear model encompassing the charge transfer resistance (Rt). The linear resistance model was optimised with the semi-empirical quantitative structure–activity relationships approach. Regression equations, with multiple correlation coefficients superior at 0.90, were derived between 1/Rt and molecular descriptors. These significant correlations indicated that the variation of the corrosion inhibition with the structure of the inhibitors may be explained in terms of electronic properties. [ABSTRACT FROM AUTHOR]

The efficiency of 2,5-bis(n-methoxyphenyl)-1,3,4-oxadiazoles (n-MOX), as corrosion inhibitors for mild steel in 1 M HCl and 0.5 M H2SO4 have been determined by weight loss measurements and electrochemical studies. The results showed that these inhibitors revealed a good corrosion inhibition even at very low concentrations. Comparison of results among those obtained by the studied oxadiazoles showed that 2-MOX was the best inhibitor. It is found to behave better in 1 M HCl than in 0.5 M H2SO4. Polarisation curves indicate that 2-MOX is a mixed inhibitor in 1 M HCl, but in 0.5 M H2SO4, the inhibition mode of 2-MOX depends on the electrode potential and acts essentially as a cathodic inhibitor. The inhibition efficiency slightly increases with temperature in the range from 25 to 60 °C, the associated activation energy have been determined. The addition of 2-MOX leads to decrease this activation energy. The adsorption of 2-MOX on the mild steel surface in both acidic media follows a Langmuir isotherm model. Significant correlations are obtained between inhibition efficiency with the calculated chemical indexes, indicating that variation of inhibition with structure of the inhibitors may be explained in terms of electronic properties. [ABSTRACT FROM AUTHOR]

The effect of isomers of 2,5-bis(n-pyridyl)-1,3,4-thiadiazole (n-PTH) on the corrosion of mild steel in perchloric acid solution (1 M HClO4) has been investigated using weight loss, electrochemical impedance spectroscopy and potentiodynamic polarisation. Inhibition efficiency is dependent upon the thiadiazole structure, with 3-PTH serving as a better inhibitor than 2-PTH. 2-PTH stimulates corrosion at low concentrations. Polarisation curves indicate that 3-PTH acts as an anodic-type inhibitor whereas 2-PTH is of the cathodic-type. X-ray photoelectron spectroscopy were also carried out to establish the mechanism of corrosion inhibition of 3-PTH and stimulation corrosion of 2-PTH of mild steel in acid solution. Adsorption of 3-PTH on the mild steel surface in 1 M HClO4 follows the Langmuir adsorption isotherm model. [Copyright &y& Elsevier]

Abstract: The inhibitive action of some thiadiazole derivatives, namely 2,5-bis(2-thienyl)-1,3,4-thiadiazole (2-TTH) and 2,5-bis(3-thienyl)-1,3,4-thiadiazole (3-TTH) against the corrosion of mild steel in 0.5M H2SO4 solution has been investigated using weight loss measurements, Tafel polarisation and electrochemical impedance spectroscopy (EIS) techniques. The experimental results obtained revealed that these compounds inhibited the steel corrosion in acid solution. The protection efficiency increased with increasing inhibitors concentration and the ability of the molecule to adsorb on the steel surface was dependent on the position of the sulphur atom on the thienyl substituent. Inhibition efficiency values obtained from various methods employed were in reasonable agreement. Potentiodynamic polarisation studies clearly showed that 2-TTH and 3-TTH acted as mixed inhibitors. Adsorption of these inhibitors on steel surface obeyed to Langmuir adsorption isotherm. X-ray photoelectron spectroscopy and the thermodynamic data of adsorption showed that inhibition of steel corrosion in normal sulphuric solution by n-TTH is due to the formation of a chemisorbed film on the steel surface. Molecular modelling was used to gain some insight, about structural and electronic effects in relation to the inhibiting efficiencies. [Copyright &y& Elsevier]

The inhibition of the corrosion of mild steel in 1 M HCl by 3,5-di(m-tolyl)-4-amino-1,2,4-triazole (m-DTAT) and 3,5-di(m-tolyl)-4H-1,2,4-triazole (m-DTHT) has been investigated at 30 °C using electrochemical and weight loss measurements. Polarisation curves reveal that m-DTHT is a mixed type inhibitor whereas m-DTAT is a cathodic type inhibitor. Inhibition efficiencies up to 95% for m-DTAT and 91% for m-DTHT can be obtained. Adsorption of these inhibitors on the mild steel surface is found to obey the Langmuir adsorption isotherm. The effect of temperature on the corrosion behaviour with addition of 4×10−4 M of m-DTAT and m-DTHT added was studied in the temperature range 30–60 °C. The associated activation corrosion and free adsorption energies have been determined. The comparative study of triazole derivatives indicates that the corrosion inhibition efficiency of the aminotriazole is greater than that of the 4H-triazole. [Copyright &y& Elsevier]

Abstract: The new 2,5-disubstituted 1,3,4-thiadiazoles were investigated as corrosion inhibitors of mild steel in 1M HCl using AC impedance technique. Four of these compounds exhibit good inhibition properties, while two of them, 2,5-bis(4-nitrophenyl)-1,3,4-thiadiazole and 2,5-bis(4-chlorophenyl)-1,3,4-thiadiazole, stimulate the corrosion process especially at low concentrations. The experimental data obtained from this method show a frequency distribution and therefore a modelling element with frequency dispersion behaviour, a constant phase element (CPE) has been used. Possible correlations between experimental inhibition efficiencies and quantum chemical parameters such as dipole moment (μ), highest occupied (E HOMO) and lowest unoccupied (E LUMO) molecular orbitals were investigated. The models of the inhibitors were optimised with the Density Functional Theory formalism (DFT) using hybrid B3LYP/6-31G (2d,2p) as a higher level of theory. The Quantitative Structure Activity Relationship (QSAR) approach has been used and composite index of some quantum chemical parameters were constructed in order to characterize the inhibition performance of the tested molecules. [Copyright &y& Elsevier]

Abstract: New macrocyclic polyether compounds containing a 1,3,4-thiadiazole moiety have been prepared to study the corrosion inhibitive effect of mild steel in normal hydrochloric acid solutions. The salient features obtained from weight loss and electrochemical impedance spectroscopy (EIS) have been discussed. The results of these investigations have shown enhancement in inhibition efficiencies with the extent of the polyethylene glycol unit that forms a cavity. Data obtained from EIS show a frequency distribution and therefore a modelling element with frequency dispersion behaviour, a constant phase element (CPE) has been used. Display Omitted Adsorption of n-MCTH was found to follow the Langmuir’s adsorption isotherm. The thermodynamic functions of adsorption process were calculated from experimental ac impedance data and the interpretation of the results are given. Molecular modelling has been conducted in an attempt to correlate the corrosion inhibition properties with the calculated quantum chemical parameters. [Copyright &y& Elsevier]

Abstract: This study examines the use of some 4H-triazole derivatives, namely 3,5-diphenyl-4H-1,2,4-triazole (DHT), 3,5-bis(4-pyridyl)-4H-1,2,4-triazole (4-PHT) and 3,5-bis(4-methyltiophenyl)-4H-1,2,4-triazole (4-MTHT) for corrosion and dissolution protection of mild steel in normal hydrochloric acid solution. The inhibiting efficiency of the different additives is evaluated by means of weight loss and electrochemical techniques such as ac impedance measurements and polarisation curves. The experimental results obtained reveal that 4-MTHT is the best effective inhibitor and the inhibition efficiency is found to be in the following order: 4-MTHT>4-PHT>DHT. The variation in inhibitive efficiency mainly depends on the type and nature of the substituents present in the inhibitor molecule. Polarisation curves show that theses triazoles are mixed-type inhibitors in 1M HCl. The inhibition efficiency increases with 4H-triazole derivatives concentration and attains the maximum value of 99.6% in the case of 4-MTHT at 5×10−4 M. The results obtained from weight loss electrochemical studies were in reasonable agreement. The adsorption of 4H-triazole derivatives on the steel surface obeys to the Langmuir isotherm model. The thermodynamic data of adsorption and activation are determined and discussed. The fundamental thermodynamic functions were used to glean important information about the 4H-triazoles inhibitory behaviour. Molecular modeling was used to get better insight, about structural and electronic effects in relation to the inhibition efficiencies. [Copyright &y& Elsevier]

Abstract: The inhibition effect of the new pyridazine derivative, namely 1,4-bis(2-pyridyl)-5H-pyridazino[4,5-b]indole (PPI) against mild steel corrosion in 1M HCl solutions was evaluated using weigh loss and electrochemical techniques (potentiodynamic polarisation curves and impedance spectroscopy). The experimental results suggest that PPI is a good corrosion inhibitor and the inhibition efficiency increased with the increase of PPI concentration, while the adsorption followed the Langmuir isotherm. X-ray photoelectron spectroscopy (XPS) and theoretical calculation of electronic density were carried out to establish the mechanism of corrosion inhibition of mild steel with PPI in 1M HCl medium. The inhibition action of this compound was, assumed to occur via adsorption on the steel surface through the active centres contained of the molecule. The corrosion inhibition is due to the formation of a chemisorbed film on the steel surface. [Copyright &y& Elsevier]