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The influence of thiophene and five of its substituted derivatives on the corrosion inhibition of steel in 0.5 m H2SO4 solution was studied using weight-loss, electrochemical polarisation, and impedance measurements. 5-tert-butoxythiophene-2-carbaldehyde phenylhydrazone (TBCP) is the best inhibitor and its inhibition efficiency increases with increase in concentration to attain 87% at 5×10−3 m. Potentiodynamic polarisation studies clearly reveal that it acts essentially as a cathodic inhibitor. The partial π-charge on atoms has been calculated. A correlation between the highest occupied molecular orbital E HOMO and inhibition efficiencies was sought. The inhibition efficiency of TBCP is not affected by rise in temperature in the range 298–353 K. E% values obtained from weight-loss and electrochemical methods were in good agreement. Adsorption of TBCP on steel has an S-shaped adsorption isotherm.

Inhibitory effect of some new synthesized bipyrazole compounds, namely, N,N-bis[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-N-(4-methylphenyl)amine (P1) and methyl-1-[((methylphenyl){[3-(methoxycarbonyl)-5-methyl-1H-pyrazol-1-yl]methyl}amino)methyl]-5-methyl-1H-pyrazole-3-carboxylate (P2) on corrosion of pure iron in 1 M HCl solution has been studied using chemical technique as weight loss and electrochemical techniques as potentiodynamic polarization, linear polarization and impedance. The inhibition efficiencies obtained from gravimetric, cathodic Tafel plots, linear polarization resistance and EIS methods are in good agreement. The results obtained reveal that these compounds are efficient inhibitors. The inhibition efficiency increases with the increase of inhibitor concentration to reach 93% for P1 at 2.5 × 10−4 M. Potentiodynamic polarisation studies clearly reveal that P1 and P2 act as mixed-type inhibitors without change of the mechanism of hydrogen evolution. The temperature effect on the corrosion behaviour of iron in 1 M HCl without and with the inhibitors at 2.5 × 10−4 M has been studied in the temperature range from 298 to 353 K. EIS measurements show that the increase of the transfer resistance with the inhibitor concentration. The inhibitors are adsorbed on the iron surface according to the Langmuir adsorption isotherm model. From the adsorption isotherm some thermodynamic data for the adsorption process (K and Δ G ads ° ) are calculated and discussed.

Inhibition by some newly synthesised pyridazine compounds of corrosion of pure iron in 1 M HCl solution has been studied using weight loss measurements, polarisation and impedance spectroscopy methods. The inhibiting action is more pronounced with stet-5-benzyl-6-methylpyridazin-3-yl thioethanoic (P1) and (5-benzyl-6-methylpyridazin-3-yl) ethyl thioethanoate (P3) and their inhibition efficiency increases with concentration to attain the maximum value of 85% and 81% at the 10−4 M, respectively. We note good agreement between gravimetric and electrochemical methods (potentiodynamic polarisation and impedance spectroscopy (EIS)). Polarisation measurements show also that the pyridazines act essentially as cathodic inhibitors. The cathodic curves indicate that the reduction of proton at the pure iron surface happens with an activating mechanism. The presence of the sulphur atom increases the inhibition efficiency changing the adsorption mechanism.

The inhibition of pure iron in 1 M HCl by new synthesised pyridazine compounds has been studied by weight loss, electrochemical polarisation and electrochemical impedance spectroscopy (EIS) measurements. The results obtained reveal that these compounds are efficient inhibitors. The inhibition efficiency increases with the increase of inhibitor concentration and reached 98% at 10 −4 M for 5-benzyl-6-methyl pyridazine-3-thione. Potentiodynamic polarisation studies clearly reveal that the presence of pyridazines does not change the mechanism of hydrogen evolution and that they act essentially as cathodic inhibitors. The temperature effect on the corrosion behaviour of pure iron in 1 M HCl without and with the pyridazines at 10 −4 M was studied in the temperature range from 298 to 353 K. EIS measurements show that the increase of the transfer resistance with the inhibitor concentration.

The corrosion behaviour of mild steel in 1M HCl containing organic compounds such as: 2,2′-bipyridine (inh1), 1,10-phenanthroline (inh2), 4,4′-6,6′-tetramethyl-2,2′-bipyridine (inh3) and 2,9-chloromethyl-1,10-phenanthroline (inh4), was studied by weight loss and electrochemical polarization methods. Polarization data indicate that all tested compounds are mixed-type inhibitors affecting both cathodic and anodic processes. The inhibition efficiency of these additives increases in the order: inh3