The Corrosion Inhibition of Pure Zinc in NH4Cl Aqueous Solutions by NalkylQuaternary Ammonium Bromides

摘要

The corrosion of zinc in NH4Cl solutions of different concentrations has been studiedrnby potentiostatic and potentiodynamic methods and by impedance measurements. The resultsrnobtained in a potential region near the zinc corrosion potential showed that the cathodicrnreaction of hydrogen discharge does not fit a simple exponential law because the Taffelrncoeficient appears to be electrode potential dependent. At low overvoltages, in the anodicrnregion, the corrosion process is under activation control, while at high overvoltages thernprocess is under diffusion control. N-dodecyl, n-tetradecyl and n-hexadecyl ammoniumrnbromides were used as organic inhibitors. The potentiostatic and potentiodynamic polarizationrnstudies and EIS measurements showed that each quaternary ammonium salt inhibits therncorrosion of pure zinc in 1M NH4Cl at 30℃. The inhibition efficiency of the cations wasrndiscussed on the basis of coulombic adsorption behaviour, using molecular coverage areasrnand polar substituent constant. The inhibition efficiency obtained from the corrosion currentsrnwas found to increase linearly with an increase in the molecular coverage area (A) for thernseries of tetralkyl ammonium ions. The efficiency of the cations increased with an increase inrnthe positive charge of the nitrogen atom, which was due to the inductive effect of the electronattractiverngroups. This effect on the coulombic adsorption of the cations was, however, farrnless than that of the electron donating groups on the chemisorption of unprotonated amines.rnThe steric requirements for the adsorption of the quaternaries with branched alkyl chains werernsmall.rnEach organic compound exhibits Langmuirian behaviour and inhibition increases withrnincreasing alkyl chain length. This is attributed to cohesive Van der Waals forces between thernpositive head groups co-adsorbed with bromide ions on the positively charged zinc surface.