AN INVESTIGATION OF ZINC CORROSION INHIBITION INAQUEOUS SOLUTIONS OF NH4Cl BY ORGANIC COMPOUNDS

摘要

Polarization and impedance measurements were carried out on a zinc electrode in aqueoussolution of NH4Cl containing organic corrosion inhibitors. This measurements have been realised bymeans of analytical and electrochemical methods. The inhibitors investigated were:dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide andhexadecyltrimethylammonium bromide. These compounds suppressed both cathodic and anodicprocesses of zinc corrosion in the NH4Cl aqueous solutions. The double layer capacitances obtained fromthe impedance data for the electrode inhibited with organic inhibitors were markedly lower in thepotential region of the cathodic polarization curves than those for the uninhibited electrode.At the cathodic potentials, the calculated impedance values fit the experimental ones only if the Tafelcoefficient for hydrogen discharge is taken to be potential dependent. This fact suggests that the cathodicprocess is multistage with the rate of the overall electrochemical reaction controlled in turn by one of thepartial reactions according to the value of the overpotential. The analysis of the impedance measurementsshowed that the organic inhibitors considered affect both the kinetic coefficients of the anodic process andthe H2 discharge Tafel coefficients.Considering all the results and taking into account the dispersion of the data we came to conclusion thatall the organic inhibitors show a well detectable inhibition of zinc corrosion. A poorly soluble layerconsisting of the inhibitor and zinc corrosion products is proposed to account for protective effect inchloride solutions. On the other hand, pyrrole was successfully electropolymerized at a zinc electrode in anear neutral sodium oxalate solution when a homogenous and adherent polypyrrole film is obtained. Thegrowth of this film was facilitated by the initial pre-treatment of zinc electrode in an aqueous solution of0,2M Na2S to generate a sulphide pseudo-passive layer. This layer was sufficiently protective to inhibitfurther dissolution of zinc electrode and sufficiently conductive to enable the electropolymerization ofpyrrole at the interface and generation of an adherent polypyrrole film. This film remained stable andexhibited significant corrosion protection properties in acidified and neutral chloride solutions even onpolarization to high anodic potentials.