AN INVESTIGATION OF ZINC CORROSION INHIBITION IN AQUEOUS SOLUTIONS OF NH4Cl BY ORGANIC COMPOUNDS

摘要

Polarization and impedance measurements were carried out on a zinc electrode in aqueous solution of NH4Cl containing organic corrosion inhibitors. This measurements have been realised by means of analytical and electrochemical methods. The inhibitors investigated were: dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide. These compounds suppressed both cathodic and anodic processes of zinc corrosion in the NH4Cl aqueous solutions. The double layer capacitances obtained from the impedance data for the electrode inhibited with organic inhibitors were markedly lower in the potential 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 Tafel coefficient for hydrogen discharge is taken to be potential dependent. This fact suggests that the cathodic process is multistage with the rate of the overall electrochemical reaction controlled in turn by one of the partial reactions according to the value of the overpotential. The analysis of the impedance measurements showed that the organic inhibitors considered affect both the kinetic coefficients of the anodic process and the H2 discharge Tafel coefficients. Considering all the results and taking into account the dispersion of the data we came to conclusion that all the organic inhibitors show a well detectable inhibition of zinc corrosion. A poorly soluble layer consisting of the inhibitor and zinc corrosion products is proposed to account for protective effect in chloride solutions. On the other hand, pyrrole was successfully electropolymerized at a zinc electrode in a near neutral sodium oxalate solution when a homogenous and adherent polypyrrole film is obtained. The growth of this film was facilitated by the initial pre-treatment of zinc electrode in an aqueous solution of 0,2M Na2S to generate a sulphide pseudo-passive layer. This layer was sufficiently protective to inhibit further dissolution of zinc electrode and sufficiently conductive to enable the electropolymerization of pyrrole at the interface and generation of an adherent polypyrrole film. This film remained stable and exhibited significant corrosion protection properties in acidified and neutral chloride solutions even on polarization to high anodic potentials.