Preparation of Self-Healing Protective Films on Zinc by Treatments with Environmentally Acceptable Inhibitors

摘要

Chromate-free, self-healing protective films were prepared on a zinc electrode by treatments with environmentally acceptable inhibitors, cerium(III) nitrate Ce(NO_3)_3, sodium phosphate Na_3PO_4 and zinc nitrate Zn(NO_3)_2. After the electrode coated with the film was scratched using a knife-edge crosswise and immersed in an aerated 0.5 M NaCl solution at 30°C for many hours, polarization measurements of the electrode in the solution and observation of the pit formation at the scratches were carried out in order to examine the self-healing ability of the film. Self-healing mechanism of the film was discussed using results of polarization measurement, X-ray photoelectron spectroscopy and electron-probe microanalysis. The cathodic process of zinc corrosion in aerated 0.5 M NaCl was suppressed by coverage of the surfaces other than the scratches with a hydrated Ce_2O_3 layer prepared by treatment in the Ce(NO_3)_2 solution and at the scratches with a layer of Zn(OH)_2 deposit. Precipitate of Zn_3(PO_4)_2 of which ions migrated from the film into the scratches inhibited the anodic process occurring at the scratched surface, resulting in the self-healing activity of the film. Hence, no pit was observed at the scratches during immersion in the solution for many hours.