XPS and EPMA studies on self-healing mechanism of a protective film composed of hydrated cerium(III) oxide and sodium phosphate on zinc

Abstract

A thin, protective film was prepared on a zinc electrode by coverage with a hydrated Ce₂O₃ layer and 0.0552 mg/cm² of a Na₃PO₄$\text{·}$12H₂O deposit layer and by drying the layers at 90 °C for 23 h, as has been reported in the preceding paper. This film was highly self-healing at the scratched electrode in an aerated 0.5 M NaCl solution for many hours. The present investigation deals with elucidation on self-healing mechanism of this film by X-ray photoelectron spectroscopy and electron-probe microanalysis. The scratched surface was covered with deposits of Zn(OH)₂, ZnO and small amount of Zn₃(PO₄)₂. Both cathodic and anodic processes of zinc corrosion were markedly suppressed by coverage of the scratches with the deposits, resulting in the self-healing activity at the scratched electrode in 0.5 M NaCl.

Introduction

Chromate-free, self-healing protective films were prepared on a zinc electrode by previous treatment in an aqueous solution of cerium(III) nitrate Ce(NO₃)₃ and coverage of sodium phosphate Na₃PO₄ in the preceding paper [1]. Zinc corrosion at a scratched surface is prevented without further treatment due to the “self-healing” activity of the film. The self-healing ability of the film was examined by polarization measurement and observation of pit formation after the electrode was scratched with a knife-edge crosswise and immersed in an aerated 0.5 M NaCl solution for many hours.

Cerium(III) ion is an effective inhibitor to suppress the cathodic process of zinc corrosion in aerated 0.5 M NaCl by the formation of a hydrated cerium(III) oxide Ce₂O₃ layer [2]. This layer is highly protective against zinc corrosion in the inhibitor-free NaCl solution but has no self-healing activity at the scratched zinc surface [3]. Phosphate ion suppresses the anodic process by the formation of zinc phosphate Zn₃(PO₄)₂ precipitate at the surface [4].

A film comprising 0.0552 mg/cm² of Na₃PO₄$\text{·}$12H₂O was prepared on the zinc electrode previously treated in 1×10⁻³ M Ce(NO₃)₃ at 30 °C for 30 min and was dried at 90 °C for 23 h [1]. The thickness of the film including the hydrated Ce₂O₃ layer was 0.35±0.05 μm. This film was highly protective against zinc corrosion in aerated 0.5 M NaCl at the scratches, the values of the protective efficiency, P (see Eq. (4)) being 95.4% and 95.3% after the immersion for 24 and 72 h, respectively. A pit-like anodic dissolution feature (plf) appeared but a pit arising from growth of the plf was not observed at the scratches after 72 h. It was thus concluded that this film is highly self-healing for preventing zinc corrosion at the scratched surface in 0.5 M NaCl.

In this work, the self-healing activity of this protective film on the scratched zinc electrode is investigated by X-ray photoelectron spectroscopy (XPS) and electron-probe microanalysis (EPMA) for the zinc surface covered with the film after the electrode was scratched crosswise and immersed in the NaCl solution for many hours. Self-healing mechanism of the film on zinc was discussed using polarization curves and results of XPS and EPMA.