Parameters of Oxide Films Anodically Formed on Ag-Zn Alloys with Different Concentrations of Vacancy Defects in the Surface Layer

Abstract

The nature and the properties of oxide fi lms anodically formed on metals and alloys depend on the chemical composition and energetic state of the electrode surface. This study was aimed at revealing the composition and the parameters of oxide fi lms formed in 0.1 M KOH on silver-zinc
alloys (up to 30 at % of Zn) with different concentrations of vacancy defects in the surface layer.
The enhanced concentration of vacancy defects in the alloys surface layer was created by preliminary selective dissolution of zinc at different potentials in 0.01 M HNO₃ + 0.09 M KNO₃. These alloys were moved into 0.1 M KOH for the formation of oxide fi lms. The composition of fi lms was monitored by cathodic voltammetry. The fi lm thickness was calculated from the anodic coulometry taking into account the current effi ciency. The latter was determined by cathodic coulometry. The morphology of the electrode surface was monitored by scanning electron microscopy (SEM).
It was revealed that the concentration of superequilibrium vacancies formed in the surface layer depended on the potential of the selective dissolution of zinc from the alloy. The peak of Ag(I) oxide reduction was registered on the cathodic voltammograms of the alloys after their anodic oxidation. The current effi ciency of oxide formation on Ag-Zn alloys was less than 100%. It decreased with zinc concentration in the alloys. The thickness of Ag(I) oxide on alloys did not exceed 25 nm. SEM images showed the uniform distribution of oxide particles along the electrode surface. The shape of the particles was close to spherical.
The Ag(I) oxide was the main product of oxidation of silver-zinc alloys (up to 30 at % of Zn) with different vacancy defects of the surface layer. The current effi ciency and the thickness of Ag(I) oxide fi lms formed on alloys with an increased concentration of vacancy defects were less as compared with the current effi ciency and the thickness of Ag(I) oxide fi lms formed on pure silver. However, these parameters were higher as compared with the parameters of Ag(I) oxide fi lms formed on alloys with an equilibrium concentration of vacancy defects. It was found that the diameter of the Ag(I) oxide particles
decreased, and their quantity on the unit of the electrode surface increased with an increase of the concentration of vacancy defects in the alloys surface layer.

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