Kinetics and Mechanism of β-Brass Dealloying in Aqueous 0.5 M Sodium Chloride Solution Derived from Combined Scanning Tunneling Microscopy and Electrochemical Data

J. Morales,R. C. Salvarezza,L. Vazquez,A. J. Arvia,S. Gonzalez,P. Esparza

doi:10.1021/la950409f

Abstract

Scanning tunneling microscopy (STM) combined with conventional electrochemical techniques was used to determine the kinetics and mechanism of β-brass dealloying in aqueous neutral solutions containing NaCl. In the potential range where selective dissolution of Zn occurs, electrochemical data suggest that the corrosion process is controlled by surface diffusion of Cu atoms. STM imaging of β-brass after dealloying shows the development of an irregular surface which attains a stationary regime. The irregular surface topography consists of faceted islands with atomically smooth terraces. For a constant dealloying time, the extent of faceting increases as the potential is moved positively, and at a constant applied potential it increases with electrodissolution time. The proportionality ξ ∝ Lα was established, where ξ is the interface width related to L, the sample size. The exponent α is related to the degree of surface disorder. The value of α derived from STM is in the range 0.7 ≤ α ≤ 0.8, i.e. close to the predictions of aggregation models including surface diffusion.

Highlights

The stability of binary alloys in aqueous environments[1,2,3,4] depends on the alloy composition at the surface,[5] the solution composition,[6] the surface alloy pretreatment, and temperature.[7,8] Dealloying appears as a complex process related to different problems such as metallic corrosion, including stress corrosion cracking, and heterogeneus catalysis
For E > -0.4 V, a considerable increase in the anodic current (A(III)) was produced. Both A(I) and A(II) and the anodic current plateau remained insensitive to the electrode rotation (ω) in the range 0 < ω < 2000 rpm
Anodic peaks A(I)′ and A(I)′′ are related to the electroformation of a ZnO‚xH2O layer, and Zn electrodissolution leading to a depletion of Zn atoms at the alloy surface and the formation of a Cu-rich layer.[2,13]

Summary

Introduction

The stability of binary alloys in aqueous environments[1,2,3,4] depends on the alloy composition at the surface,[5] the solution composition,[6] the surface alloy pretreatment, and temperature.[7,8] Dealloying appears as a complex process related to different problems such as metallic corrosion, including stress corrosion cracking, and heterogeneus catalysis. Ec, the electrodissolution of the most active species results in a protective metal layer of the most noble constituent which in turn, under favorable conditions, hinders the corrosion process. At this stage no remarkable changes in the corroding surface roughness can be observed.

Results

Discussion

Conclusion