Surface characterization and corrosion behavior of a novel gold-imitation copper alloy with high tarnish resistance in salt spray environment

doi:10.1016/j.corsci.2013.05.026

Corrosion Science

Volume 76, November 2013, Pages 42-51

https://doi.org/10.1016/j.corsci.2013.05.026 Get rights and content

Highlights

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A novel imitation-gold copper alloy (CuZnAlNiSnBRe) was designed.
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The new alloy has good tarnish resistance and corrosion resistance performance.
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A multi-layer corrosion film formed on the surface of the new alloy.
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The corrosion products was a mixture of CuO, Cu₂O, ZnO, Al₂O₃ and Al(OH)₃.
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A transition of the product from Cu₂O to CuO occurred during the corrosion process.

Abstract

A novel gold-imitation copper alloy (CuZnAlNiSnBRe) was designed and its corrosion behavior in salt spray environment was investigated. The new alloy has better tarnish resistance and corrosion resistance than the current coinage alloy used in China (H7211). A multi-layer film formed on the surface of the new alloy after a period of exposure to salt spray was responsible for the good resistance of the alloy. The corrosion products were a mixture of CuO, Cu₂O, ZnO, Al₂O₃ and Al(OH)₃, with the transition from Cu₂O to CuO occurring during the corrosion process.

Introduction

Copper-based alloys are widely used in many industries [1], [2], [3]. One of the applications is for manufacturing coins and medals. Many copper-based alloys, such as Cu–Zn [4], [5], Cu–Al [6] and Cu–Ni [7], have been developed as mintage and ornamental materials, due to their nobility, various beautiful colors, remarkable cast ability and good formability [4], [5], [6], [7], [8], [9]. Table 1 lists a selection of the mintage of a few countries made from copper-based alloys [10].

The color of gold is often preferred for mintage or commemorative coins. Zinc, which is a key component of brass, is considered as the best element to increase the brightness of the golden color of gold-imitation copper alloys. However, dezincification in brass is a common problem. The alloy develops a reddish color with de-alloying, which contrasts with its original yellowish color and can be readily observed with naked eyes [11]. Generally, there are two types of de-alloying, depending on the zinc content. For alloys with high zinc content, uniform or layer de-alloying commonly occurs, with the outer layer showing a dark color. For alloys with low zinc content, plug de-alloying occurs, typified by the presence of the de-alloyed dark plugs in the otherwise unaffected matrix [2], [11], [12], [13].

In the present work, a new aluminum (Al) brass alloy with 24 k-gold color was designed and prepared. The corrosion behavior of the new alloy was studied by salt spray test and polarization curve characterization. X-ray photoelectron spectroscopy (XPS), which can provide more comprehensive and accurate information of the surface than X-ray diffraction (XRD) and energy dispersive spectroscopy (EDX), was employed to characterize the nature and the composition of the corrosion product layer on the surface as a function of exposure time. Electrochemical impedance spectroscopy (EIS) was used to obtain further insights into the mechanisms of the accelerated attack of the alloy in the corrosive environment.

Section snippets

Alloy preparation

A new gold-imitation alloy with 24 k-gold color was prepared by induction melting and mold casting. The new alloy was designed with additions of Al and rare earth and its composition is shown in Table 2. The surface defects of the ingot were first removed by mechanical milling. The ingot was then homogenized at 1023 K for 2 h and subsequently hot-rolled from a thickness of 22–4 mm, and then cold-rolled to 2 mm. The as-produced strip was annealed at 963 K for 1 h. As a comparison, the H7211 copper

Tarnish resistance

Fig. 1 shows the variations of the color difference of the new alloy and the H7211 alloy after being exposed in salt spray environment for different times. For both alloys, the color difference increased rapidly with exposure time for the first 60 h. After the 60 h exposure, the color difference of the H7211 alloy became stable while that of the new alloy continued to increase, but with a much slower rate. However, the color difference of the new alloy was always less than that of the H7211 alloy

Conclusions

(1)
The newly-designed gold-imitation CuZnAlNiBCeSn alloy has better corrosion resistance and tarnish resistance than the H7211 alloy in salt spray environment. The polarization resistance R_p of the new alloy increased with exposure time in salt spray environment and was more than three times of that of the H7211 alloy for any given exposure time. The relationship between the color difference and corrosion time obeyed the power law in the early stage.
(2)
EIS results showed that, with the extension of

Acknowledgements

The work was supported by two grants from the National Natural Science Foundation of China (51271203) and the Hunan Provincial Natural Science Foundation of China (11JJ2025).

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Surface characterization and corrosion behavior of a novel gold-imitation copper alloy with high tarnish resistance in salt spray environment

Highlights

Abstract

Introduction

Section snippets

Alloy preparation

Tarnish resistance

Conclusions

Acknowledgements

Materials and Design

Materials and Design

Journal of Archaeological Science

Corrosion Science

Corrosion Science

Corrosion Science

Corrosion Science

Corrosion Science

Corrosion Science

Applied Surface Science

Electrochimica Acta

Solid State Ionics, Diffusion and Amp Reactions

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Corrosion Science

Corrosion Science

Corrosion Science