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  • Corrosion Science 76 (2013) 42–51

    Contents lists available at SciVerse ScienceDirect

    Corrosion Science

    journal homepage: www.elsevier .com/locate /corsc i

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

    0010-938X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.

    ⇑ Corresponding author at: School of Materials Science and Engineering, Central South University, Changsha 410083, China. Tel.: +86 73188830264; fax: +86 73188876692.

    E-mail address: (Z. Li).

    Zhu Xiao a,b, Zhou Li a,c,⇑, Anyin Zhu a,d, Yuyuan Zhao b, Jinglin Chen a,d, Yuntian Zhu a a School of Materials Science and Engineering, Central South University, Changsha 410083, China b School of Engineering, University of Liverpool, Liverpool L69 3GH, UK c State Key Laboratory of Powder Metallurgy, Changsha 410083, China d Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Changsha 410083, China

    a r t i c l e i n f o a b s t r a c t

    Article history: Received 28 December 2012 Accepted 29 May 2013 Available online 2 July 2013

    Keywords: A. Alloy B. Polarization B. EIS B. XPS C. De-alloying C. Passive films

    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 cor- rosion 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.

    � 2013 Elsevier Ltd. All rights reserved.

    1. Introduction In the present work, a new aluminum (Al) brass alloy with 24 k-

    Copper-based alloys are widely used in many industries [1–3]. 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 mate- rials, due to their nobility, various beautiful colors, remarkable cast ability and good formability [4–9]. 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 commemora- tive 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 al- loys with high zinc content, uniform or layer de-alloying com- monly 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 unaf- fected matrix [2,11–13].

    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 spec- troscopy (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 spectros- copy (EIS) was used to obtain further insights into the mechanisms of the accelerated attack of the alloy in the corrosive environment.

    2. Experimental procedures

    2.1. Alloy preparation

    A new gold-imitation alloy with 24 k-gold color was prepared by induction melting and mold casting. The new alloy was de- signed with additions of Al and rare earth and its composition is shown in Table 2. The surface defects of the ingot were first re- moved 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 alloy (a currency coinage alloy with 24 k-gold color) provided by Shenyang Mint, China, was also tested.

  • Table 1 Selected mintage alloys [10].

    Currency Denomination Materials

    UK pound 1-Penny, 2-pence Copper-plated steel 5-, 10-, 50-pence and £5 B25 20-pence B16 £1 HNi70–5.5 £2 HNi76–4 (outer), B25 (inner)

    Euro 10-, 20- and 50-cent CuAl5Zn5Sn1 €1 CuZn20Ni (outer), B25/7%Ni/B25 (inner) €2 B25 (outer), CuZn20Ni5/12%Ni/CuZn20Ni5 (inner)

    US dollar 1-cent Copper-plated zinc 2-cent Nickel-brass 10-, 25- and 50-cent B25-plated B17 $ 1 Copper with manganese brass clad

    Australian Dollar 5-, 10-, 20- and 50-cent B25 $ 1 and $ 2 Cu92Al6Ni2

    Chinese Yuan 50-cent H7211 Commemorative coin made by Poongan Co. B25,Cu89Zn5Al5Sn1,H65,HNi70–12,HNi75–5

    Z. Xiao et al. / Corrosion Science 76 (2013) 42–51 43

    2.2. Salt spray test and color difference measurement

    The salt spray test was carried out in a LYW-025 salt spray test chamber with 5% sodium chloride solution at a constant temperature of 35 �C. The specimens with dimensions of 10 mm � 10 mm � 2 mm were first cut from the strips of the two alloys with a high speed cutting machine. The surfaces of the spec- imens were ground carefully using 1200 grit paper and polished. The specimens were subsequently dehydrated by 90% ethanol for 5 min and then dried by hot air for testing. A group of specimens were exposed in the salt spray environment for different times up to 240 h. The color differences between the specimens after dif- ferent exposure times and gold were measured by a CC-6801 col- orimeter (BYK, German) to investigate the tarnish resistance of the alloys.

    The color difference, DE, was calculated according to the CIE1976LAB standard:

    DE ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðLt � LgÞ2 þ ðat � agÞ2 þ ðbt � bgÞ2

    q ð1Þ

    where L is lightness, a is red–green opponent, and b is yellow–blue opponent; the subscript t designates the tested sample, and g the calibrating sample, i.e. gold (Lg = 36.97, ag = 5.22, bg = 24.04).

    2.3. Electrochemical test

    Electrochemical characterization was carried out on an IM6ex electrochemical workstation using Pt as the auxiliary electrode and the saturated calomel electrode (SCE) as the reference elec- trode. The surface of each specimen for the electrochemical test was sealed with paraffin, except an uncovered area of 1 cm2. The specimens were exposed in salt spray environment for 6 h, 12 h, 24 h, 60 h, 120 h and 240 h and then tested. All electrochemical measurements were performed after the open circuit potential (OCP) was stabilized. The potentiodynamic polarization measure- ments were performed by scanning from �200 mV (SCE) to 600 mV (SCE) with a speed of 2 mV/s and the data were analyzed by the CHI660C software. In the EIS measurements, the AC voltage signal amplitude was 10 mV (peak to zero), the frequency was be-

    Table 2 Composition of the new alloy.

    Alloy elements Cu Al Ni Sn B Re Zn

    Content (wt.%) 76 2 0.5 0.2 0.01 0.05 Balance

    tween 100 kHz and 10 mHz, and 101 points were measured. The experimental data were analyzed by the Zview software.

    2.4. X-ray photoelectron spectroscopy (XPS) analysis

    The XPS analysis of the specimens of the new alloy after ex- posed in salt spray environment for 6 h and 60 h was performed on a K-Alpha 1063 X-ray photoelectron spectroscope (Thermo Fisher Scientific), with a monochromotized Al Ka X-ray source in vacuum of 10�9 mBar. The spot diameter was 400 lm and the step lengths were 1 eV for wide scanning and 0.1 eV for narrow scan- ning. The output of the analysis was recorded as binding energy vs. intensity count plots through a data logger system. The speci- mens were immersed in deionized water for 5 min and then dried in vacuum drying oven for 15 min before testing. The high resolu- tion photoelectron spectra were recorded for Cu2p, Zn2p, Al2p and O1s. The data reduction was performed by deconvoluting the high resolution composite XPS peaks of the individual species having different oxidation states using the XPSPEAK software based on a non-linear least square regression method. The peak position and peak width (FWHW) of the individual species obtained from Ref. [14] were used as input parameters in deconvoluting the spectra. The criteria for the best fit were judged by the smallest Chi-Square value. The final results consisted of the area under each of the re- solved peaks for the individual oxidation states, the total area un- der raw data and the total area under the convoluted curves.

    2.5. Microstructure analysis by SEM

    The samples of the new alloy exposed in salt spray environment for 6 h and 60 h were rinsed with deionized water for three times and then air dried. The morphology and the thickness of the corro- sion layer were


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