Applied Surface Science 346 (2015) 158–164 Contents lists available at ScienceDirect Applied Surface Science journal h om epa ge: www.elsevier.com/locate/apsusc Unravelling the composition of the surface layers formed on Cu, Cu-Ni, Cu-Zn and Cu-Ni-Zn in clean and polluted environments Nasser K. Awad a , E.A. Ashour a , Nageh K. Allam a,b,∗ a National Research Centre, Electrochemistry and Corrosion Lab., Dokki, Cairo 12422, Egypt b Energy Materials Laboratory (EML), Physics Department, School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt a r t i c l e i n f o Article history: Received 26 January 2015 Received in revised form 24 March 2015 Accepted 29 March 2015 Available online 4 April 2015 Keywords: Mixed oxides XPS XRD Dissolution Surface a b s t r a c t The performance of copper and copper-based alloys in working environments is controlled by the compo- sition of the layers formed on their surfaces. Herein, we report the detailed structural and compositional analyses of the layers formed on the surface of Cu, Cu-Ni, Cu-Zn and Cu-Ni-Zn upon their use in both NaCl and Na 2 S-polluted NaCl solutions. In clean NaCl environments, X-ray photoelectron spectroscopy (XPS) analysis revealed that Cu 2 O is the major compound formed over the surfaces of pure Cu and Cu-Ni, whereas mixed oxides/hydroxides were detected over the surfaces of Cu-Zn (Cu 2 O and ZnO) and Cu- Ni-Zn alloy (CuO, ZnO, Cu(OH) 2 and Ni(OH) 2 ). However, in Na 2 S- polluted NaCl environments, sulphide compounds (such as Cu 2 S) were detected on the surfaces of Cu-Ni and Cu-Zn. X-ray diffraction (XRD) analysis confirmed the XPS findings, where Cu 2 O was confirmed in case of Cu and CuO in case of Cu-Ni- Zn in pure NaCl solutions. However, in sulphide-polluted media, compounds such as Cu 4 (S 2 ) 2 (CuS )2 were identified in case of Cu-Ni, and CuS in case of Cu-Zn. Further, the morphology of the surface of Cu-Ni-Zn tested in Na 2 S-polluted NaCl solution looks compact and has a wide band gap (4.47 eV) as revealed from the UV–vis absorption measurements. Therefore, the formation of mixed oxides/hydroxides and/or sulp- hides on the surface of Cu-Ni-Zn alloy is ultimately responsible for the enhancement of its dissolution resistance. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The electronic, mechanical and chemical properties of copper- based alloys are better than those of pure copper due to the existence of alloying elements such as nickel and zinc [1–4]. There- fore, they have been extensively used in various industries [5]. Specifically, the stability of Cu-Ni and Cu-Zn binary systems has been intensively investigated in clean and polluted marine envi- ronments. It has been proven that Cu 2 O [6–12] and ZnO [13] are the major oxides formed in clean-chloride media [6–12], which consti- tute nonconductive islands on the alloy surface. However, upon the use of Cu-Zn and Cu-Ni-Zn in sulfide-polluted chloride media, ZnS (a wide band gap p-type semiconductor) is usually formed, leading to a downturn in the ionic and electronic properties of the sur- face and consequently a decrease in the corrosion rate [3,4]. Recent work done by Awad et al. [14] showed that the dissolution rate of Cu-Ni-Zn (1.105 mm/year) in 40 ppm Na 2 S polluted NaCl was ∗ Corresponding author at: The American University in Cairo, Energy Materials Laboratory, New Cairo 11835, Egypt. Tel.: +20 2 2615 2568; fax: +20 2 2615 6009. E-mail address: nageh.allam@aucegypt.edu (N.K. Allam). much lower than that measured for pure Cu (5.732 mm/year). This was attributed to the formation of mixed oxides on the alloy sur- face that could potentially prevent the ion penetration and thus decrease the dissolution rate. In photoelectrochemical water split- ting, Allam et al. [15] reported that Ti-Nb-Zr alloy showed ∼17.5% improvement in the overall photoconversion efficiency compared to pure Ti as a result of the formation of mixed oxides with bet- ter semiconducting properties [15]. Therefore, it is evident that the performance of many materials is controlled by the composition of the surface layers. To this end, X-ray photoelectron spectroscopy (XPS) has been proven a distinctive technique to investigate the composition of such surface layers [16–18]. The aim of the present work is to investigate the structure and composition of the lay- ers formed on the surface of Cu-based alloys in both clean and sulfide-polluted seawater. 2. Experimental 2.1. Materials and solution preparation Experiments were performed using as-received commercial specimens of the tested alloys of the compositions listed in Table 1. http://dx.doi.org/10.1016/j.apsusc.2015.03.200 0169-4332/© 2015 Elsevier B.V. All rights reserved.