Discoloration Process of Minted Copper-Nickel Alloys in Chloride Ion-Containing Environments: Experimental and DFT Research

doi:10.1007/s40195-025-01819-z

Abstract

Abstract:

A corrosion discoloration model for copper-nickel alloys in Cl⁻ environments was established using CIE-Lab, UV-VIS absorption spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The corrosion discoloration process and the corresponding main corrosion products can be summarized as follows: silver-white (Cu + Ni) → green (NiO) → reddish-brown (NiO + Cu₂O) → black (NiO + Cu₂O + CuO). Density functional theory was employed to explain the corrosion process of copper-nickel alloys and the detrimental effect of Cl⁻. The results indicate that adsorbates preferentially bind to nickel, leading to the preferential formation of NiO, which imparts a green appearance to the surface. Furthermore, the difficulty in forming nickel cation vacancies and the higher diffusion barrier for nickel inhibit the migration of species within the oxide layer. Notably, nickel also suppresses carrier migration within the oxide layer, reducing the charge transfer rate. In contrast, the promotion of corrosion by Cl⁻ is primarily attributed to the reduction in surface work function and the formation energy of cation vacancies.

Key words: Composite chemical conversion, Ti/Zr/epoxy silane coatings, Aluminum alloys, Galvanized steel, Corrosion behavior

Chenzhi Xing, Ming-Hsien Lee, Gongwang Cao, Yuwei Liu, Quanzhong Guo, Zhenyao Wang, Chuan Wang. Discoloration Process of Minted Copper-Nickel Alloys in Chloride Ion-Containing Environments: Experimental and DFT Research[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(6): 925-945.

Figures/Tables 23

Fig. 1 Macroscopic morphology, L* values a, and UV-VIS absorption spectrum c of the samples immersed in 0.05 mol/L Cl-environment. The a* and b* values b of the sample after immersing in solutions of different concentrations

Fig. 2 OCP values a of copper-nickel alloys samples immersed for 16 h under different Cl− concentrations. Potentiodynamic polarization curve measured in 0 mol/L b, 0.05 mol/L c, 0.1 mol/L d Cl− environment

Fig. 3 Nyquist a, d, g and Bode diagrams b, c, e, f, h, i of copper-nickel alloys immersed in a-c 0 mol/L, d-f 0.05 mol/L and g-i 0.1 mol/L Cl− environment. The equivalent circuits j, k used to fit the EIS spectra of copper-nickel alloys samples

Fig. 4 Resistance value of Rct a, Rf b, Rp c with immersion time

Fig. 5 Surface morphologies of corrosion products immersed in 0 mol/L a1-a3, 0.05 mol/L b1-b6, 0.1 mol/L c1-c3 Cl− environment

Fig. 6 Variation of the particle size of corrosion products versus immersion time

Fig. 7 Oxygen content a and the ratio of Ni content to Cu content b under different Cl− concentration

Table 1 Elemental analysis of the specimen surface by EDS

Solution-time	O (wt%)	Cl (wt%)	Ni (wt%)	Cu (wt%)
0 mol/L-1 h	0.18	0	19.56	80.27
0 mol/L-9 h	0.54	0	19.12	80.34
0 mol/L-16 h	0.55	0	19.2	80.25
0.05 mol/L-1 h	0.91	0	19.14	79.95
0.05 mol/L-3 h	1.89	0	17.89	80.22
0.05 mol/L-6 h	2.47	0	16.91	80.62
0.05 mol/L-9 h	4.47	0	13.5	82.03
0.05 mol/L-12 h	4.32	0	15.14	80.54
0.05 mol/L-16 h	4.95	0.11	13.87	81.07
0.1 mol/L-1 h	1.16	0	17.94	80.9
0.1 mol/L-9 h	5.76	0.11	11.15	82.98
0.1 mol/L-16 h	6.78	0.18	9.26	83.79

Fig. 8 XRD pattern a of the corrosion products on copper-nickel alloys immersed in 0.05 mol/L Cl− solution. The morphology of corrosion products b actually observed and corresponding schematic diagram c

Fig. 9 Elementary composition of the specimens’ surface varies with the sputtering time immersed in the condition of 0.05 mol/L Cl−

Fig. 10 High-resolution spectra of Cu2p a1-a3, Cu LMM b1-b3, Ni2p c, O1s d for the corrosion products immersed in 0.05 mol/L Cl− solution

Fig. 11 Relative content of corrosion products on the surface of copper-nickel alloys with immersion time

Fig. 12 Surface work functions of clean metal a, adsorption structures b, and three kinds of corrosion products f. Adsorption energy c of Cl and O on metal surfaces. Surface energy density d of pure copper and Cu3Ni alloys and d-band centers e of pure copper and Cu3Ni alloys

Fig. 13 Fitting curve of the relationship between the surface work function of pure copper with the coverage of chloride ions and the open circuit potential with the concentration of chloride ions

Fig. 14 Models of vacancy formation energy a-d, transfer activation energy e-h

Table 2 Vacancy formation energy

	E_VCu (eV)		E_VNi (eV)
E_VCu1	4.38337	E_VNi1	5.17386
E_VCu2	4.88267	E_VNi2	6.08696
E_VCu3	4.23967	E_VNi3	5.67346
E_VCu4	3.74727	E_VNi4	5.30506
E_VCu5	3.70717	E_VNi5	4.98496

Fig. 15 Transition state search results of Cu in model 1 a, b, model 2 c, d and model 4 g, h. Transition state search results of Ni in model 2 e, model 3 f and model 4 i, j. Transition state confirmation results k, l which indicates that f, h are indeed transition states

Table 3 Migration barrier

	Barrier from reactant (eV)	Barrier from product (eV)
M4_Cu_Oxygen-sharing	0.85185	0.82296
M4_Cu_Non-oxygen-sharing	0.31061	0.31585
M4_Ni_Oxygen-sharing	1.48924	1.48854
M4_Ni_Non-oxygen-sharing	0.36995	0.36305
M2_Cu_Oxygen-sharing	1.13370	1.13359
M2_Cu_Non-oxygen-sharing	0.38777	0.39420
M2_Ni_Oxygen-sharing	1.63745	1.63727
M3_Ni_Non-oxygen-sharing	0.49776	0.49838
M1_Cu_Oxygen-sharing	0.67167	0.67126
M1_Cu_Non-oxygen-sharing	0.21859	0.21862

Table 4 Calculated results of diffusion coefficient

	E_a (eV)	Q (eV)	v (s⁻¹)	D (m² s⁻¹)
M4_Cu_Oxygen-sharing	0.85185	4.59912	2.66394 × 10¹²	4.04878 × 10⁻⁸⁵
M4_Cu_Non-oxygen-sharing	0.31061	4.05788	1.60861 × 10¹²	3.47874 × 10⁻⁷⁶
M4_Ni_Oxygen-sharing	1.48924	6.7943	3.6647 × 10¹²	4.1888 × 10⁻¹²²
M4_Ni_Non-oxygen-sharing	0.36995	5.67501	1.82653 × 10¹²	1.7723 × 10⁻¹⁰³

Fig. 16 Band structure of pure Cu2O a, b, nickel-doped Cu2O c, d, the ratio of copper to nickel is 3:1

Fig. 17 Schematic diagram of composition and color change of corrosion products on the surface of copper-nickel alloys

Fig. 18 Schematic diagram of competitive adsorption of corrosion factors on the substrate surface

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