Abstract: By comparing the corrosion behavior of B10 cu-ni alloy in natural seawater and seawater containing 10 mg/l NH4 + , the corrosion mechanism of B10 cu-ni alloy with NH4 + was studied. The average corrosion rate was measured by weight loss method, the electrochemical characteristics of interface corrosion were studied by potentiodynamic polarization analysis and electrochemical impedance spectroscopy (Eis) , and the morphology of corrosion products was characterized by Scanning electron microscope The corrosion products were analyzed by energy dispersive spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS) . The results show that the addition of NH4 + decreases the content of CU2O in corrosion products, increases the corrosion rate of B10 cu-ni alloy in seawater and promotes the pitting corrosion.

Key words: NH4 pollution; The sea; B10 copper-nickel alloy; Cu2O ; The corrosion product film B10 copper-nickel alloy has excellent corrosion resistance in seawater, and the addition of alloying elements Ni and Fe greatly enhances the corrosion resistance of the alloy [1,2]. The Cu2O in the corrosion products of B10 copper-nickel alloy has better film-forming protection, which is an important reason for the excellent corrosion resistance of copper-nickel alloy [3-7]. The ni-rich surface film is formed by doping the alloying element Ni into the structural defects of the protective Cu2O film in the corrosion products, which changes the structural defects of the film and improves the binding force and self-healing ability between the film and the matrix [8]. The corrosion rate of B10 copper-nickel alloy in seawater mainly depends on the properties of the corrosion product film on its surface. The average corrosion rate of B10 copper-nickel alloy in seawater is about 6 μm/a[9]. Can affect the corrosion resistance of copper-nickel alloy in seawater [11]. Terrigenous pollution injects a large amount of nutrients into seawater, making inorganic nitrogen content in local sea waters such as offshore ports 10 times higher than natural sea water [12,13]. NH4 content in natural seawater is 0.04 mg/L on average and rarely exceeds 0.40 mg/L[14,15]. In this paper, the concentration of 10 mg/L was selected to study the influence of NH4 pollution on the corrosion of B10 copper-nickel alloy in seawater. As for the influence of ammonium ion on copper alloy corrosion, Pugh et al. [16] believed that the composite ion formed by the combination of NH4 and copper ion would lead to intergranular fracture of pure copper under stress conditions. Lobnig et al. [17] studied the influence of ammonium sulfate on Cu corrosion, and detected Cu2O and basic copper sulfate in corrosion products. Al-hashem et Al. [18] believed that ammonia and ammonium salts enhanced the corrosion ability of the alloy through the complexation of copper ions. Wang Jihui et al. [19] believed that the addition of NH4 significantly increased the corrosion rate of HA177 and BFe30 copper alloys and accelerated the corrosion wear. Zhang Juan et al. [20] believed that when NH4Cl content is up to 27 g/L, aluminum-brass can detect the hydrate of copper ammonia complex in corrosion products in simulated seawater. The corrosion behavior and mechanism of B10 copper-nickel alloy in natural seawater have been studied in detail, but the effect of NH4 pollution on the corrosion of B10 copper-nickel alloy in seawater environment needs further discussion. Therefore, this paper studied the influence of NH4 pollution on the corrosion of B10 copper-nickel alloy in seawater, in order to provide a reference for the extensive application and anti-corrosion design of B10 copper-nickel alloy in seawater environment.

1. Methods the chemical composition (mass fraction,%) of B10 Cu-Ni alloy is SI 0.15, p 0.02, s 0.02, Zn 0.3, MN 1.0, Zn 0.15, Ni 10.0, Fe 1.0, Cu allowance. The experimental seawater was taken from the sea area of Shazikou, Qingdao, Shandong Province. The temperature of the corrosion medium was (25 ± 0.2) °C and the initial Ph was (7.9 ± 0.1) . Adding Analytical Pure NH4CL into natural seawater to get NH4 + in seawater, where the concentration of NH4 + is 10mg/l. The weight loss test cycle was 30 days, and 3 groups of parallel samples were set up. Rust removal reference GB/t 16545-2015. The GAMRY Reference 600 type electrochemical workstation is used for the electrochemical test. The working area of the sample is 1cm2. The platinum electrode is used as auxiliary electrode and the Reference electrode is Ag/AgCl/electrode. Electrochemical impedance spectroscopy (Eis) is used to measure the voltage at 105 ~ 10-2 Hz. The high frequency is scanned to the low frequency, and a sine voltage signal with amplitude of 10 MV is applied on the basis of open-circuit potential. The scanning rate of the polarization curve is 0.334 mV/s, and the scanning range of the polarization curve is OCP ± 800 MV. The surface morphology of samples was observed by Tescan Vega3 Scanning electron microscope, and the corrosion products were analyzed by energy dispersive diffraction (EDS) and Themo-Scientific Escalab 250Xi x-ray photoelectron spectroscopy (XPS) .

2. Results and discussion 2.1 weightlessness test results Fig. 1 shows weightlessness test results of corrosion 30 days. The average corrosion rate of B10 cu-ni alloy in natural seawater for 30 days is 0.013 G m-2 h-1, and 0.016 G m-2 h-1 in high NH4 + seawater. The average corrosion rate of B10 cu-ni alloy in seawater increased by 23% after 30 days of NH4 + corrosion. 2.2 The open-circuit potential of B10 cu-ni alloy in two kinds of seawater system is shown in Fig. 2. The open-circuit potential of B10 cu-ni alloy in two kinds of seawater system is decreased first and then increased, the General Law of corrosion is similar, but the state of corrosion product is different with the time of corrosion product forming.

Figure 3 shows the electrochemical impedance of B10 cu-ni alloy in seawater. Figure 4 shows the equivalent circuit used to fit the EIS data. Table 1 shows the EIS data. The results of electrochemical impedance spectroscopy (Eis) showed that NH4 + pollution did not change the corrosion phase of B10 cu-ni alloy in seawater. This is consistent with the open-circuit potential, and corresponds to the formation of protective corrosion product film and the weakening failure of cu-ni alloy in seawater. Comparing with the radius of capacitive arc resistance before and after adding NH4 + , it can be seen that the radius of capacitive arc resistance under the condition of adding NH4 + is obviously smaller than that of natural seawater. From Fig. 3B, it can be seen that the slope of the mode values in high NH4 + seawater is smaller, which indicates that the adding of NH4 + reduces the compactness of the corrosion product layer. The phase angle peak of B10 cu-ni alloy in natural seawater is wider, and there are two apparent time constants, but they are not obvious. Two more obvious time constants can be observed in high NH4 + seawater, which indicates that the existence of NH4 + makes the difference between corrosion product film and substrate more obvious.

Fig. 5 shows the polarization curves of B10 cu-ni alloy in two seawater systems. As can be seen from the diagram, the addition of NH4 + does not change the self-corrosion potential of B10 cu-ni alloy. Under the two systems, the anodic polarization curves have three distinct oxidation peaks, which presumably involve three anodic reaction processes, the non-typical “Passivation”behavior indicates that the anodic reaction product covers the surface of B10 cu-ni alloy and prevents the corrosion.

The fitting data of polarization curves show that B10 cu-ni alloy has higher self-corrosion current density in seawater with NH4 + , which shows that NH4 + can promote the corrosion of B10 cu-ni alloy.

2.3 characterization and composition analysis of corrosion products 2.3.1 SEM observation figure 6A and B are the 30-day corrosion products of B10 cu-ni alloy and the surface topography of the derusting substrate. It can be seen that there are more corrosion products on the surface in both environments, and as a result of surface loss of water, can see chapped fine lines. After removing the corrosion products, it can be seen from figures 6C and D that the B10 cu-ni alloy has more pitting corrosion and pits, and the local corrosion is more serious in high NH4 + environment. The results show that NH4 + promotes the local corrosion of B10 cu-ni alloy.

2.3.2 EDS analysis results Fig. 7 shows EDS spectra of corrosion products of B10 cu-ni alloy immersed in two seawater systems for 30 days, and Fig. 3 shows EDS data of corrosion products of B10 cu-ni alloy immersed in two seawater systems for 30 days. The results show that the corrosion products of B10 Cu-Ni alloy in two seawater systems are mainly composed of o, Cu and Ni. In addition, trace elements such as S, CL, MN and Fe were also detected, which may come from alloy components or seawater. N was not detected, probably because the corrosion product of NH4 + reaction dissolved in seawater and did not appear in the product.

2.3.3 XPS component analysis Fig. 8 shows the XPS spectrum of the corrosion products of B10 cu-ni alloy immersed in corrosive medium for 30 days. The combination energy of XPS elements was fitted to the peak at 284.8 eV by using the c bond as a reference. The quantitative result of the product is shown in Fig. 9. The main forms of Cu, O and Ni in corrosion products are Cuo, Cu2O, Nio and Ni (OH)2. The difference lies in the content of Cu2o in the corrosion products. The content of Cu2o in the oxide of Cu decreases from 37% to 22% with the increase of NH4 + concentration. It can be seen that the addition of NH4 + significantly reduced the content of Cu2o in the corrosion products and promoted the corrosion of B10 cu-ni alloy.

2.4 The mechanism of the effect of NH4 + on the corrosion of B10 cu-Ni alloy in seawater is discussed from the coordination property of NI2 + , it can be seen that under the condition of NH4 + concentration of 10mg/l, the complex ion [ Ni (NH3)6]2 + is not easy to be formed, and Ni (OH)2 is its stable existence form [22] . The two processes of B10 cu-ni alloy in the early stage of corrosion are the establishment of protective passivation film and the adsorption and erosion of Cl-et al. . Reaction (1) is the most important CATHODIC depolarization process in the system, and reaction (2) reflects the promotion of Cl-on the ANODIC process. Reaction (3) reflects the establishment of Cu2o thin film, which has high density and plays an important role in corrosion resistance of Cu alloy with its high integrity. The reaction (4) is the formation process of Cuo, the CuO layer lies above the CU2O layer and is relatively loose.

The reactions (5) and (6) show that NH4 + weakens the compactness and integrity of CU2O membrane by consuming Cu2O to form complex ions. The existence of reaction (6) will make Cu2o film-protective film thin and produce cracks/defects, resulting in its protective performance greatly reduced. At the same time, the reaction (7) consumes the O2 near the interface, thus inhibiting the repair of the CU2O membrane. In general, NH4 + can destroy CU2O protective films in corrosion products. The mechanism of NH4 + promoting the corrosion of the alloy can be explained as follows: On the one hand, NH4 + can form copper-ammonia complex with copper ion, which promotes the process of electron loss of Cu, on the other hand, NH4 + can reduce the protective product Cu2O.

3. Conclusion (1) NH4 + promoted the corrosion of B10 cu-ni alloy in seawater, and promoted the local corrosion of B10 cu-ni alloy, which mainly showed pitting corrosion. (2) the corrosion products on the surface of B10 Cu-Ni alloy in seawater containing NH4 + are mainly Cuo, Cu2O and NiO oxides of Cu and Ni, but the proportion of CU2O is small. NH4 + can destroy CU2O protective film in corrosion products. 

Source: Chinese Journal of Corrosion and protection, author: Wang Jiaming, Yang Haodong, Du Min, Peng Wenshan, Chen Hanlin, Guo Weimin, Lin Cunguo

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