Digital Holographic Study of pH Effects on Anodic Dissolution of Copper in Aqueous Chloride Electrolytes

Abstract

The anodic dissolution of copper in chloride electrolytes with different pH has been investigated by using polarization measurements and digital holography. In acidic and neutral NaCl solutions, the oxidation processes of copper are almost the same: copper firstly dissolves as cuprous ions, which then produces the CuCl salt layer. The dissolution rate in the acidic solution is a little higher than that in the neutral. However, the mechanism is quite different in the alkaline NaCl solution: copper turns passive easily due to the formation of a relatively stable Cu₂O film which results in pitting, and the dissolution rate of copper decreases before pit initiation.

1. Introduction

Copper is one of the most important materials in the industry owing to its relatively noble properties. The anodic polarization of copper in acidic, neutral, and alkaline chloride media has received considerable attention in the literature [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. As the electro-dissolution of Cu is found to be complex, the various mechanisms proposed can be applied for chloride electrolytes. In 2004, Kear et al., summarized copper corrosion in chloride electrolytes and concluded three different models of the initial electro-dissolution reactions of bare copper [1]. There are three possible initial reaction cases of the anodic polarization behavior of pure copper in acidic and neutral chloride media [1,10,12].

$$(\mathrm{I})\mathrm{Cu}\stackrel{}{\rightleftarrows }{\mathrm{Cu}}^{+}+{\mathrm{e}}^{-}$$

(1)

$${\mathrm{Cu}}^{+}+2{\mathrm{Cl}}^{-}\stackrel{}{\rightleftarrows }{{\mathrm{CuCl}}_2}^{-}$$

(2)

$$(Ⅱ)\mathrm{Cu}+{\mathrm{Cl}}^{-}\stackrel{}{\rightleftarrows }\mathrm{CuCl}+{\mathrm{e}}^{-}$$

(3)

$$\mathrm{CuCl}+{\mathrm{Cl}}^{-}\stackrel{}{\rightleftarrows }{{\mathrm{CuCl}}_2}^{-}$$

(4)

$$(Ⅲ)\mathrm{Cu}+2{\mathrm{Cl}}^{-}\stackrel{}{\rightleftarrows }{{\mathrm{CuCl}}_2}^{-}+{\mathrm{e}}^{-}$$

(5)

In recent years, the development and improvement of inhibitors of copper and its alloys in chloride media is of great interest to corrosion scientists and applied electrochemists [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]. The explanation of corrosion inhibition in presence of chloride ions are based on the three different models. Case I presents the direct formation of cuprous ions [15,16,18,22,30], whereas cases II [17,19,20,21,23,24,25,26,29,31] and III [27,28] involve the formation of cuprous chloride. It means that the reaction pathway is under debate up to the present in acidic and neutral chloride media [10,12,18,19,20,21,22,23,24,25,26,27,28,29,30,31].

Holographic interferometry has been widely employed to study dynamic electrochemical processes for recent decades [32,33,34,35,36,37,38,39,40,41,42,43,44,45]. The basis of holographic method for the observation of an electrode/electrolyte interface in an electrochemical system, is the generation and measurement of gradients in the refractive index of the solution adjacent to the electrode [32,33,35,36,37,38,39,40,41,42,43,44,45]. A common holographic setup of a solid/liquid interface is illustrated in Figure 1, which is based on a Mach-Zehnder interferometer. In the fields of electrochemistry and corrosion, the changes of concentration caused by electrochemical reactions at an electrode/electrolyte interface, lead to the changes in the refractive index and thus the variations of the phase of the object beam when it goes through the surface of the electrode, as shown in Figure 1. The phase changes can be encoded in the holograms via a CCD (charge coupled device) image sensor and then reconstructed by image processing algorithms [39,40]. The principle of the experiment is that the phase changes ∆Ф (usually called phase differences in optics) have a linear relationship with the ion concentration changes of electrolyte at the electrode/electrolyte interface [6,32,39,40,41]. As a non-destructive testing technology, the holographic method is sensitive to the high rate changes of the concentration of aqueous solutions. The dynamic concentration changes can be in situ monitored during electrochemical processes and the direct information concerning the electrode/electrolyte interface would provoke new insights into the mechanisms of electrochemical reactions.

The anodic processes of copper in chloride electrolytes are surprisingly complex as both soluble and insoluble products are involved. In our previous studies [6], anodic dissolution of copper in neutral NaCl solution was investigated with digital holography and potentiodynamic polarization. The results only confirmed that the first step is the formation of cuprous ions. In addition, the formation and dissolution of species may be depended upon the pH of the solution and the electrode potential. M. Antonijevic et al., investigated the pH effects on electrochemical behavior of copper [4] and its alloy [7] in presence of chloride ions with the open circuit potential and linear voltammetry. H. Curkovic et al., studied the pH influence on the efficiency of corrosion inhibiters of copper in chloride media [19]. J. Scully’s group found [46] and elucidated [47] the pH dependence of copper pitting in chlorinated and aerated Edwards synthetic drinking waters. Ni et al., found out that the pH of Cl⁻ containing solution played a crucial role in the polarity reversal of the Cu–304 stainless steel galvanic couple [48]. It can be concluded that the pH change in the solution has a significant effect on the copper corrosion and protection.

In order to verify the anodic dissolution mechanisms of copper in acidic, neutral, and alkaline chloride media, the electro-dissolution processes of copper in NaCl solutions with different pH have been investigated by monitoring the diffusion layers at the interface. We present detailed, time-resolved, in situ visualizations of the dynamic processes of anodic dissolution of copper directly at the electrode/electrolyte interface, using digital holography combined with potentiostatic and potentiodynamic polarization. The anodic dissolution mechanisms of copper in acidic, neutral, and alkaline chloride electrolytes are discussed, respectively.

2. Materials and Methods

The electrochemical cell and the holographic measurement system were the same with our previous study [9]. The material used in the experiment was pure copper (99.99%, Aldrich). The working electrode was in the form of copper rod, with a diameter of 2 mm. The exposed area of Cu electrode to the solution was 3.14 mm². The observed area in the holographic measurement is shown in Figure 2. Before each experiment, the copper electrode was mechanically abraded with #1500 and #3000 emery papers to a mirror-like surface and then cleaned by alcohol and triply distilled water in an ultrasonic bath. Three following electrolytes were used: 0.5 M NaCl, 0.5 M NaCl + 0.01 M HCl, and 0.5 M NaCl + 0.01 M NaOH. The voltammetric measurements were performed by means of the CHI 660B electrochemical station at room temperature under non-deaerated condition. All potentials reported were reported vs. SCE (saturated calomel electrode). Before the polarization measurements, the copper electrode was kept at −1200 mV vs. SCE for 10 min in the corresponding electrolyte to reduce the oxides on the surface. All of the scan rates were 10 mV/s in the potentiodynamic polarization measurements.

3. Results and Discussion

3.1. Neutral and Acid Chloride Electrolytes

It has been reported that the polarization behavior of copper in acid chloride solution is very similar to that in neutral chloride solution within the apparent Tafel region [1]. Firstly, anodic dissolution of copper in neutral and acid NaCl solutions was investigated by use of potentiodynamic polarization and digital holography.

3.1.1. Potentiodynamic and Holographic Results

The cyclic voltammograms of copper electrode in 0.5 M NaCl and 0.5 M NaCl + 0.01 M HCl solutions are shown in Figure 3. There are two anodic current density peaks in the curves. According to the E-pH diagram [49], the first peak is associated with the oxidation of Cu to Cu(I) and the second is related to the oxidation of Cu(I) to Cu(II) species. It is evident that the curve in neutral media is very similar to the one in acidic solution.

Figure 4 and Figure 5 present the phase maps reconstructed at the given potential points (a–h) in the forward scan curves of Figure 3a,b, respectively. In each curve, eight remarkable points are chosen and the corresponding phase maps are obtained with the digital holography. In each phase map, the left is electrode part and the right is solution. The copper/solution interface is marked as a black line in Figure 4a and Figure 5a. The color changes indicate ion concentration changes at the area, where turn yellow/red means increase while turn blue means decrease [6]. It is evident that the ions concentration at the interface experiences dramatic changes during the potentiodynamic polarization processes. The evolution of the diffusion layer in acidic solution is similar to that in neutral. According to the phase maps, both the anodic processes are divided into three sections, as shown in Figure 3a,b. In agreement with previous investigations [6], the holographic results indicate the following points: (1) the increase of concentration at section I (Figure 4b–d and Figure 5b–d) is caused by the formation of cuprous ions; (2) the formation of CuCl salt layer consumes amounts of chloride ions which lead to the decrease of concentration at the interface at section II (Figure 4e–f and Figure 5e–f); (3) the dissolution of CuCl salt layer ceases the decrease of the phase difference ∆Ф (Figure 4g and Figure 5g), then amounts of cupric ions are formed which lead to the sharp increase of the phase difference in section III (Figure 4h and Figure 5h). The holographic results confirm that the mechanisms proposed in neutral solution can be applied for in an acidic solution [1,4].

3.1.2. Potentiostatic and Holographic Results

Potentiostatic current-time tests were performed to give more insights into the reaction mechanisms during the anodic processes of copper. Since the anodic dissolution of copper in neutral and acid NaCl solutions can share the same reaction mechanisms, only the results in neutral media are exhibited in the following.

Three typical potentials (−60 mV, 180 mV, and 400 mV vs. SCE) were chosen and the corresponding holographic results were obtained. All the electrolytes were 0.5 M NaCl solution in the potentiostatic polarization measurements. In each experimental, the potential of copper electrode was jump to the selected potentials after 10 min reduction under −1200 mV vs. SCE. Figure 6 shows the i-t curve of the Cu/NaCl system and the corresponding phase maps under the potential of −60 mV vs. SCE. There is a peak in the curve before the current density comes to a stable value. The concentration at the interface increases at point b firstly, as shown in Figure 6b, then it decreases gradually at points c and d (Figure 6c,d). Finally, it increases again at points e and f (Figure 6e,f). Since the applied potential (−60 mV vs. SCE) was lower than the first peak potential in Figure 3, it is reasonable to believe that the current increased first due to the formation of the cuprous ions (point b), then began to decrease after the current peak for the formation of the CuCl film (points c and d). Finally, CuCl dissolves as CuCl₂⁻ and the current tended to be stable (point f).

Figure 7 and Figure 8 show the i-t curve and phase maps under the potential of 180 mV vs. SCE and 400 mV vs. SCE, respectively. The current density drops directly in the curves. From these phase maps, it can be seen evidently that the concentration at the interface firstly decreases and then increases at the plateau current region. At the initial stage, there is no concentration increase, indicating the phase maps cannot trace cuprous ions in the solution. The formation of cuprous chloride salt layer consumes chloride ions and leads to the decrease of the phase difference (Figure 7b–d and Figure 8b–c). At these relatively high potentials, there may not be enough time for cuprous ions to diffuse into the solution. When the current density is relatively stable, the cuprous chloride salt layer dissolves as cupric ions and the concentration at the interface increases sharply (Figure 7e–f and Figure 8d). According to Figure 7f and Figure 8d, more cupric ions are formed at higher potential when the current density is stable. Compared with these phase maps (Figure 6, Figure 7 and Figure 8), when the potential increases, the cuprous chloride layer forms faster but its time of duration becomes shorter.

In neutral NaCl solution, copper dissolves as cuprous ions near the corrosion potential. With the increase of the electrode potential, a large amount of CuCl salt layer forms and covers on the electrode surface at the first oxidation peak. The phase difference begins to grow after the peak. Before the turn over point, part of CuCl dissolves as cuprous dichloride complex. If the potential is persistent at a point in the region of the first peak, the formation and dissolution balance of CuCl is formed and the current density is limited by the transport of CuCl₂⁻. When the potential continues to rise, the CuCl layer dissolves into cupric ions and chloride ions, which lead to a great growth of the phase difference and thus a steep concentration gradient layer at the interface. The reaction pathway of the initial stage of copper anodic dissolution in neutral NaCl solution is:

$$\mathrm{Cu}\stackrel{}{\rightleftarrows }{\mathrm{Cu}}^{+}+{\mathrm{e}}^{-}$$

(6)

$${\mathrm{Cu}}^{+}+{\mathrm{Cl}}^{-}\stackrel{}{\rightleftarrows }\mathrm{CuCl}$$

(7)

$$\mathrm{CuCl}+{\mathrm{Cl}}^{-}\stackrel{}{\rightleftarrows }{{\mathrm{CuCl}}_2}^{-}$$

(8)

The proposed reaction mechanisms are also applied for acid NaCl solution.

3.2. Alkaline Chloride Electrolyte

In alkaline chloride media (0.5 M NaCl + 0.01 M NaOH), it can be seen evidently that the electrochemical behavior of copper electro-dissolution is entirely different from that in acidic and neutral solutions, as shown in Figure 9. The current density begins to increase at a higher potential value compared with the former.

In Figure 10, the evolution of the diffusion layer at the interface is quite different from that in acid and neutral solutions as well. At an early stage of the experiment, there is hardly any observable change at the interface in the phase maps when the current density is low (Figure 10a). It is evident that small amounts of cuprous ions or cuprous dichloride complex dissolves into the solution when the electrode potential is lower than 200 mV vs. SCE. As it was reported that Cu(I) species were detected during this region [4], the authors propose that the main component of Cu(I) species is probably to be Cu₂O film in the alkaline environment. The consumption of hydroxide ions leads to a slight decrease of the phase difference. If the Cu⁺ or CuCl₂⁻ were released into the solution, the concentration at the interface would increase as found in neutral/acid solutions. If the salt film CuCl was the main formation, the concentration at the interface would decrease for the consumption of chloride ions in the solution. Compared to the situation in acid and neutral solutions, the Cu₂O is favored as OH⁻ concentration increases in the alkaline environment [4],

$$2\mathrm{Cu}+2{\mathrm{OH}}^{-}\stackrel{}{\rightleftarrows }{\mathrm{Cu}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$$

(9)

The current density increases sharply when the electrode potential is above 200 mV vs. SCE. Meanwhile, two yellow spots appear at the electrode/electrolyte interface (Figure 10b), indicating that the concentration in those areas is higher due to the formation of soluble species. The Cu₂O film is broken down firstly at these yellow spots. It means that localized corrosion has been initiated near the yellow spots on the copper surface. This phenomenon confirms the point that pitting of copper is evident only after the formation of a protective film of Cu₂O [2,4]. Then, these pits propagate and new pits continue to be induced as shown in Figure 10c–d. With the increase of the current density, more and more pits are induced on the entire surface of the electrode. At last, the whole electrode is activated. During this process, plenty of soluble cupric species dissolve into solution and a steep concentration gradient layer forms at the interface (Figure 10e–f). The pitting dynamic process of copper is visually presented in Video S1. According to the phase maps, it is deduced that a protective film of oxide film exists on the surface of copper and plays an important role in the anodic processes of copper. The surface films in alkaline media decrease the dissolution rate of copper before the pitting occurs.

3.3. Effects of pH

Both the cyclic voltammograms and the phase maps exhibit the pH effects on the anodic dissolution of copper in aqueous chloride electrolytes.

There are three differences between the cyclic voltammograms in acidic and neutral solutions. First, two peak potentials in acidic solution are shifted to a negative direction. Second, in acidic medium, the plateau current density is bigger than that in neutral solution. Third, in the reverse scan, the current density peak in former is larger than that in the later. All of these changes in acidic solution indicate a marked increase of the anodic dissolution processes of copper electrode.

The phase maps directly exhibit the pH effects on concentration changes at the electrode/electrolyte interface. Compared with Figure 4 and Figure 5, there are some evident differences in the phase maps between the acidic and the neutral. When the current density starts to increase, there is more deep yellow area in Figure 5c than Figure 4c, indicating acidic environment is in favor of the formation of cuprous ions at this region. In the first oxidation peak, more deep blue area at the interface (Figure 5f and Figure 4f) indicates more Cl⁻ ions are consumed, which means that the formation of CuCl salt layer on the electrode surface is favored in acidic solution. In the plateau region involving the second oxidation peak, more deep red area at the interface (Figure 5h and Figure 4h) means more cupric ions dissolve into the solution, indicating that the rate of electro-dissolution increases in an acidic solution. It is concluded that the concentration gradient in acid solution is steeper than that in neutral solution during the three sections, indicating a marked increase of the anodic processes in acidic environment, which is in agreement with the polarization curves.

Compared to surface film in the alkaline media, the oxide film is difficult to exist steadily and likely to be corroded by chloride ions in acid and neutral environments. The main component of the surface film is CuCl salt layer at the first oxidation peak. When the potential is more positive than 150 mV vs. SCE, the CuCl film dissolves and soluble cupric ions appear, which bring about a sharp concentration gradient layer. In contrast, the formation of Cu₂O film is favored in alkaline chloride solution and a relatively stable oxide film exists on the surface of copper. When the potential is higher than 200 mV vs. SCE, the protective film damages and dissolves as soluble species. The pitting dynamic process of copper is evident at the electrode/electrolyte interface in the alkaline chloride media from the phase maps.

4. Conclusions

The pH effects on the anodic dissolution of copper in aqueous chloride electrolytes have been investigated with the polarization measurements and digital holographic method. In acidic and neutral NaCl solutions, the holographic results support that the anodic dissolution mechanisms in the initial stage of copper are,

$$\mathrm{Cu}\stackrel{}{\rightleftarrows }{\mathrm{Cu}}^{+}+{\mathrm{e}}^{-}$$

$${\mathrm{Cu}}^{+}+{\mathrm{Cl}}^{-}\stackrel{}{\rightleftarrows }\mathrm{CuCl}$$

$$\mathrm{CuCl}+{\mathrm{Cl}}^{-}\stackrel{}{\rightleftarrows }{{\mathrm{CuCl}}_2}^{-}$$

Compared to neutral electrolyte, acidic environment accelerates the copper corrosion in chloride solutions: the steeper concentration gradient layer in the different stages means more formation of cuprous ions, CuCl salt layer, and cupric ions, respectively. In an alkaline chloride environment, the dissolution rate of copper decreases for the formation of a protective oxide film,

$$2\mathrm{Cu}+2{\mathrm{OH}}^{-}\stackrel{}{\rightleftarrows }{\mathrm{Cu}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$$

However, when the electrode potential is positive enough, the pitting dynamic process is observed which lead to an abrupt increase of corrosion current density.