Real-Time Imaging of Brass Cross-Section with Dezincification Corrosion by Electrochemical Measurement Combined with Video Observation

Abstract

A real-time imaging electrochemical measurement system was developed for an in-situ observation of brass dezincification corrosion. This system allows for a video recording of brass cross-section under galvanostatic polarization. The progress of erosion associated with the dezincification corrosion in the depth direction on the brass cross-section was successfully observed from the video recording during the measurement. The erosion depth at each measurement time could be estimated from the cross-section images, indicating that the erosion depth was drastically increased at arbitrary time under galvanostatic polarization. It suggested that the erosion rate in the depth direction is strongly related to the dissolution behavior of zinc and copper from brass surface, namely, the preferential dissolution and the simultaneous dissolution. The relation between the dissolution behavior and the electrode potential of brass under galvanostatic polarization was discussed.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 61 (2022) 168–172.

Fig. 2 Results of (a) changes of electrode potential under galvanostatic polarization of brass and cross-sectional images of brass (b) before measurement, (c) immediately after starting measurement, at (d) 3 h, (e) 6 h, (f) 9 h, (g) 12 h, (h) 15 h, and (i) 18 h. The dotted line and the Cu_layer in the images (c)–(i) denote the position of electrode surface before starting measurement and the copper-enriched layer formed on the brass surface.

1. Introduction

Brass, which has excellent mechanical properties, is used as parts in cold and hot water supply piping systems owing to its good ability for fine cutting. However, dezincification corrosion of brass may occur depending on the water quality, and the breakage failure due to the decrease in the stress of brass caused by this corrosion.^1,2) In dezincification corrosion, a copper-rich layer is formed on the brass surface owing to the preferential dissolution of Zn. To analyze this corrosion phenomenon, the hydrodynamic method and quantitative analysis method of dissolved metal ions in the electrolyte solution were employed.^3–9) Itagaki et al.^4–7) analyzed the anodic dissolution of brass and dezincing-resistant brass using a channel-flow double electrode (CFDE). In this method, the dissolved ions from the working electrode can be determined in situ by arranging a detection electrode downstream from the working electrode. They reported potential regions of preferential Zn dissolution and simultaneous dissolution of Zn and Cu by detecting Cu ion dissolution current with the anodic polarization curve of brass. Zhou et al.^8,9) analyzed dissolved metal ions from brass in situ by applying an anodic current through a flowing electrolyte using atomic emission spectroscopy. They determined the corrosion rate of Cu and Zn dissolved from brass using this method and investigated the further dissolution of Zn or Cu from the underlayer related to the formation of a Cu-rich layer. We^10–12) developed an electrochemical imaging system with real-time imaging, combining simultaneous electrochemical measurements and video observations of metal surfaces. This system allows the analysis of reaction mechanisms based on the correspondence between the electrochemical data and images of metal dissolution or oxide formation. In the present study, an electrochemical measurement system was developed for the video observation of brass cross-sections. This system allows observation of the progress and cross-section of dezincification corrosion of brass in the depth direction during anodic dissolution and susceptibility test under galvanostatic polarization, respectively. The phenomenon of dezincification corrosion from brass cross-section during susceptibility test of dezincification corrosion of brass under galvanostatic polarization was observed by the developed system in the present study.

2. Experimental Procedure

2.1 Real-time imaging electrochemical measurement system

Figure 1 shows the schematic of the real-time imaging electrochemical measurement system developed in this study. An electrochemical cell was fabricated using a transparent acrylic plate with a thickness of 5 mm. Electrochemical measurements were performed using a three-electrode system. The working electrode was a free-cutting brass rod (φ = 20 × 100 mm) C3601 (Cu 59.2 mass%, Pb 3.24 mass%, Fe 0.07 mass%, Sn 0.37 mass%, Zn 37.12 mass%), the counter electrode was a Pt wire. A KCl-saturated Ag/AgCl electrode (SSE) with a double junction was used as the reference electrode. The working electrode samples were fabricated using the following procedure. First, the rod was cut lengthwise. In this case, the cross-section surface area of the rod is 1.57 cm². The surface, excluding the test surface used for the measurement, was then covered with transparent epoxy resin. The test surface was dry-polished with emery paper up to #2000, and ultrasonically cleaned with distilled water. The electrode was placed at the wall of the electrochemical cell and fixed by placing a silicon rubber sheet and the wall of the electrochemical cell in a vise, as shown in Fig. 1. This system allows the monitoring of the dissolving cross-section of the electrode through a transparent acrylic plate during electrochemical measurements using a digital microscope. The images obtained in the study are shown with the electrode surface on the top because the dissolution was captured on video from the end face (bottom side) of the electrode. Furthermore, susceptibility tests of the dezincification corrosion of brass by electrochemical measurement¹³⁾ and video recording were conducted using the following procedure.

Fig. 1

Real-time imaging electrochemical measurement system developed in the present study.

• ①    A test solution of 0.5 M NaCl containing 5 mM NaHCO₃ adjusted with double distilled water, with a resistivity of 21.0 Ω cm, was used for the measurement.
• ②    The electrochemical cell was placed in a constant-temperature bath filled with distilled water, which was controlled to maintain the temperature of the test solution prepared at ① in the electrochemical cell at 60°C.
• ③    Galvanostatic polarization at a current density of 1.0 mA cm⁻² for 18 h was performed using a potentiostat/galvanostat (Bio-Logic Science Instruments, SP-200). The time variation of the electrode potential was monitored during the measurements.
• ④    The penetration of the dissolution in the depth direction of the electrode cross-section was observed using a digital microscope (SELMIC, SEL-80) during electrochemical measurements.

2.2 Anodic polarization measurements

Anodic polarization measurements of brass were performed in the test solution prepared in Section 2.1. The brass composition and the test solution’s temperature were the same as those described in Section 2.1. Anodic polarization was initiated immediately after the immersion of the brass electrode in the test solution at a scan rate of 0.1 mV s⁻¹.

3. Results and Discussions

3.1 Real-time imaging of brass cross-section during galvanostatic polarization

In the present study, galvanostatic polarization of brass was performed for 18 h using a real-time imaging electrochemical measurement system, as shown in Fig. 1. The dissolving brass cross-section was recorded using a digital microscope during the measurement. Figure 2 shows the changes in the electrode potential of brass under galvanostatic polarization and cross-sectional images of brass at arbitrary times. The dotted line indicates the electrode surface position before the measurement, and the Cu-rich layer formed on the brass surface denotes Cu_layer in the images. The electrode potential of brass (Fig. 2(a)) was polarized from the rest potential (−0.18 V vs. SSE) to approximately −0.15 V vs. SSE immediately after starting the galvanostatic polarization. The electrode potential was then shifted to a noble potential and became stable after 9 h. Surface roughness from the brass dissolution was compared using the brass cross-sectional images, before (Fig. 2(b)) and after galvanostatic polarization of brass (Fig. 2(c)), which corresponded to the shifting of the electrode potential in the noble direction. The Cu-rich layer formation progressed with time in the depth direction under galvanostatic polarization after 3 h (Fig. 2(d)) and 6 h (Fig. 2(e)), indicating the preferential dissolution of Zn from brass. The thickness of the Cu-rich layer with a pore structure was approximately 50 µm in the depth direction at 9 h (Fig. 2(f)). In the images at 12 h (Fig. 2(g)), 15 h (Fig. 2(h)), and 18 h (Fig. 2(i)), the Cu-rich layer formation rate in the depth direction increased with the galvanostatic polarization time, demonstrating that the progress of dissolution was inhomogeneous in the depth direction.

Fig. 2

Results of (a) changes of electrode potential under galvanostatic polarization of brass and cross-sectional images of brass (b) before measurement, (c) immediately after starting measurement, at (d) 3 h, (e) 6 h, (f) 9 h, (g) 12 h, (h) 15 h, and (i) 18 h. The dotted line and the Cu_layer in the images (c)–(i) denote the position of electrode surface before starting measurement and the copper-enriched layer formed on the brass surface.

3.2 Dezincification corrosion behavior of brass

We confirmed the penetration of dezincification corrosion and the formation process of the Cu-rich layer by real-time monitoring of the cross-section of the dissolving brass under galvanostatic polarization. The dezincification corrosion of brass in this study is discussed as follows.

Figure 3 shows the anodic polarization curve of the brass. The current increased with a large gradient owing to polarization from the rest potential to the noble potential. The gradient of current was small around −0.17 V vs. SSE and then became large. Itagaki et al.⁴⁾ clarified the potential regions of preferential Zn dissolution and simultaneous dissolution of Cu and Zn in the anodic polarization curve of brass and detected dissolved copper ions using CFDE. They indicated that the potential region in which the current rise due to the polarization from the rest potential to noble potential and the small current rise observed thereafter was related to the preferential dissolution of Zn from brass by the following reaction:   

\begin{equation} \text{Zn} \to \text{Zn$^{2+}$} + \text{2e$^{-}$} \end{equation} (1)

Fig. 3

Anodic polarization curve of brass in the solution containing 0.5 M NaCl and 5 mM NaHCO₃ at 60°C.

Furthermore, the observed large current increase in the noble potential region (approximately 0.10 V vs. SSE) was associated with the simultaneous dissolution of Zn by reaction (1) and Cu by reaction (2).   

\begin{equation} \text{Cu} \to \text{Cu$^{2+}$} + \text{2e$^{-}$} \end{equation} (2)

In Fig. 2(a), the electrode potential of brass was larger than −0.10 V vs. SSE after 6 h. Because the noble potential from −0.10 V vs. SSE in the anodic polarization curve of Fig. 3 corresponds to the simultaneous dissolution of Zn and Cu,⁴⁾ the simultaneous dissolution of Zn and Cu may occur after 6 h in Fig. 2. Figure 4 presents the maximum¹³⁾ and minimum erosion depths of brass estimated from cross-sectional images at each measurement time in Fig. 2(c)–(i), and these values were plotted against the time of the images obtained in Fig. 2. The smallest erosion values in the depth direction were defined as the minimum erosion depth. In the figure, ○ and ● denote the maximum and minimum erosion depths, respectively. The results indicated that the values of the maximum and minimum erosion depths increased with time during galvanostatic polarization. The difference between the maximum and minimum erosion depths was almost constant at 3 h and 6 h, and it increased after 9 h. Moreover, the preferential Zn dissolution occurred at 3 h and 6 h, and Zn dissolution from the substrate occurred simultaneously with the dissolution of the Cu-enriched layer, leading to the progress of inhomogeneous dissolution in the depth direction. Thus, the electrode potential shift of brass from noble to less noble potential after 6 h under galvanostatic polarization may be related to the transition from the preferential Zn dissolution (formation of a Cu-rich layer) to the simultaneous dissolution of Zn and Cu. In this case, the simultaneous dissolution may be composed of reactions, including the progress of inhomogeneous dissolution of Zn in the depth direction, and the newly formed Cu-rich layer dissolution due to the preferential Zn dissolution.

Fig. 4

Plots of erosion depth vs. time. The white and black circles denote maximum erosion depth and minimum erosion depth estimated from cross-section images at each measurement time in Fig. 2(c)–(i).

In this study, the dissolution behavior of brass was analyzed by real-time imaging of the dezincification of brass, combining electrochemical measurements and video observations. The developed system can be applied to metal-containing materials to analyze the relationship between alloy composition and the progress of dissolution in the depth direction, leading to the development of a new corrosion susceptibility test for copper alloys.

4. Conclusions

A new electrochemical measurement system was developed for in-situ observation of the dezincification corrosion of brass during electrochemical measurements. The findings of this study are as follows:

1. (1)    Video observation of brass cross-sections under galvanostatic polarization allows real-time monitoring of dezincification corrosion of brass cross-sections.
2. (2)    Changes in erosion depth can be monitored from cross-sectional images at an arbitrary time.
3. (3)    Preferential Zn dissolution and the simultaneous dissolution of Zn and Cu can be analyzed by the changes in the electrode potential of brass under galvanostatic polarization and cross-sectional images of brass at an arbitrary time.

Acknowledgments

This study was supported by a 2019 research grant from the Japan Institute of Copper.

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