Effective Corrosion Inhibition of Galvanic Corrosion of Cu Coupled to Au by Sodium Dodecyl Sulfate (SDS) and Polyethylene Glycol (PEG) in Acid Solution

Abstract

This study investigates the effects of sodium dodecyl sulfate (SDS) and polyethylene glycol (PEG) on the galvanic corrosion behavior of copper (Cu) coupled to gold (Au) in a printed circuit board (PCB) etching solution. Galvanic corrosion tests using ZRA (zero resistance ammeter) were performed to determine the optimal SDS concentration for corrosion inhibition. The corrosion current between Cu and Au decreased significantly with the addition of SDS, from 3.26 mA/cm² to 0.248 mA/cm² at 4 mM SDS, achieving an inhibitor efficiency (IE) of 92.3%. However, at 15 mM SDS, the corrosion current increased, and IE decreased to 80.5%. This phenomenon is attributed to the critical micelle concentration (CMC) of SDS, where surfactant molecules aggregate and reduce surface adsorption properties. Similarly, ZRA tests were conducted to analyze the effects of PEG on galvanic corrosion. The corrosion current significantly decreased with PEG addition, achieving 98.1% IE at 1 g/L and 99.5% IE at 2 g/L. Beyond this concentration, no significant change in IE was observed, indicating saturation. Potentiodynamic polarization tests were also conducted to study the individual effects of SDS and PEG on Cu and Au. The results showed that SDS effectively inhibited Cu corrosion but had a minimal impact on Au. In contrast, PEG significantly reduced the corrosion current density for both Cu and Au, with reductions of 99.5% and 95.1%, respectively.

1. Introduction

Printed Circuit Boards (PCBs) are utilized as substrates in various electronic devices. These PCBs consist of a multilayer structure composed of copper, nickel, gold, and other metals [1,2,3,4]. During the PCB manufacturing process, including an etching process, galvanic corrosion occurs due to the electrochemical potential differences between the metals. In particular, significant galvanic corrosion occurs in patterns where copper and gold are electrically connected, which leads to an over-etching problem of copper pads [5,6,7,8]. To prevent galvanic corrosion problems, researchers investigated corrosion inhibitors such as organic surfactants, which function by physically and/or chemically adsorbing onto the metal surface to form a protective film [9,10,11,12,13,14,15,16,17,18]. Sodium dodecyl sulfate (SDS) functions effectively as a corrosion inhibitor due to its ability to adsorb onto metal surfaces, forming a protective layer that impedes oxidative and reductive processes. This surfactant’s adsorption mechanism enhances its capacity to shield metals from corrosive environments, making it highly effective in diverse applications [19,20,21,22]. Honggun Song’s research group [19] explored the use of SDS as a corrosion inhibitor for Mg alloys in NaCl environments, achieving a corrosion inhibition efficiency of 89.1% with the addition of 0.06 M SDS. X. Lu [22] and colleagues demonstrated that sodium dodecyl sulfate (SDS) serves as an effective corrosion inhibitor for AZ91 magnesium alloys. Their findings showed that when the alloy was immersed in a 3.5 wt.% NaCl solution with 0.05 M SDS for 48 h, the volume of hydrogen generated decreased from 9.1 to 0.3 mL/cm². Ali Yousefi [20] employed SDS as a corrosion inhibitor for mild steel in hydrochloric acid solutions. They conducted experiments using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, atomic force microscopy (AFM), and dynamic light scattering, and compared the results with DFT calculations. Jianhong Tan [21] investigated the synergistic inhibition of carbon steel corrosion in acidic media by the combined addition of sodium dodecyl sulfonate (SDS) and potassium iodide (KI). The results indicated that adding 5 mM KI achieved approximately 75% corrosion inhibition, which was significantly enhanced to 96.9% with the inclusion of SDS, demonstrating the effective maximization of corrosion inhibition through the synergistic action of these two inhibitors. Polyethylene glycol (PEG) is also an effective corrosion inhibitor, as it forms a stable protective film on metal surfaces, significantly reducing corrosion rates [22,23,24,25,26]. Its versatility allows it to perform well in various acidic, alkaline, and neutral environments. Inhibition of PEG is further enhanced when combined with other inhibitors, such as cerium nitrate, providing robust protection for metals [27,28]. Fouda et al. examined the effectiveness of polyethylene glycol (PEG) in inhibiting the corrosion of α-brass alloy in a 1 M nitric acid solution, noting that higher concentrations of PEG resulted in greater protection efficiency [23]. H. Boudellioua [24] investigated the impact of polyethylene glycol (PEG) on the corrosion inhibition of mild steel using cerium nitrate in a 0.1 M NaCl solution. They discovered that adding PEG to the cerium nitrate solution improved protection by forming stable corrosion products and reducing cracks in the protective film on the mild steel surface. Jwaher M. AlGhamdi [25] achieved a 97.6% corrosion inhibition efficiency for mild steel in a 1 M HCl environment using PEG-BEC as an inhibitor. The efficiency increased with the rising inhibitor concentration but decreased with increasing temperature (25 °C to 65 °C) and under hydrodynamic conditions. The research group of X.Y. Wang [27] improved the corrosion resistance of aluminum in an alkaline zincate solution by adding PEG. The optimal electrolyte composition, containing 0.2 M ZnO and 2.0 mM PEG, exhibited the highest corrosion inhibition efficiency at 98.8%. Studies on the use of corrosion inhibitors such as SDS and PEG to prevent galvanic corrosion between copper and various dissimilar metals have been published, and there remains a gap in the research specifically examining the inhibition of galvanic corrosion in copper–gold pairs within PCB etching solutions using these inhibitors. This study examined the effectiveness of SDS and PEG as corrosion inhibitors of the galvanic corrosion of copper coupled with gold in PCB etching solutions through electrochemical experiments to elucidate the inhibition mechanisms.

2. Experimental Specimen Preparation

Copper foil on a copper-clad laminate (CCL) was utilized for the copper specimen to mimic a typical printed circuit board configuration. To avert the formation of an intermetallic compound between copper and gold, nickel was plated onto the copper substrate. Nickel electroplating was performed for 15 min at 40 °C using a cathodic current density of 100 mA/cm² from a solution consisting of 0.45 M NiSO₄·6H₂O, 0.126 M NiCl₂·6H₂O, and 0.8 M H₃BO₃. Subsequently, gold was electroplated on the nickel substrate for 10 min and 30 s at 50 °C with a cathodic current density of 1.5 mA/cm² from a solution containing 50 mM Kau (CN)₂. The working area exposed on the gold specimen was 0.196 cm² (diameter of 0.5 cm, circular shape), maintaining a 1:1 ratio of exposed areas between copper and gold. The etching solution for the PCB was formulated with 0.28 M H₂SO₄, 0.42 M H₂O₂, 0.28 M CuCl₂·2H₂O, and deionized water, with a resultant pH of 0.47. These compounds were directly incorporated into the etching solution at varying concentrations.

Electrochemical Experiments

Electrochemical tests were conducted at 25 °C using a Versastat4 electrochemical workstation (Ametek). To examine the corrosion behavior of copper and gold, a potentiodynamic polarization test was executed, with a three-electrode system comprising the test specimens (Cu, Au) as the working electrode (WE), Pt as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode (RE). The open circuit of the working electrode was measured until it sufficiently stabilized in the solution for 30 min, after which polarization testing was conducted. After the open circuit potential (OCP) was stabilized, the polarization test was conducted. The polarization test was measured with a scan rate of 1 mV/s and the scan potential range was from −200 mV to +400 mV (OCP). To investigate the galvanic current density (i_{couple, (Cu-Au)}) between the copper and gold electrodes in the etching solution, a galvanic corrosion test (zero resistance ammeter test) was performed using a two-electrode system. The copper specimen served as the working electrode, while the gold specimen functioned as both the counter and reference electrode. The test was carried out for 600 s, varying the type and concentration of the corrosion inhibitor.

3. Results and Discussion

Figure 1 shows the effects of SDS contents on the galvanic corrosion behavior of Cu coupled to Au through the ZRA test. The contents of SDS were verified from 4 mM to 15 mM to confirm the optimum contents for the corrosion inhibition of Cu coupled to Au.

Table 1. Galvanic corrosion rate and inhibitor efficiency of Cu coupled to Au with the change in concentration of SDS and PEG.

	Concentration (g/L)	icouple, (Cu-Au) (mA/cm2)	Inhibitor Efficiency (%)
No inhibitor	-	3.26
SDS	4	0.248	92.3
	8	0.211	93.5
	15	0.636	80.5
PEG	1	0.06	98.1
	2	0.017	99.5
	4	0.02	99.4

presents the corrosion current values according to the SDS and PEG concentration and the inhibitor efficiency (IE) calculated from these values. The galvanic corrosion test results indicated that the galvanic corrosion current between Cu and Au significantly decreased with the addition of SDS in etching solution. The galvanic corrosion current (i_{couple, (Cu-Au)}) was decreased from 3.26 mA/cm² to 0.248 mA/cm² with the addition of 4 Mm SDS. The corrosion inhibitor efficiency of the 4 Mm SDS was calculated to be 92.3%. With the increase in SDS contents to 8 mM, the galvanic corrosion current was decreased to 0.211 mA/cm² and the corrosion inhibitor efficiency was increased to 93.5%.

With further increase in the SDS contents to 15 mM, the galvanic corrosion current was measured to be 0.636 mA/cm², which is an increased value compared with the contents under 8 mM SDS. The inhibitor efficiency was also decreased to 80.5% compared with the results of contents under 8 mM SDS. Figure 2 depicts the IE values according to SDS concentration. The comparison of the inhibitor efficiency with varying SDS concentrations reveals that the IE increased up to 8 mM but decreased beyond this concentration.

Upon the addition of SDS to a solution, the molecules initially disperse and adsorb individually onto metal surfaces. As the concentration of SDS increases and exceeds the critical micelle concentration (CMC), self-aggregation occurs, resulting in the formation of micelles. In these micellar structures, the hydrophobic tails converge within the core, while the hydrophilic heads orient outward, interacting with the aqueous environment. This self-assembly plays a crucial role in SDS’s behavior as a corrosion inhibitor [17]. The CMC of SDS is known to be 8.2 mM. Below 8.2 mM of SDS, SDS molecules effectively inhibit corrosion reactions by adsorption on metal surfaces. Beyond the CMC, the molecules begin to form micelles rather than adsorbing directly onto the metal surface. While micelle formation helps in surfactant stabilization in solution, it can reduce the availability of individual SDS molecules for surface adsorption [24]. Additionally, micelles may act as a barrier, altering the surface chemistry and reducing the effectiveness of the protective layer, which can re-expose the metal to corrosive agents, resulting in diminished inhibition [29,30]. To compare the galvanic corrosion inhibition effects of polyethylene glycol (PEG) content on copper coupled to gold, ZRA tests were conducted. The PEG concentrations ranged from 1 g/L to 4 g/L and were added to the etching solution to observe the effect on the galvanic corrosion inhibition of copper coupled to gold. Figure 3 shows a significant decrease in the copper galvanic corrosion current when PEG is added to the etching solution.

Table 1 presents the corrosion current values and the inhibitor efficiency calculated from the corrosion current for different PEG concentrations. In the galvanic test without any corrosion inhibitor, the galvanic corrosion current of copper and gold was 3.26 mA/cm². With 1 g/L PEG, the corrosion current decreased to 0.06 mA/cm², showing a 98.1% inhibition efficiency compared to the solution without the inhibitor. For 2 g/L PEG, the corrosion current was 0.017 mA/cm², with an inhibition efficiency of 99.5%. At 4 g/L PEG, the corrosion current measured 0.02 mA/cm², resulting in a 99.4% inhibition efficiency. The comparison of inhibitor efficiency with PEG content indicated an increase in IE values up to 2 g/L, enhancing the corrosion inhibition effect. However, beyond 2 g/L, the IE values showed no significant change, indicating saturation. Figure 4 illustrates the IE values with varying PEG concentrations. This phenomenon suggests that PEG, due to its non-polar molecular structure, anchors through physical adsorption or weak chemical interactions, reducing the active area available for corrosion reactions.

This adsorption creates a protective film that isolates the metal, thus inhibiting both the anodic and cathodic reactions responsible for corrosion. To study the influence of SDS as a corrosion inhibitor on the corrosion inhibition behavior of single copper and gold, a potentiodynamic polarization test was conducted in a PCB etching solution containing 8 mM SDS (Figure 5).

The potentiodynamic polarization test results for copper exhibit a corrosion potential of −7.01 mV and a corrosion current of 0.49 mA mA/cm². When SDS was added at its critical micelle concentration of 0.8 mM, the corrosion potential of copper increased to 89 mV, and the corrosion current density decreased to 0.0178 mA/cm², showing a significant reduction of approximately 96%. This result indicates that the addition of SDS as a corrosion inhibitor substantially decreased the corrosion potential of copper by acting on the copper surface. To investigate the effect of SDS on the corrosion behavior of gold, polarization tests were conducted on gold specimens in an 8 mM SDS solution. The results showed that, in the absence of SDS, the corrosion current density of gold was 0.0992 mA/cm², whereas with SDS, it was 0.114 mA/cm², indicating no significant change in corrosion current density. However, the corrosion potential increased from 91 mV to 168 mV upon SDS addition. This indicates that the adsorption of SDS onto copper specimens effectively inhibits corrosion, leading to an increase in the corrosion potential and a corresponding decrease in the corrosion rate. Conversely, for gold, the addition of SDS resulted in a relatively smaller increase in corrosion potential compared to copper, and the corrosion current values remained almost unchanged before and after the SDS treatment. These results indicate that SDS adsorbs less effectively onto gold than on copper, demonstrating its relatively lower efficacy as a corrosion inhibitor for gold. Figure 6 shows that a potentiodynamic polarization test was performed in a PCB etching solution with 2 g/L PEG to investigate the effect of PEG on the corrosion inhibition properties of individual copper and gold. The results of the polarization test demonstrated that the corrosion current density of copper without the addition of PEG measured to be 0.49 mA/cm². However, when 2 g/L of PEG was introduced into the solution, the corrosion current density dramatically decreased to 0.0022 mA/cm². This significant reduction of approximately 99.5% indicates the effectiveness of PEG as a corrosion inhibitor for copper. Furthermore, the addition of PEG resulted in a slight increase in the corrosion potential of copper from −7.01 mV to 31 mV.

These observations suggest that PEG adsorbs onto the copper surface in the etching solution, thereby reducing the reaction surface area and slowing down the corrosion rate, while having a minimal impact on the corrosion potential. To further investigate the impact of PEG on the corrosion behavior of gold, polarization tests were conducted on gold specimens in a solution containing 2 g/L of PEG. The results revealed that the corrosion current density of gold without PEG was 0.0992 mA/cm². In contrast, with the addition of PEG, the corrosion current density decreased to 0.0048 mA/cm², indicating a reduction of about 95.1%. Moreover, the corrosion potential of gold increased from 91 mV to 159 mV upon the addition of PEG. These findings clearly indicate that PEG effectively adsorbs onto both copper and gold specimens, enhancing their corrosion resistance. The non-polar molecular structure of PEG allows it to adhere to the surfaces of both copper and gold, thereby improving their corrosion resistance. The ability of PEG to reduce the corrosion current density and slightly alter the corrosion potential demonstrates its potential as an effective corrosion inhibitor in various applications. The adsorption of PEG onto the metal surfaces reduces the available reaction area, thus slowing down the overall corrosion process. This dual functionality of PEG in enhancing corrosion resistance without significantly affecting the corrosion potential is particularly beneficial in environments where both copper and gold are used.

4. Conclusions

This study evaluated the corrosion inhibition performance of SDS (sodium dodecyl sulfate) and PEG (polyethylene glycol) on copper coupled with gold, focusing on their galvanic corrosion behavior. Experimental results from ZRA tests and potentiodynamic polarization revealed clear trends in inhibitor efficiency with varying concentrations of SDS and PEG. For SDS, the galvanic corrosion current between copper and gold significantly decreased with increasing SDS concentration, reaching a maximum inhibitor efficiency (IE) of 93.5% at 8 mM. However, further increasing the concentration to 15 mM resulted in a decrease in IE due to micelle formation, as indicated by the reduction in surface adsorption properties. This behavior aligns with the critical micelle concentration (CMC) of SDS at 8.2 mM, where surfactant molecules begin to self-aggregate, reducing the direct interaction with the copper surface, as evidenced by the increase in galvanic corrosion current. PEG exhibited a different inhibition behavior. The galvanic corrosion current dropped significantly with the increasing PEG concentration, achieving a maximum IE of 99.5% at 2 g/L. Beyond this concentration, no further significant improvement in IE was observed, indicating surface saturation. The potentiodynamic polarization tests further validated these findings, showing that PEG effectively reduced the corrosion current density in both copper and gold, with reductions of 99.5% and 95.1%, respectively, confirming its broad efficacy across both metals.