Fig. 1 shows the polarization curves of PCB-Cu after exposure to different SO₄^2– concentrations for 2 hours at a temperature of 25°C and relative humidity of 60%. The cathodic polarization curve consists of two regions, I and II. Region I is a weak polarization area near the open-circuit potential where oxygen reduction occurs. Region II represents the limiting diffusion zone of oxygen, where both oxygen and corrosion products undergo reduction.

Based on the cathodic polarization curves shown in Fig. 1, the values of the cathodic current density at – 0.24 V (SCE) for PCB-Cu exposed to different SO₄^2– concentrations for 2 hours were presented in Table 1. The cathodic current density of PCB-Cu improved with the increasing of SO₄^2– concentration. This improvement was due to the effects of the adsorbed liquid film thickness and corrosion products. Na₂SO₄ has a tendency to absorb H₂O from the air, leading to the deliquescence.

Therefore, under constant environmental relative humidity, the increasing of SO₄^2– concentration facilitated the thickening of the liquid film on the surface of PCB-Cu. This condition promoted the formation of soluble sulfates such as NaCu₂(SO₄)₂OH, Cu₄SO₄(OH)₆ and CuSO₄. As shown in Fig. 2a, the corrosion products on the sample after electrochemical testing mainly adhere to the surface in a flocculent manner. EDS test results indicate that the main chemical elements of the corrosion products were C, O, Na, Cu, and S. The XRD result (Fig. 2b) indicated that the corrosion products mainly included Cu₄SO₄(OH)₆·H₂O and CuSO₄. The presence of these corrosion products expanded the effective area for oxygen reduction on the electrode surface, thereby enhancing the cathodic current density [10]. Fig. 3 shows the SEM and EDS test results of the PCB-Cu under SO₂ atmosphere with different humidity. The corrosion products of the samples were also showing a flocculent manner and denser. The EDS results show that the corrosion products were consist of C, O, S and Cu. Except of Na which was induced by NaSO₄ solution, the main chemical elements of the corrosion products of the samples in SO₂ atmosphere and after electrochemical test were same.

Fig. 4 and 5 illustrate the impedance changes of PCB-Cu after exposure to varying concentrations of SO₄^2– for 2 hours, with Fig. 4 displaying the Nyquist impedance spectra and Fig. 5 showing the Bode impedance spectra. As shown in Fig. 4, two capacitive arcs were observed at SO₄^2– concentrations of 0.01 mol L^–1 and 0.1 mol L^–1. The high-frequency capacitive arc corresponds to the capacitance and resistance of the film formed on the PCB-Cu surface, which includes the oxide layer and corrosion products of copper [11]. The low-frequency capacitive arc was because of the double-layer capacitance and charge transfer resistance [12]. At the SO₄^2– concentration increasing to 0.5 mol L^–1, only one capacitive arc is visible, indicating that the corrosion process is predominantly controlled by the charge transfer resistance. And the center of the capacitive arc deviates from the real axis, suggesting a dispersion effect.

The Bode plot in Fig. 5 indicates the uniformity of current distribution. When the SO₄^2– concentration was 0.01 mol L^–1, the phase angle remained above –45o, implying a uniform current distribution over the working electrode surface in the low-frequency region. However, with the SO₄^2– concentration increased to 0.1 mol L^–1 and 0.5 mol L^–1, the phase angle fell below –45o, indicating that higher SO₄^2– concentration reduced the uniformity of current distribution on the PCB-Cu surface.

Fig. 6 provided a schematic diagram of the equivalent circuit for PCB-Cu under thin liquid film, in which R_s represents the solution resistance, CEP represents the constant phase angle element of the liquid film, R_f is the film resistance, CPE_dl represents the double-layer capacitance associated with the charge transfer resistance, and R_ct represents the charge transfer resistance.

The EIS fitting results obtained under different SO₄^2– concentrations are shown in Table 2. R_s represents the solution resistance, which gradually decreased from 9 Ωcm² to nearly 0 with increasing of the SO₄^2– concentration. n₂ represents the dispersion coefficient, and the greater the value of n, the less the dispersion effect. R_ct is the charge transfer resistance, indicating the resistance encountered by the charge transfer during the electrode process, and its reciprocal can characterize the corrosion rate of PCB-Cu under varying conditions. When the SO₄^2–-concentrations were 0.01, 0.1 and 0.5 mol L^–1, the corresponding corrosion rates of PCB-Cu were 6.76×10^–5 Ω^–1cm^–2, 12.36×10^–5 Ω^–1cm^–2 and 16.05×10^–5 Ω^–1cm^–2, respectively. During the early stages of corrosion (after exposure for 2 hours), the increasing of SO₄^2– concentration made PCBCu more susceptible to corrosion, significantly accelerating the corrosion rate, which is consistent with the change in the cathodic current density in the polarization curves. The increasing of SO₄^2– concentration accelerated the adsorption of water vapor from the atmosphere, leading to an increased thickness of the liquid film on the surface of PCB-Cu. Additionally, the formation of soluble sulfates on the PCB-Cu surface due to high concentrations of SO₄^2– also expedites the corrosion of PCB-Cu [13].

At a temperature of 25°C and relative humidity of 60%, the impedance changes of PCB-Cu over exposure time under different concentrations of SO₄^2– are shown in Fig. 7 to 9. In the Nyquist impedance spectra, the radius of the capacitive arcs noticeably decreases with increasing exposure time across all SO₄^2–-concentrations, indicating that the corrosive medium more easily penetrated through the layer of corrosion products to react with the substrate surface, thereby reducing the corrosion resistance of the corrosion products. Compared to the R_ct, the value of the R_s is smaller, suggesting that despite the very thin thickness of the adsorbed liquid film, the ohmic drop between the working electrode and the reference electrode was relatively small.

As shown in Fig. 10, 1/R_ct can characterize the corrosion rate of PCB-Cu under different conditions. In the initial stage of corrosion (exposure for 2 hours), fewer corrosion products generated, and the primary influencing factor of the corrosion process is the cathodic reduction process. The increase of SO₄^2– concentration enhanced the cathodic current density, thus increasing the corrosion rate of PCB-Cu. As exposure time increased, corrosion products continuously accumulated on the surface of PCB-Cu, significantly affecting the corrosion process. Fig. 8 shows that the corrosion rate of PCB-Cu in different SO₄^2– concentrations exhibits a steady growth trend throughout the entire corrosion period. This indicates that the corrosion products formed on PCB-Cu in Na₂SO₄ solution have a porous structure, offering poor protection against corrosion of the PCB-Cu substrate.

The reducible corrosion products participate in the corrosion reaction and accelerate the further dissolution of the substrate, which also increased the corrosion rate. Notably, when the exposure time reached to 72 hours, the corrosion rate at an SO₄^2– concentration of 0.01 mol L^–1 markedly increased, while at 0.5 mol L^–1 SO₄^2–, the corrosion rate tend to stabilize. This phenomenon may be related to the morphology, distribution, and composition of the corrosion products under different SO₄^2– concentrations. As shown in Fig. 11, the thickness of the corrosion products of the 24h-tested sample was obviously more than which of the 2 h-tested sample. While the thickness of the corrosion products did not increase significantly after 24-hour test. Especially for the sample tested with high sulfate concentration, which had a high corrosion rate, the fast generation of the corrosion products would effectively reduce the contact between the corrosive medium and the metal surface and obstruct the diffusion paths of corrosive ions, thereby stabilizing the corrosion rate. After prolonged exposure, the layer of corrosion products accumulated on the surface of PCB-Cu at high SO₄^2– concentrations might inhibited the diffusion of reactive substances, thereby suppressing the increase of the corrosion rate.