Anti-corrosion application of parylene C film for stainless steel fasteners in electroplating industry Open Access

Electroplating is an electrochemical process in which a metal film is coated onto a surface to improve its aesthetics or protect its appearance. Many of the tools used in electroplating, such as plating hangers, baskets, or fasteners, are continually immersed in a corrosive acid or alkaline electroplating solution, limiting their service lifetime. Some such tools are coated with a polyvinyl chloride (PVC) film or are made of certain materials, such as titanium or stainless steel, to improve their service life.¹ 

In the printed circuit board industry, copper electroplating is commonly performed.^2,3 Using an electroplating solution mainly comprising sulfuric acid (H₂SO₄) and copper sulfate (CuSO₄·5H₂O) at concentrations of ∼60–200 and 200–260 g/L, respectively.^3–5 

Fasteners specifically made for electroplating are often used to hold workpieces during the electroplating process. Stainless steel is a common material for such fasteners, but it is not immune to corrosion while immersed in an electroplating solution; hence, such fasteners are considered consumable in the electroplating industry. However, coating these fasteners with an anti-corrosion material can reduce the exposure of the metal to the corrosive environment, extending its service life. For a 1-mm-thick 304 stainless steel plate in a copper sulfate electroplating solution with a sulfuric acid concentration of ∼60–200 g/L at a temperature of 25–30 °C.³ The stainless steel workpiece will form holes due to corrosion within 2–5 years.⁶ 

Many scholars have coated metal surfaces with inorganic and organic films and studied their corrosion resistance.^7–10 Lu et al.⁷ produced aluminum coatings on LA43M magnesium alloy by spraying and shot peening and immersed the samples in a 3.5 wt. % NaCl solution for 1000 h. They reported that shot peening was effective for eliminating coating porosity and preventing the corrosive solution from penetrating the coating. Mahato et al.⁸ coated a Ti–Si–B–C coating on 304 stainless steel. After coated stainless steel was immersed in a 3.5 wt. % NaCl solution for 70 days, electron dispersive x-ray spectroscopy revealed only Ti, S, B, and C element peaks on the coated surface, indicating that no corrosion had occurred. Jeong et al.⁹ used a liquid crystal polymer to encapsulate neural electrodes and performed accelerated lifetime tests. The electrodes survived in 87 °C saline for 158 days, indicating an equivalent lifetime of 14 years at the human body temperature of 37 °C. Rubehn et al.¹⁰ soaked a polyimide (PI)-coated sample in phosphate-buffered saline (PBS) at 60 °C for 20 months and found no change in the PI chemical structure or crystallinity, indicating that the coating has an equivalent lifetime of 8 years at human body temperature (37 °C).

Some studies have also reported that poly-para-xylylene (parylene) can be used as a coating to extend its service life. The glass transition temperature (Tg) of parylene C is 55–95 °C, and vapor deposition can be used to polymerize it on surfaces.^11,12 Parylene is acid resistant, alkali resistant, and transparent. By contrast with inorganic films, parylene can be conformally coated onto surfaces of any shape, including those with sharp corners, gaps, or pinholes, and its thickness can be precisely controlled. Therefore, it is widely used to protect circuit boards, sensors, or medical instruments from moisture and corrosion.¹³ Fasteners have complex shapes; hence, electroplating a uniform coating on these fasteners is challenging. Conventional coating with organic films (other than parylene) usually cannot achieve a uniform coating on the inner side of the fastener or areas with a large curvature by the spray coating method. In addition, PVC has weak adhesion to metal, and sandblasting and primer are required to enhance adhesion.¹ The inner side of the fastener or areas with a large curvature may have a risk of coating detachment. Therefore, parylene is superior as a fastener coating. Table I summarizes results from the literature for various accelerated aging tests of parylene-coated test pieces.

In this study, parylene C film was coated on 304 stainless steel, and its corrosion resistance while immersed in a copper sulfate solution was evaluated. In addition, ASTM F1980 is an international standard for accelerated lifetime aging tests.²² It is applicable for heat-accelerated aging sterile barrier systems and medical device polymers. The 10° rule (the reaction rates double for every 10 °C increase in temperature) can be used to evaluate polymer lifetimes.^{9–11,14–17} In addition, various researchers have noted that polymers should not be heated above 60 °C to avoid reaching the glass transition temperature of parylene C.^9–11,17,22 This glass transition temperature may change nonlinearly with temperature; therefore, tests above this temperature may be inaccurate for lifetime predictions.^11,22 Hence, this experiment is based on the life evaluation method of ASTM F1980, and the coating's failure time at 60 °C was used to evaluate the coating's failure time at room temperature.

A 304 stainless steel (Gloria Material Technology Corp., Taiwan) sample was chosen (Fe base with 0.08 wt. % C, 0.1 wt. % Si, 18.07 wt. % Cr, 1.36 wt. % Mn, 8.01 wt. % Ni, and 2.2 wt. % Mo) as the coated substrate, and its size was 60 × 20 × 1 mm³ (length × width × thickness). The surface was ground with No. 2000 grinding sandpaper and polished using an alumina suspension. Finally, acetone was used to remove all pollutants from the sample surface.

To enhance the interface adhesion between parylene C and the metal, the surface was immersed in a silane-based solution before chemical vapor deposition (CVD) coating.^{11,16,17,20,21} The silane-based solution was prepared with 50% deionized water, 50% isopropanol, and 0.5% silane A174.²¹ Before being coated, the test piece was immersed in the solution, soaked for 30 min., and finally blow dried with high-pressure air. Finally, parylene C was coated on the surface of 304 stainless steel through CVD. The 30 g parylene C dimer was heated to its vaporization temperature of 140 °C and then passed through a high-temperature zone at 650 °C to pyrolyze it into monomers. The coating time is 240 minutes. Subsequently, these parylene C monomers were polymerized onto the substrate surface at room temperature. The Alpha-step surface profilometer was used to confirm that the film thickness was 10 µm. Furthermore, the test piece was analyzed through atomic force microscopy (AFM) to inspect its surface quality (a scan area of 1 µm² on the test piece). For the parylene C film, the root mean square roughness (Rq) was 4.816 nm, and the average roughness (Ra) was 3.805 nm. The AFM images of the pre-CVD, the 10-μm-thick parylene C coating on the surface of the 304 stainless steel, and an SEM photograph of the parylene C coating are shown in Fig. 1. The surface roughness of the test pieces can be reduced from a Ra value of about 3.8-13 nm by the deposition of parylene.

The copper sulfate electroplating solution was prepared.^3–5 An electronic balance was used to measure 200 g of copper sulfate powder (CuSO₄·5H₂O), which was placed into 1000 ml of deionized water and stirred, and 32.6 ml (60 g/L) of sulfuric acid was slowly added. The concentrations of the copper sulfate solution components are listed in Table II.

The coated test piece was immersed in the copper sulfate solution, and a plate was used to heat the solution to 60 °C to accelerate aging. An electrochemical analyzer (CHI614D, CHI Instrument, Austin, TX) was used to observe the impedance of the test piece in the solution. The detection frequency was 0.1–100 000 Hz, and the AC voltage was 5 mV.²³ The experimental configuration is displayed in Fig. 2.

This electrochemical impedance experiment was performed using a three-electrode system comprising a working electrode connected to the test piece, a mesh platinum counter electrode (area is 6 × 7 mm²), and a silver–silver chloride reference electrode. Finally, a thermometer was placed in the container for temperature monitoring.

In the accelerated aging test, groups of 12 samples were soaked in solution at 60 °C; three samples were removed and evaluated at each time point.

When the test piece is initially immersed, the film blocks the external solution; the equivalent circuit model is as shown in Fig. 3(a).^21,23 However, as the immersion time increases, the solution gradually penetrates the surface, and mass transfer begins. At this time, the insulation of the film begins to decrease; hence, the Z_w is added in series with the R_film, resulting in the equivalent circuit model in Fig. 3(b). In the Nyquist plot, the trailing line segments in the lower frequencies are affected by the Z_w. In the initial immersion stage, the solution penetrates from the surface due to the low concentration of the solution at the film's interface; the Z_w presents a high-impedance 45° oblique straight line, indicating a strong diffusion phenomenon.²⁶ However, with the increase in penetration time, the film's solution concentration increased. The smaller the concentration difference between the film and the solution, the weaker the diffusion phenomenon, and the Z_w presents a 45° oblique straight line with low impedance. When the concentration gradient between the film and the solution is minimal, the Z_w impedance will approach zero, indicating that the diffusion phenomenon has almost stopped (the constant of the Z_w slope is fixed), meaning that the film's solution concentration is similar to the external solution's concentration and the solution has diffused to the entire film; therefore, the film's protection was invalidated. If the solution continues to penetrate, it will reach the metal surface, forming the appearance of the second electric double-layer capacitance. Eventually, the penetration of the film surface by the solution causes bubbles to accumulate at the interface of the film and the substrate surface, which indicates that the surface film has been delaminated. In this case, the equivalent circuit model is shown in Fig. 3(c).

Figure 4 presents Nyquist plots for the ALST. At 0.5 h of immersion, it can be observed that the high impedance of the arc line on the complex plane; both the maximum real and imaginary impedances are on the megaohm scale. Hence, the film can resist the solution until this point. After 4 h, the semicircular capacitive impedance and an oblique straight line appear together on the complex plane, indicating that the solution proceeded with the mass transfer and penetrated the film. The impedance of the low-frequency oblique line on the Nyquist plot is affected by Z_w, and the impedance of the tail end of Z_w decreases from the immersion period of 4 h to the 66th day of the immersion period. On the 73rd day of the immersion period, the solution has already penetrated into the surface of the metal, forming the second electric double layer phenomenon, and the complex plane produces the second semicircular line segment. Since the solution exists between the film and the metal interface, it can be used as a basis for determining the failure of the film package, as shown in Figs. 4(c) and 4(d). Based on the experimental data, it was determined that the solution penetrated the metal surface between days 66 and 73, with a conservative estimate of 66 days as the benchmark for film failure.

Figures 5(a) and 5(b) present the total impedance and phase angle, respectively, of the test pieces at various frequencies. At low frequencies, the total impedance can be expressed as the sum of the film impedance and the solution impedance. If the film impedance is much larger than the solution impedance, the total impedance is nearly equal to the film impedance. As the immersion time increased, the film impedance decreased. According to Fig. 4, as the immersion time increased, both the imaginary and real impedances on the Nyquist plot decreased; hence, the total impedance tended to decrease as the immersion time increased.

Figure 5(b) reveals that, at low frequencies (logarithmic frequency from log 1 to log −1), the phase angle increases because of Z_w. As the frequency increased, the effect of Z_w was smaller, and the phase angle decreased. At intermediate frequencies (from log 1 to log 3), the phase angle began to increase because of the greater capacitive reactance of the film at these frequencies. When the frequency increased and reached the maximum phase angle, the point was the point of the tangency on the tangent of the circular arc on the Nyquist plot. At high frequencies (from log 3 to log 5), the phase angle decreased as the frequency increased because the film's capacitive reactance was small at high frequencies. This was attributed to the increasing frequency, causing the resistance value to approach R_s. Therefore, regardless of the final detection frequency, the phase angle value tends to be constant at high frequencies.

Moreover, as the immersion time increased, the maximum value of the phase angle tended to be at a higher frequency. Because the maximum phase angle is the angle formed by the circular arc and the tangent to the origin, if the circular arc becomes smaller, its tangent point moves to a higher frequency, resulting in this phenomenon.

The EIS fit results for the experimental parameters are shown in Table III. n, which characterizes the imperfect capacitance of the film due to roughness or inhomogeneous surface charge distribution, decreases as the immersion time increases, affecting the CPE capacitance.⁸ This decrease (n < 1) indicates that the film is a nonideal capacitor; hence, the surface has become uneven because the solution has penetrated into the film, causing deformation.

Table III reveals that Y₀ was lowest after 0.5 h of immersion. Y₀ increased as the immersion time increased because the solution gradually permeated the film, increasing its capacitance. This is consistent with Fernández-Sánchez et al.²³ and Toorani et al.²⁵ reported that Y₀ increases with immersion time.

In addition, when the test piece was immersed in the solution for 4 h, the y₀ was small (the σ is large), resulting in a high-impedance 45° oblique line at the end of Z_w. Because of the interfacial concentration at the start of immersion, the interface's concentration is a significant difference, which results in mass transfer by diffusion, and the Warburg impedance is high. However, as the immersion time prolongs, the y_o becomes large (the σ becomes small), resulting in a low-impedance 45° angled straight line at the end of Z_w. Because the film's solution concentration was increased by diffusion, the concentration difference with the external solution was slight, the mass transfer phenomenon of diffusion decreased, and the Warburg impedance became smaller. Chun et al.¹⁷ and Davis et al.²⁷ have reported that diffusion is initially rapid when parylene film is immersed in a solution but slows as the immersion time increases.

The results of the ALST at 60 °C indicate that the failure time of the film was 66 days, with a conservative estimate. At a typical electroplating temperature of 25 °C, the acceleration factor A_T can be calculated to be 11.3 by using Eq. (1), and the MTTF is 11.3 × 66 (days) = 745.8 (days) = 2.04 years; hence, it can be estimated that the effective lifetime of the 10-µm film would be 2.04 years. The effective lifetime could be extended by increasing the film thickness; increasing the film thickness by 10 µm should increase its effective lifetime by ∼2 years.

Immediately after immersion, the equivalent circuit model of the sample is as in Fig. 3(a). The parylene C film blocked the external solution. After continuous immersion, the equivalent circuit model of the sample is as in Fig. 3(b). The copper sulfate solution then penetrated the film. The immersion process can be divided into three stages, but the SEM images of film rupture can prove that parylene C occurred during the delamination stage. When the test piece reaches the state corresponding to the equivalent circuit model in Fig. 3(c), the copper sulfate solution penetrates continuously into the surface film, causing bubbles on the surface of the film to gradually grow and causing delamination of the film to occur. This phenomenon is shown in the SEM images in Fig. 6.

This study investigated the resistance of parylene C to corrosion in a copper sulfate solution, and the goal was to identify a method to extend the service life of electroplating fasteners. Electrochemical impedance analysis was used to evaluate the coating health of an ALST at 60 °C. Based on the experimental data, it was determined that the solution penetrated the metal surface between days 66 and 73, with a conservative estimate of 66 days as the benchmark for film failure. Hence, according to the 10° rule, the effective life of the film would be 2.04 years at a typical electroplating temperature of 25 °C. These results suggest that parylene C can effectively protect a fastener in a copper sulfate electroplating bath. Moreover, increasing the film thickness by 10 µm could increase the fastener lifetime by ∼2 years. Practitioners could use parylene C in an attempt to improve the efficiency of electroplating and reduce associated costs.

This study was supported by the Ministry of Science and Technology (Grant No. MOST 109-2221-E-992-033-MY3).

The authors have no conflicts to disclose.

Chien-Hao Chung: Conceptualization (equal); Data curation (equal); Methodology (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Wen-Cheng Kuo: Conceptualization (equal); Data curation (equal); Methodology (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.