Zn–Ni Alloy Plating with Trivalent Chromate: Effects of NaF Additive Concentration and Treatment Time on Film Color, Thickness, and Electrochemical Properties

Abstract

Zn–Ni alloy plating is widely applied in manufacturing of automobile and construction material components because it provides better corrosion resistance and wear resistance than Zn plating. Furthermore, chromate coating treatment is gaining attention with respect to improving the corrosion resistance of Zn–Ni alloys. In this study, we investigated the effects of NaF additive concentration and treatment time on trivalent chromate coating, which has been developed as an alternative to hexavalent chromate coating. The chromate post-treatment solution used in this study comprises Cr(NO₃)₃·9H₂O (360 g/L), CoSO₄·7H₂O (40 g/L), and HNO₃ (35 mL/L), to which NaF is added in the concentration range of 0–30 g/L. The as-formed coating films at 1.6 pH and 60 °C treatment temperature for deposition times ranging from 30 to 120 s demonstrated a decreasing corrosion rate as the NaF concentration increased. The electrochemical and morphological analyses inferred that NaF acted as a catalyst, enhancing the rate of film formation. Furthermore, the film thickness increased with the treatment time, and the film color changed in the order of yellow, purple, and green.

1. Introduction

Zn–Ni alloy plating is widely applied in manufacturing of automobile and construction material components because it provides better corrosion resistance and wear resistance than Zn plating [1,2,3,4]. In general, Zn–Ni alloy plating is conducted in sulfuric acid baths and chloride baths, but alkali plating is being considered to achieve uniform electrodeposition. Furthermore, chromate coating treatment is gaining attention with respect to improving the corrosion resistance of Zn–Ni alloys [5,6]. Through this treatment, a self-repairing film is generated via immersion of the alloys in a hexavalent chromium solution for passivation.

Hexavalent chromate coatings are typically used to improve the corrosion resistance of Zn–Ni alloy plating. However, their usage has been prohibited owing to their hazardousness. As a result, eco-friendly coatings, such as trivalent chromate coatings, have been developed as alternatives to hexavalent chromate coatings. However, there are limited studies on the application of trivalent chromates in Zn–Ni alloy plating. In this regard, we investigated the effects of NaF additive concentration and trivalent chromate treatment time on the corrosion rate, surface structure, and film color of Zn–Ni alloy plating with colored chromate.

2. Materials and Methods

2.1. Zn–Ni Alloy Plating and Chromate Coating Process

Figure 1 shows the flowchart of the Zn–Ni alloy plating process followed in this study [7]. The chromate coating solution composed of a trivalent chromium source, a metal compound, an oxidizing agent, and a fluoride was used. The specimens were iron substrates plated with a Zn–Ni alloy (thickness: 3 μm, Ni content: 13.7%) with the dimensions 25 mm × 20 mm × 0.5 mm. The chromate post-treatment solutions were prepared using fixed amounts of Cr(NO₃)₃·9H₂O (360 g/L), CoSO₄·7H₂O (40 g/L), and HNO₃ (35 mL/L) at various NaF concentrations. NaF was slowly added at concentrations of 0, 10, 20, and 30 g/L while being stirred using a magnetic stirrer (DAIHAN, Gangwon-do, Korea) at 250 RPM. The prepared solutions were diluted to 120 mL/L with deionized water and were stirred at 1.6 pH and 60 °C treatment temperature for 2 h to stabilize the solutions. At each NaF concentration, the Zn–Ni alloy-plated iron specimens were activated in a 5% hydrochloric acid solution for 5 s, followed by the chromate coating process in the prepared chromate solutions under the aforementioned pH and temperature conditions for immersion times of 30, 60, 90, and 120 s.

2.2. Electrochemical, Morphological, and Chemical Characterization

For the corrosion potential–current analysis, a Pt mesh (5 cm × 5 cm) was used as the anode, and the Zn–Ni-alloy-plated iron substrate specimens subjected to the chromate post-treatment under various NaF concentrations and treatment times were used as the cathodes (diameter 0.25 cm). A 3% NaCl solution supersaturated with dissolved oxygen served as the electrolyte. For obtaining the polarization curve, a Ag/AgCl electrode (saturated KCl, 0.199 V vs. NHE, 298 K) was used as the reference electrode. Potentiodynamic polarization curves were obtained using a potentiostat/galvanostat (BioLogic, Seyssinet-Pariset, France), whose speed scan was performed from −1.4 to −0.4 at 1 mV/s. The surface structure after the chromate coating was observed by scanning electron microscopy (SEM, Thermo Fisher, Phenom ProX, Waltham, MA, USA). In addition, the exterior color was evaluated by SCE L *(D65), a *(D65), and b *(D65) color spaces using a spectrophotometer (Konica Minolta, CM-2600d/25,000d, Tokyo, Japan). The dependence of the film thickness on film color was evaluated by field-emission transmission electron microscopy (FE-TEM, Hitachi, SU8230, Tokyo, Japan). The composition and the chemical bonding of the specimen surface were determined by X-ray photoelectron spectroscopy (XPS, Thermo Fisher, NEXSA G2, Waltham, MA, USA) via binding energy measurements of Zn, Ni, Cr, O, C, F, and Co.

3. Results

3.1. Effects of NaF Concentration and Treatment Time on Corrosion Rate of Zn–Ni Alloy Plating with Colored Chromate

Because NaF affects the film formation, the effect of NaF concentration on chromate film formation was investigated [8,9]. Figure 2 shows the potentiodynamic polarization curves at various NaF concentrations and treatment times. Figure 3 shows the Tafel slopes at various NaF concentrations and treatment times by calculating the potentials and current densities from the polarization curves in Figure 2. Figure 3 illustrates that the potential increased with the NaF concentration owing to the cracks formed on the Zn–Ni alloy plating layer in the chromate film. However, the current density decreased with increasing NaF concentration. This indicates that the chromate film exists in a passive state, which suppresses the current density and improves the corrosion resistance by delaying corrosion, as determined in previous studies [10,11,12].

3.2. Effects of NaF Concentration and Treatment Time on Changes in the Surface Structure of Zn–Ni Alloy Plating with Colored Chromate

Figure 4, Figure 5, Figure 6 and Figure 7 present the SEM images of the surface structures of Zn–Ni flakes subjected to different chromium conversion treatment times at various NaF concentrations. As can be seen in the figures, the cracks in the chromate film became finer with the addition of NaF.

Figure 8 shows the SEM images of the cracks in the NaF 100% 60 s specimen at 66,000 × magnification. Intertwined webs formed between the cracks, which is atypical of plating cracks. These cracks could be attributed to the evaporation of moisture from the chromate film in a gel state [13,14,15,16,17,18].

3.3. Effects of NaF Concentration and Treatment Time on Changes in the Film Color of Zn–Ni Alloy Plating with Colored Chromate

Table 1. SCE L *(D65), a *(D65), and b *(D65) values after chromium conversion treatment for different immersion times and NaF concentrations.

Category	NaF Concentration (%)	Immersion Time (s)	L *(D65)	a *(D65)	b *(D65)
(a)	0	30	54.72	−1.70	−10.94
(b)	50		58.64	4.74	15.19
(c)	100		62.96	1.41	17.64
(d)	150		50.02	2.12	11.54
(e)	0	60	67.77	−3.96	1.96
(f)	50		51.07	9.00	−13.97
(g)	100		52.72	10.25	−9.36
(h)	150		49.00	9.96	−4.32
(i)	0	90	59.84	−0.22	3.66
(j)	50		55.84	−9.48	−5.03
(k)	100		54.18	−3.78	−5.79
(l)	150		47.79	5.01	5.90
(m)	0	120	55.64	8.15	−15.06
(n)	50		59.32	−9.34	5.91
(o)	100		57.08	−10.19	−1.41
(p)	150		44.46	−0.28	5.01

lists the values of SCE L *(D65), a *(D65), and b *(D65) following the chromium conversion treatment for different treatment times and NaF concentrations at a chromate post-treatment temperature of 60 °C and pH of 1.6. Figure 9 shows the graphical representations of SCE L *(D65), a *(D65), and b *(D65) values listed in Table 1 and the SCE colors at each treatment time. An overetched film was formed upon 150% NaF addition, whereas a chromate film was formed at the NaF concentrations of 50% and 100%, with film formation being faster with 100% than with 50% NaF. No film was formed for 0% NaF. The chromate film color changed in the order of yellow, purple, and green with increasing treatment time. Furthermore, L *(D65) decreased with increasing NaF concentration for 60 and 90 s immersion times (Table 1).

3.4. Coating Thickness Measurement and STEM-EDS by Chromate Coating Color

Figure 10 shows the thickness value determined by FE-TEM, corresponding to each exterior film color according to the treatment time at 100% NaF concentration (20 g/L). The films resulting from chromate conversion treatment for 30, 60, and 90 s at 100% NaF concentration (20 g/L) had thicknesses of 153.71, 199.34, and 228.3 nm, respectively. Furthermore, the respective colors of the as-formed films were yellow, purple, and green, in the order of increasing thickness [19]. The resulting color from the chromate conversion treatment for 90 s was similar to that for 120 s.

3.5. Measurement of Component and Depth Distributions of Zn–Ni-Alloy-Plated Specimens with Colored Chromate by NaF Concentration

The depth profiles and binding energies of four specimens—those treated at NaF concentrations of 0%, 50%, and 100% for 60 s and a Zn–Ni alloy plating specimen as base material—were determined by XPS. The atomic percent depth profiles as a function of the etching time, in Figure 11, exhibit the same decreasing patterns as the atomic percent values of Cr and O, indicating the formation of a chromate oxide film. Additionally, the atomic percent of Cr decreased with increasing NaF concentration. In Figure 12, which shows the XPS depth profiles of atomic percent values multiplied by a factor of 50, all four samples demonstrated similar values as those of the specimens at various NaF concentrations, and no chromate film was formed. The spectrum of C reveals slight film formation in the initial stage, which is attributed to etching. However, the spectra of F and Co do not reveal any film formation. The XPS binding energies indicate the absence of peaks in the F and Co spectra.

Figure 13 shows the XPS binding energies of Zn peaks from alkali Zn-Ni alloy plating at etch times of 0, 150, and 1000 s corresponding to regions in XPS depth profiles (Figure 11). On the surface (0 s), peaks presenting Zn(OH)₂ were detected, and these peaks became more vague after either adding chromate coating or increasing NaF concentration. For regions 150 s, XPS spectrum suggested the presence of metallic Zn without NaF and chromate. However, when adding chromate and increasing NaF, the Zn 2p peaks shifted to Zn(OH)₂ peaks. The XPS spectra manifested metallic Zn peaks at 1000 s regardless of NaF and chromate. The XPS binding energies of Ni peaks are depicted in Figure 14. No peak of Ni 2p was observed on the surface (0 s) in all cases of NaF and chromate. At 150 s, peak Ni 2p shifted from metallic Ni to Ni²⁺ with the addition chromate before vanishing with the addition of NaF. XPS spectra at 1000 s showed Ni⁰ peaks regardless of NaF and chromate.

The XPS binding energies of O peaks in Figure 15 indicate the presence of CrO₃ and CrO₂ peaks. Therefore, the chromate film was formed when NaF concentration was increased. The H₂O peak is attributed to the moisture on the surface.

Figure 16 shows the XPS binding energies of Cr peaks. The peaks at 582 and 572 eV are attributed to noise caused by Zn; the composition table also indicates 0% Cr. In (b), i.e., for NaF 0% 60 s etch time, only the Zn noise peak is observed because Cr fully penetrated into the Zn–Ni alloy plating layer. In (c) NaF 50% and (d) NaF 100% 60 s etch time chromate coatings, the spectra reveal Cr₂O₃ and Cr(OH)₃ peaks and a Zn noise peak. These observations imply that the chromate thickness increases as the NaF concentration increases.

4. Theory

The formation of chromate coating is explained by reactions as follows:

$$Z{n}^{2+}+2H_{2}O+2F^{-}\to Zn{\left(OH\right)}_2+2HF$$

(1)

$$Cr{O}_3+2HF\to Cr{O}_2{F}_2+H_{2}O\left(\mathrm{hydrolysis}\right)$$

(2)

$$4Cr{O}_3\to 2C{r}_2{O}_3+3O_2$$

(3)

The formulae infer that NaF promotes the formation of chromate coatings. The influence of NaF on the formation of chromate could be investigated using color–thickness relationships. Specifically, the increase in thickness changed the color of the chromate in the order of yellow, purple, and green, as shown in Figure 10. In Figure 9a, the samples presented yellow and purple only at 90 and 120 s without NaF. With NaF (Figure 9b–d), the samples showed yellow at 30 s and purple at 60 s. When the time prolonged over 90 s, the color changed to green. As seen in L *(D65) results in Table 1, the decreasing reflection may alter the interference color, which in turn would affect the surface color [20].

Cracks are formed in the chromate as the moisture evaporates from the chromate film and is deposited on the Zn–Ni alloy plate in a gel structure during the drying process. Subsequently, the chromate film contracts owing to the internal tensile stress of the coating, forming fine cracks on the coating surface [15,21].

5. Conclusions

• The chromate film formed in the gel state was dried and subsequently contracted by the internal tensile stress to form a film.
• As the NaF concentration and chromate coating treatment time increase, the cracks on the surface become finer. This indicates that NaF influences the formation of the chromate film and acts as a catalyst, accelerating the film formation.
• The results of the electrochemical measurement indicate that the current density decreases as the NaF concentration increases. Because the chromate film exists in a passive state, the passivation via chromate conversion treatment suppresses the current density, consequently improving the corrosion resistance by delaying corrosion. However, in the case of 150%, corrosion resistance was low due to overchromating.
• In TEM cross-sectional observation, the chromate film color changes to yellow, purple, and green with increasing treatment time. The film thickness is 153.71 nm for yellow, 199.34 nm for purple, and 228.3 nm for green. This behavior agrees with the theoretical differences in the film thickness with respect to color because the film thickness increases with the treatment time.