3.1. Composition and Microstructure of the Pd-Ni Film
Figure 1 shows the cross section morphology of the Pd-Ni film that was plated on the stainless steel sample. It could be seen that the thickness of the film was about 2 μm. The contents of the Pd and Ni elements in the plated film that were determined by EDS are shown in
Table 2. It could be seen that the concentration of CeCl
3 had no significant influence on the film’s composition. It could be inferred that the effect of CeCl
3 on the electrodeposition of Pd and Ni was similar. The Ce element was not detected in the Pd-Ni alloy film. This was probably because the electronegativity of the rare earth element Ce is relatively low and the deposition potential of the rare earth element Ce is very negative (around −2.4 V), so it is difficult for Ce to be directly deposited [
15]. In the electrodeposition process of Pd and Ni, the cathode potential was higher than the deposition potential of Ce, so Ce could not enter into the Pd-Ni alloy film by a reduction reaction.
Figure 2 shows the X-ray diffraction pattern of the Pd-Ni film. The film showed a single phase FCC (Face-centered cubic) structure, indicating that a substitution solid solution formed during the electrodeposition. The addition of CeCl
3 did not change the crystal type of the film. It can be seen from
Figure 2 that the position of the Pd-Ni diffraction peak is close to but slightly shifted to the right of that of the pure Pd diffraction peak. This is because the lattice constant of Ni is smaller than that of Pd, and the Ni atom partially replaces the position of Pd in crystal, which resulted in the lattice constant of Pd-Ni decreasing compared with that of pure Pd.
Table 3 shows the crystallite sizes of the Pd alloy films, as calculated by Scherrer equation. It can be seen that the crystallite size of the Pd-Ni film was reduced after CeCl
3 addition, indicating that the addition of CeCl
3 apparently decreased the crystallite size and refined the microstructure of the Pd-Ni film, which is in agreement with the results in the literature [
16,
17].
3.2. Hardness and Adhesion Tests Results
Figure 3 shows the micro-hardness and adhesive strength of the Pd-Ni alloy film that was obtained in the electrodeposition solutions that contained different concentrations of CeCl
3. The hardness of the Pd-Ni film in the solution without CeCl
3 was 524 HV. After CeCl
3 addition, the hardness was improved, and it reached its maximum value (578.7 HV) when the CeCl
3 concentration was 1.0 g L
−1. The adhesive strength between the Pd-Ni film and the substrate was also significantly improved after the CeCl
3 addition. With the CeCl
3 concentration increasing, the adhesive strength firstly increased and then decreased; it achieved a maximum (5.64 MPa) at around 1.0 g L
−1 of CeCl
3 addition in the electroplating solution. This shows that adding proper concentration of CeCl
3 can enhance the adhesive strength between a film and a substrate, thereby increase the protective effect of a film to a substrate. It can be noted that the adhesive strength and hardness of the Pd-Ni film had the same variation tendency with the concentration of CeCl
3 in the plating solution. This was probably because that the addition of cerium salt could refine the film’s grain size, reduce the film’s pinholes during deposition, and make the surface of the film more compact and uniform [
16,
17]. These would have resulted in an enhanced bonding force [
16]
. Meanwhile, according to the Hall–Petch relationship, the micro-hardness of the film was also improved as the result of the grain refinement [
18,
19]. However, the excessive addition of cerium salt could have degraded the homogeneity of the film, which resulted in the decrease of the hardness and the adhesive force of the film. This is in agreement with the results of some of the literature [
16,
17].
3.3. Porosity of the Pd-Ni Alloy Films
Porosity is one of the key indicators of the quality of a film.
Table 4 shows the porosity of the Pd-Ni alloy films that were obtained in the solutions with different concentrations of CeCl
3. Compared with the Pd-Ni film without CeCl
3 addition, the film after adding 1.0 and 1.5 g L
−1 of CeCl
3 had a lower porosity. According to the literature, porosity is formed mainly due to the hydrogen evolution occurring on the surface of a substrate [
3,
16]. When hydrogen generation is suppressed, porosity can be significantly reduced. The adsorption of cerium ions on the surface enhanced the reduction potential of H
+ and inhibited the adsorption of hydrogen ions. Therefore, it was more difficult for H
+ to be reduced, and the reaction rate of hydrogen evolution thereby decreased, so the occurrence of pinholes in the film was inhibited to some degree [
20]. The trend of the porosity with the variation of the cerium salt concentration was similar to that in the literature [
16].
3.4. Weight Loss Tests
Weight loss tests were carried out for electroplated steel samples in a 20 wt % H
2SO
4 solution at 80 °C. After immersion for 72 h, apparent corrosion was observed on the Pd-Ni film that was obtained in the plating solution without CeCl
3, and part of the film was peeled off from the steel substrate. After the addition of CeCl
3, the adhesive force and the corrosion resistance of the film were enhanced.
Table 5 shows the corrosion rate of the Pd-Ni film in the 20 wt % H
2SO
4 solution (80 °C), as calculated from weight loss test. With the increase of the content of CeCl
3 in the plating solution, the corrosion rate first decreased and then slightly increased. The lowest corrosion rate, which was 0.0039 g m
−2 h
−1, was obtained when the content of CeCl
3 was 1.0 g L
−1, and, after 72 h of immersion, the surface film was intact.
Figure 4 shows the potentiodynamic polarization curves performed in the 20 wt % H
2SO
4 solution (80 ℃). The corrosion current density (
ic) and corrosion potential (
Ec) of the electroplated sample were obtained from polarization curves and are shown in
Table 6. It can be seen that the corrosion potentials of the samples with the Pd-Ni film ranged between 0.18 and 0.27 V, which was in the passive range of the 316L stainless steel in the 20 wt % H
2SO
4 solution at 80 °C [
12,
21], so electroplating an Pd-Ni alloy film can increase the corrosion potential of stainless steel, thereby maintaining its passivation state and present good corrosion resistance. After CeCl
3 addition, the corrosion current density of the electroplated samples decreased. At the CeCl
3 content of 1.0 g L
−1, the corrosion current density reached the lowest value (1.01 × 10
−5 A dm
−2), which was consistent with result of the weight loss test.
3.5. The Morphology of the Pd-Ni Films
From the above test results, it can be noted that the Pd-Ni film that was deposited in the solution when the CeCl
3 content was 1.0 g L
−1 presented better performances. The surface morphology of the Pd-Ni film was observed by SEM and is shown in
Figure 5, where it is compared with the morphology of the film in the solution without CeCl
3. It could be seen that after 1.0 g L
−1 CeCl
3 was added, the surface of the Pd-Ni film was more homogeneous and continuous, with less rough particles covered on the surface. As we know, the process of electroplating mainly comprises the absorption and the reduction of cations on the surface of the cathode. Cerium is a surface active element with a large atomic radium (0.1824 nm), a unique 4f electron configuration, and a strong adsorption ability. When CeCl
3 is added to a plating solution, Ce
3+ is preferentially adsorbed on the crystal defects of the cathode and obstructs the reduction of metal cations on the cathode surface [
17]; this needs a high cathodic potential to supply the reduction energy of Pd (NH
3)
22+ and Ni (NH
3)
22+, so the nucleation rate of the new crystallite is increased and then grains are refined [
19].
3.6. Throwing Power of the Electroplating Solution
The throwing power (T.P.) of the electroplating solution was evaluated, and the T.P. value was calculated with Formula (1). The results are shown in
Figure 6.
It could be seen that the addition of CeCl
3 could improve the throwing power of the bath solution, showing a tendency of first increasing and then decreasing. When the concentration of CeCl
3 was 1.0 g L
−1, the percentage throwing power, T.P.%, of the plating solution reached 73.9%, which is relatively high [
9]. It is supposed that
M (
m1/
m2) is the weight distribution ration in the base plating bath, and
M′ (
m′
1/
m′
2) is the weight distribution ration in the plating bath containing CeCl
3. Based on the result in
Figure 6, it can be inferred that:
According to the Faraday’s law, a relationship exists between the mass of a film (
m) and current density (
i), as shown in Formula (3), where
t is the plating time,
s is the sample area,
A is the atomic weight, and
F is the Faraday constant.
From Formulas (2) and (3), it can be found that:
where
i1/
i2 and
i′
1/
i′
2 are the current density ratios of the near and far cathodes in the plating solution without and with CeCl
3, respectively. Therefore, the difference between the far and near cathode current densities became smaller after the CeCl
3 addition, which means that the distribution of current in the plating solution became more uniform.
There are several factors that affect the near and far cathode current densities (
i1 and
i2), and these are expressed in Formula (5), where
l1 is the distance between the near cathode and anode, Δ
l is the distance between the near and far cathodes,
ρ is the conductivity of the plating solution, and Δ
φ/Δ
i is the degree of cathodic polarization. In the case in which the plating bath structure and the conductivity of the plating solution are basically unchanged, the main factor that affects current distribution is the degree of cathodic polarization. Based on the decrease in the near-far current density ratio after the CeCl
3 addition shown in Formula (4), it can be inferred that the cathodic polarization was strengthened because of the rare earth salt addition.
The cathodic polarization curves during the Pd-Ni electrodeposition on the steel substrates were measured, and the results are given in
Figure 7. It was observed that the polarization curves were negatively shifted with an increasing rare earth content, indicating that the degree of cathodic polarization improved. However, after the concentration of CeCl
3 was higher than 1.0 g L
−1, the increasing degree of polarization became smaller, probably because the amount of Ce
3+ that was absorbed at the cathode approached saturation. The enhanced cathodic polarization indicated that the deposition overpotential was increased. This could have supplied the reduction energy of Pd
2+ and Ni
2+, consequently accelerating the nucleation rate of the Pd-Ni alloys. As the nucleation rate exceeded the growth rate of crystallites, the crystallization of the alloys was inhibited, resulting in the refinement of the grain size of the alloys and the improvement of the performance of the film [
19].
From the above, it can be concluded that CeCl
3 improved the properties and played important roles in changing the surface morphology and microstructure of the Pd-Ni alloy films. As for the addition of the cerium salt, the hardness and the adhesive force of the film were increased, and the porosity of the film was decreased. The corrosion resistance of the electroplated stainless steel in the 20 wt % H
2SO
4 solution at 80 °C was improved. CeCl
3 increased the throwing power of the plating bath, so the current distribution was more uniform and the cathodic polarization degree was strengthened during the electroplating process. Consequently, the refinement of grain size and the surface compaction of the alloy film was improved. However, a higher concentration of CeCl
3 (>1.0 g L
−1) became deleterious for the electrodepositing film, as manifested by the decrease of the above properties and the corrosion resistance. This is probably because that if too much Ce
3+ is adsorbed on the surface of a cathode, the cathodic polarization could become so big that the deposition speed decreases [
17,
20].