Microstructure and Electrochemistry Behavior of Ni-Cu-P Ternary Nickel-Based Amorphous Coating

Abstract

Enhanced corrosion-resistant Ni-Cu-P amorphous coatings were applied to AZ91D magnesium alloy substrates using an integrated approach that included direct-current (DC) copper pre-plating and electroless Ni-Cu-P plating at varying CuSO₄ concentrations. The effects of CuSO₄ concentration on the microstructure, electrochemistry behavior and corrosion rate of Ni-Cu-P coatings and the adhesion between the Ni-Cu-P coating and the magnesium alloys substrate were investigated. The results indicated that the surface of the Ni-Cu-P ternary coating which integrated well with the magnesium alloys substrate exhibited heterogeneous cell structure with relatively flat and dense structure. When the CuSO₄ concentration was 0.8 g/L, the mass fraction of P in the coating was 9.06 wt% meaning amorphous Ni-Cu-P coating. The results of the Nyquist and Bode graph, corrosion rate, corrosion morphologies and of the Ni-Cu-P coatings with different CuSO₄ addition indicated that the corrosion resistance reached optimization with corrosion potential of -0.31 V, the corrosion current density of 0.0039 A/cm² when the CuSO₄ concentration is 0.8 g/L. Furthermore, the adhesion test confirmed that the adhesion of Ni-Cu-P coatings improved gradually as the CuSO₄ concentration increased and exhibited optimum when added 1.0 g/L CuSO₄. The obtained Ni-Cu-P coating with amorphous structure prevented effectively magnesium alloy from corrosion.

Keywords:
Direct current pre-plating copper; Electroless plating; Ni-Cu-P ternary amorphous coating; Electrochemistry

1. Introduction

Magnesium alloys are extensively utilized across various industries, including aerospace, navigation, and defense, due to their beneficial properties such as low density, high specific strength, excellent damping capabilities, and favorable recyclability¹-³. However, their widespread application is hindered by their low corrosion resistance⁴-¹⁰. To address this, surface coating techniques like anodizing, conversion coating, and plating have been introduced to bolster the durability of magnesium alloys against corrosive environments. Among these, electroless plating stands out for its cost-effectiveness, environmental friendliness, and superior corrosion resistance¹¹-¹⁵. Despite these advantages, the direct application of electroless plating on magnesium alloys is challenging due to the loose oxide/hydroxide film that naturally forms on their surface¹⁶-¹⁸. Our prior research has shown that the application of a direct current copper plating layer on magnesium alloy surfaces serves to significantly strengthen the adhesion to the subsequent outer coating. Additionally, the intermediate copper by electroless plating is essential, ensuring adhesion to magnesium alloy substrates, providing a shield against corrosive elements, and enhancing the overall quality and efficacy of the coatings¹⁹-²³.

The Ni-P coating is recognized for its exceptional corrosion resistance and wear resistance, making it an ideal protective layer for magnesium alloys to combat damage from various environments, especially those that are corrosive, thus extending their service life²⁴-²⁶. The application of electroless Ni-P coatings with the right balance of corrosion protection and mechanical properties is promising for fulfilling industrial requirements²⁷-³⁰.

In light of these considerations, the aim of this study is to develop innovative Ni-Cu-P coatings with superior anti-corrosion properties using a combination of direct-current electroplating and electroless plating methods. Furthermore, the corrosion protection performance of these Ni-Cu-P coatings is evaluated through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) tests conducted in a 3.5 wt% NaCl solution.

2. Experimental Methods

2.1. Pretreatments and electroless plating

Pressure casting AZ91D magnesium alloys, as substrates, were cut into 15mm×10mm×20mm samples by an electronic spark cutting machine. The specimens were ground with SiC sand papers from 320 to 2000 grit by sequence to polish impurities and oil contamination and rinsed with distilled water.

The substrates were then ultrasonically cleaned in acetone at room temperature for 10min and immersed in hydrochloric and hydrofluoric acid solution for pickling for 20s to eliminate oxide film. In order to keep the reaction slow and sustainable, the substrates were immediately placed into activating solution. After that, the samples were immersed in zinc dipping solution to obtain zinc dipping layer with the purpose of improving the combining between magnesium alloy substrates and the outer layer. Next, a copper undercoat was electroplated on the pretreated magnesium alloys substrates prior to the electroless plating to prevent the substrates being damaged from acid bath and to enhance the adhesion between the substrates and the coating. Table 1 lists the electroplating copper bath composition and plating conditions.

Table 1
Electroplating copper bath composition and parameters.

Chemical composition	Plating conditions
CuSO4·5H2O	50 g/L
KNaC4H4O6·4H2O	140 g/L
NaOH	40 g/L
Temperature	25°C
Time	1 h

At the end, the pretreated samples were immersed in electroless plating bath, which was carried out at 60°C for 1h with fixed pH of 7.0-7.5. The Electroless bath composition and concentration are exhibited in Table 2.

Table 2
Electroless bath composition and concentration.

Bath composition	Concentration(g/L)
NiSO4·6H2O	30
CuSO4·5H2O	0.2 - 1.0
NaH2PO2	50
Na3C6H5O7·2H2O	10
NH4F	20

2.2. Characterization

The surface and cross-sectional morphologies of Ni-Cu-P composite coatings were investigated by scanning electron microscopy (SEM) equipped with an energy dispersive spectroscope. The phase composition of the Ni-Cu-P coating was analyzed by X-ray diffractometer with Cu kα (XRD-7000, Shimadzu, Japan). The deposition rate of the Ni-Cu-P composite coating was measured by the method of weight increase during deposition process. The adhesion between the Ni-Cu-P coatings and the magnesium alloy substrates was characterized by WS-2005 Automatic Adhesion Tester carried out with a load of 0.1 N and a speed of 5 mm/min. The indenter increased to 100 N at a speed of 50 N/min with 4 mm scratch length approximately.

2.3. Electrochemical measurements

Electrochemical test is conducted in 3.5 wt% NaCl solution to simulate the environment of seawater. Electrochemical Workstation (Autolab) is used to measure electrochemical impedance spectroscopy and potentiodynamic polarization curves. The tests were all implemented using a conventional three-electrode system. As working electrodes, the samples coated with Ni-Cu-P with 1cm² surface exposed to the corrosive solution were sealed with rosin and paraffin mixed. The platinum (Pt) plate and saturated KCl were used as auxiliary electrode and reference electrode separately during the tests.

The EIS measurements were conducted at a sine wave voltage with amplitude of 10 mV in the frequency range between 0.01 Hz and 10 KHz. The PDP tests were performed immediately after the EIS tests at a scan rate of 1 mV/s and the potential scanning range from -1.8 V to 0 V. The impedance spectra were calculated and fitted using ZSimp Win software.

The immersion experiment was executed in 3.5 wt% NaCl solution for 72 h at room temperature. The corrosion resistance of the coating was characterized by the surface morphology and the weight changes before and after immersion.

3. Results and Discussion

3.1. Morphologies and composition of Ni-Cu-P coating

Figure 1 shows SEM micrographs of the Ni-Cu-P coatings with different CuSO₄ concentration being 0.2, 0.5, 0.8, 1.0 g/L respectively. It is observable that the pre-plated copper layers were completely covered with Ni-Cu-P coatings. Furthermore, the morphologies of the coatings without visible defects such as cracks exhibit cellular structure and uniform formation when add different CuSO₄ concentration. Notably, the surface of the coatings become smoother, the cellular structure become gradually fine and uniform with increasing the CuSO₄ concentration. It is presumably responsible that the co-deposition reaction of Ni and Cu affects the refinement of the surface cellular structure of Ni-Cu-P coating to a certain extent.

Figure 1
Surface morphologies of Ni-Cu-P coatings with different CuSO₄ concentration. (a) 0.2 g/L (b) 0.5 g/L (c) 0.8 g/L (d) 1.0 g/L.

The EDS graphs of Ni-Cu-P coatings with different CuSO₄ concentration are demonstrated in Figure 2 (a-d). It is noteworthy that the detected elements of the composite coating are Ni, Cu and P. The elemental composition of the Ni-Cu-P coating measured by energy spectrum analysis is shown in Table 3. As it is obvious, the mass fraction of copper in the coating increases slowly as increases CuSO₄ concentration, whereas the mass fraction of phosphorus decreases gradually. A conclusion drawn above that the increasing of copper mass fraction results in the decreasing of phosphorus mass fraction, indicates that the addition of Cu²⁺ impede the reaction between Ni and P, and change the crystal structure of Ni-Cu-P coating accordingly. Moreover, the coatings present amorphous structure when the mass fraction of P exceeds 9%, while the coatings appear microcrystalline structure evolved from crystalline to amorphous state when the content of p is between 7-8%. Therefore, Ni-Cu-P coating exhibit amorphous structure when the concentration of CuSO₄ is 0.2, 0.5, 0.8 g/L. Since the content of P is lower than 9%, the Ni-Cu-P coating present microcrystalline structure.

Figure 2
Spectral analysis results of Ni-Cu-P coatings with different CuSO₄ concentration. (a) 0.2 g/L (b) 0.5 g/L (c) 0.8 g/L (d) 1.0 g/L.

Table 3
Energy spectrum analysis of Ni-Cu-P coating.

CuSO4 concentration	Mass fraction of P (%)	Mass fraction of Cu (%)
0.2	9.80	2.57
0.5	9.32	3.05
0.8	9.06	4.50
1.0	7.62	7.10

The cross-sectional images of the samples coated Ni-Cu-P presented in Figure 3(a) show that magnesium alloy substrate was covered with copper layer, and both substrate and copper layer interlock to each other strongly with clear interface. The Ni-Cu-P coating exhibited uniform and dense structure with the thickness of 18μm approximately according to the result of line scan. Moreover, no evident defects such as holes and impurities are observed on the coated surface. The EDS result presented in Figure 3(b) shows that the composite coating is mainly consisted of Ni, P elements and a little Cu.

Figure 3
The cross-sectional morphology and line energy spectrum analysis.

XRD patterns of Ni-Cu-P coatings shown in Figure 4 exhibit a broad bread peak which corresponds to cubic crystalline Ni (111) from 0.2 g/L to 0.8 g/L. And no sharp diffraction peak appears on the coating indicating amorphous structure of three types coating. Whereas, the broad diffraction peak of the Ni-Cu-P coating adding 1.0 g/L CuSO₄ narrows, sharp diffraction peaks appear at the same time, which indicates that the coating covered Ni-Cu-P with 1.0 g/L CuSO₄ added is mixed structure of crystallization superposition, calling microcrystalline structure. In conclusion, the crystal structure of Ni-Cu-P coatings evolves from amorphous to microcrystalline state with CuSO₄ concentration varying from 0.2 g/L to1.0 g/L. It can be deduced that the mass percentage of P in the coating determines the phase composition of the nickel-based alloy in a large extent.

Figure 4
XRD patterns of the Ni-Cu-P coatings with different CuSO₄ concentration.

3.2. The deposition rate for Ni-Cu-P coating

The deposition rate for Ni-Cu-P coatings is exhibited in Figure 5. Owing to a certain difference in the redox property between copper and nickel, the deposition rate of the Ni-Cu-P coatings decreases gradually with increasing the CuSO₄ concentration. Cu preferentially deposited and precipitated during the reaction because copper is more oxidizing than nickel. Moreover, copper deposited firstly on the magnesium alloys substrate to reduce the catalytic activity of the surface impeding the occurrence of chemical reactions in the bath. Therefore, the CuSO₄ concentration in the solution will prevent the co-deposition of Ni-P and reduce the deposition reaction rate.

Figure 5
The deposition rate for Ni-Cu-P coatings.

3.3. The adhesion of Ni-Cu-P composite coatings

The adhesion strength between the Ni-Cu-P composite coating and the AZ91D magnesium alloys substrate was examined by the relationship between acoustic emission signal intensity and loading (K-L) presented in Figure 6. When the surface of the ternary alloy coating is cracked and peeled off, the displayed load is the critical load named Lc. We can analyze the damage of the coatings according to the critical load Lc that the intensity of acoustic emission signal increase abruptly when the coating is broken. As we know, the greater the Lc value, the better adhesion of the coating combined to the substrate. As can be seen visually, the Lc value becomes larger with increasing the addition of CuSO₄. Apparently, the Lc reaches a maximum at 1.0 g/L CuSO₄ concentration. As compared with four types of Ni-Cu-P composite coating, the adhesion between Ni-Cu-P plating/substrate of exhibits the maximum in the condition of 1.0 g/L CuSO₄. The higher value of the Lc in the Ni-Cu-P composite plating may be due to the increase of CuSO₄ concentration.

Figure 6
Curves of Ni-Cu-P coatings obtained during scratch tests.

3.4. Electrochemistry

The PDP curves of AZ91D magnesium alloy substrate as well as four types of Ni-Cu-P coatings measured when the open circuit potential reached a stable value in corrosive medium of 3.5 wt% NaCl solution are presented in Figure 7. To the best of our knowledge, the different corrosion potential and corrosion current density of the samples symbolize different corrosion behavior. Furthermore, in theory, a higher corrosion potential and a lower corrosion current density of the polarization curve signify a lower corrosion rate. As it is clear, the self-corrosion potential of Ni-Cu-P coatings exhibit a great degree of right offset and the corrosion current density all low two order magnitudes compared with the substrate, indicating better corrosion resistance of Ni-Cu-P coating at a certain extent. To obtain corrosion density (Icorr) and corrosion potential (Ecorr), the Tafel extrapolation technique for the anodic and the cathodic Tafel slops of the polarization curves may was used to determine³¹. The fitting results of polarization curve for are listed in Table 4. It is noteworthy that the AZ91D magnesium alloy exhibits the most negative E_corr (-1.71V) and highest I_corr (23.188 A/cm²) among the samples. Additionally, with increasing CuSO₄ concentration the self-corrosion potential of Ni-Cu-P coatings present an upward trend compared with magnesium alloy substrate. The corrosion potential of the Ni-Cu-P coating is the most positive when the concentration of CuSO₄ added in the bath is 0.8 g/L, indicating optimum corrosion resistance of Ni-Cu-P coating. Furthermore, as the CuSO₄ added in the bath increases, the corrosion current density firstly decreases and then increases. From polarization curve and fitting results, we can draw a conclusion that four types of Ni-Cu-P coatings added CuSO₄ serving as the protective layer for magnesium alloys to tailor its corrosion rate effectively in corrosive environment.

Figure 7
Polarization curves of substrate and Ni-Cu-P coatings in 3.5 wt% NaCl.

Table 4
Fitting parameters for polarization curves.

Sample	CuSO4 (g/L)	Ecorr (V)	Icorr (A/cm2)
Substrate	/	-1.71	23.188
No.1	0.2	-0.46	0.086
No.2	0.5	-0.41	0.010
No.3	0.8	-0.31	0.004
No.4	1.0	-0.39	0.010

The EIS maps of the substrate and copper layer are shown in Figure 8 as Nyuist (a), Bode modulus (b). As revealed in Figure 8, both impedance spectra of magnesium alloy substrate and the pre-plated copper layer are characterized by one capacitance semicircle. What is more, the capacitive arc of pre-copper layer is markedly larger than that of bare magnesium alloys, indicating a lower corrosion rate of the copper layer. Moreover, the value of impedance at low frequency is critical for analyzing the corrosion behavior of Mg alloys. In Figure 8(b), the |Z| value of the pre-plated copper layer in low frequency is higher compared to AZ91D magnesium alloys substrate, which can be owing to the copper layer provides better resistance to the aggressive media. Moreover, Bode phase angel plots in Figure 8(c) show that the phase angels are under 0° at low frequency, indicating pitting corrosion appeared on the samples surface.

Figure 8
(a) Impedance spectra of the copper layer and magnesium alloy; (b) Bode diagram of the copper layer and magnesium alloy.

The Nyquist and Bode graph of the substrate and Ni-Cu-P coatings are shown in Figure 9. A common feature of all curves presented in Figure 9(a) that there is a single capacitive arc in the entire test can be observed. In general, larger radium of capacitive arc means large transfer resistance and reduces the corresponding corrosion rate, indicating superior corrosion resistance. From Figure 9(a) we can see, as increases the CuSO₄ concentration, the radium of capacitive arc from increases gradually. Moreover, the radium of capacitive arc of magnesium alloy reaches the minimum, the Ni-Cu-P coating adding 0.8 g/L CuSO₄ reaches the maximum. And in Figure 9(b), the |Z| becomes larger with increasing the CuSO₄ concentration under the condition of the CuSO₄ concentration is below 0.8 g/L. However, the value of |Z| decreases than before as the CuSO₄ concentration exceeds 0.8 g/L. The above discussion demonstrated that the corrosion resistance of Ni-Cu-P plating is the optimum when the CuSO₄ concentration is 0.8 g/L. Additionally, as shown in Figure 9(c), the curves of Ni-Cu-P coating appear single narrow peaks whose time constant corresponds with corrosive medium/plating surface, which means that there is a time constant in the corrosion process, manifesting that Ni-Cu-P coating incorporated with Cu coated magnesium alloys exhibit superior anticorrosion performance in corrosive medium comparatively.

Figure 9
Impedance spectra of substrate and Ni-Cu-P coatings in 3.5 wt% NaCl. (a) Nyquist diagram. (b) The amplitude frequency plot of Bode diagram. (c) Phase frequency plot of Bode diagram.

The appropriate equivalent electrical circuit depicted in Figure 10 is used to extract the impedance parameters form the experimental data. The equivalent circuit is composed of solution resistance R_s, charge transfer resistance R_ct and constant phase element Q_dl of electric double layer controlled by R_ct. Table 5 lists the fitted experimental EIS data using ZSimp Win software. It is noticed that the R_s value of all five samples are close on account of the same test solution and relatively fixed position of the working electrode and reference electrode. With respect to R_ct, obvious variation in the fitting parameters can be observed. Compared to magnesium alloy substrate, a remarkably higher R_ct value of Ni-Cu-P composite coatings reveals that the Ni-Cu-P composite coatings play a protective role in the corrosion test. This finding is consistent with previous conclusion.

Figure 10
Appropriate equivalent circuits for Ni-Cu-P composite coating.

Table 5
The EIS parameters for the Ni-Cu-P composite coatings and substrate.

Sample	CuSO4 concentration	Rs (Ω·cm2)	YQdl (Sα·Ω-1·cm-2)	n	Rct (Ω·cm2)
Substrate	/	16.62	2.54×10-5	0.82	375.9
1	0.2	13.14	1.33×10-5	0.82	1.90×104
2	0.5	11.63	1.58×10-5	0.91	2.17×104
3	0.8	12.69	9.17×10-5	0.82	3.25×104
4	1.0	13.86	1.44×10-5	0.81	3.00×104

The corrosion rate of four types of Ni-Cu-P coatings in 3.5 wt% NaCl solution is showed in Figure 11. The corrosion rate of the coatings was calculated by the equation presented in Equation 1, where m₁ and m₀ represent the mass of the sample before and after corrosion distinctly, S₀ signifies the surface of immersion, t is the immersion time. As revealed, the corrosion rate of samples initially decreases and then increases, and exhibits the lowest value in the condition of 0.8 g/L CuSO₄. Nevertheless, the corrosion resistance of Ni-Cu-P adding 1.0 g/L CuSO₄ reaches 1.6424 g/m²·h, indicating inferior corrosion resistance compared with the rest samples. From the above it can be concluded that adding CuSO₄ in the plating bath within a certain range decreased the corrosion rate of the coating and improve the corrosion resistance. Meanwhile, the corrosion rate of Ni-Cu-P coating reaches the lowest and its corrosion resistance reaches optimum when adding 0.8 g/L CuSO₄. However, adding excessive CuSO₄ will cause to decrease the corrosion resistance of the coating.

Figure 11
The corrosion rate of Ni-Cu-P coatings with different CuSO₄ addition.

$V=\frac{m_1-m_0}{s_0\times t}$ (1)

Figure 12(a)-(d) show the corrosion morphologies of Ni-Cu-P coatings after being immersed in 3.5 wt% NaCl solution for 72 h. As it can be observed in Figure 12(a), the sample adding 0.2 g/L CuSO₄ has suffered the worst corrosion. And the whole surface is corroded and accompanied by serious corrosion cracks so that the surface can be observed hardly. Meanwhile, large areas of peeling can be distinguished from its topography. For the sample adding 0.5 g/L CuSO₄ corresponding to Figure 12(b), there are several obvious pitting holes can be observed on the surface. The corrosion degree of the coating adding 0.8 g/L is the least with a small amount of corrosion pits. And the morphology of Ni-Cu-P coating can be seen clearly. Nevertheless, the Ni-Cu-P coating adding 1.0 g/L CuSO₄ with some cracks and shedding is corroded more seriously than the Ni-Cu-P coating adding 0.8 g/L CuSO₄. As can be seen in Figure 12(d), the as-deposited Ni-Cu-P coating is ruptured in some areas with more serious corrosion, and magnesium alloys substrate eroded by corrosion medium, causing corrosion occurs to the magnesium alloy substrate. In summary, the most superior corrosion resistance of Ni-Cu-P coating was obtained in the condition of 0.8 g/L CuSO₄.

Figure 12
Corrosion morphologies of Ni-Cu-P coatings with different CuSO₄ concentration (a) 0.2 g/L (b) 0.5 g/L (c) 0.8 g/L (d) 1.0 g/L.

A small amount of oxygen existed in electroless plating bath so that corrosive micro-cell formed on the surface of the coating and then electrochemical reaction occurred. Copper electrode potential is higher than that of nickel. Copper in the solution acted as cathode and nickel acted as anode, which would produce a layer of Ni(OH)₂. The Ni(OH)₂ will slow down the corrosion process, which prevents the magnesium alloy substrate from damaging at a certain extent. Moreover, phosphorus in the coating can accelerate the formation of passivation film on the alloy coating. Additionally, the passivation is stronger with the percent of P becoming higher. The corrosion resistance of passivation film becomes more excellent ascribes to low porosity, which leads to the composite coating staying more stable in corrosive environment, the corrosion resistance of the plating being better accordingly.

Energy spectrum analysis and XRD patterns of the Ni-Cu-P coating morphology immersed 72h are presented in Figure 13. It can be seen that the surface of the Ni-Cu-P coating covered with crystal NaCl. The corrosive substances of Ni-Cu-P coating after corrosion are still composed of Ni, P, Cu elements. As it can be observed from the XRD patterns that the corrosion product of Ni-Cu-P coating are Ni₇P₃, MgCuP, saline solution and Na₃P produced by plating, besides are exposed Mg substrate after peeling. Furthermore, there are more diffuse diffraction peak in XRD patterns, which indicates that the amorphous structure of Ni-Cu-P coating in some areas did not change during immersion, preventing magnesium alloy from corrosion correspondingly.

Figure 13
(a) Energy spectrum analysis of Ni-Cu-P coatings immersed for 72 h; (b) XRD patterns of Ni-Cu-P coatings immersed for 72 h.

4. Conclusions

1. Ni-Cu-P ternary nickel-based amorphous coatings with enhanced corrosion resistance were prepared on the AZ91D magnesium alloys substrates by an integrative method involved direct-current (DC) pre-plating copper and electroless plating Ni-Cu-P coatings with various CuSO₄ concentration.

2. When the CuSO₄ concentration was 0.8 g/L, the mass fraction of P in the coating was 9.06 wt%, the Ni-Cu-P composite coatings exhibited uniform and dense amorphous structure.

3. The results of the Nyquist and Bode graph, corrosion rate, corrosion morphologies and of the Ni-Cu-P coatings with different CuSO₄ addition indicated that the corrosion resistance reached optimization with corrosion potential of -0.31 V, the corrosion current density of 0.0039 A/cm² when the CuSO₄ concentration is 0.8 g/L.

4. The adhesion of Ni-Cu-P coatings improved gradually as the CuSO₄ concentration increased and exhibited optimum when added 1.0 g/L CuSO₄.

5. The obtained Ni-Cu-P coating with amorphous structure prevented effectively magnesium alloy from corrosion.

5. Acknowledgments

This work was supported by Service Local Special Program of Education Department of Shaanxi Province (23JC051), the National Natural Science Foundation of China (Grant No.52175184).

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Publication Dates

• Publication in this collection
  18 Oct 2024
• Date of issue
  2024

History

• Received
  04 June 2024
• Reviewed
  18 Aug 2024
• Accepted
  20 Sept 2024