Novel copper immersion coating on magnesium alloy AZ91D in an alkaline bath.

Copper immersion coating of magnesium alloys has, to date, been conducted only in acidic baths. This article describes a novel alkaline bath for copper immersion coating on AZ91D magnesium alloy. Prior to the coating process, a chemical etching process of the magnesium substrate was optimized using orthogonal experimental methodology. The copper immersion coating was then investigated with regard to the effect of pH and fluoride content in the deposition bath. It was revealed during the coating process that an increase of pH and fluoride content led to a surface film formation on the magnesium substrate. The surface film formation occurred simultaneously with copper reduction, rendering a controlled magnesium dissolution, thereby a controlled copper deposition. With optimized conditions of chemical etching and immersion coating processes, uniform copper deposits were achieved.

Keywords: Copper immersion coating, alkaline bath, magnesium alloy

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Magnesium is known as a difficult metal to plate due to its high reactivity. (1-3) A special procedure for its surface coating is therefore required, which includes a chemical etching step to remove the oxide film on magnesium surface, and an immersion coating process to form an under layer for protecting the magnesium substrate for the subsequent electroless/electro depositions. (4-5) While each step in the procedure is essential, the immersion coating process is critical.

Immersion coating is a simple process involving what are generally referred to as "metal displacement reactions." (6) When a metal (e.g., magnesium substrate) is immersed in a solution containing a second metal ion (e.g., [Cu.sup.++]), magnesium atoms (less noble) dissolve in the anodic reaction and are spontaneously replaced by the copper cations from the solution, resulting in a copper deposition on the magnesium surface. The most common immersion coating process for magnesium alloy is the zincate process. (1,3,7-8) To enable the subsequent electro/electroless deposition, however, a copper cyanide strike must be applied after the zincate process, which has been of concern for a number of reasons. (2) A copper immersion coating process was therefore developed in our labs, (9-12) which successfully enabled subsequent electroless Ni deposition from an acidic bath. However, challenges still remain, such as the higher than desired level of porosity in the copper deposits.

The present communication focuses on a simple copper immersion coating in an alkaline bath. In this study, prior to the copper immersion coating, a chemical etching process of the magnesium alloy substrate was optimized. The coating process was then investigated in terms of pH effect and the role of fluoride content in the alkaline immersion coating bath. Based on the analysis of experimental results, the mechanism of copper immersion coating in the alkaline bath is discussed.

EXPERIMENTAL

Pretreatment of Magnesium Alloy Substrate

The magnesium alloy substrate used throughout the experiments was AZ91D, a high-purity alloy with excellent corrosion resistance commonly used as a die-casting alloy. The pretreatment of the substrate was specified elsewhere. (9) Generally, the substrate was cut into 1.5 x 10 x 10 mm pieces and subsequently glass-beaded for 10 sec at a pressure of 450 kPa. The substrate was then cleaned in isopropanol for six minutes in the presence of sonication, followed by a three-minute alkaline degreasing in a solution containing 60 g/L NaOH + 10 g/L [Na.sub.3]P[O.sub.4] at 75[degrees]C. After thoroughly rinsing the substrate in deionized water, two parallel processes, i.e., a copper immersion coating and a blank immersion process, were carried out as described below.

Copper Immersion Coating and Blank Immersion Process

For the copper immersion coating, the magnesium substrate was further subjected to a chemical etching process in a bath containing 100 g/L [K.sub.4][P.sub.2][O.sub.7] + 30 g/L [Na.sub.2]C[O.sub.3] and then immediately transferred into an alkaline coating bath for copper deposition. The coating bath contained 100 g/L [K.sub.4][P.sub.2][O.sub.7], 30 g/L [Na.sub.2]C[O.sub.3], 12.5 g/L CuS[O.sub.4].5[H.sub.2]O plus various concentrations of NaF. The surface of the coated samples was then examined by scanning electron microscopy (SEM) using the back-scattered electron (BSE) mode. The resultant BSE images were processed with Image-Pro Plus software (Media Cyberhetics Inc.) to obtain copper surface coverage.

With regard to the blank immersion process, it is performed by immersing the pretreated substrate into a solution that contains 100 g/L [K.sub.4][P.sub.2][O.sub.7], 30 g/L [Na.sub.2]C[O.sub.3] plus various concentrations of NaF, i.e., the same composition as the copper immersion coating solution, except for the copper sulfate. Investigation of this blank immersion process is expected to prove beneficial in understanding the copper immersion coating.

Electrochemical Impedance Spectroscopy 

Electrochemical impedance spectroscopy (EIS) measurements were performed during the blank immersion process, as stated above. The surfaces of magnesium substrate were sealed using epoxy resin, except for a 1 x 1 [cm.sup.2] effective working area to be exposed to the solutions. The working area was polished using 320 grit wet emery paper, followed by the same pretreatment procedure as specified in the "Pretreatment of Magnesium Alloy Substrate" section, i.e., glass-beading, isopropanol cleaning under sonication, and alkaline degreasing. After thoroughly rinsing in deionized water, the sample was immediately transferred into the blank immersion solution for EIS measurement.

A cell fitted with three electrodes, i.e., a working electrode, a reference electrode, and a counter electrode (Pt gauze with a surface area of ~ 10 [cm.sup.2]) was employed for the EIS tests. The reference electrode used was Ag/AgCl/KCl(3M) connected to the electrolyte through a salt solution bridge of 3M KCl. The distance between the reference electrode and the working electrode was maintained at ~ 0.3 cm. In this article, all potential values reported are referenced against the Ag/AgCl/KCl (0.21V vs. RHE at 25[degrees]C).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

An EG & G 273A potentiostat was employed, coupled with an M5210 lock-in amplifier. The frequency range used in this work was set to be between 100 kHz and 1 Hz. For all the EIS tests a bias potential of 0.0 V (vs. OCP) and a 5 mV AC perturbation were used.

Chemicals

All the chemicals (AR grade), including NaOH, [Na.sub.2]C[O.sub.3], [Na.sub.3]P[O.sub.4], NaF, and CuS[O.sub.4] were supplied by Sigma. Deionized water (> 15 M[ohm] [cm.sup.-1]) prepared with Millipore Elix 10 water deionization system was used for solution preparations.

RESULTS AND DISCUSSION

Optimization of Chemical Etching Process

It is generally recognized that chemical etching of substrates plays an important role in surface coating. Given the complex effect of different parameters from the etching process, it is necessary to consider a systematic approach to study the etching process for the copper immersion coating. For this purpose, orthogonal analysis is known to be an effective tool. (13-15) In this study the orthogonal design was therefore performed by selecting four factors, at four levels each (Table 1), to evaluate the effect of the chemical etching process based on copper surface coverage. The experimental arrangements and the obtained results are shown in Table 2, in which each process parameter was assigned to a column and 16 parameter combinations (i.e., 16 treatments) were available. For more reliable results, each treatment was repeated three times to obtain the average surface coverage.

Based on average surface coverage, the mean level response of each factor, [R.sub.pl], was calculated as:

[R.sub.pl] = [1/k] [k.summation over.(i=1)] [bar.[eta].sub.i] (1)

where k (= 4) is the number of treatments involving level, l (l = 1 to 4), for factor, p (p = A, B, C, and D). The average coverage corresponding to each treatment is [bar.[eta].sub.i].

According to equation (1), the mean level response was calculated and plotted in Figure 1. The mean level response suggests that, within the investigated level response suggests that, within the investigated level ranges, the optimal combination of the chemical etching process to obtain the high surface coverage for the subsequent copper immersion coating is [A.sub.2], [B.sub.1], [C.sub.1], and [D.sub.1] (Figure 1), i.e., etching magnesium substrate in 100 g/L [K.sub.4][P.sub.2][O.sub.7] + 30 g/L [Na.sub.2]C[O.sub.3] ([A.sub.2]) at a temperature of 50[degrees]C ([B.sub.1]) for 10 sec ([C.sub.1]) without water rinsing after the chemical etching process ([D.sub.1]).

The optimized conditions are easily acceptable because both pyrophosphate and carbonate used in the etching solution were also used in the subsequent alkaline copper immersion coating bath. It is also noted that under the optimized condition, rinsing in water after etching process was not necessary. Therefore, the surface state of the magnesium substrate after chemical etching under the optimal conditions directly facilitates the copper immersion coating process. The above conclusion is consistent with Sakata (2) in terms of the facilitation of chemical etching to the subsequent coating. The obtained optimal conditions of the chemical etching process were employed in the following studies.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Optimization of the Alkaline Copper Immersion Coating Bath

Potential-pH diagrams of copper-water and magnesium-water (16) can be used to thermodynamically determine the theoretical condition of the copper immersion coating. For copper reduction to occur, the potential of magnesium substrate must be in the region where metallic copper is stable and magnesium dissolves, to provide the driving force for copper deposition. According to the potential-pH diagrams, (16) there is a broad pH region (pH between 2 to 9) in which the theoretical conditions of copper immersion deposition are satisfied. However, a quality coating cannot be achieved by a simple immersion of magnesium substrate into a copper ions-containing solution within the pH range. It was observed that during the simple immersion coating process, violent reactions of magnesium dissolution, and copper reduction with hydrogen evolution took place, and the resultant coating was dark and spongy. The optimization of the copper immersion coating bath was therefore carried out in terms of pH effect and the role of fluoride content in the coating bath, as will be discussed below.

EFFECT OF PH ON THE COPPER SURFACE COVERAGE: It is of note that with further increase of pH beyond 9, the thermodynamically stable region of Mg(OH)[.sub.2] in the potential--pH diagram of magnesium-water (16) was gradually entered. Accordingly, the dissolution of magnesium may have decreased due to the formation of a stable magnesium hydroxide film, (17) thus making it possible to render a controlled magnesium dissolution, and, in turn, a controlled copper reduction.

To investigate the pH effect on magnesium dissolution, a blank immersion process was performed in a bath containing [K.sub.4][P.sub.2][O.sub.7] 100 g/L + [Na.sub.2]C[O.sub.3] 30 g/L + NaF 5 g/L. Prior to the blank immersion process, the magnesium substrate was pretreated by glass beading, followed by isopropanol cleaning and alkaline degreasing, as stated in the "Pretreatment of Magnesium Alloy Substrate" section. The SEM images before (Figure 2d) and after (Figures 2a to 2c) the blank immersion process are presented. It is interesting to note that the glass-beading process resulted in erosion marks (18) on the surface of the magnesium alloy substrate, as is clearly observable on the blank sample (Figure 2d). Such erosion marks were lessened but remained on the substrate surface after the subsequent immersion process (Figures 2a to 2c). The remaining erosion marks could be interpreted as an indication of the dissolution of magnesium induced by the immersion process, i.e., the less obvious the erosion marks, the more the magnesium dissolution. As such, a comparison of Figures 2a to 2c clearly indicates that with an increase in pH, the dissolution of magnesium decreases.

To obtain more information during the blank immersion process, EIS measurements were conducted. Shown in Figure 3 are the Nyquist spectra which are characterized by a capacitive loop in the frequency range studied. A simple, one-time constant equivalent circuit may be applied to represent the capacitive loop, i.e., a solution resistance, [R.sub.s], in series with a parallel RC circuit with a charge transfer resistance, [R.sub.ct], and a double layer capacitance, [C.sub.d] (Figure 3, the insert). Figure 3 shows that with increasing pH of the immersion solution, the charge transfer resistance, [R.sub.ct], measured from the diameter of the Nyquist arc, is significantly increased, reflecting the decrease of magnesium dissolution. This is consistent with the SEM results.

The back-scattered electron images of the copper immersion coating are presented in Figure 4, showing the effect of pH. The surface coverage derived from these images is shown as an insert. It indicates that with increasing pH, the copper surface coverage first increases then decreases after reaching a maximum point at a pH of 10.3. As is evidenced from the SEM (Figure 2) and EIS (Figure 3), the dissolution of magnesium at a pH of 9.3 is high, which results in a high driving force for the copper reduction. A high copper coverage was therefore expected. However, this was not the case. The observed surface coverage at pH 9.3 was even lower than those at higher pH (e.g., pH 10.3). The analysis of collected data at pH 9.3 shows that the data scattered widely, as indicated by the standard deviation (the insert, Figure 4). This is because the dissolution of magnesium at pH 9.3 was violent, resulting in nonadherent and spongy copper deposits. Such spongy deposits were subsequently partially removed from the substrate surface by violent hydrogen evolution. With increasing pH, a possible film formation on the magnesium surface mediated/limited the magnesium dissolution, which not only prevented magnesium substrate from violent dissolution but also resulted in a uniform coating (Figure 4, pH 10.6). With the further increase in pH, however, the dissolution of magnesium was slowed down and a decrease in copper coverage was observed (Figure 4, pH 10.9 and 11.2), which might be attributed to a reduced driving force due to a reduced dissolution of magnesium.

Based on the above discussion, a pH range of 10.3-10.9 (preferably 10.6) should be employed to achieve a uniform copper immersion coating.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

ROLE OF FLUORIDE: Shown in Figure 5 is the effect of fluoride concentration on the copper immersion coating. Figure 5a shows that without fluoride in the coating bath, the obtained coating is spongy and a quality coating is not achievable. After the addition of fluoride, the observed surface coverage decreased with increasing fluoride content, attributable to a possible formation of fluoride-containing surface film. (19,20) Similar to the pH effect discussed in the prior section, the effect of fluoride concentration on the dissolution of magnesium was also manifested by SEM images for the magnesium substrate surface processed in the immersion solution (Figure 6) and the change of the charge transfer resistance, [R.sub.ct], measured from the diameter of the Nyquist arc (Figure 7). It is clear that the dissolution of magnesium in the immersion bath without fluoride is high (Figure 6a). After the addition of fluoride, the dissolution of magnesium substrate was mediated/limited by a possible fluoride-containing surface film formation. As a result, the copper surface coverage decreased with an increase in fluoride concentration (Figure 5). Based on the above discussion, a fluoride content of 5 g/L to 10 g/L should be added to the coating solution to achieve a uniform copper immersion coating.

CONCLUSIONS

The optimization of a chemical etching process for magnesium substrate was performed, resulting in an optimal experimental condition for the subsequent copper immersion coating.

Both pH and fluoride content in the alkaline coating bath had significant effects on achieving a quality coating. During the coating process, an increase of pH and fluoride content led to a surface film formation on magnesium substrate, which rendered a controlled magnesium dissolution, thereby a controlled copper deposition.

With the optimal process conditions of chemical etching and the immersion coating, uniform and finely grained copper deposits were achieved.

ACKNOWLEDGMENT

The present work was sponsored by the Natural Science & Engineering Research Council (NSERC) of Canada. The authors are grateful to Mike Meinert for his work on SEM measurement, and William Wells for sample preparations.

References

(1) Dennis, J.K., Wan, M.K.Y.Y., and Wakes, S.J., Trans. Inst. Met. Finish, 63, 74 (1985).

(2) Sakata, Y., 74th AESF Technical Conference, 15 (1987).

(3) Chen, J.H., Chang, C.C., and Lee, T.S., AESF SUR/FIN' 91, 754 (1991).

(4) ASTM Standard Designation B 480-88.

(5) Gray, J.E. and Luan, B., J. Alloys Comp., 336, 88 (2002).

(6) Electroless Plating: Fundamentals and Applications, Mallory, G.O. and Hajdu, J.B. (Eds.), American Electroplaters and Surface Finishers Society, 1990.

(7) Such, T.E. and Wyszynski, A.E., Plating, 52, 1027 zincating on Al (1965).

(8) Wyszynski, A.E., Institute of Metal Finishing, 45, 147, zincate on Al (1967).

(9) Yang, L., Luan, B., Cheong, W.J., and Jiang, J., "Optimization and Performance Analysis of Copper Immersion Coating on AZ91 Magnesium Alloy," J. COAT. TECHNOL. RES., 2, No. 6, 493 (2005).

(10) Yang, L., Luan, B., Cheong, W.J., and Shoemith, D., J. Electrochem. Soc., 152, C131 (2005).

(11) Yang, L. and Luan, B., J. Electrochem. Soc., 152, C474 (2005).

(12) Luan, B. and Gray, J., Acousto-Immersion Coatings for Magnesium and Its Alloys, U.S. Patent 6,669,997, 2003.

(13) Han, Q., Liu, K., Chen, J., and Wei, X., Int. J. Hydrogen Energy, 28, 1345 (2003).

(14) Lin, T.R., J. Mater. Processing Technol., 127, 1 (2002).

(15) Shaji, S. and Radhakrishnan, V., J. Mater. Processing Technol., 141, 51 (2003).

(16) Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solution, NACE, Houston, TX, 1974.

(17) Ambat, R., Aung, N.N., and Zhou, W., J. Appl. Electrochem., 30, 865 (2000).

(18) Possart, W., Bockenheimer, C., and Valeskem B., Surf. Interface Anal., 33, 687 (2002).

(19) Dennis, J.K., Wan, M.K.Y.Y., and Wake, S.J., Trans. IMF., 63, 81 (1985).

(20) Fairweather, W.A., Trans. IMF., 75, 113 (1997).

Lianxi Yang, Ben Luan, ([dagger]) and John Nagata--Integrated Manufacturing Technologies Institute*

* The National Research Council Canada, 800 Collip Circle, London, ON, Canada N6G 4X8.

([dagger]) Author to whom correspondence should be addressed. Voice: 519.430.7043, fax: [phone], email: [e-mail].

Table 1 -- Factors and Their Levels for the Optimization of Chemical
Etching Process (a) of Magnesium Substrate for the Subsequent Copper
Immersion Coating (b)

          Factors
        Chemical Etching                   Levels
Symbol  Parameter         1           2           3           4

A       Etching bath      [EB.sub.1]  [EB.sub.2]  [EB.sub.3]  [EB.sub.4]
          (EB) (c)
B       Etching           50          60           70          80
          temperature
          ([degrees]C)
C       Etching time      10          60          120         240
          (sec)
D       Rinsing time       0          10           20          40
          after the
          etching (sec)

(a) General coating procedure: AD91D substrate--glass beading--
isopropanol cleaning under sonication for three minutes--degreasing in
60 g/L NaOH + 10 g/L [Na.sub.3]P[O.sub.4] at 75[degrees]C for 3 min--
rinsing--chemical etching process--rinsing--copper immersion coating.
(b) The immersion coating bath contains: 100 g/L
[K.sub.4][P.sub.2][O.sub.7] + 30 g/L [Na.sub.2]C[O.sub.3] + 10 g/L
NaF + 12.5 g/L CuS[O.sub.4],5[H.sub.2]O (pH = 10.6, temperature:
22[degrees]C, coating time: 15 min).
(c) Chemical etching baths (EB) are [EB.sub.1]: 60 g/L NaOH + 10 g/L
[Na.sub.3]P[O.sub.4]; [EB.sub.2]: 100 g/L
[K.sub.4][P.sub.2][O.sub.7] + 30 g/L [Na.sub.2]C[O.sub.3]; [EB.sub.3]:
100 g/L [K.sub.4][P.sub.2][O.sub.7] + 30 g/L [Na.sub.2]C[O.sub.3] + 5
g/L NaF; and [EB.sub.4]: 100 g/L [Na.sub.4][P.sub.2][O.sub.7] + 50 g/L
NaN[O.sub.3].

Table 2 -- Experimental Arrangements and the Results for the Orthogonal
Design for the Chemical Etching Process

                A           B             C             D
No. of          Etching     EB Temp       Etching Time  Rinsing Time
Treatments      Bath        ([degrees]C)  (sec)         (sec)

 1              [EB.sub.1]  50             10            0
 2              [EB.sub.1]  60             60           10
 3              [EB.sub.1]  70            120           20
 4              [EB.sub.1]  80            240           40
 5              [EB.sub.2]  50             60           20
 6              [EB.sub.2]  60             10           40
 7              [EB.sub.2]  70            240            0
 8              [EB.sub.2]  80            120           10
 9              [EB.sub.3]  50            120           40
10              [EB.sub.3]  60            240           20
11              [EB.sub.3]  70             10           10
12              [EB.sub.3]  80             60            0
13              [EB.sub.4]  50            240           10
14              [EB.sub.4]  60            120            0
15              [EB.sub.4]  70             60           40
16              [EB.sub.4]  80             10           20
[R.sub.p1] (b)  40          44             45           41
[R.sub.p2]      45          39             43           37
[R.sub.p3]      38          36             37           40
[R.sub.p4]      36          40             34           41

                Coverage (a) (%),  Average
No. of          [[eta].sub.i]      Coverage,
Treatments      1   2   3          [bar.[eta].sub.i]

 1              54  56  55         55
 2              43  41  44         43
 3              33  35  35         34
 4              30  30  32         30
 5              43  51  47         47
 6              50  51  48         50
 7              40  34  40         38
 8              42  46  45         44
 9              44  42  39         42
10              35  33  38         35
11              33  30  30         31
12              41  44  43         43
13              33  32  32         32
14              32  28  27         29
15              42  40  39         40
16              37  47  45         43
[R.sub.p1] (b)
[R.sub.p2]
[R.sub.p3]
[R.sub.p4]

(a) Coating coverage was obtained by processing the back-scattered
electron images, using Image-Pro Plus software (Media Cyberhetics Inc.).
Three groups of data listed here were obtained from the three-repeated
test for each treatment in the orthogonal array.
(b) [R.sub.pl] is the mean level response for factor, p, at level, I
(I = 1-4, p = A, B, C, or D).