novel copper immersion coating on magnesium alloy az91d in an alkaline bath.

by:ZHENHUA     2020-09-02
So far, the copper-dipped coating of magnesium alloy has only been carried out in acidic solutions.
In this paper, a new type of alkaline bath for copper dipping coating of magnesium alloy is introduced.
Before the reduction process, the chemical etching process of magnesium matrix was optimized using an orthogonal experimental method.
The copper immersion coating was then studied in relation to the effects of ph and fluoride content in the deposition bath.
During the reduction process, it was found that the increase in pH and fluoride content led to the formation of the facial mask on the magnesium substrate.
With the reduction of copper, the formation of the surface film occurs at the same time, so that the dissolution of magnesium is controlled, thus forming a controlled copper deposition.
Uniform copper deposition was obtained by optimizing the process conditions of chemical corrosion and dipping coating.
Key words: copper dipping coating, alkaline bath, aluminum alloy, magnesium, is called hard-to-plating metal due to its high reactivity. (1-3)
Therefore, a special surface coating procedure is required, including chemical etching steps to remove oxide films on the surface of magnesium, as well as a immersion coating process, used to protect magnesium substrates for subsequent chemical/electro-deposition. (4-5)
While every step of the process is essential, the immersion coating process is critical.
The immersion coating is a simple process, often referred to as a \"metal replacement reaction \". \" (6)When ametal (e. g.
Magnesium substrate)
Immersed in a solution containing a second metal ion (e. g. , [Cu. sup. ++])
Magnesium atom (less noble)
Dissolved in the anode reaction and spontaneously replaced by copper positive ions in the solution, resulting in copper deposition on the surface of magnesium.
The most common dip coating process for magnesium alloy is zinc salt process. (1,3,7-8)
However, in order to achieve the subsequent electric/chemical deposition, the cyanide impact of copper must be applied after the zinc salt process, which has always been a concern for many reasons. (2)
Therefore, a copper-coated process has been developed in our laboratory ,(9-12)
This successfully achieved the subsequent chemical deposition of nickel from an acidic solution.
However, challenges remain, such as the higher hole rates in copper deposits than expected.
The current exchange is concentrated on the simple copper immersion coating in the alkaline bath.
In this study, the chemical etching process of the magnesium alloy substrate was optimized before the copper dipping coating.
The effects of pH effect and fluoride content in alkaline immersion coating Bath were then studied.
Based on the analysis of the experimental results, the mechanism of copper coating in alkaline bath was discussed.
Experimental pretreatment of magnesium alloy substrate the magnesium alloy substrate used in the whole experiment is high
Pure alloys with excellent corrosion resistance are commonly used as moldscasting alloy.
Pretreatment of substrates is specified elsewhere. (9)
The substrate is generally cut into 1.
5x10x10mm pieces, subsequent glass-
Bead for 10 seconds under pressure of 450 kPa.
Then, under the action of ultrasound, clean the substrate with methanol for 6 minutes, and then use three-
10g/L sodium hydroxide in solution containing 60g/L [10g/L]Na. sub. 3]P[O. sub. 4]at 75[degrees]C.
After thoroughly washing the foundation in deionised water, there are two parallel processes, I . E. e.
Copper dipping coating and blank dipping process were carried out as described below.
Blank dipping process for copper dipping coating and copper dipping coating, magnesium substrate further chemical etching process in bath containing 100g/L [K. sub. 4][P. sub. 2][O. sub. 7]+ 30 g/L [Na. sub. 2]C[O. sub. 3]
Then immediately transfer to the alkaline coating bath for copper deposition.
100g/L for coating tank [K. sub. 4][P. sub. 2][O. sub. 7], 30 g/L [Na. sub. 2]C[O. sub. 3], 12. 5 g/LCuS[O. sub. 4]. 5[H. sub. 2]
O plus various concentrations of NaF.
Then the surface of the coating sample is examined by scanning electron microscopy (SEM)using the back-
Scattering electrons (BSE)mode.
Bseimage of the result is processed with Image-
Pro Plus software (
Media Network News Center)
Get copper surface coverage.
Regarding the blank soaking process, it is carried out by immersing the pre-treated substrate in a solution containing 100g/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, Ie.
, The same as the composition of the copper coated solution, except for copper sulfate.
The study of this blank dipping process is expected to prove beneficial in understanding the copper dipping coating.
Electrical impedance spectrum (1)EIS)
As mentioned above, measurements were made during blank soaking.
In addition to 1x1 [the surface of the magnesium base is sealed with epoxy resincm. sup. 2]
Exposed to the effective work area of these solutions.
Use 320 sand wet King Kong sandpaper to polish the work area and then use the same pre-treatment procedure as specified in the section \"aluminum alloy Substrate pretreatment\"e. ,glass-
Beads, methanol cleaning and alkali degreasing under Ultrasonic Action.
After the sample is thoroughly rinsed in deionised water, it is immediately transferred to a blank immersion solution for eis measurement.
A battery with three electrodes. e.
Working electrode, reference electrode and reverse electrode (
Pt gauze with a surface of about 10 [cm. sup. 2])
Used for EIS testing.
The reference electrode used is Ag/AgCl/ck (3M)
Connect to the electrolyte through a salt water bridge of 3M potassium chloride.
The distance between the reference electrode and the working electrode is maintained ~ 0. 3 cm.
In this article, the potential values of all reports are referenced0. 21V vs. RHE at 25[degrees]C). [
Figure 1 slightly][
Figure 2:[
Figure 3 slightly]
Constant potential meter with EG & G 273A plus m52 10 lock-in amplifier.
The frequency range used in this work is set to between 100 kHz and 1Hz.
The bias potential is 0 for all EIS tests. 0V (vs. OCP)
5 mV ac disturbances were used.
All the chemicals (AR grade)
Including [·Na. sub. 2]C[O. sub. 3],[Na. sub. 3]P[O. sub. 4], NaF, and CuS[O. sub. 4]
Provided by Sigma.
Deionized water (> 15 M[ohm][cm. sup. -1])
Preparation with milliporeelix 10 water Deion system for solution preparation.
Results and discussion of optimization of chemical corrosion process it is generally believed that the chemical corrosion of the underlying layer plays an important role in the surface coating.
In view of the complex effects of different parameters and etching processes, it is necessary to consider a systematic approach to study the etching process of copper-immersed coatings.
For this reason, Orthogonal analysis is considered to be an effective tool. (13-15)
Therefore, in this study, orthogonal design is performed at four levels by selecting four factors (Table 1)
, Evaluate the effect of a chemical etching process based on copper surface coverage.
Table 2 shows the experimental arrangement and the results obtained, where each process parameter is assigned to a column and a combination of 16 parameters (i. e. , 16treatments)were available.
To obtain more reliable results, the treatment is repeated three times per treatment to obtain an average surface coverage.
According to the average surface coverage, the average horizontal response of each factor ,[R. sub. pl]
The calculation is :[R. sub. pl]= [1/k][k. summation over. (i=1)][bar. [eta]. sub. i](1)where k (= 4)
Level, l (l = 1to 4)
For the factor, p (
P = A, B, C, D).
The average coverage rate for each treatment is [bar. [eta]. sub. i].
According to the equation (1)
, The average horizontal response is calculated and plotted in figure 1.
The average horizontal response indicates that in the horizontal response studied, the optimal combination of the chemical etching process in the horizontal range studied is [A. sub. 2], [B. sub. 1], [C. sub. 1], and [D. sub. 1](Figure 1),i. e.
, Etching magnesium substrates at 100g/L [K. sub. 4][P. sub. 2][O. sub. 7]+ 30 g/L [Na. sub. 2]C[O. sub. 3]([A. sub. 2])
Temperature is 50 [degrees]C ([B. sub. 1])for 10 sec ([C. sub. 1])
Do not rinse with water after chemical etching ([D. sub. 1]).
The optimized conditions are easy to accept because the pyrophosphate and carbonated roots used in the etching solution are also used in subsequent Alkaline copper immersion coating baths.
It was argued that it was unnecessary to rinse in water after etchingprocess under optimized conditions.
Therefore, the surface state of the magnetic substrate after chemical corrosion under optimal conditions directly promotes the copper dipping coating process.
The above conclusions and sak fields (2)
In terms of the promotion effect of chemical corrosion on subsequent coatings.
In the following study, the best conditions for the obtained chemical etching process were adopted. [
Figure 4 slightly][
Figure 5 Slightly]
Optimization of slot potential of Alkaline copper immersion coating-
PH diagram of copper
Water and magnesiumwater (16)
It can be used to determine the theoretical conditions of copper-immersed coatings from thermodynamics.
For copper reduction, the potential of the magnesium substrate must be located in an area where the metal copper is stable and the magnesium is dissolved to provide the driving force for copper deposition.
According to potential-pH diagrams, (16)
There are pH areas abroad (
PH between 2 and 9)
The theoretical conditions for copper immersion deposition are met.
However, by simply dipping the magnesium substrate into copper ions, a high quality coating cannot be achieved
Solution containing pH range.
It was observed that during the simple soaking coating process, the intense reaction of magnesium dissolution and copper reduction occurred with the evolution of hydrogen, and the resulting coating was dark and sponge-like.
Therefore, under the action of pH effect and fluorine content in the paint bath, the copper-immersed paint bath was optimized as described below.
The effect of PH on copper surface coverage: it is worth noting that with the further increase of pH over 9, the thermodynamic stable region of Mg (OH)[. sub. 2]
Potential-
PH chart of magnesiumwater(16)
Gradually enter.
Therefore, due to the formation of a stable magnesium hydroxide membrane, the dissolution of magnesium may be reduced ,(17)
Therefore, it is possible to present a controlled dissolution of magnesium, thus achieving a controlled reduction of copper.
In order to study the effect of pH value on the dissolution of magnesium, blank soaking 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]
30g/L NaF5 g/L.
Before the blank soaking process, as described in the section \"pretreatment of magnesium alloy substrate\", the magnesium substrate is pre-treated by glass beads, followed by methanol cleaning and alkaline degreasing.
Previous SEM images (Figure 2d)andafter (
Figure 2a to 2c)
The blank soaking process is introduced.
What\'s interesting is that glass
Erosion marks in the process of beads (18)
On the surface of the magnesium alloy substrate, it is clearly visible on the blank sample (Figure 2d).
This erosion trace is reduced during subsequent immersion, but remains on the surface of the substrate (
Figure 2a to 2c).
The remaining erosion markers can be interpreted as an indication of the induction of magnesium dissolution during immersion, I . E. e.
The less obvious the erosion marks are, the more magnesium dissolves.
Therefore, the comparison of Figure 2a to 2c clearly shows that the dissolution of magnesium decreases with the increase of pH value.
In order to obtain more information during blank soaking, eis measurements were performed.
What is shown in Figure 3 is the quest spectrum, which is characterized by a capacitor loop in the frequency range. A simple, one-
The capacitor circuit can be represented by a time constant equivalent circuit. e.
Solution resistance ,[R. sub. s]
In series with a parallel RC circuit with a charge transfer resistance ,[R. sub. ct]
, And double-layer capacitors ,[C. sub. d](
Figure 3, insert.
Figure 3 shows the charge transfer resistance as the pH value of the immersion solution increases ,[R. sub. ct]
Measured from the diameter of the naiquest arc, a significant increase reflects a reduction in magnesium dissolution.
This is consistent with the SEM results. The back-
Figure 4 shows the scattered electron image of the copper-immersed coating, showing the effect of pH value
The surface coverage exported from these images is displayed as an insert.
This shows that with the increase of pH value, the copper surface coverage rate reaches the maximum value at pH value of 10 and then increases first and then decreases. 3.
From SEM (Figure 2)and EIS (Figure 3)
, Dissolution of magnesium at pH 9.
3 is high, which leads to a high reduction driving force for copper.
Therefore, copper coverage is expected to be high.
This is not the case, however.
Surface coverage observed at pH 9.
3 people who are even lower than pH (e. g. , pH 10. 3).
The collected data of pH 9 was analyzed.
3 shows that the data is widely distributed, as shown in the standard deviation (
Insert of figure 4).
This is due to the dissolution of magnesium at pH 9.
3 is violent, resulting in non-stick and sponge copper deposits.
This sponge-like sediment was partially removed from the substrate surface by violenthydrogen evolution.
With the increase of pH value, the film that may be formed on the magnesium surface guides/limits the dissolution of magnesium, which not only prevents the intense dissolution of the magnesium substrate, but also produces in a uniform coating (Figure 4, pH 10. 6).
However, with the further increase of pH value, the dissolution rate of magnesium slows down and the decrease of copper coverage rate is observed (Figure 4, pH 10. 9 and11. 2)
This may be due to the reduction of the driving force by the dissolution of magnesium.
Based on the above discussion, the pH range is 10. 3-10. 9 (preferably10. 6)
It should be used to achieve a uniform copper dipping coating. [
Figure 6 slightly][
Figure 7 Slightly]
Effect of fluorine: as shown in figure 5, the effect of fluorine concentration on copper-immersed coating.
Figure 5a shows that there is no fluorine in the coating bath, the obtained coating is sponge-like, and the quality coating cannot be realized.
After the addition of fluorine, the observed surface coverage decreased with the increase of fluorine content, which may be due to the formation of fluorine
Surfacefilm is included. (19,20)
Similar to the pH effect discussed in the previous section, the effect of fluorine concentration on magnesium dissolution is also represented by SEM images of the surface of the magnesium substrate treated in a immersion solution (Figure 6)
Change in transfer resistance ,[R. sub. ct]
, The diameter of the arc from the nquest (Figure 7).
Obviously, the dissolution of magnesium is high in a soaking bath without fluorine (Figure 6a).
Dissolution of magnesium substrate is mediated/restricted by possible fluorine after fluorine addition
Contains surface film formation.
Therefore, the surface coverage of copper decreases with the increase of fluorine concentration (Figure 5).
Based on the above discussion, fluoride from 5g/L to 10g/L should be added to the solution to achieve a uniform copper immersion coating.
Conclusion The Chemical etching process of the magnetic substrate was optimized, providing the best experimental conditions for the subsequent copper dipping coating.
The pH value and fluoride content in the alkaline coating Bath have a significant effect on obtaining a high quality coating.
During the coating process, an increase in pH and fluoride content resulted in the formation of a surface film on a magnesium substrate, resulting in a controlled magnesium dissolution, resulting in a controlled copper deposition.
Through the best process conditions of chemical corrosion and dipping coating, uniform and fine-grained copper deposition is achieved.
Recognizes that the current work is sponsored by the Commission for Natural Science and Engineering Research (NSERC)of Canada.
The author thanks Mike Meinert for his work on SEM measurements and William Wells for his work on sample preparation. References (1)Dennis, J. K. , Wan, M. K. Y. Y. , and Wakes, S. J. , Trans. Inst. Met. Completed, 63,74 (1985). (2)Sakata, Y.
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Yang Lianxi, Ben Luan ,([dagger])
John NATA. -
Institute of Integrated Manufacturing Technology ** National Research Council of Canada, 800 collipu circle, London, N6G 4x8, Canada. ([dagger])
The author who should communicate. Voice: 519. 430. 7043, fax: 519. 430.
7064, Email: BenLuan@nrc. gc. ca.
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