2020 Volume 60 Issue 9 Pages 2031-2037
The use of alkaline electroplating baths is the essential requirement to deposit Cu directly onto steels because of non-adherent Cu formation by replacement reaction between Cu2+ and Fe in acidic solution. For the development of such an electroplating bath, complexing agents to form soluble Cu complex in alkaline pH is necessary at first. Secondary, the soluble Cu complex must be reduced electrochemically. Cyanide-based baths meet these requirements, but the bath is toxic. In this study, the survey of complexing agents revealed that citric and tartaric acids form soluble copper complex solutions in alkaline pH, and electroplating is possible. The cathodic current density range to obtain smooth and adhesive electroplating with citrate complexed bath was extensive than that with a tartrate bath. It was found that 0.1 mol dm−3 CuSO4 - 0.5 mol dm−3 citric acid baths with pH of 9–11 are optimum to obtain adhesive and uniform Cu layer. Copper electroplating with an acidic CuSO4–H2SO4 bath was possible on 1 µm Cu layer with the alkaline citrate bath. Because the plating rate is high with the acidic bath, the multilayer Cu electroplating from the citrate bath and then an acidic sulfate bath gives a reasonable way for Cu coating onto steels. Elongation test of the steel sheet electroplated with the multilayer Cu showed that detachment of the Cu layer was limited in the vicinity of the broken part of the sheet. It is concluded that the toxic cyanide Cu plating bath can be replaced with a citrate bath.
Copper plating on steels is used to form Cu-plated steel tubes, an intermediated layer beneath Ni or Cr electroplating for the enhancement of adherence, or partial diffusion barrier of C during the carburization of steels. Because the standard electrode potential of Cu2+/Cu couple is nobler than that of Fe2+/Fe, a displacement reaction between Fe and Cu2+ occurs to form Fe2+ and Cu at pH = 4–7 where Fe actively dissolves.1) The Cu deposit formed in this way gives a porous and non-adherent copper layer on iron and steels.2,3) Therefore, baths at pH>7 are necessary for adherent Cu electrodeposition. Copper cyanide - potassium cyanide solution at pH 10 is used to form an adherent Cu layer on steel substrates without the replacement reaction. It is mainly for two reasons. Firstly, copper forms a stable soluble complex with cyanide ions at pH 10. Secondary, the replacement reaction is slow at this pH.4) Electroplating bath containing cyanide ion is toxic. It can have a severe effect on human and animal health. Therefore, a strict wastewater treatment and air pollution control are required leading to an increase in plating costs.5) For these reasons, the copper electroplating baths without cyanide ion is needed in industries.
Cu electroplating baths containing copper pyrophosphate are used for electroplating on steel. The bath is non-toxic, but careful wastewater treatment is also needed because the emission of phosphor to river water is strictly restricted. Pyrophosphoric acid decomposes with time to form orthophosphoric acid. It gives rise to poor adhesion and surface quality. Because the selective removal of orthophosphoric acid from the bath is difficult, electroplating bath must be replaced after a finite number of the orthophosphate accumulated in the bath.6)
Carboxyl acids release their protons to the solution with increasing pH resulting lone-pair of electrons within the molecules. Metal ions in the solution form complex with the proton-dissociated carboxyl acids then dissolve in the solution at pH where metal hydroxides precipitate.7,8) If the stability constant of the soluble complexion is not so high, the electrodeposition of metal complexion will be possible. Hosokawa et al. studied the electrodeposition of Cu from solutions containing some carboxylic acids. They found that Cu electroplating is possible if citric acid, lactic acid, and tartaric acid were used as complexing agents.9,10)
However, a more detailed study on the type of complexing agent, their concentration and the solution pH, and on the effect of current density is needed to establish a Cu plating method on steels. Moreover, the evaluation of the adhesion of the Cu plating layer to substrates was unclear in the reports.
Therefore in this study, the authors (1) surveyed a series of carboxylic acids to determine which can be used for Cu electroplating on low carbon steel sheet, then (2) optimized plating bath composition and operation conditions for citric and tartaric acids complexed bath, and finally (3) tested the adherence of layered Cu electroplating with citrate complexed bath and acid copper sulfate bath.
A cold-rolled steel sheet of 0.3 mm thickness containing 0.045 mass% carbon was used as the substrate for tests. It was abraded with a #600 grade SiC paper in distilled water before tests. Dimensions of the specimen were 100 × 50, 20 × 50, 3 × 3 mm2 for the Hull-Cell test, constant current electroplating, and polarization test, respectively. For an elongation test, cold stripped steel sheets (JIS G3141, t = 0.3 mm) were machined into 13B test pieces (JIS Z2201).11,12) The elongation test of oxygen-free copper (JIS C1020) was also made using a 14B test piece for comparison.
2.2. Electroplating BathCu complex electroplating baths were composed of 0.1 mol dm−3 CuSO4 and a series of complexing agents at 0.1–1 mol dm−3. The agents used were EDTA-2Na (Na2C10H14N2O8), alanine (C3H7NO2), citric acid (C6H8O7), glycine (C2H5NO2), tartaric acid (C4H6O6), malonic acid (C3H4O4), aspartic acid (C4H7NO4), ascorbic acid (C6H8O6), glutamic acid (C5H9NO4), or lactic acid (C3H6O3). Concentrated H2SO4 and 5 mol dm−3 NaOH solution was used to adjust pH of baths. A Cu sulfate plating bath containing 0.75 mol dm−3 CuSO4 and 0.3 mol dm−3 H2SO4 was also used. The solution condition was summarized in Table 1.
| reagent | concentration/mol dm−3 | |
|---|---|---|
| bath | complex bath | acid bath |
| CuSO4 | 0.1 | 0.75 |
| H2SO4 | 0.3 | |
| complexing agent | 0.1–1.0 | |
| pH | 4–11 | |
Hull-cell test at room temperature was carried out to study the effect of complexing agents on electroplating. The current and the time for the test was I = 1 A and t = 15 min, respectively. Cyclic voltammograms were obtained at room temperature with a Pt counter and an Ag–AgCl standard electrode at a sweep rate of 1 mV s−1. Constant current electrolysis was carried in a 250 cm3 beaker at 40°C. Cu plate was used as an anode except for the polarization test. A magnet bar was used to stir baths for all the tests.
2.4. Evaluation of ElectroplatingCathodic current density ic at A dm−2 of the Hull-cell test was calculated by Eq. (1).13)
| (1) |
Electroplating of Cu was impossible, or the range of available current density was very narrow when complexing agents except citric and tartaric acids were used. Figure 1 shows the upper limit (iUL) and the lower limit (iLL) current densities to obtain smooth electroplating with citric and tartaric acids as a function of solution pH. The value of r in the figure indicates the mole ratio of the complexing agent (ligand) to total Cu ion in the solution. For tartaric acid addition at r = 3, the iUL was about 2 A dm−2 at pH = 7, and it slightly increased with increasing pH. No electroplating was available below pH = 7 for this solution. The shape of the curve at r = 1 was almost the same. For citric acid at r = 1 and pH = 5, no electroplating layer was obtained below i = 0.8 A dm−2. Burnt electroplating was obtained above i = 2.8 A dm−2. Values of iUL and iLL decreased with pH at r = 1. When r = 3, the iUL was about 0.5 A dm−2 at pH = 5, and it increased with increasing pH. Electroplating was possible at a very low current density with this solution. Although the shape of the curve for r = 10 was similar to that of r = 3, iUL value was small.
Effect of solution pH on upper limit (iUL) and lower limit (iLL) current densities to obtain smooth electroplating with citrate and tartrate complex baths. Value r indicates the mole ratio of complexing agent to total Cu ion. (Online version in color.)
The effect of ligand type to total Cu ion concentration on iUL and iLL is shown in Fig. 2. The iUL for the citric acid-containing bath was large at low r value for all pH, and it decreased with increasing r value. Despite Cu deposition below iLL was impossible at r = 1, lower current density side of Hull-cell test panel was covered with Cu at r higher than 3. The iUL values for tartaric acid at r = 1 and 3 were similar to those for citric acid at pH 9 and 11, whereas the value was small at pH 7. Cu plating was possible at low pH independent of r for this ligand.
Effect of solution the ratio of the concentration of ligand to total Cu, r, on upper limit (iUL) and lower limit (iLL) current densities to obtain smooth electroplating with citrate and tartrate complex baths. (Online version in color.)
Figure 3 shows the effect of pH on the fraction of Cu complex in Cu plating baths. Here, L assigns ligand. Cu2+ means aquo -complex, CuLH 1:1 complex with excess proton, Cu2L2 2:2 complex, Cu2L2H−1 and Cu2L2H−2 2:2 complexes dissociating one and two protons respectively.7,8) For both ligands, Cu2L2H−2 type complex is predominant at pH>7. By comparing the experimental results in Fig. 2 and the calculated curves in Fig. 3, it is indicated that the electrodeposition of Cu from Cu2L2H−2 type complex is likely to occur. For the bath containing tartaric acid, Cu2L2 type complex is stable at pH range 1–6 where no Cu electroplating was obtained. Soluble Cu2L2 is seemed to be inactive to the electrochemical reduction.
Fraction of Cu complexes with pH for citrate and tartrate solutions. Cu2+ means Cu aquo complex, CuLH 1:1 complex with excess proton, Cu2L2 2:2 complex, Cu2L2H−1 and Cu2L2H−2 2:2 complexes dissociating one and two protons respectively. (Online version in color.)
The result of the adhesion test is summarized in Table 2. Grades 25 and 0 mean complete and no adhesion of Cu electroplating. For the tartrate complex bath, the electroplating layer well attached to the substrate at pH≥7, ic≥5 A dm−2, and r≤3. Electroplating at the cathodic current density lower than ic<5 A dm−2 resulted in poor adherence. In the case of citric acid, adhesion was satisfactory at r>3 and pH≥9, and it was independent of cathodic current density. From these comparisons, it can be said that the use of citric acid is better than the use of tartaric acid to obtain an adherent Cu electroplating on steel strips.
| pH | 7 | 9 | 10 | 11 | ||||||||||
| current density/A dm−2 | 0.1 | 5 | 10 | 0.1 | 5 | 10 | 0.1 | 5 | 10 | 0.1 | 5 | 10 | ||
| ratio [ligand]/[Cu]T | citrate | 1 | 22 | 3 | 25 | 0 | 0 | 25 | 0 | 0 | 25 | |||
| 3 | 25 | 25 | 25 | 25 | 25 | 25 | 1 | 25 | 25 | |||||
| 5 | 25 | 25 | 25 | 25 | 25 | 25 | 25 | 20 | 25 | |||||
| 6 | 25 | 25 | 25 | 25 | 25 | 25 | 25 | 25 | 25 | |||||
| 7 | 25 | 25 | 25 | 25 | 25 | 25 | 25 | 25 | 25 | |||||
| 10 | 25 | 25 | 25 | 25 | 25 | 25 | ||||||||
| tartrate | 1 | 0 | 25 | 25 | 0 | 0 | 25 | 0 | 0 | 25 | ||||
| 3 | 0 | 25 | 25 | 0 | 25 | 25 | 0 | 25 | 25 | |||||
The result of elemental analysis by XRF after the replacement reaction test at pH = 4–10 for 24 h is shown in Fig. 4. For the tartrate bath of r = 1, the surface color changed to reddish, and the Cu content was more than 10 at% at pH = 6. The content decreased with an increase in pH. At r = 3, Cu content was lower than that at r = 1 for all pH. For citrate plating bath, the copper contents were lower than those for tartrate baths of r = 1 and 3. The content and pH dependence were almost identical at r = 3 and 10. At pH 10, it was small for all conditions except tartrate complex bath at r = 1. Mizuki et al. reported that no displacement reaction occurred for 0.1 mol dm−3 CuSO4 solution with 0.2 and 0.06 mol dm−3 of EDTA and glycine, respectively. Although the type of the complexing agent is different, the effect of complexing agent addition and pH to the displacement reaction is similar to the present work.
Result of surface analysis of steel specimens dipped in complex Cu electroplating baths containing tartrate or citrate as ligand with various concentration ratio of ligand to total Cu ion, r and pH. (Online version in color.)
Tanabe et al. reported that the displacement reaction took place when pure iron was dipped in an acidic copper ammine complex solution. A compact Cu film was well attached to Fe within 3 s, and then the adherence became weak for more prolonged immersion because of the formation of porous and needle-like Cu deposits.2) Ogata has found an adherent Cu layer can form on Fe by the displacement reaction if the solution pH is lower than 2.5 and Cu ion concentration is smaller than 0.03 mol dm−3.3)
These two examples indicate that the adherent Cu layer is available by controlling bath and process conditions. However, these requirements are not suitable for applying to the industry.
Cu ion forms 2:2 complex with citric acid in alkaline solutions, as shown in Fig. 3.17) Therefore, the baths with r larger than 2 contain excess citrate ions, which can form soluble iron-citrate complexes. A reason for non-adherent Cu layer formation by the displacement reaction is the precipitation of iron sulfate or iron hydroxide. These compounds precipitate in the porous Cu layer formed and may decrease the adherence. The baths with high r values may avoid the precipitation, and consequently improve adherence of Cu electroplating.
It is still unknown that the reason for slower displacement reaction in citrate bath compared with that in tartrate bats. However, it can be summarized that the citrate baths with r = 5 and more at pH = 9–11 are preferable to obtain an adherent Cu electroplating layer on the low carbon steel.
3.3. Electrochemical Measurement with Citrate BathCyclic voltammogram of steel specimen in 0.1 mol dm−3 CuSO4 - 0.5 mol dm−3 citric acid solution (r = 5) is shown in Fig. 5. The potential was set at the immersion potentials for each test, and it was swept toward the negative direction. At pH = 9, the reduction current appeared at about the electrode potential E = −0.8 V vs. SHE. It increased by decreasing the electrode potential. Although the polarization curve was smooth until the potential reached to E = −1 V vs. SHE, the curve became noisy in more negative potentials. Gas bubbles formed on the cathode at these potentials, and the gas must be hydrogen. Alternatively, no hydrogen reduction might occur above E = −1 V vs. SHE. The onset potential of the cathodic current in pH = 11 solution was similar to that at pH = 9. The cathodic current increased with decreasing potential and showed a small peak around E = −0.8 V vs. SHE, and then the current became flat. The current increased again below E = −1.2 V vs. SHE. The curves were smooth throughout the sweep. At pH = 10, the shape of the cathodic polarization curve was similar to that found at pH = 9 for the first scan. However, a decrease in cathodic current occurred around E = −1.1 V vs. SHE, and the curve became smooth in the second scan. The cathodic current in the third scan was smaller than that of the first scan, and the noise on the curve was low in the entire potential range. The polarization curves at pH = 9 were different from those obtained at pH = 11.
Cyclic voltammogram of steel in 0.1 mol dm−3 CuSO4 - 0.5 mol dm−3 citric acid solution (r = 5) at pH = 9, 10, and 11 in ambient temperature with the sweep rate of 1 mV s−1. (Online version in color.)
The reduction of Cu ion and proton competes in solution 1. The activity of Cu ion must be the same at all pH because the types of the Cu complex is the same for all the solutions. However, proton activity at pH = 9 is two orders of magnitude higher than at pH = 11. Therefore, Cu ion preferentially reduced to form the Cu layer quickly at pH = 11. The smooth curve with low cathodic current observed at pH = 11 may reflect the electrochemical reduction of Cu ion on Cu covered steel surface.
On the contrary, the noisy curve with the high current at pH 9 may be caused by H2 gas formation of the steel surface. At pH = 10, the polarization curve changed from pH 9 type to pH 11 type during the second scan. The reason for the transition may be the difference in the exchange current density of H2 evolution on Cu and Fe surface. Kita reviewed that the exchange current density for the hydrogen evolution on a variety of metals.18) The exchange current density on the Cu electrode is at least one order of magnitude smaller than that on the Fe electrode. With an increase in surface coverage of the Cu, the rate of H2 evolution reaction becomes small. Finally, total cathodic current decreases from pH 9 type to pH 11 type. Hydrogen gas formed on the steel surface can diffuse into the steel substrate leading to hydrogen embrittlement. Therefore, it is concluded from this experiment that a high solution pH = 11 is favorable.
3.4. Constant Current ElectrodepositionA δ = 5 μm thickness Cu electroposition test was done with a constant current density. A uniform and smooth electroplating was obtained from the bath of r = 5 at pH = 9–11 with ic = 0.5–1 A dm−2. At 2 A dm−2, a part of the electroplating was burnt. Figure 6 shows SEM microstructures of Cu deposition. Burnt deposits were composed of fine grains, and there were spaces between the grains. The uniform electroplating parts with large and fine grains and grains contacted each other. At ic = 0.5 A dm−2, grooves in which separate grains were observed, particularly at pH = 10 and 11. The relation between current efficiency and the solution pH is shown in Fig. 7. Cathodic current efficiency ηc was almost 100% independent of current density and the solution pH. Anodic current efficiency ηa was 100% or slightly higher than 100% because of the remained water within the pore of the anode formed during the anode reaction. The effect of pH and current density on ηa is not apparent.
Surface appearance of Cu electroplating from citrate bath (r = 5) at pH = 9, 10, and 11 at 40°C. Plating thickness was 5 μm.
Cathodic and anodic current efficiency of Cu electroplating from citrate bath (r = 5) at pH = 9, 10, and 11 at 40°C. Plating thickness was 5 μm. (Online version in color.)
The density of pinholes np in the electroplating of δ = 5 μm was measured and plotted against cathodic current density in Fig. 8. The density is small at 1–1.5 A dm−2 for all pH where dense electroplating was observed in Fig. 6. The number was large at lower or higher current densities, at which Cu grains were separated from each other. The density is high at high pH and low at pH = 9.
Number of pinholes in unit area of Cu electroplating from citrate bath (r = 5) at pH = 9, 10, and 11 at 40°C. Plating thickness was 5 μm. (Online version in color.)
It was found that the maximum current density for Cu deposition from the citrate complex bath is small as 1.5 A dm−2. On the other hand, the current density available for sulfate Cu deposition is higher at least one order of magnitude than the citrate Cu electroplating.3) Effective additives to obtain smooth electroplating and high-throwing power have been found for sulfate Cu electroplating baths. Therefore, it is rational to use the citrate Cu bath as a Cu strike bath. If there are a large number of pinholes within the citrate Cu plating layer, the dissolution of iron from substrate steel is expected. It is important to estimate the minimum thickness of citrate Cu electroplating (undercoat) and sulfate Cu electroplating (overcoat). The undercoat was obtained with a citrate bath at pH = 9, r = 5, and ic = 1 A dm−2.
Figure 9 shows the surface appearance of Cu electroplated steel specimens. Scratches found on the undercoat disappeared after the overcoat. The size of Cu grains observed on specimens with δO = 10 μm overcoat layer increased with the thickness of the undercoat layer.
Surface microstructure of Cu electroplated steel specimen from citrate bath at pH 9, r = 5, and ic = 1A dm−2 (upper), and appearance of a 10 μm Cu overcoat from an acidic sulfate Cu plating bath on the citrate Cu plating (bottom).
The effect of the undercoat thickness on the number of pinholes in the undercoat and the two-layered specimens is shown in Fig. 10. Without Cu overcoat, there were more than 150 pinholes to δU = 0.25 μm Cu undercoat layer. The number decreased with the thickness of Cu δU to 1 μm; then it reached zero at δU = 5 μm. For two-layered Cu plating, the number of pinholes at δU = 0.25–0.5 μm was about half of the undercoat only, and it reached zero at δU = 1 μm. These results indicate that most of the pinholes in the undercoat could be filled by the overcoat layer, and δU = 1 μm of the undercoat layer is enough as a strike plating for pinhole-free δO = 10 μm Cu electroplating with acidic sulfate bath.
Effect of the thickness of citrate Cu plating on the pinhole density of Cu plating from citrate bath at pH 9, r = 5, and ic = 1A dm−2 (under coat) with and without a 10 μm Cu plating from a sulfate bath (overcoat). (Online version in color.)
Surface appearances of 180° bent Cu plating are shown in Fig. 11. Cu undercoat layer of δU = 0.25–5 μm was electroplated onto a 0.3 mm thick steel plate, and then a δO = 10 μm Cu was overcoated. Although cracks normal to the stretched direction were found, the Cu layers adhered to substrates. The opening of the Cu layer was wide for the thicker undercoat. For δU = 5 μm undercoat specimen, Cu electroplating remained under the opening, meaning that the crack is limited in the overcoat layer.
Appearance of 180° bent Cu electroplating on 0.3 mm thick steel plate. Electroplating layers consist of 0.25–5 μm undercoat and 10 μm overcoat. The former was electroplated with citrate bath at pH 9, r = 5, and ic = 1A dm−2, and the latter with a sulfate bath.
Figure 12 shows the macro- and micro-morphologies of two-layered Cu plated steel after linear elongation test. Independent of the thickness of the undercoat layer, electroplating layers well adhered to substrates at points A and C. Deformation of the Cu grains at these points were not significant. In the vicinity of broken ends B and D, deformation of Cu grains was observed, and breaks of Cu electroplating layer were observed. The break seemed to happen at the grain boundary of Cu at point D, where an undercoat Cu layer was visible through the crack. Elongation of specimens was 24 and 19% for upper and bottom cases in Fig. 12. It was reported that the elongation of sulfate Cu electroplating is about 20%, and the break of this layer is reasonable.
Surface appearances of Cu electroplating on 0.3 mm thick steel plate. Electroplating layers consist of 0.25 and 5 μm undercoat and 10 μm overcoat. The former was electroplated with citrate bath at pH 9, r = 5, and ic = 1A dm−2, and the latter with a sulfate bath. Elongation of specimens were 24 and 19% for upper and bottom cases. The direction of elongation for micrograph A–D is up to down. (Online version in color.)
There is no regulation on the elongation of the SPCC steel sheet. The minimum elongation of SPCCT steel, which has a similar composition to SPCC steel is 28% for 0.3 mm thickness sheet (JIS G3141). The elongation obtained in this experiment was slightly smaller than that of SPCCT steel. Elongation of oxygen-free copper sheet (JIS C1020) was 49.5% in this experiment. The elongation of Cu electroplating in this experiment is about half of that of the C1020.
The elongation of removed Cu foils after electrodeposition was reported. Mizuki et al. found the elongation of the foils of 10 μm thickness from copper sulfate bath was less than 10%.18) The addition of ethylenediamine, glycine, and thioglycolic acid improved the elongation to 20%.19,20) Takahashi reviewed the elongation of electroformed Cu foils. Annealed Cu foil of 20 μm thickness was 20%.21) It is summarized that the two-layered copper electroplating on SPCC steel was found to show the elongation comparable to electroformed Cu foils reported in literatures.
The electroplating of copper on low carbon steel from alkaline citrate complex baths was studied, and the following conclusion was obtained.
Citric acid was found to be an effective additive among a series of additives tested. Citric acid dissociates to form copper complexion at an alkaline solution. The addition improved the displacement deposition of Cu, and the current density range to obtain smooth Cu plating was wide.
The optimum condition of the Cu electroplating bath composition was found. 0.1 mol dm−3 CuSO4, 0.5 mol dm−3 citric acid (C6H8O7) at pH = 10. Because the limiting current density with this bath is smaller than that of acidic Cu electroplating bath, it is reasonable to deposit a thin Cu layer with an alkaline citrate bath, then deposit thick Cu with conventional 0.75 mol dm−3 CuSO4 and 0.3 mol dm−3 H2SO4. (two-layer Cu electroplating).
Two-layer Cu electroplating contained a small number of pinholes and showed good adhesion and good elongation similar to commercial Cu foils.
