Electroplating of Copper on Low Carbon Steel from Alkaline Citrate Complex Baths - J-Stage
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DOI: 10.2355/isijinternational.ISIJINT-2019-747
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Electroplating of Copper on Low Carbon Steel from Alkaline
Citrate Complex Baths
Isao SAEKI,1)* Takuma HARADA,2) Isamu TANAKA,1) Tetsuya ANDO,1) Lu GAN3,4) and Hideyuki MURAKAMI3,4)
1) Division of Materials Science and Engineering, Graduate School of Engineering, Muroran Institute of Technology, 27-1
Mizumoto, Muroran-shi, 050-8585 Japan.
2) Department of Materials Science and Engineering, Muroran Institute of Technology, 27-1 Mizumoto, Muroran-shi, 050-8585
Japan.
3) School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555 Japan.
4) Research Center for Structural Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba-shi, 305-0047 Japan.
(Received on November 20, 2019; accepted on February 20, 2020; J-STAGE Advance published date:
May 18, 2020)
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 complex-
ing 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.
KEY WORDS: Cu electroplating; complex bath; steel sheet.
trol are required leading to an increase in plating costs.5) For
1. Introduction
these reasons, the copper electroplating baths without cyanide
Copper plating on steels is used to form Cu-plated steel ion is needed in industries.
tubes, an intermediated layer beneath Ni or Cr electroplat- Cu electroplating baths containing copper pyrophosphate
ing for the enhancement of adherence, or partial diffusion are used for electroplating on steel. The bath is non-toxic,
barrier of C during the carburization of steels. Because the but careful wastewater treatment is also needed because the
standard electrode potential of Cu2+/Cu couple is nobler than emission of phosphor to river water is strictly restricted.
that of Fe2+/Fe, a displacement reaction between Fe and Cu2+ Pyrophosphoric acid decomposes with time to form ortho-
occurs to form Fe2+ and Cu at pH = 4–7 where Fe actively phosphoric acid. It gives rise to poor adhesion and surface
dissolves.1) The Cu deposit formed in this way gives a porous quality. Because the selective removal of orthophosphoric
and non-adherent copper layer on iron and steels.2,3) There- acid from the bath is difficult, electroplating bath must be
fore, baths at pH>7 are necessary for adherent Cu electrode- replaced after a finite number of the orthophosphate accu-
position. Copper cyanide - potassium cyanide solution at pH mulated in the bath.6)
10 is used to form an adherent Cu layer on steel substrates Carboxyl acids release their protons to the solution with
without the replacement reaction. It is mainly for two reasons. increasing pH resulting lone-pair of electrons within the
Firstly, copper forms a stable soluble complex with cyanide molecules. Metal ions in the solution form complex with
ions at pH 10. Secondary, the replacement reaction is slow the proton-dissociated carboxyl acids then dissolve in the
at this pH.4) Electroplating bath containing cyanide ion is solution at pH where metal hydroxides precipitate.7,8) If the
toxic. It can have a severe effect on human and animal health. stability constant of the soluble complexion is not so high,
Therefore, a strict wastewater treatment and air pollution con- the electrodeposition of metal complexion will be possible.
Hosokawa et al. studied the electrodeposition of Cu from
* Corresponding author: E-mail: isaos@mmm.muroran-it.ac.jp solutions containing some carboxylic acids. They found that
DOI: https://doi.org/10.2355/isijinternational.ISIJINT-2019-747 Cu electroplating is possible if citric acid, lactic acid, and
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tartaric acid were used as complexing agents.9,10) Cu plate was used as an anode except for the polarization
However, a more detailed study on the type of complex- test. A magnet bar was used to stir baths for all the tests.
ing agent, their concentration and the solution pH, and on
the effect of current density is needed to establish a Cu 2.4. Evaluation of Electroplating
plating method on steels. Moreover, the evaluation of the Cathodic current density ic at A dm − 2 of the Hull-cell test
adhesion of the Cu plating layer to substrates was unclear was calculated by Eq. (1).13)
in the reports.
ic I c (5.10 5.24 logL ) ..................... (1)
Therefore in this study, the authors (1) surveyed a series
of carboxylic acids to determine which can be used for Cu Here, Ic is the total cathodic current for the test (A), L is
electroplating on low carbon steel sheet, then (2) optimized the distance from the right side (high current side) of the
plating bath composition and operation conditions for citric Hull-cell test specimen (cm).12) Upper and lower limits of
and tartaric acids complexed bath, and finally (3) tested the current density (iUL, iLL) for Cu electroplating was estimated
adherence of layered Cu electroplating with citrate com- by the observation of Hull-cell test plates. The microstructure
plexed bath and acid copper sulfate bath. of the plating was observed by a scanning electron micro-
scope (SEM, JEOL LV-6110). The number of pinholes in Cu
electroplating was evaluated by ferroxyl test according to JIS
2. Experimental Procedure
H8617.14,15) Adhesion of Cu layer was assessed by cross-cut
2.1. Specimen test following JIS K5600-5-6 code.16) Also, a 180° bending
A cold-rolled steel sheet of 0.3 mm thickness containing and elongation tests were employed to visualize adhesion and
0.045 mass% carbon was used as the substrate for tests. It ductility of the electroplating. Displacement reaction between
was abraded with a #600 grade SiC paper in distilled water Cu ion in electroplating baths and Fe substrate was tested by
before tests. Dimensions of the specimen were 100 × 50, immersion of the substrate in the baths for 24 hrs. Cu content
20 × 50, 3 × 3 mm2 for the Hull-Cell test, constant current in the surface layer after the immersion test was evaluated
electroplating, and polarization test, respectively. For an with an X-ray fluorescent analyzer (XRF, JEOL JSM-3202).
elongation test, cold stripped steel sheets (JIS G3141, t = 0.3
mm) were machined into 13B test pieces (JIS Z2201).11,12)
3. Results and Discussion
The elongation test of oxygen-free copper (JIS C1020) was
also made using a 14B test piece for comparison. 3.1. Survey of Electroplating Baths and Result of
Hull-cell Test
2.2. Electroplating Bath Electroplating of Cu was impossible, or the range of avail-
Cu complex electroplating baths were composed of able current density was very narrow when complexing agents
0.1 mol dm − 3 CuSO4 and a series of complexing agents except citric and tartaric acids were used. Figure 1 shows the
at 0.1–1 mol dm − 3. The agents used were EDTA-2Na upper limit (iUL) and the lower limit (iLL) current densities to
(Na2C10H14N2O8), alanine (C3H7NO2), citric acid (C6H8O7), obtain smooth electroplating with citric and tartaric acids as a
glycine (C2H5NO2), tartaric acid (C4H6O6), malonic acid function of solution pH. The value of r in the figure indicates
(C3H4O4), aspartic acid (C4H7NO4), ascorbic acid (C6H8O6), the mole ratio of the complexing agent (ligand) to total Cu
glutamic acid (C5H9NO4), or lactic acid (C3H6O3). Concen- ion in the solution. For tartaric acid addition at r = 3, the iUL
trated H2SO4 and 5 mol dm − 3 NaOH solution was used to was about 2 A dm−2 at pH = 7, and it slightly increased with
adjust pH of baths. A Cu sulfate plating bath containing 0.75 increasing pH. No electroplating was available below pH = 7
mol dm − 3 CuSO4 and 0.3 mol dm − 3 H2SO4 was also used. for this solution. The shape of the curve at r = 1 was almost
The solution condition was summarized in Table 1. the same. For citric acid at r = 1 and pH = 5, no electroplating
2.3. Electrolysis
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.
Table 1. Composition of Cu electroplating baths.
reagent concentration/mol dm − 3
bath complex bath acid bath
CuSO4 0.1 0.75
H 2SO4 0.3 Fig. 1. Effect of solution pH on upper limit (iUL) and lower limit
(iLL) current densities to obtain smooth electroplating with
complexing agent 0.1–1.0 citrate and tartrate complex baths. Value r indicates the
mole ratio of complexing agent to total Cu ion. (Online
pH 4–11
version in color.)
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layer was obtained below i = 0.8 A dm−2. Burnt electroplat- trodeposition of Cu from Cu2L2H − 2 type complex is likely
ing was obtained above i = 2.8 A dm−2. Values of iUL and iLL to occur. For the bath containing tartaric acid, Cu2L2 type
decreased with pH at r = 1. When r = 3, the iUL was about complex is stable at pH range 1–6 where no Cu electroplat-
0.5 A dm−2 at pH = 5, and it increased with increasing pH. ing was obtained. Soluble Cu2L2 is seemed to be inactive to
Electroplating was possible at a very low current density with the electrochemical reduction.
this solution. Although the shape of the curve for r = 10 was The result of the adhesion test is summarized in Table 2.
similar to that of r = 3, iUL value was small. Grades 25 and 0 mean complete and no adhesion of Cu elec-
The effect of ligand type to total Cu ion concentration on troplating. For the tartrate complex bath, the electroplating
iUL and iLL is shown in Fig. 2. The iUL for the citric acid- layer well attached to the substrate at pH ≥ 7, ic ≥ 5 A dm − 2,
containing bath was large at low r value for all pH, and it and r ≤3. Electroplating at the cathodic current density lower
decreased with increasing r value. Despite Cu deposition than ic < 5 A dm − 2 resulted in poor adherence. In the case
below iLL was impossible at r = 1, lower current density of citric acid, adhesion was satisfactory at r > 3 and pH ≥ 9,
side of Hull-cell test panel was covered with Cu at r higher and it was independent of cathodic current density. From
than 3. The iUL values for tartaric acid at r = 1 and 3 were these comparisons, it can be said that the use of citric acid
similar to those for citric acid at pH 9 and 11, whereas the is better than the use of tartaric acid to obtain an adherent
value was small at pH 7. Cu plating was possible at low pH Cu electroplating on steel strips.
independent of r for this ligand.
Figure 3 shows the effect of pH on the fraction of Cu 3.2. Displacement Reaction between Iron and Copper
complex in Cu plating baths. Here, L assigns ligand. Cu2 + Ion
means aquo -complex, CuLH 1:1 complex with excess The result of elemental analysis by XRF after the replace-
proton, Cu2L2 2:2 complex, Cu2L2H − 1 and Cu2L2H − 2 2:2 ment reaction test at pH = 4–10 for 24 h is shown in Fig.
complexes dissociating one and two protons respectively.7,8) 4. For the tartrate bath of r = 1, the surface color changed
For both ligands, Cu2L2H − 2 type complex is predominant at to reddish, and the Cu content was more than 10 at% at
pH > 7. By comparing the experimental results in Fig. 2 and pH = 6. The content decreased with an increase in pH. At
the calculated curves in Fig. 3, it is indicated that the elec- r = 3, Cu content was lower than that at r = 1 for all pH.
Fig. 2. Effect of solution the ratio of the concentration of ligand to Fig. 3. Fraction of Cu complexes with pH for citrate and tartrate
total Cu, r, on upper limit (iUL) and lower limit (iLL) current solutions. Cu 2 + means Cu aquo complex, CuLH 1:1 com-
densities to obtain smooth electroplating with citrate and plex with excess proton, Cu 2L2 2:2 complex, Cu 2L2H −1
tartrate complex baths. (Online version in color.) and Cu 2L2H − 2 2:2 complexes dissociating one and two
protons respectively. (Online version in color.)
Table 2. Summary of cross-cut adhesion test (JIS K5600-5-6). Grade 25 means that there is no peel-off of Cu electro-
plating in 25 sections of 1 × 1 mm 2 square after the test.
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
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
citrate
ratio 6 25 25 25 25 25 25 25 25 25
[ligand]/[Cu]T 7 25 25 25 25 25 25 25 25 25
10 25 25 25 25 25 25
1 0 25 25 0 0 25 0 0 25
tartrate
3 0 25 25 0 25 25 0 25 25
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For citrate plating bath, the copper contents were lower than in Fig. 5. The potential was set at the immersion potentials
those for tartrate baths of r = 1 and 3. The content and pH for each test, and it was swept toward the negative direc-
dependence were almost identical at r = 3 and 10. At pH tion. At pH = 9, the reduction current appeared at about the
10, it was small for all conditions except tartrate complex electrode potential E = − 0.8 V vs. SHE. It increased by
bath at r = 1. Mizuki et al. reported that no displacement decreasing the electrode potential. Although the polarization
reaction occurred for 0.1 mol dm − 3 CuSO4 solution with curve was smooth until the potential reached to E = − 1 V
0.2 and 0.06 mol dm − 3 of EDTA and glycine, respectively. vs. SHE, the curve became noisy in more negative poten-
Although the type of the complexing agent is different, the tials. Gas bubbles formed on the cathode at these potentials,
effect of complexing agent addition and pH to the displace- and the gas must be hydrogen. Alternatively, no hydrogen
ment reaction is similar to the present work. reduction might occur above E = − 1 V vs. SHE. The onset
Tanabe et al. reported that the displacement reaction potential of the cathodic current in pH = 11 solution was
took place when pure iron was dipped in an acidic copper similar to that at pH = 9. The cathodic current increased
ammine complex solution. A compact Cu film was well with decreasing potential and showed a small peak around
attached to Fe within 3 s, and then the adherence became E = − 0.8 V vs. SHE, and then the current became flat. The
weak for more prolonged immersion because of the forma- current increased again below E = − 1.2 V vs. SHE. The
tion of porous and needle-like Cu deposits.2) Ogata has curves were smooth throughout the sweep. At pH = 10, the
found an adherent Cu layer can form on Fe by the displace- shape of the cathodic polarization curve was similar to that
ment reaction if the solution pH is lower than 2.5 and Cu found at pH = 9 for the first scan. However, a decrease in
ion concentration is smaller than 0.03 mol dm − 3.3) cathodic current occurred around E = − 1.1 V vs. SHE, and
These two examples indicate that the adherent Cu layer the curve became smooth in the second scan. The cathodic
is available by controlling bath and process conditions. current in the third scan was smaller than that of the first
However, these requirements are not suitable for applying scan, and the noise on the curve was low in the entire poten-
to the industry. tial range. The polarization curves at pH = 9 were different
Cu ion forms 2:2 complex with citric acid in alkaline from those obtained at pH = 11.
solutions, as shown in Fig. 3.17) Therefore, the baths with The reduction of Cu ion and proton competes in solu-
r larger than 2 contain excess citrate ions, which can form tion 1. The activity of Cu ion must be the same at all pH
soluble iron-citrate complexes. A reason for non-adherent because the types of the Cu complex is the same for all the
Cu layer formation by the displacement reaction is the solutions. However, proton activity at pH = 9 is two orders
precipitation of iron sulfate or iron hydroxide. These com- of magnitude higher than at pH = 11. Therefore, Cu ion
pounds precipitate in the porous Cu layer formed and may preferentially reduced to form the Cu layer quickly at pH =
decrease the adherence. The baths with high r values may 11. The smooth curve with low cathodic current observed
avoid the precipitation, and consequently improve adher- at pH = 11 may reflect the electrochemical reduction of Cu
ence of Cu electroplating. ion on Cu covered steel surface.
It is still unknown that the reason for slower displacement On the contrary, the noisy curve with the high current at
reaction in citrate bath compared with that in tartrate bats. pH 9 may be caused by H2 gas formation of the steel sur-
However, it can be summarized that the citrate baths with face. At pH = 10, the polarization curve changed from pH 9
r = 5 and more at pH = 9–11 are preferable to obtain an type to pH 11 type during the second scan. The reason for
adherent Cu electroplating layer on the low carbon steel. the transition may be the difference in the exchange current
density of H2 evolution on Cu and Fe surface. Kita reviewed
3.3. Electrochemical Measurement with Citrate Bath that the exchange current density for the hydrogen evolution
Cyclic voltammogram of steel specimen in 0.1 mol dm − 3 on a variety of metals.18) The exchange current density on the
CuSO4 - 0.5 mol dm − 3 citric acid solution (r = 5) is shown Cu electrode is at least one order of magnitude smaller than
Fig. 4. Result of surface analysis of steel specimens dipped in Fig. 5. Cyclic voltammogram of steel in 0.1 mol dm − 3 CuSO4 -
complex Cu electroplating baths containing tartrate or 0.5 mol dm − 3 citric acid solution (r = 5) at pH = 9, 10, and
citrate as ligand with various concentration ratio of ligand 11 in ambient temperature with the sweep rate of 1 mV s −1.
to total Cu ion, r and pH. (Online version in color.) (Online version in color.)
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that on the Fe electrode. With an increase in surface coverage been found for sulfate Cu electroplating baths. Therefore, it
of the Cu, the rate of H2 evolution reaction becomes small. is rational to use the citrate Cu bath as a Cu strike bath. If
Finally, total cathodic current decreases from pH 9 type to there are a large number of pinholes within the citrate Cu
pH 11 type. Hydrogen gas formed on the steel surface can plating layer, the dissolution of iron from substrate steel is
diffuse into the steel substrate leading to hydrogen embrittle- expected. It is important to estimate the minimum thick-
ment. Therefore, it is concluded from this experiment that a ness of citrate Cu electroplating (undercoat) and sulfate Cu
high solution pH = 11 is favorable.
3.4. Constant Current Electrodeposition
A δ = 5 μm thickness Cu electroposition test was done
with a constant current density. A uniform and smooth elec-
troplating 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 electro-
plating 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 solu-
Fig. 7. Cathodic and anodic current efficiency of Cu electroplat-
tion pH. Anodic current efficiency ηa was 100% or slightly
ing from citrate bath (r = 5) at pH = 9, 10, and 11 at 40°C.
higher than 100% because of the remained water within the Plating thickness was 5 μm. (Online version in color.)
pore of the anode formed during the anode reaction. The
effect of pH and current density on ηa is not apparent.
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.
3.5. A Two-layers Cu Electroplating with Citrate and
Sulfate Baths
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 magni-
Fig. 8. Number of pinholes in unit area of Cu electroplating from
tude than the citrate Cu electroplating.3) Effective additives to
citrate bath (r = 5) at pH = 9, 10, and 11 at 40°C. Plating
obtain smooth electroplating and high-throwing power have thickness was 5 μm. (Online version in color.)
Fig. 6. Surface appearance of Cu electroplating from citrate bath (r = 5) at pH = 9, 10, and 11 at 40°C. Plating thick-
ness was 5 μm.
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electroplating (overcoat). The undercoat was obtained with a Deformation of the Cu grains at these points were not sig-
citrate bath at pH = 9, r = 5, and ic = 1 A dm−2. nificant. In the vicinity of broken ends B and D, deformation
Figure 9 shows the surface appearance of Cu electro- of Cu grains was observed, and breaks of Cu electroplating
plated steel specimens. Scratches found on the undercoat layer were observed. The break seemed to happen at the
disappeared after the overcoat. The size of Cu grains grain boundary of Cu at point D, where an undercoat Cu
observed on specimens with δO = 10 μm overcoat layer layer was visible through the crack. Elongation of speci-
increased with the thickness of the undercoat layer. mens was 24 and 19% for upper and bottom cases in Fig. 12.
The effect of the undercoat thickness on the number of It was reported that the elongation of sulfate Cu electroplat-
pinholes in the undercoat and the two-layered specimens is ing is about 20%, and the break of this layer is reasonable.
shown in Fig. 10. Without Cu overcoat, there were more There is no regulation on the elongation of the SPCC steel
than 150 pinholes to δ U = 0.25 μm Cu undercoat layer. The sheet. The minimum elongation of SPCCT steel, which has
number decreased with the thickness of Cu δU to 1 μm; then a similar composition to SPCC steel is 28% for 0.3 mm
it reached zero at δ U = 5 μm. For two-layered Cu plating, the thickness sheet (JIS G3141). The elongation obtained in this
number of pinholes at δ U = 0.25–0.5 μm was about half of experiment was slightly smaller than that of SPCCT steel.
the undercoat only, and it reached zero at δ U = 1 μm. These Elongation of oxygen-free copper sheet (JIS C1020) was
results indicate that most of the pinholes in the undercoat
could be filled by the overcoat layer, and δ U = 1 μm of the
undercoat layer is enough as a strike plating for pinhole-free
δO = 10 μm Cu electroplating with acidic sulfate bath.
3.6. Elongation of Two-layered Cu Plating
Surface appearances of 180° bent Cu plating are shown
in Fig. 11. Cu undercoat layer of δ U = 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 δ U = 5 μm undercoat specimen, Cu
electroplating remained under the opening, meaning that
the crack is limited in the overcoat layer.
Figure 12 shows the macro- and micro-morphologies Fig. 10. Effect of the thickness of citrate Cu plating on the pinhole
density of Cu plating from citrate bath at pH 9, r = 5, and
of two-layered Cu plated steel after linear elongation test.
ic = 1A dm − 2 (under coat) with and without a 10 μm Cu
Independent of the thickness of the undercoat layer, electro- plating from a sulfate bath (overcoat). (Online version in
plating layers well adhered to substrates at points A and C. color.)
Fig. 9. 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).
Fig. 11. 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.
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Fig. 12. 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.)
49.5% in this experiment. The elongation of Cu electroplat-
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9) K. Hosokawa and K. Suematsu: J. Met. Finish. Soc. Jpn., 17 (1964),
4. Conclusion 404 (in Japanese). https://doi.org/10.4139/sfj1950.15.404
10) K. Hosokawa and T. Inui: J. Met. Finish. Soc. Jpn., 17 (1966), 138
The electroplating of copper on low carbon steel from (in Japanese). https://doi.org/10.4139/sfj1950.17.138
alkaline citrate complex baths was studied, and the follow- 11) JIS G 3141: 2017, Cold-reduced carbon steel sheet and strip (in
ing conclusion was obtained. Japanese).
12) JIS Z 2201: 2011, Test pieces for tensile test for metallic materials
Citric acid was found to be an effective additive among (in Japanese).
a series of additives tested. Citric acid dissociates to form 13) K. Koiwa and W. Yamamoto: J. Surf. Finish. Soc. Jpn., 63 (2012),
489 (in Japanese). https://doi.org/10.4139/sfj.63.489
copper complexion at an alkaline solution. The addition 14) T. Ashizawa: J. Met. Finish. Soc. Jpn., 9 (1958), 383 (in Japanese).
improved the displacement deposition of Cu, and the current https://doi.org/10.4139/sfj1950.9.383
15) JIS H 8617: 1999, Electroplated coatings of nickel and chromium (in
density range to obtain smooth Cu plating was wide. Japanese).
The optimum condition of the Cu electroplating bath 16) JIS K 5600-5-6: 1999, Testing methods for paints − Part 5: Mechani-
composition was found. 0.1 mol dm − 3 CuSO4, 0.5 mol cal property of film − Section 6: Adhesion test (Cross-cut test) (in
Japanese).
dm − 3 citric acid (C6H8O7) at pH = 10. Because the limiting 17) H. Kita: Nippon Kagaku Zassi, 92 (1971), 99 (in Japanese). https://
current density with this bath is smaller than that of acidic doi.org/10.1246/nikkashi1948.92.99
18) S. Mizumoto, H. Nawafune and M. Kawasaki: J. Met. Finish. Soc. Jpn.,
Cu electroplating bath, it is reasonable to deposit a thin Cu 35 (1984), 149 (in Japanese). https://doi.org/10.4139/sfj1950.35.149
layer with an alkaline citrate bath, then deposit thick Cu 19) S. Mizumoto, H. Nawafune, T. Kojima, M. Haga and T. Sonoda:
with conventional 0.75 mol dm − 3 CuSO4 and 0.3 mol dm − 3 J. Surf. Finish. Soc. Jpn., 41 (1990), 156 (in Japanese). https://doi.
org/10.4139/sfj.41.156
H2SO4. (two-layer Cu electroplating). 20) S. Mizumoto, H. Nawafune, T. Hiroo, Y. Zhang and M. Haga: J. Surf.
Two-layer Cu electroplating contained a small number of Finish. Soc. Jpn., 43 (1992), 978 (in Japanese). https://doi.org/10.4139/
sfj.43.978
pinholes and showed good adhesion and good elongation 21) N. Takahashi: J. Surf. Finish. Soc. Jpn., 49 (1998), 1272 (in
similar to commercial Cu foils. Japanese). https://doi.org/10.4139/sfj.49.1272
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