TECHNICALLY
speaking

trolyte salts [1]. When a diaphragmtype electrolytic apparatus is used forelectrolysis, anolyte (anodically electrolyzed water) and catholyte (cathodically electrolyzed water) are generated in the anode compartment andthe cathode compartment, respectively. Sodium chloride (NaCl) is
commonly used as an electrolyte forgenerating electrolyzed water. At theanode, oxygen (O2) and hydrogen ions(H+) are formed by the oxidation ofwater, as well as chlorine (Cl2) by oxidation of chloride ions (Cl-). Dissolved chlorine (Cl2 (aq)) consequently reacts with water (H2O) toyield hypochlorous acid (HOCl) andhydrochloric acid (HCl). As a result,anolyte exhibits an increased redox
potential and acidity and is also called
acidic electrolyzed water. Meanwhile,at the cathode, hydrogen (H2) andhydroxyl ions (HO-) are formed by the
reduction of water and dissolved oxygen. As a result, catholyte exhibits anincrease in dissolved hydrogen concentration, a decrease in dissolved
oxygen concentration, a remarkably
lowered redox potential and alkalinity, and can be referred to as alkaline
electrolyzed water. Chemical reactions occurring at this time are shownas equations (1)–(5). When sodium
sulfate (Na2SO4) is used as an electrolyte, Cl2 is not generated at the anode,
resulting in an elevated concentrationof dissolved oxygen in the anodewater.

BY TOSHIKAZU TAKENOUCHI, SHINKO ELECTRIC INDUSTRIES CO., LTD.

Characteristics of AnodicallyElectrolyzed Water —Dissolution Behavior of Copperor Copper Oxide Filmdissolved oxygen / ×10-4 mol dm-3

nodically electrolyzed dilute
aqueous solution of NaCl (NaClAEW), as well as Na2SO4 (Na2SO4AEW), shows high redox potential
and dissolved oxygen concentration,with additional generation of freechlorine for NaCl-AEW. The objectiveof this study is to evaluate the characteristics of both AEWs and to compare the dissolution rates of oxygenfree copper, which is immersed in
AEW, HCl or H2SO4 solution at thesame pH (=2.48). The result showedthat the dissolution rate of oxygenfree copper immersed in NaCl-AEW
was 4.2 times as high as that in HCl
solution, and that in Na2SO4-AEW
was 3.0 times as high as that in H2SO4
solution. The explanation is that freechlorine in NaCl-AEW accelerates thedissolution rate and increases the surface roughness of copper, and dissolved oxygen in Na2SO4-AEW accelerates copper dissolution.
Additionally, when copper withthermal-oxide film was immersed in
NaCl-AEW or Na2SO4-AEW, copper
thermal-oxide film, as well as theunderlying copper, was dissolved toform a natural-oxide film on its surface, which was similarly formed
when copper with thermal-oxide film
was immersed in HCl or H2SO4 solution. To conclude, AEW has the possibility to apply to etching or the pretreatment of copper as an alternativeto HCl or H2SO4 solution.
Electrolyzed water is a generic term
for solutions obtained by electrolyzing diluted aqueous solutions of elec-

Do and free chlorine
20time/mintime/sacidic electrolyzed
Na2SOwaterelectrolyzed
chlorine water
4 acidicfree
18
0
0
4,7
6,25
1,1
1
60
4,1
5,62
1,06
16
▲ free chlorine
2
120
3,8
5,31
1,02
14
5
300
3,65
5
0,99
15
900
3,56
4,84
0,958
12
30
1800
3,5
4,68
0,916
Dissolved
10452700 oxygen; 3,43
4,53
0,873
● Na2SO4-AEW
60
3600
3,37
4,37
0,845
8
○ NaCl-AEW
90
5400
3,28
4,21
0,805
6
120
7200
3,21
4,06
0,775
180
10800
3,12
3,9
0,72

1,2
1
0,8
0,6
0,4

4

0,2

2
0

0
0

2

4

time / ×103 s

6

8

10

Fig. 1 Time course of dissolved oxygen and free chlorine concentrations after the end ofelectrolysis.
Fig. 1 Time course of dissolved oxygen and free chlorine concentrations after the end of electrolysis.

20 | metalfinishing | November/December 2013

free chlorine / x 10-3 mol dm-3

A[Anode]H2O  1/2O2 + 2H+ + 2e• • • (1)• • • (2)2Cl-  Cl2 + 2eCl2 (aq) + H2O  HCl + HOCl • • • (3)[Cathode]
2H2O + 2e-  H2 + 2OH1/2O2 + H2O + 2e- 2OH-

• • • (4)• • • (5)The features of electrolyzed water
are its ability to generate thermodynamically unstable substances in aqueous solutions by electrolysis and toensure work safety during the generation of acidic and alkaline solutions bymeans of electrochemical procedures
without using hazardous acids andbases. Because anodically electrolyzed
water (AEW) has a pH value of aroundwww.metalfinishing.com

100

C12

80
Ratio / m ol %TECHNICALLY
speaking

60

Dissolution rate / g m-2 h-1

HClO
C1O2.5 and a high redox potential, it is40
contemplated that the dissolution
reaction of metals proceeds faster in
20
AEW than in HCl or sulfuric acid
(H2SO4) solution at the same pH.
0
Oxygen-free copper, which is usedas material for precision electronic
2
2
3
3
4
4
5
5
6
6
7
7
9
9
12
12
8
8
10
10
11
11
components, forms oxide film on thepHsurface by heat treatment and naturaloxidation. When oxygen-free copper
Fig.2 dependence of molar concentration ratios Cl2, HOCl, and OCl- on pH.
is plated, HCl or H2SO4 solution isused for removal of oxide film, scale
1,2
and rust on the surface [2]. It is also
Fig.2 dependenceof molar concentration ratios Cl2, HOCl, and OCl- on pH.
used in other processes such as etching. However, because HCl and H2SO4
1
solution are hazardous substances,
the use of AEW alternative to these0,8solutions has a great advantage overwork safety and environment.
0,6
In this study, first the properties of
AEW containing sodium chloride
0,4(NaCl-AEW) or sodium sulfate(Na2SO4-AEW) as an electrolyte were
measured. Next the dissolution rates
0,2
and surface morphology change ofoxygen-free copper immersed in0
AEWs were compared with those inA
BCD
HCl or H2SO4 solution at the same
pH, and the factors affecting the disFig. 3 Dissolution rate of oxygen-free copper immersed in NaCl-AEW (A), Na2SO4-AEW
solution rate was discussed. Further(B), HClrateof pH
2.48 (C), orcopper
H2SO4immersedof pH 2.48
(D).
Fig. 3 Dissolutionof oxygen-freein NaCl-AEW(A), Na2SO4-AEW (B), HCl of pH 2.48
more, the surface observation and(C), or H2SO4 of pH 2.48 (D).
analysis of copper with thermal-oxide
film immersed in those solutions
A
B
were performed when both its thermal-oxide film and underlaying copper were simultaneously dissolved, as
well as determination of the dissolution loss of thermal-oxide film. Basedon these results, the potential of AEW
as alternative to HCl or H2SO4 solution in etching and the pretreatment
10 μm
10 μm
of oxygen-free copper was discussed.

2. EXPERIMENTAL

Fig. 4 SEM images of surface of oxygen-free copper which was immersed for 900 s in
NaCl-AEW at a free chlorine concentration of 1.6×10-3 mol/dm3 (A) or 3.9×10-3 mol/dm3 (B).

2.1 Materials and Instrumentation
Oxygen-free copper plates (50 x 50 x
lytes for preparing electrolyzed water.
at pH7.4 was used. A diaphragm-type
0.20 mm, Cu: 99.9 wt%; Hakudo Co.
HCl, H2SO4, and sodium hydroxide
electrolytic apparatus (JED-007; Altec
Ltd., Tokyo, Japan) were used
testimages(NaOH)
solutions(analyticalreagentCo., Ltd.,Tokyo,
Japan) wasfor concentr
Fig. as
4 SEMof surfaceof oxygen-freecopper which
was immersed
for 900
s in NaCl-AEW
at a used
free chlorine3
3 (A)0.1
specimens. NaCl and anhydrous
grade;; Wako3 (B).
Pure Chempreparing the electrolyzed water. Aof 1.6×10
1 6×10-3 mol/dm
or 3.9×10
3mol/dm
9×10-3 mol/dm(B)
Na2SO4 (both analytical reagent grade;
pH meter (Model F-22; Horiba Ltd.,
ical Industries, Ltd.) were used as acids
Wako Pure Chemical Industries Ltd.,and a base. Deionized water having an
Kyoto, Japan), an ORP meter (TRXOsaka, Japan) were used as electroelectrical conductivity of 10 MΩ∙cm,
90; Toko Kagaku Co. Ltd., Tokyo,
www.metalfinishing.com

November/December 2013 | metalfinishing | 21

TECHNICALLY
speaking

Corrosion rate / g m-2 h-1

2,5
2
1,5

2.2. Preparation of AEW
An aqueous solution of 1.0 x 10-2 mol/dm3 NaCl or Na2SO4 was used to prepare AEW. The electrolytic apparatus
comprised an anode tank (0.7 dm3)
and a cathode tank (0.7 dm3), whichwere separated by a diaphragm. Platinum-coated titanium plates of 74
mm x 113 mm were used as the electrodes. The electrolyzing conditions
were at 0.6 A and 12 V for 15 min.
2.3 Determination of the dissolution
rate of copper
Specimens were degreased with acetone
and ethanol. Moreover the specimen

Dissolution rate/g m-2 h-1

Surface roughness, Ra / µm

Japan), and a dissolved oxygen meter(MO128; Mettler Toledo Interna1
tional Inc., Columbus, OH) were used
for measuring the pH, redox poten0,5
tial, and dissolved oxygen concentration of AEW, respectively. A residual
0
chlorine analyzer (PC compact MT;
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
AquaLytic Co. Ltd., Dortmund, GerFree chlorine / ×10-3 mol dm-3many) was used for measuring freeFig. 5 Effects of free chlorine concentration on the dissolution rate of oxygen-free copper.
chlorine concentrations. Free chlorineis the total amount of Cl2 (aq), HOCl Fig. 5 Effects of free chlorine concentration on the dissolution rate of oxygen-free copper.
and hypochlorous ions (ClO-). The
0,8principle of residual chlorine measurement is a colorimetric method bypotassium iodine. An inductively0,6
coupled plasma (ICP) emission spectrometer (iCAP 6000; Thermo Fisher
0,4
Scientific, Waltham, MA) was used forthe quantification of copper. A scanning electron microscope (SEM,
0,2S-4300SE; Hitachi Ltd., Tokyo, Japan)
was used for surface observations. A
surface roughness meter (Surfcom
0480A; Tokyo Seimitsu Co. Ltd., Tokyo,
0
1
2
3
4
Japan) was used for the measurement
Free chlorine / ×10-3 mol dm-3
of surface roughness of the samples.
An Auger electron spectroscopy anaFig. 6 Effects of free chlorine concentration on the surface roughness of oxygen-free copper.
lyzer (Model 680; Physical Electronics
Fig. 6 Effects of free chlorine concentration on the surface roughness of oxygen-free copper.
Inc., Chanhassen, MN) was used for
conducting a surface analysis of the
1,4
specimen. A focused ion beam (FIB)
machining and observation instru1,2
ment (FB-210; Hitachi Ltd.) and a1scanning ion microscope (SIM) wereused for the processing and observa0,8tion of the cross section, respectively.
0,6
0,4
0,2
0
1

2

3

4

5

6

7

8

9

10

11

pH

Fig.7 Effect of pH of NaCl-AEW on the dissolution rate of oxygen-free copper.

Fig.7 Effect of pH of NaCl-AEW on the dissolution rate of oxygen-free copper.

surface was polished with sandpaper(#4000) to remove natural-oxide film.
Then the specimen was immersed in
200 dm3 of NaCl-AEW or Na2SO4AEW, as well as HCl (3.3 x 10-3 mol/dm3) or H2SO4 solution (1.7 x 10-3 mol/22 | metalfinishing | November/December 2013dm3) at the same pH of 2.4, for 1 hour.
The dissolution rate was calculated
from the weight loss of oxygen-freecopper before and after immersion byequation (6). Simultaneously, it wascalculated from the copper ion concenwww.metalfinishing.com

TECHNICALLY
speakingA

B

tration in the solution determinedthrough a ICP analysis by equation(7).
Dissolution rate (g/m2 h) =[wb (g)– wa (g)]/[S (m2) x 1 (h)] • • • (6)Dissolution rate (g/m2 h) =wd (g) /[S (m2) x 1 (h)]

10 μm

10 μm

Fig. 8 SEM images of surface of oxygen-free copper which was immersed for 900 s in NaClAEW at a free chlorine concentration of 1.0×10-3 mol/dm3 and at pH 2.20 (A) or pH 4.60 (B).

• • • (7)a saturated concentration of 2.5 x 10-4
ified by adding either water and HCl-2
3
where wb is weight of the specimen(1.0 x 10 mol/dm ) or water andmol/dm3 (25 °C) in 3 hours. So AEW
Fig. 8 SEM images of surface of oxygen-free-2copper which
was immersed for 900 s in NaCl-AEW at -2a free chlorine conc
3
before immersion, wa is weightofof
the -3 mol/dm
NaOH
solution(1.0
10pHmol/dm
from Na2SO4 solution (1.0 x 10 mol/
3 and
1.0×101 0×10at pH 22.20
20(A)x or
44.60
60 (B)(B). )
specimen after immersion, S is totaldm3) was generated by electrolysis
to shift pH to 2.2 – 9.9 and free chlo-3
surface area of the specimen and wd
and was left for 24 hours at room
rine concentration to 1.0 x 10 mol/is weight of dissolved copper in thedm3. Next the dissolution rate wastemperature. And then a dissolution
solution.
evaluated by the immersion of specirate was evaluated by immersion of amens in these solutions.
specimen in the resulting AEW with
2.4 Evaluation of the effect of freea saturated oxygen concentration for
2.5 Evaluation of the effect of
1 hour. In addition, a dissolution rate
chlorine
The following tests were performed towas evaluated by immersion of specidissolved oxygen
evaluate the effect of free chlorine on
Oxygen generated by electrolysis ismens in H2SO4 solution, whose oxythe dissolution and morphology of
gen concentration was enriched overdissolved in AEW in oversaturated
conditions. AEW releases excessively
the saturated concentration by blowthe oxygen-free copper surface in
NaCl-AEW. NaCl solutions that havdissolved oxygen gradually and reaching oxygen, at pH of 2.48.
ing concentrations of 1.0 x 10-2, 2.5 x
10-2, and 1.0 x 10-1 mol/dm3 were electrolyzed under the same condition
described in section 2.2 to yield NaClAEWs with free chlorine concetrationsof 8.6 x 10-4, 1.6 x 10-3 and 3.9 x 10-3
mol/dm3, respectively. All the NaClAEWs had the same pH of 2.48. The
surface roughness of specimens were
measured after they were immersed in
the AEWs for 15 min. Also the dissolution rates were evaluated by an
immersion of the specimens in the
AEWs for 1 hour. The correlation
between free chlorine concentrations
and the roughness of the oxygen-freecopper surface was investigated, as
well as free chlorine concentrations
and the dissolution rates.
Furthermore, the following tests
were performed to investigate theeffect of pH of NaCl-AEWs on therate of copper dissolution. NaCl solutions of concentrations of 2.5 x 10-2
mol/dm3 was electrolyzed under thesame condition described in section
2.2 to yield NaCl-AEWs (pH 2.48)
with free chlorine concentrations of
1.6 x 10-3 mol/dm3, which were mod-www.metalfinishing.com

November/December 2013 | metalfinishing | 23

2.7 Dissolution loss of thermal-oxide
film
Specimens were oxidized by heat in
the air at 200°C for 3 hours to forma thicker thermal-oxide film. These
specimens were immersed in NaClAEW, Na2SO4-AEW, HCl or H2SO4
with a pH of 2.48 for 10 min. The dissolution loss of thermal-oxide film
was evaluated by the change in weightof a specimen before and after immersion.
24 | metalfinishing | November/December 2013

Cu

60
80
100
40
100
60
80

1

2

3

4

C
Cu
C
5 O

A
F
A
F
A8

6

7

9

6

7

8

9

6

7

8

9

6

7

8

9

Sputtering time / x 102 sO

0

1

2

3

4

5

Sputtering time / x 102 s
1
1

2
2

3

4

5

2
Sputteringtime/ x 103
4
5 s

Sputtering time / x 102 s

60
80
100

Cu

40
100
60
80

Cu
C

20
80
40
60

O
CCu

B

O

0
60
20
40 0

1

2

3

4

O
Cu
5C
O

B
B
6

7

8

6

7

8

9

6

7

8

9

6

7

8

9

B

0
20
0
100 0
0
80 0
100

1

2

3

4

5

Supettering time / x 102 s
1
1

2
2

3

4

5

2
Supetteringtime/ x 103
4
5 s

Supettering time / x 102 s
Cu

60
80
100

O

C

Cu
C

40
100
60
80
20
80
40
60

0

1

2

3

4

O
CCu

C

O
Cu
C
O5

C
C

6

7

8

9

6

7

8

9

6

7

8

9

6

7

8

9

C2 s
Sputtering time / x 10

40
0
20
0

1

2

20
0

3

4

5

Sputtering time / x 102 s

0
0
80 0

1

2

1

2

3

4

5

2
Sputteringtime/ x 103
4
5 s

Sputtering time / x 102 s
Cu

60
80

D

O

40
60
80

C
Cu

20
80
40
60

O
CCu

D

0
60
20
40 0

O
Cu
C
O5

D

1

2

3

4

D

6

7

8

9

6

7

8

9

6

7

8

9

6

7

8

9

Sputtering time / x 10
C2s

40
0
20
0

1

2

20
0
0

9

2
Supettering time / x 10
C s

40
0
20

3

4

5

Sputtering time / x 102 s00Concentration
Concentration/ at% / at% / at%Concentration/ at%Concentration

0

0
20
0
100 0
0

0
60
20
40

F

C
Cu
O

40
0
20

80
100

A

Cu
O

20
80
40
60
0
60
20
40

F

C

Concentration
Concentration/ at% / at% / at%Concentration/ at%Concentration

80
100

1
1

2
2

3

4

5

2
Sputteringtime/ x 103
4
5 s

Sputtering time / x 102 s

www.metalfinishing.com

Concentration
Concentration/ at% / at% / at%Concentration/ at%Concentration

Concentration
Concentration/ at% / at% / at%Concentration/ at%Concentration
Concentration
Concentration/ at% / at% / at%Concentration/ at%Concentration
Concentration
Concentration/ at% / at% / at%Concentration/ at%Concentration

2.6 Surface analysis and observationof the specimens with thermaloxide film immersed in AEW
The surface analysis and observationof specimens with thermal-oxide film
immersed in AEWs were performed
to investigate the changes in theirsurfaces when the thermal-oxide film
and underlaying copper were simultaneously dissolved.
Specimens were oxidized by heat in
the air at 200°C for 1 hour to form
thermal-oxide film on the surface.
Specimens exhibited a brass color
after the heat treatment. When thethermally-oxidized specimens were
immersed in NaCl-AEW, Na2SO4AEW, HCl or H2SO4 solution with a
pH of 2.48 for 2 min, the brass color
faded gradually followed by theappearance of a lusterless surface ofcopper in each case. Then these specimens were immersed in each solution for 10 min, washed with water,and dried by nitrogen gas. Auger electron spectroscopy analysis was performed for conducting a surface
analysis of these specimens, as well as
specimens immediately after drypolishing with sandpaper (#4000) forcomparison.
Furthermore, surface analysis by
SEM was performed with specimens
with thermal-oxide film after immersion in each solution for 15 min. The
immersion time was extended inorder to accelerate the change in surface morphology because there wasno difference in surface appearance
when the immersion time was short.

100

Concentration
Concentration/ at% / at% / at%Concentration/ at%Concentration

TECHNICALLY
speaking

Fig.9 Depth profile of specimens by Auger electron spectrometry. Oxygen-free copperwith thermal-which were heated in the air at 2000C for 1 h and immersed in NaCl-AEW(A), HCl of pH 2.48 (B), Na2SO4-AEW H2SO4 of pH 2.48 (D) for 600 s, without immersion(E), oxygen-free copper degreased with acetone and ethanol copper degreased
with acetone and ethanol and polished by sandpaper #4000 (G).

its generation because of efficiency,
AEW immediately after its generation
was used in this study.
The properties of NaCl-AEW,
Na2SO4-AEW, HCl and H2SO4 solution used in this study are shown in
Table 1. Free chlorine concentrations
and dissolved oxygen concentrationsof AEWs are measured values 5 minafter the end of electrolysis because it
takes 5 min to get AEW out and tomeasure. Na2SO4-AEW exhibited highdissolved oxygen concentration andredox potential. This result showsthat the increased redox potential is
attributed to the high dissolved oxygen concentration. Meanwhile chlorine was generated in NaCl-AEW in
concurrence with oxygen. It appears
to be due to the lower chlorine overpotential compared to the oxygen
overpotential in electrolysis of NaClsolution. The standard electrodepotentials (E°, vs. SHE) for generated
chlorine (1.36 V) and HOCl (1.63 V)
are high, suggesting that the higher
redox potential of NaCl-AEW compared to Na2SO4-AEW is attributed
to the hybrid potential of generated
chlorine and HOCl. Regarding thedifference in concentrations of cations and anions, it is due to that Cl- or
sulfate ions (SO42-) were electrophoresed to an anode during electrolysis, resulting in the elevated concentrations of these ions in the anodechamber.
The equilibrium composition offree chlorine in NaCl-AEW shifts
dependent on the pH of solution. At
acidic pH a part of Cl2 (aq) becomesgaseous chlorine (Cl2 (g)) to yield
chlorine gas. At weakly acidic to neutral pH, undissociated HOCl isformed. At weakly alkaline pH, ClOis formed. The dissociation equilibrium is shown as equation (8). The
acid dissociation constant of HOCl(pKa) is 7.5 at 25°C [3].

3. RESULTS AND DISCUSSION

Cl2 (aq) + H2O  HOCl +Cl- + H+  ClO- + Cl-+ 2H+

Concentration
Concentration
Concentration/ at%/ at% / at%

100
80
100
60
80
100
40
60
80
20
40
60
0
20
40 0
0
20 0

Cu

E

O
C
Cu

E

O
C
Cu

E

O

1

2

3

4

5

6

7

8

6

7

8

9

6

7

8

9

C

9

Sputtering time / x 102 s
1

2

3

4

5

Sputtering time / x 102 s
1000

Concentration
Concentration
Concentration/ at%/ at% / at%

0

1

2

60
80
100
40
60
80
20
40
60
0
20
40 0

4

5

0
20 0

Cu

F

O
C
Cu

F

O
C
Cu
O

1

2

3

4

C

5

F

6

7

8

9

6

7

8

9

6

7

8

9

Sputtering time / x 102 s
1

2

3

4

5

Sputtering time / x 102 s

1000Concentration
Concentration
Concentration/ at%/ at% / at%3

Sputtering time / x 102 s

80
100

0

1

2

80
100

3

4

5

Sputtering time / x 102 s

60
80
100
40
60
80
20
40
60
0
20
40 0
0
20 0

Cu
O

G

C
Cu
O

G

C
Cu
O

1

2

3

4

5

C

G

6

7

8

9

6

7

8

9

6

7

8

9

Sputtering time / x 102 s
1

2

3

4

5

Sputtering time / x 102 s
0
0

1

2

3

4

5

Sputtering time / x 102 s

3.1 Characteristics of AEW
The time course of free chlorine concentrations and dissolved oxygen
concentrations in the solutions isshown in Fig.1. The concentrations of
www.metalfinishing.com

free chlorine and dissolved oxygen
decreased markedly within 15 min
immediately after the end of electrolysis, and stabilized relatively around
1 hour after the end of electrolysis.
However, since in a manufacturing
setting AEW is used immediately after• • • (8)The molar concentration ratios ofCl2 (aq), HOCl and ClO- at each pH
are shown in Fig.2 [4]. Cl2 (aq), HOCl
and ClO- are held in equilibrium in
NaCl-AEW. The lower the Cl- concen-

November/December 2013 | metalfinishing | 25

TECHNICALLY
speaking
tration is, the larger the region ofexistence for HOCl is [5]. Since theCl2 (aq) concentration fulfill the equilibrium with the Cl2 (g) concentration, the higher the Cl2 (aq) concentration is, the higher the Cl2 (g)concentration is. That is, the Cl2 (aq)concentration is equilibrated with H+
concentration in solution, while theCl2 (g) concentration is dependent on
the Cl- concentration and pH. Cl2 (g)in NaCl-AEW is released to the air sothat free chlorine concentration shifts
to the equilibrium concentration.

A

C

B

5 μm

5 μm

E

D

5 μm

5 μm

F

5 μm

5 μm

Fig. 10 SEM images of surfaces of specimens. Oxygen-free copper with thermal-oxide film,
which were heated in the air at 2000C for 1 h and were immersed in NaCl-AEW (A), HClof pH 2.48 (B), Na2SO4-AEW (C) or H2SO4 of pH 2.48 (D) for 900 s, without immersion
SEM imagesof surfacesof specimens.
(E),Fig.
or10without
heat
treatment(F).hi h were hheatedd iin the
O
Oxygen-free
f copper withi h thermal-oxidehl id filfilm, whichh air
i at 200Ԩ forf 1 h andd were immersedid iin NaCl-AEW
N Cl AEW (A)(A), HClof pH 2.48 (B), Na2SO4-AEW (C) or H2SO4 of pH 2.48 (D) for 900 s, without immersion (E), or without heat treatment (F).

Weight loss / %3.2 Dissolution rate
A
B
The dissolution rate (n = 5) of oxygenfree copper is shown in Fig.3. The dissolution rate was 1.10 g/m2 h (SD =0.18) and 0.26 g/m2 h (SD = 0.08)
when oxygen-free copper was
immersed in NaCl-AEW and in HCl
solution, respectively. The dissolution
10 μm
10 μm
rate in NaCl-AEW was 4.2 times fasterthan that in HCl solution. It is considered that free chlorine in NaCl-AEW
Fig. 11 Comparison of surface morphology of specimens. Oxygen-free copper withthermal-oxide film, heated in the air at 2000C for 1 h (A) , or oxygen-freeaccelerated the dissolution of oxygencopper without heat treatment (B) was immersed in NaCl-AEW for 900 s, respectively.
free copper, which is investigated further in section 3.3. The dissolution
Fig. 11 Comparison of surface morphology of specimens.
rate was 0.72 g/m2 h (SD = 0.21)
andOxygen-free copper with thermal-oxide film, which were heated in the air at 200Ԩ for 1 h (A) , or oxygen-free coppe
0.24 g/m2 h (SD = 0.11) when oxygen0,1
heat treatment (B) was immersed
in NaCl-AEW for 900 s, respectively.
free copper was immersed in Na2SO40,08
AEW and in H2SO4 solution, respectively. The dissolution rate in
0,06
Na2SO4-AEW is 3.0 times faster thanthat in H2SO4 solution.
0,04
The solutions, which were used for
immersion of specimens and shown in
0,02
Table 1, have no buffering effect, suggesting the pH of the solutions shift
0
after immersion. Therefore the pH ofA
BCDthe solutions was measured after specimens were immersed for 1 hour. As a
Fig. 12 Weight loss of copper with thermal oxide film, which was immersed in NaCl-AEW(A), Na2SO4-AEW (B), 2.48 (C) or H2SO4 of pH 2.48 (D) for 600 s.
result, there was no change in the pH
Fig.
12 Weight loss of copper with thermal oxide film, which was immersed in NaCl-AEW (A), Na2SO4-AEW (B)of HCl or H2SO4 solution, while it wasof pHchlorine
2.48 (D) for 600 s.
2.48(C)Effect
or H2SOobserved that the pH of NaCl-AEW and
3.3
of4 freeshape with an increase in the freein Na2SO4-AEW was elevated to 2.52.
The surface SEM images of the specchlorine concentration.
Regarding the dissolution behavior
imen immersed in NaCl-AEWs, when
The relationship between free chloof oxygen-free copper in electrolyzed
the free chlorine concentrations wererine concentration and dissolution ratewater, the measurements of the corprepared at 1.6 x 10-3 and 3.9 x 10-3of oxygen-free copper in NaCl-AEW isrosion potential and the polarizingshown in Fig. 5. The dissolution rate
mol/dm3, for 15 min are shown inpotential remain to be solved for aincreased with increase in the free chloFig.4. The specimen surface wascomprehensive discussion.
roughened in a more concavo-convex
rine concentration. The relationship
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TECHNICALLY
speaking
between free chlorine concentrationand the surface roughness of the specimen in NaCl-AEW is shown in Fig. 6.
The surface roughness increased alongwith the increase in free chlorine concentration. These results show that freechlorine dissolves oxygen-free copper
surface in a concavo-convex shape andelevates the dissolution rate and surface roughness.
The effect of the pH on the dissolution rate of oxygen-free copper when
free chlorine exists in the NaCl-AEWs
is shown in Fig. 7. The free chlorine
concentration was 1.0 x 10-3 mol/dm3
for each AEW. The dissolution rateincreased with a decrease in the pH of
AEW and maintained nearly constantwhen AEW was in neutral and alkaline
regions. It was found that oxygen-freecopper dissolved even in theseregions.
The surface appearances of the specimens immersed in NaCl-AEWs at pHof 2.20 and 4.60, when the respective
dissolution rates were 1.28 and 0.22
g/m2 h, for 15 min were observed. The
surface SEM images of the specimens
are shown in Fig.8. When a specimen
was immersed in the former, theroughening of the surface in a concavo-convex shape increased compared to that in NaCl-AEWs at pH of
2.48 (refer to Fig.4A). The hydrogenion concentration was elevated by theadded HCl to lower the pH value,
resulting in the increase in the concentration of Cl2(aq) (refer to theequation (6)). As a result, rougheningof the surface was accelerated. The pH
of NaCl-AEW shifted from 2.20 to2.30 after immersion, suggesting that
it is due to the progress of dissolution.
Meanwhile, when a specimen was
immersed in the latter with a pH of
4.60, nearly all of free chlorine existed
as HOCl. The roughening of the surface in a concavo-convex shape did
not occur significantly, nor was foundthe shift of the pH.
Moreover, when free chlorine in thegenerated NaCl-AEW was removed by
the addition of ascorbic acid (50www.metalfinishing.com

mmol/dm3), the appearance of oxygenfree copper immersed in the solution
was similar to that in the HCl as previously described (not shown). The dissolution rate was 0.23 g/m2 h, whichis almost the same as that in the HCl.
From these results, it was foundthat free chlorine in NaCl-AEW
increased the dissolution rate and had
an effect on oxygen-free copper tometamorphose its surface in a concavo-convex shape. This phenomenonis also observed when iron or nickel
are immersed in NaCl-AEW [6, 7].

therm aloxide film
voiid
oxygen-free copper

2 μm
Fig. 13. SIM of thermal-oxide film milled

by FIB.SIM image of oxygen-free copper with thermal oxideFig.13 Cross sectional

3.4 Effect of sulfate ions
To investigate the effect of the total
sulfate ions on the dissolution rate ofoxygen-free copper, H2SO4 at pH 2.48
with a sulfate ions concentration of
0.1 mol/dm3 was prepared by adding
Na2SO4 to the solution, and then the
dissolution rate was measured in the
resulting solution. As a result, the dissolution rate was 0.24 g/m2 h, which
is identical to that in the H2SO4 at pH
2.48. Therefore the result shows that
sulfate ions do not have an effect on
the dissolution rate.
3.5 Effect of dissolved oxygen
The dissolution rates of oxygen-freecopper in Na2SO4-AEW and H2SO4
solution with the same pH and different dissolved oxygen concentrations
are shown in Table 2. The dissolution
rates were equal for Na2SO4-AEW andH2SO4 solution with a dissolved oxygen concentration nearly equal to thesaturation concentration. The dissolution rates were equal for Na2SO4AEW immediately after generationand H2SO4 solution with an increased
dissolved oxygen concentration of 6.3
x 10-4 mol/dm3. Moreover, the dissolution rate decreased with thedecrease in dissolved oxygen with theprogress of time in Na2SO4-AEW, andincreased with the increase in dissolved oxygen in the H2SO4 solution(not shown). The E°(vs. SHE) for copper and oxygen are 0.34 V and 1.23 V,
respectively, so oxygen-free copper isdissolved with the consumption ofoxygen in acidic solutions. The results
show that the increased dissolution
rate in Na2SO4-AEW is due to dissolved oxygen.

3.6 Surface analysis of underlaying
copper after dissolved thermaloxide film
The result of the Auger electron spectroscopy analysis of the surface ofunderlaying copper after a thermaloxide film was dissolved is shown in
Fig. 9. It was found that copper oxide
was thickly-formed on the specimen
with thermal-oxide film (E). Formation of natural-oxide film was observedon the top of the specimen degreased
with acetone and ethanol (F). Oxygen
was observed on the top of the specimen (G), which was prepared from
specimen (F) by polishing with sandpaper to remove natural-oxide film,suggesting that the natural-oxide film
was formed immediately after polishing. The thickness of the natural-oxide
film was equal for specimens (F) and(G), so specimens with natural-oxide
film used in this study were substantially reproducible.
When specimens were immersed in
NaCl-AEW (A) and in HCl (B), the oxygen concentration decreased from the
topmost surface to the bulk layer andthe copper concentration increased
contrastingly. A strong peak of oxygen
appeared on the uppermost surface ineither case, which is assigned to natural-oxide film formed on the undelaying copper surface after removal ofthermal-oxide film. Also, the significant peak of oxygen was attenuated by
the sputtering time of about 60 seconds in each case. The etching speed ofcopper by Argon sputtering in Auger
electron spectroscopy analysis is 88
nm/min [9], suggesting that oxygen
Continued on page 42

November/December 2013 | metalfinishing | 27

TECHNICALLY
speaking
Continued from page 27
diffused as deep as 88 nm. The proﬁlesof oxygen concentration for specimens (A) and (B) are similar to thosefor specimens (F) and (G), suggestingthat the oxidation states of specimens(A) and (B), when the thermal-oxide
ﬁlm was removed, are similar to thoseof specimens (F) and (G) with thenatural-oxide film. The results ofspecimens (C) and (D), which were
immersed in Na2SO4-AEW and H2SO4
solution, were similar to those of
specimens (A) and (B).

3.7 Surface observation of underlaying copper after dissolved
thermal-oxide ﬁlm
SEM images of the specimen are shownin Fig. 10. Because the brass color ofthermal-oxide ﬁlm was not observedto the naked eye, the SEM images are
the bulk surface of the specimens after
both the thermal-oxide ﬁlm and theunderlaying copper were dissolved andremoved by the immersion. When aspecimen was immersed in NaCl-AEW(A), its surface morphology undergoesa change in a signiﬁcant concavo-convex shape. When specimens were
immersed in HCl (B), Na2SO4-AEW (C)and H2SO4 solution (D), smaller concave dissolutions (pitting corrosion),compared to (A), were found in eachcase. The difference in surface morphology of specimens with and without
thermal-oxide ﬁlm after immersed in
NaCl-AEW is shown in Fig.11.
3.8 Dissolution loss of thermal-oxide
ﬁlm
The dissolution loss in NaCl-AEW andNa2SO4-AEW was approximately 1.7
times larger that that in HCl and H2SO4
solutions at the same pH (Fig. 12).
AEW may easily dissolves thermaloxide ﬁlm. To examine this phenomenon, thermal-oxide ﬁlm was milled by
FIB and then the SIM image of its
cross-section surface was observed. As
shown in Fig. 13, it was found that
thermal-oxide ﬁlm was not a dense
coat. In addition, a void on the bound-ary of copper was observed, suggestingthat the adhesiveness between the thermal-oxide ﬁlm and copper is weak. It isdemonstrated that oxygen nanobubbles with a diameter of several dozen
nm exist stably in AEW [10]. Moreover,
it is also shown that hydrogen ionstend to concentrate on the gas-liquidinterface of oxygen nanobubbles [11].
The dissolution and removal of thermal-oxide ﬁlm is due to the dissolutionof copper oxide by hydrogen ions andetching of underlaying copper.

3.9 Use of AEW alt. to HCl and H2SO4
These results show that AEW is a moreoxidative solution than HCl andH2SO4 solutions at the same pH anddissolution rate for AEW is larger thanthose for HCl and H2SO4 solutions. It
is proposed that AEW is likely to be
used for etching of oxygen-free copper
surface. Furthermore, the oxidative
state of the surface of underlaying
copper after removal of thermal-oxide
ﬁlm is identical to that of copper withnatural-oxide ﬁlm. This suggests that
AEW is likely to be used for pretreatment, such as scaling and rust removal,
as an alternative to HCl and H2SO4
solutions. In addition, it would appear
that roughening of the oxygen-freecopper surface treated with NaClAEW, as shown in Fig. 11, may improve
the adhesiveness of plated ﬁlms.
Cathodically electrolyzed water,which is simultaneously generated onelectrolysis, exhibits a redox potentialof -700 mV (vs. SHE) and a pH ofapproximately 11.5. Cpper under theabove-mentioned condition appears
in the region of immunity in the
Pourbaix Diagram [12], and is noteroded unless corrosive ions coexist.
Although cathodically electrolyzed
water of NaCl or Na2SO4 contains
chloride ions or sulfate ions, when theconcentrations of these ions are low,copper almost never erodes by theimmersion of these solutions. It was
shown that corrosion did not occur
when iron was immersed in cathodically electrolyzed water of sodium
carbonate [13]. Therefore, it isthought that a stepwise method,which consists of acid treatmentusing AEW followed by anti-corrosive
treatment by the immersion in

42 | metalﬁnishing | November/December 2013

cathodically electrolyzed water, may
be possible.

ACKNOWLEDGMENTS
Thanks to Hiroko Katagiri and Ayako
Sakai at Shinko Electric Industries
Co. Ltd., for their assistance with
SEM observation and Auger electron
spectroscopy analysis.
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[2] K. Maruyama (1983) Mekki Jitsumu Dokuhon (Reading from
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Shinbunsya, Tokyo, p.58
[3] J. C. Morris, J. Phys. Chem. 70(1966) 3798.
[4] A comprehensive Guide to
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[5] N. Tagomori, H. Matsuda, Y.
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Bousyoku Gijutsu (Corrosion
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