D222
Journal of The Electrochemical Society, 162 (6) D222-D228 (2015)
0013-4651/2015/162(6)/D222/7/$33.00 © The Electrochemical Society
Evaluating the Performance of
2-Mercapto-5-Benzimidazolesulfonic Acid
in Controllable Electro-Healing Cracks in Nickel
X. G. Zheng, Y.-N. Shi,z and K. Lu
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, People’s Republic of China
The performance of 2-Mercapto-5-benzimidazolesulfonic acid (MBIS) in a Watts bath in crack electro-healing was investigated.
Potentiostatic and galvanostatic voltammetry, current efficiency measurements were performed for electrochemical investigation.
The morphology of the healed crack indicates that the healing crystals grow in a controllable manner with the addition of MBIS
under forced convection with the healing crystals fill preferentially in the crack tip to the crack sidewalls. No obvious defects were
observed along the interface between the substrate and the healing crystals, which is attributed the much higher current efficiency of
MBIS. Annual-ring like defects are observed but with a much longer width compared with PEI. The formation mechanism of annual
ring is discussed. Possible approach to get defect-free crack healing is also prospected.
© 2015 The Electrochemical Society. [DOI: 10.1149/2.1021506jes] All rights reserved.
Manuscript submitted February 16, 2015; revised manuscript received March 6, 2015. Published March 19, 2015.
Healing or repairing cracks in materials is of significant importance for the sustainable development of the world. As the mobility
of metal atoms is quite limited at service temperature, it remains a
challenge to realize crack healing in metallic materials.1–4 Recently,
an electrochemical process named electro-healing was introduced to
heal cracks in nickel.5 Compared with other attempts made in the crack
healing in metals, electro-healing take the advantage of utilizing the
satisfied mobility of metallic ions in the healing solution and make
it possible for the metallic ion to reach the crack site, get reduced to
metal atoms and finally heal (fill) the crack. However, to get a satisfactory recovery of strength, it is critical to minimize pores or voids that
left after electro-healing which might be realized by controlling the
growth mode of the healing crystals, i.e., making the healing crystals
grow preferentially from the crack tips to the crack sidewalls.6
Lately, it is found6 that controllable electro-healing cracks is
achieved by the addition of polyethyleneimine (PEI) and saccharin
to Watts bath under free convection, which exhibit an inhibition effect to the reduction of Ni. The reason behind is that the distribution
of these inhibitors shows a convection-dependent adsorption, which
means that in the crack tips where the diffusion of PEI and saccharin
is restricted, there are very few inhibitors. The diffusion condition
gets better toward the center of the crack results in more inhibitors
on sidewalls. The inhibitor concentration difference on the crack tip
and the crack sidewall leads to a differentiated growth velocity from
the tips and the sidewalls: the healing crystals grow much faster from
the tips than from the sidewalls. Nevertheless, some visible defects
were found along the interfaces between the substrate and the healing
crystals after the healing process, especially under forced convection.
It is recognized7 that with the presence of additives, the most obvious secondary reaction on the cathode is hydrogen evolution, the
rate of which is believed to be altered by the adsorption of additives.
This secondary reaction will definitely decrease the current efficiency
of metallic ion reduction. In a work on organic additives in the electrochemical processing of CoNiFe film, Tabakovic8 reported that the
current efficiency of the cathode varies with additives. A decrease
in current efficiency is generally observed with the increase of the
additive concentration. In our previous work on PEI and saccharin,
it is exhibited that both the inhibition and the current efficiency with
additives are convection dependent. When the convection becomes
stronger, the current efficiency of Ni reduction gets much reduced. On
crack sidewalls, where the convection condition of additives is much
better, the corresponding current efficiency must be low. Therefore,
the visible defects observed on the interface between the substrate and
the healing crystals came from an increased rate of hydrogen evolution, agreeing well with the reported result by Tabakovic.8 To better
z
E-mail: [email protected]
the healing efficiency, additives with higher current efficiency has to
be introduced.
2-Mercapto-5-benzimidazolesulfonic acid (MBIS), a benzimidazole derivative, exhibits acid-base nature that can be protonated in
Watts solution, with which the pH value normally in the range of
3.5–4. Very recently, the behavior of MBIS in the superconformal deposition in trenches or vias has been explored in electroless deposited
Cu9 and in electrodeposited ferromagnetic materials such as Ni, Co,
Ni-Fe and Co-Fe alloy.10,11 It is demonstrated in the deposition of
these ferromagnetic materials, MBIS is adsorbed and forms an inhibition layer to the reduction of the metallic ions, void-free trench filling
was realized in the present of MBIS, which is characterized by the
preferential deposition at the bottom and the subsequent geometric
leveling. In a paper concerning the superconformal Ni electrodeposition by using another benzimidazole derivative MBI, Lee12 found
that the filling performance when MBI is added to a sulfate/chloride
bath actually contains the passivation and the activation process corresponding to the formation of MBI layer and the breakdown of the
layer, respectively. In the passive state, the current efficiency is quite
low while in the activation stage, the current efficiency of the cathode
is close to 90%, which is similar to that of MBI-free electrolyte.
Although similarity exist between the concept of electro-healing
cracks and superfilling of vias or trenches, the dimension of cracks
we are interested in is usually with a length of millimeters and an
opening in a few tens of micrometers or larger. Cracks with these
specific features provide quite different electrochemical condition in
additive transfer, additives distribution, additive consumption, etc. In
the present work, the performance of MBIS in controllable electrohealing crack in nickel will be evaluated. Voltammogram and galvanic
sweep with the addition of different concentration of MBIS to conventional Watts bath will be carried out to systematically evaluate
the inhibition behavior of MBIS to the reduction of Ni. A Pt rotating disc electrode with rotation speeds of 50 rpm, 300 rpm and 2000
rpm will be employed to examine the influence of convection on the
voltammetry of the MBIS-added electrolyte and also to simulate the
electrochemical behavior from crack tips approaching crack center.
Variation of current efficiency with addition of MBIS on current density, MBIS concentration and convection will be measured. Scanning
electron microscope will be used to observe the healed crack after
electro-healed by MBIS-added electrolyte. To better illuminate the
correlation between the electrochemical behavior of additives and the
morphology of the healed cracks with additives, some results of PEI
will also be included for comparison.
Experimental
The measurements of additive effect on nickel reduction were
performed on the electrochemical workstation (PAR 273A) in a
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Journal of The Electrochemical Society, 162 (6) D222-D228 (2015)
three-electrode thermostatic cell at 30 ± 0.5◦ C. A platinum rotating disk electrode (Pt-RDE, Pine) with a diameter of 5 mm was used
as the working electrode (WE). Rotation speeds of 50 rpm, 300 rpm
and 2000 rpm were employed for each test to investigate the convection dependency. A saturated calomel electrode (SCE) and a nickel
rod were used as the reference electrode (RE) and counter electrode
(CE), respectively. The electrolyte contained 125 ml Watts bath solution which composed of 1 M NiSO4 · 6H2 O, 0.2 M NiCl2 · 6H2 O,
and 0.5 M H3 BO3 . MBIS (98%, Aldrich) with concentrations of
10 μM, 20 μM and 50 μM were investigated. Cyclic potential sweep
voltammetry was recorded between −0.5 V (SCE) and −1.2 V (SCE)
at a scan rate of 5 mV / s, whereas the galvanic sweep voltammetry
was recorded between −0.5 mA/cm2 and 20 mA/cm2 with a scan rate
of 10 μA/s. Before each test, a thin film of additive-free Ni layer was
plated on Pt RDE with a constant current density of 50 mA/cm2 for
50 s in Watts bath.
Current efficiency measurements were conducted in a galvanostatic electrodeposition process on Pt RDE with the total charge of
2 C in the studied bath under the required current density, followed
by a deplating process in the solution containing a mixture of concentrated phosphoric acid (H3 PO4 ), concentrated sulfuric acid (H2 SO4 ),
and distilled water (H2 O) with the mass fraction of 63%, 15%, and
22%, respectively. The deplating process was performed at a constant
potential of 0.4 V in a two-electrode-system with a Pt sheet as the
counter electrode. The nickel layer on the Pt electrode was considered
to be dissolved completely when the deplating current attenuated below 10−6 A. The current efficiency equaled to the ratio between the
charges transferred in the deplating process and the charges (2C) in
the deposition process.
The preparation of samples with through thickness crack was
detailed in our previous work.5 Before electro-healing, the cracked
samples with a thickness of 300 μm were pre-treated by alkaline
degreasing and followed by an electro-polish procedure in the abovementioned H3 PO4 -H2 SO4 -H2 O solution. A constant current density
of 10 mA/cm2 was adopted in electro-healing in 50 μM PEI or MBIS
contained in Watts bath under forced convection condition at a constant temperature of 30 ± 0.5◦ C. Post healing morphology observation
at a depth of about 150 μm from the sample surface was performed
80
on an FEI Nova scanning electron microscope (Nano-SEM) operated
at 10 to 15 kV. Chemical compositional analysis was carried out on
healed crack by Energy dispersive spectrometry (EDS) attached to
Quanta 600 SEM operated at 15 kV.
Results
Potentiostatic voltammetry.— The linear voltammograms with different MBIS concentration and a RDE rotation speed of 2000 rpm,
with 50 μM MBIS at different RDE rotation speeds are presented
in Fig. 1a–1c and 1d–1f respectively, voltammetry for additive free
bath is also included for comparison. With the addition of MBIS,
the onset potential of the reduction of Ni ions moves to more negative values, hysteresis loops form in the positive-scan and negativescan voltammetry. The loop becomes larger with the increment of
MBIS concentration (Fig. 1a–1c). When the rotation speed of RDE is
2000 rpm, the onset potential of Ni deposition is around −0.7 V in the
additive-free solution, the value goes to −0.92 V when 10 μM MBIS
is added, indicating that MBIS is an inhibitor to the Ni deposition.
When the concentration of MBIS gets higher from 10 μM to 20 μM
and 50 μM, the onset potential of the breakdown of inhibition moves
to −0.98 V and −1.08 V respectively. This illustrate that the inhibition
of MBIS is concentration dependent, which is in accordance with the
reported result.10
In the potential-sweep voltammetries for 50 μM MBIS contained
Watts bath with the rotation speed of RDE of 50 rpm, 300 rpm and
2000 rpm (Fig. 1d–1f), it is observed that in additive-free bath, no
observable difference in current density is detected in the positivegoing scan and the negative-going scan. Besides, the rotation speed
of RDE has little effect on the LSV of the additive-free bath, as the
red and gray lines exhibit.
The onset potential for 50 μM MBIS-added solution is about
−0.91 V when the rotation speed of RDE is 50 rpm. When the rotation speeds increase to 300 rpm and 2000 rpm, the characteristic value
changes to −1.0 V and −1.1 V respectively, speaking of a convection
dependent feature of MBIS inhibition. When potentials are more negative than the critical value, the current density will rise steeply and
overlap the positive-scan curve which is close to the additive-free case
80
(a)
Rotating speed: 2000 rpm
Foreward scan
Reverse scan
60
40
(d)
MBIS 50 µM:
Foreward scan
Reverse scan
60
40
Additive free
Additive free
20
10 µM
-2
0
Current density / mA·cm
Current density / mA·cm
-2
20
(b)
60
40
Additive free
20
20 µM
0
(c)
60
40
0
-1.2
50 µM
-1.1
-1.0
-0.9
50 rpm
0
(e)
60
40
Additive free
-0.8
Potential / V vs. SCE
-0.7
0
(f)
60
Additive free
2000 rpm
20
-0.6
300 rpm
20
40
Additive free
20
D223
0
-1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
Potential / V vs. SCE
Figure 1. Potential sweep voltammetry in Watts bath with the addition of 50 μM MBIS and the RDE at rotation speeds of 50 rpm, 300 rpm and 2000 rpm
respectively (a)–(c) showing a convection dependent inhibition. (d)–(f) are voltammetries with the RDE at a rotation speed of 2000 rpm but with the concentration
of MBIS varies from 10 μM to 50 μM. The black curve in each illustration is from the additive-free bath.
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D224
Journal of The Electrochemical Society, 162 (6) D222-D228 (2015)
not less than 90% when MBIS is added, which we believe is good
enough to get a satisfied interface when electro-healing the crack. The
high CE of MBIS is in accordance with that reported by Lee12 and
Liang.15 Reasons behind might be attributed to the protonation of its
charged group16 and the variation of interface pH value,17 etc.
100
80
60
40
10 µM
Current efficiency / %
20
100
80
60
40
20 µM
20
100
80
60
40
50 µM
20
0
0
10
20
30
40
50
-2
Current density / mA·cm
PEI:
50 rpm,
300 rpm,
2000 rpm
MBIS:
50 rpm,
300 rpm,
2000 rpm
Figure 2. Variation of cathodic current efficiency of Watts bath with the addition of various concentration of MBIS (solid lines + solid symbols) and PEI
(dotted lines + open symbols) with the current density at different rotation
speed. The condition of each curve is specified.
in all conditions. This critical value is believed to correspond to the
breakup of the inhibitor layer formed on the surface of the cathode.
One interesting phenomenon in the voltammetry of MBIS-added
solution is the transition between the breakdown of inhibition (active)
and the rebuilt of inhibition (passive). The phenomenon only occurs
when the MBIS concentration or the rotation speed of RDE is higher
than a certain value (50 μM and 300 rpm). The phenomenon has not
been observed with the presence PEI at a given concentration.13
Cathode current efficiency.— In our previous work,6 we found that
with the addition of PEI or saccharin, the current efficiency (CE) is
reduced over a large extent depending on potential and convection
condition. CE even dropped to 0 when 100 μM PEI and 5.5 mM
saccharin were introduced with a current density of 10 mA/cm2 and
a rotation speed of 300 rpm. Under the condition, a large defect was
found along the interface between the original crack surface and the
healing crystals, which is believed to be the result of strong parasitic
hydrogen evolution14 due to the quite low CE. It is expected that the
addition of MBIS will promote CE. The cathode CE of MBIS added
solution with different MBIS concentration under various rotation
speed of RDE over a certain current density is presented as the solid
lines in Fig. 2. The corresponding results of PEI are also included for
comparison, as the dashed lines designate. Overall, CE of MBIS is
higher than that of PEI over the entire tested current density under
a given condition. Take 50 μM as an example, when current density
is 20 mA/cm2 , CE of MBIS is approximately 90% when the rotation
speed is 2000 rpm, while that of PEI is only 10% at the same condition.
Besides, CE is dependent upon current density, additive concentration
and rotation speed. Generally speaking, on one hand, CE increase
with the increase of current density. On the other hand, CE decrease
with the increase of additive concentration or the increase of rotation
speed of RDE. In other words, CE decreases with the increases of the
magnitude of inhibition, as the inhibition of PEI and MBIS are both
concentration dependent and convection dependent. Quite obviously,
when current density is higher than 10 mA/cm2 , CE remains a value
Crack healing morphology.— We chose 50 μM and 10 mA/cm2
to carry out the crack electro-healing under forced convection, the
obtained crack-healing morphology with PEI and MBIS are presented
in Fig. 3a, 3b and Fig. 3c, 3d, respectively. It is evident that the healing
crystals grew following a controllable growth mode6 with the healing
crystals grew preferentially from the crack tip to the crack sidewalls.
This is attributed to the difference in inhibitor distribution in the crack
tip and the crack sidewalls.
Although the growth mode with PEI and MBIS are similar, big
differences exist between the morphology of these two inhibitors.
First, there is a line-like void defect (black arrow in Fig. 3a) along the
interface between healing crystals and substrates in the PEI contained
case, but this defect is not observed when MBIS is present. This is an
expected result. Second, despite the fact that annual-ring like defects
(white arrows in Fig. 3a and Fig. 3c) are observed in both cases, the
annual-ring with PEI is more dense than that with MBIS. In other
words, the width of the annual-ring with MBIS addition is larger
than with PEI. Enlarged images of the marked area in Fig. 3a and
Fig. 3c are shown in Fig. 3b and Fig. 3d respectively, demonstrating
that the annual-ring consists of many light regions (width of ring) and
dark regions (ring itself) that present alternatively. On the light region
the growth of healing crystals can be traced. The histograms of the
distribution of the width of these annual rings from Fig. 3a and Fig. 3c
are given in Fig. 3e and Fig. 3f respectively. The width of annul-ring
in PEI are all less than 0.5 μm while that in MBIS ranges from a few
micrometer to 30 μm, much larger than PEI.
In our previous work,6 annual-ring pattern was also observed with
the addition of PEI and saccharin both under free convection and
forced convection. Apparently, the annual-ring pattern seems to be a
common phenomenon when controllable electro-healing of crack is
achieved with the presence of inhibitor(s). From the close observation
of annul-ring (Fig. 3b and Fig. 3d), the light region between the rings
seems to correspond to the active growth mode of healing crystals,
while the dark ring may relate to the passive inhibition state with
negligible Ni deposition. Therefore, the annual-ring pattern actually
reflects a transition of the growing front of healing crystals from active
to passive and then back to passive. This phenomenon reminds us the
transition in the voltammetries in Fig. 1 when MBIS concentration
is high and the convection is strong (the magnitude of inhibition is
large enough). Formation mechanism of annual-ring will be further
discussed in the ensuing discussion part.
Galvanic sweep.— To further understand the phenomenon, galvanic sweep method is employed to compare the inhibition effect
of PEI and MBIS on nickel reduction. Unlike the potential sweep
method, the voltammetry recorded by galvanic sweep will give detailed information when there is a dramatic change in current density,
as the case of inhibition breakdown region shown in Fig. 1. The
galvanic sweep voltammetry for PEI and MBIS with different concentration under various rotation speed of 50 rpm, 300 rpm and 2000
rpm are presented in Fig. 4a–4c and Fig. 4d–4f, respectively. Included
are curves from additive-free bath for comparison. Generally, the galvanic sweep voltammetries for PEI and MBIS have similar inhibition
effect by moving the onset potential of Ni deposition to more negative
values. But clearly, big differences are evident in the inhibition effect
of the two inhibitors.
The galvanic sweep voltammetry of PEI can be divided into three
segments: the initial suppressed segment, active segment after inhibition break down and the reverse scan segment, as shown in
Fig. 4a–4c.
For the first segment, in the lower current density range, overpotential increases rapidly as the increases of cathodic current density,
which is attributed to the inhibition effect of PEI on nickel reduction.
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Journal of The Electrochemical Society, 162 (6) D222-D228 (2015)
( b)
Substrate
(e) 50
Relative Frequency / %
(a)
Healing crystals
10 µm
Relative Frequency / %
(f)
Healing crystals
20 µm
20
10
0.1
0.2
0.3
0.4
0.5
50
MBIS 50 µM
40
30
20
10
0
5 µm
Substrate
30
Width of annual ring / µm
(d)
(c) Substrate
PEI 50 µM
40
0
0.0
2 µm
Substrate
D225
0
10
20
30
40
50
Width of annual ring / µm
Figure 3. SEM cross-section images of the healed cracks by using Watts bath in the presence of 50 μM PEI (a–b) and 50μM MBIS (c–d) at a current density of
10 mA/cm2 under forced convection. (a) and (c) are images under lower magnification. Black arrow in (a) indicates the line-like void defect generated along the
interface while the white arrows in (a) and (c) indicate the annual rings generated during healing process. (b) and (d) are enlarged images of (a) and (c) respectively,
showing the annual rings. (e) and (f) are histograms of the distribution of the width of annual rings in (a) and (c) respectively.
(a)
15
10
50 rpm
20
Additive-free
5
0
20
(b)
15
10
5
Additive-free
300 rpm
0
20
(c)
15
10
Additive-free
0
50 rpm
20
(e)
15
10
5
Additive-free
0
300 rpm
20
(f)
15
10
10
5
MBIS 10 µM
MBIS 20 µM
MBIS 50 µM
(d)
15
-2
Current density / mA·cm
-2
5
PEI 10 µM
PEI 20 µM
PEI 50 µM
Current density / mA·cm
20
Additive-free
2000 rpm
0 2000 rpm
0
-1.1
-1.0
Additive-free
5
-0.9
-0.8
Potential / V vs. SCE
-0.7
-1.1
-1.0
-0.9
-0.8
-0.7
Potential / V vs. SCE
Figure 4. Galvanic sweep voltammetry in Watts bath with the addition of PEI (a)–(c) and MBIS (d)–(f) at concentrations of 10 μM, 20 μM and 50 μM with the
RDE rotates at speeds of 50 rpm (a),(d), 300 rpm (b),(e) and 2000 rpm (c),(f). Voltammetries are recorded with a scan rate of 10−5 A/s. Solid black curves are
from additive-free Watts solution.
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D226
Journal of The Electrochemical Society, 162 (6) D222-D228 (2015)
(b )
Inhibition break-down potential / V vs. SCE
Inhibition break-down potential / V vs. SCE
(a)
-0.70
50 rpm
300 rpm
2000 rpm
-0.75
-0.80
-0.85
-0.90
-0.95
-1.00
-1.05
-1.10
-1.15
-1.20
10
20
30
40
50
PEI concentration / µM
-0.70
50 rpm
300 rpm
2000 rpm
-0.75
-0.80
-0.85
-0.90
-0.95
-1.00
-1.05
-1.10
-1.15
-1.20
10
20
30
40
50
MBIS concentration / µM
Figure 5. The inhibition breakdown potential from the galvanic sweep voltammetries varying with the concentration of PEI (a) and MBIS (b) with the RDE at
rotation speeds of 50 rpm, 300 rpm and 2000 rpm. In (a) big difference in slope is observed while in (b) three line are nearly parallel to each other.
Subsequent increase in cathodic current density leads to the breakdown of inhibition. The inhibition breakdown potential is a weak
function of either additive concentration when the rotating speed is
as low as 50 rpm or convection when the additive concentration is
as low as 10 μM, all of which is about −0.9 V, as exhibited in
Fig. 4a–4c. However, an enhanced additive concentration dependency
and convection dependency is observed when a strong convection
(2000 rpm) or a high additive concentration (50 μM) is employed.
This means that an increase in rotating speeds or PEI concentration
would lead to significant increase in the overpotential of inhibition
breakdown in the high PEI concentration or high rotating speeds region. It suggests that when mass-transfer is favorable, i.e. in the condition of high PEI concentration and high rotating speed, the blocking
layer on the cathode surface is stable and would not easily be broken.
In other words, complete inhibition is reached and mass-transfer is
not the rate-determining step of this process.
One thing that should be noted is the initial suppressed segment
for 50 μM and 2000 rpm in Fig. 4c, which exhibits a continuous
increase in overpotential with current density. When recalling the
current efficiency result under the similar condition (Fig. 2) that is
only about 10%, it is obvious that the dominant Faradic process is
hydrogen evolution rather than active Ni deposition.
For the second segment, a potential shift to additive free curve is
observed after the occurrence of inhibition breakdown. The increase
in current causes a drop in cathodic overpotential, i.e. a negative
resistance of the electrolyte is observed, which is reported in the
literature as ‘s’ shaped negative differential resistance (s-NDR).18,19
This uncommon phenomenon of s-NDR has been proved to relate
to the competing process between the mass-transfer of inhibitor and
its inclusion into the deposits according to the model of Roha.20 In
the case of PEI, the initially blocked cathode surface is cleaned off
by the inclusion mechanism when the inhibition breakdown occurs.
The subsequent adsorption state depends on the mass-transfer and the
inclusion rate of PEI. It is reasonable that when PEI concentration
is low, 10 μM for example, the diffusion flux is negligible after the
initially adsorbed PEI blocking layer breakdown, which causes the
cathode potential shifts positively, as shown in Fig. 4a. The increase
in PEI concentration and rotating speeds would enhance the diffusion
flux of PEI, which may cause the positive shift of cathode potential less
pronounced or even vanish. Further increasing in PEI concentration
or rotating speeds to the extent that mass-transfer is not the ratedetermining step, a complete inhibition state would achieved, as the
curve of PEI 50 μM shows in Fig. 4c.
For the third segment, the reverse voltammetry scan, the inhibition
is not observed in the presence of 10 μM PEI at a rotating speed of
50 rpm, as shown in Fig. 4a. The inhibition recurs at a potential of
about −0.8 V when the rotating speed increases to 300 rpm. Further
increasing to 2000 rpm causes the inhibition recurs at a potential of
about −0.85 V. A trend of decreasing potential of inhibition recurrence
continues until PEI concentration increase to 20 μM and the rotating
speed reaches 300 rpm. After that the whole reverse voltammetry scan
is in inhibited state.
The galvanic sweep voltammetry for solution with the presence of
MBIS is presented is shown in Fig. 4d–Fig. 4f. Except the similarity in
inhibition effect, some differences are easily spot. First, the onset potential for inhibition breakdown varies with MBIS concentration and
also the rotating speed. In Fig. 4d, with a rotation speed of 50 rpm,
the value is about −0.80 V when the concentration of MBIS is 10 μM,
it decrease to −0.85 and −0.90 V when the concentration increases
to 20 μM and 50 μM, respectively. This increment is the most pronounced when the rotation speed is promoted to 2000 rpm (Fig. 4f),
where the onset value increases from −0.92 V to −1.08 V. For a given
concentration, say 50 μM, the critical potential for inhibition breakdown varies from −0.9 V to −1.08 V when the rotation speed is raised
from 50 rpm to 2000 rpm (Fig. 4d–4f). It indicates that a well-defined
concentration and convection dependent inhibition is achieved in the
tested experimental condition. This is quite different from PEI where
the critical value is not concentration sensitive or convection sensitive
at a low rotating speed and low PEI concentration range but is greatly
influenced by concentration and convection at high rotating speed and
high PEI concentration.
Secondly. The inhibition of MBIS is reinforced either by the increase of MBIS concentration or the rotation speeds, which implies
that the rate-determining-step of adsorption is mass transfer. Hysteresis loop of the voltammetry exists in all the tested MBIS concentration
and convection range, which corresponds to a different surface coverage of MBIS between the forward scan and reverse scan and suggests
a mass-transfer controlled adsorption process of MBIS.
To present the result more clearly, the inhibition-breakdown potential dependence on the concentration of MBIS and the rotation speed
of RDE are presented in Fig. 5a and 5b respectively. As discussed
above, the inhibition breakdown potential is about −0.9 V when PEI
concentration is 10 μM at the tested rotating speed, or a rotation speed
of 50 rpm at the tested PEI concentration. It becomes a strong function of PEI concentration and rotation speeds when rotation speed
increases to 2000 rpm or PEI concentration increases to 50 μM. The
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Journal of The Electrochemical Society, 162 (6) D222-D228 (2015)
Annual ring
D227
Complete inhibition
Active growth stage when complete inhibition is broken
Figure 6. A diagram illustrating the formation process of annual ring, where
the dark annual rings correspond to a complete inhibition state (passive) while
the light gray regions between the annual rings stand for the active growth state
when the complete inhibition is broken.
Convection intensity
Passive growth stage under complete inhibition
M
P
C0
dramatic slope change may indicate a transition in the rate determining step, which will cause a shift of the surface state between the
active growth state and the complete passive inhibition state.
When the breakdown of inhibition occurs, the onset value for
MBIS decrease linearly with the increase of concentration (Fig. 5b),
the slope of the line is nearly parallel to each other. A similar trend is
observed in the onset potential and the rotating speed at each concentration of MBIS in Fig. 5b. It is deduced that the adsorption of MBIS
is consistently dependent on its concentration and the convection condition. No transition in the rate determining step is observed under the
present tested condition.
The occurrence in the transition of rate-determining step of Ni
reduction with the presence of PEI and MBIS might be the main reason
for the different behavior in the crack electro-healing morphology
between these two inhibitors.
Discussion
The formation of annual ring.— With regard to the annual ringlike defects, the formation of these defects seems unavoidable in the
controllable crack electro-healing when inhibitor (or inhibitors) is
(or are) presented. Inhibitors, like PEI or MBIS, normally exhibit a
concentration dependent or convection dependent inhibition, which
make it possible to get a differentiated inhibition from crack tips and
the crack sidewalls. Fig. 6 schematically illustrates the formation of
annual rings in the healing process. As to the crack sidewalls, where
mass transfer of additives is favorable, a completely inhibition state
is generated. The healing crystals grow following a conformal growth
mode with an extremely low growth rate. On the other hand to the
crack tips where the mass transfer for additive is unfavorable, the
growth of healing crystals from the crack tip is much less inhibited,
exhibiting an active growth stage. During the period, the additives
will be progressively accumulated through mass transfer, once the
(a)
Concentration
— Average convection intensity of the electrolyte near the crack
— Amplitude of the convection intensity that deviated from the mean value of 0
— Critical convection intensity for PEI / MBIS with a concentration of C0 for a
complete inhibition
Figure 8. A schematic illustration presenting the complete inhibition of MBIS
and PEI varying with the concentration of inhibitors and the convection intensity. It illustrates that the state of a complete inhibition is a function of
convection and inhibitor concentration. The meaning of the symbols are indicated in the Figure. The density of the horizontal lines between 0 − and
0 + represents the distribution of the convection intensity of the electrolyte near the crack in the electro-healing process under forced convection.
The probability for the convection intensity to reach P is much higher than
that to M , suggesting that it is more likely to get a complete inhibition in the
presence of PEI than that of MBIS.
surface coverage of inhibitors exceeds a critical value, a complete
inhibition will be accomplished, the healing crystals will exhibit a
passive growth where an annual ring is formed. After the passive
growth stage, the surface coverage of inhibitors will be reduced due
to inhibitor removal20 or interface pH shifts,11,17 when the inhibition
is broken and the growth of healing crystals goes back to the active
stage. The annual ring is actually a transition between the active
growth and the passive inhibition, where the rings correspond to a
complete inhibition while the interval between the rings relates to the
accumulation of inhibitors.
EDS result carried out on the healed crack (Fig. 7) verifies our
assumption of inhibitor inclusion during the passive growth stage.
Besides dominant amount of Ni, O and S are detected in the annual
ring (Fig. 7a), which apparently came from the inclusion of MBIS.
While in the active growth stage between the rings, only the diffraction
peak of Ni is found without detectable O and S (Fig. 7b).
(b)
Annual-ring
Between annual-rings
Figure 7. EDS results carried out on the healed crack showing detectable O and S in the annual-ring region (a) and only Ni were detected in the region between
the annual ring (b) with the addition of 50 μM MBIS to the Watts bath under forced convection.
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D228
Journal of The Electrochemical Society, 162 (6) D222-D228 (2015)
Factors that affect the annual ring.— As significant difference is
observed on the morphologies of the healed crack with the addition
of PEI and MBIS (Fig. 3), one factor that will affect the formation of
annual ring is the inhibitor per se. Because there is little difference
in the passive inhibition stage, the significant effect of inhibitor may
lie on the active growth stage of the annual-ring defects. As shown in
Fig. 3c, 3f, the width of annual ring of MBIS is much longer than that of
PEI, presenting a much longer active growth stage of MBIS. Analysis
on the difference in the inhibition of PEI and MBIS is schematically
illustrated (Fig. 8), which is mostly based on the galvanic sweep results
(Fig. 4). Two curves in the illustration represents a complete inhibition
for PEI and MBIS respectively. This complete inhibition corresponds
to a fully surface coverage blocking layer of adsorbed additive on
the cathode when Ni reduction is negligible. Considering that the
electro-healing is carried out under forced convection in a solution at
a given inhibitor concentration of C0 , the convection intensity near
the crack have the form of = 0 ± , where 0 is the average
convection intensity and is the amplitude of fluctuated convection
intensity due to the turbulence of electrolyte flow. The distribution is
not uniform and is represented by the density of horizontal lines
in the figure. The variation in convection intensity will lead to a
transition between partly inhibition and a completely inhibition. When
the convection intensity is lower than P or M , the growth of healing
crystals will continue until the intensity reaches P or M . Then
the growth front of healing crystals would stop, an annual ring is
generated. Apparently, it is easier for convection intensity to reach P
than to reach M , so the width of annual rings of PEI is smaller than
that of MBIS.
Other factors, like additive incorporation,21 may also affect the
active growth stage in the annual ring pattern, which need more
explorations.
Prospect.— With single inhibitor (MBIS, PEI) or inhibitors
(PEI+saccharin), the growth mode of healing crystals seems be controlled by the magnitude of inhibition of inhibitor(s). When the magnitude is small, as the case for PEI or saccharin addition without
agitation,6 the healing crystals will not exhibit a controllable growth
pattern, instead, they will grow following a general growth mode,5
similar to that without inhibitor. When inhibition magnitude is consistently high, as in the case of PEI or PEI and saccharin under forced
convection, controllable growth mode is reached but both interface
defects and annual ring pattern are generated on the healed crack. The
addition MBIS shows an improved cathode current efficiency, thereby
eliminates the interface defects. Despite the differences in the performance between MBIS and PEI, annual ring pattern still presented with
the addition of MBIS. The formation of annul ring-like defects seems
unavoidable when only inhibitor(s) is (are) added to the solution.
The addition of accelerator will depolarize the reduction of Ni ions,
with which the growth of healing crystals from the sidewalls will be
enhanced,13 while the growth from the crack tips will be less faster
considering the convection-dependent distribution of accelerator.13 In
the meantime, the depolarization effect of accelerator will weaken
the inhibition of inhibitors in the crack tip, therefore may eliminate
the formation of annual ring-like defects. The combination addition
of inhibitor and accelerator might be a promising approach to realize
controllable crack electro-healing with minimized defects.
Conclusions
By the addition of MBIS to a Watts bath, controllable electrohealing has been achieved in pure nickel under forced convection.
MBIS has an inhibition effect on Ni deposition, which is both concentration dependent and convection dependent. No defects is observed
along the interface between the original crack surfaces and the healing
crystals due to a much improved current efficiency compared with PEI
under the investigated concentration. Similar to PEI, annual-ring-like
defects are observed on the morphology of the healed crack with the
addition of MBIS. Nevertheless, the width between the annual rings
with the presence of MBIS is much larger than that of PEI. It is believed that under a given concentration, it is more likely for PEI to get
a complete inhibition than MBIS.
Acknowledgments
The authors are grateful for financial support from the Ministry
of Science & Technology of China (grant Nos. 2012CB932201), National Science Foundation of China (grant No. 51231006), the International Cooperation Program funded by National Science Foundation
of China (grant No. 51261130091), the Key Research Program of Chinese Academy of Sciences (grant No. KGZD-EW-T06) and Shenyang
National Laboratory for Materials Science (grant No. Y40660).
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