Abstract This paper deals with the preparation and characterization
of thin Ni layers The electrodeposition was carried out
galvanostatically from a Watts bath at different current densities in
the range from 1 to 10 A [dm.sup.-2] and for deposition times between
900 and 7200 s. The structure and the morphology of the nickel coatings
were investigated by SEM and XRD techniques. The microhardness of
deposited layers, the electrochemical behavior and the corrosion
properties of the deposits were investigated by means of Vickers
microhardness, polarization measurements, and electrochemical impedance
spectroscopy (EIS). The uniform deposits showed fine grains and good
protection against corrosion.
Keywords Nickel, Electrodeposition, Surface morphology,
Microhardness, Corrosion
Introduction
Since the first Ni electrodeposition more than 170 years ago (see
Chapter 3 in Modern Electroplating, 5th edition, eds. M. Schlesinger and
M. Paunovic, p. 79), many working groups researched this subject) The
properties of Ni deposits, according to Brenner et al. dramatically
depend on the bath composition?
The Watts bath was first introduced in 1916 for industrial purposes
and it is still widely used. A nickel Watts bath is suitable for
high-speed electrodeposition. (3), (4) Nickel electrodeposition from a
Watts bath has been used in many functional applications to modify or
improve the corrosion resistance, hardness, wear resistance, and
magnetic properties of materials; or to build up worn or undersized
parts for salvage purposes. (5-7) These properties of the Ni coatings
depend on the surface structure of deposits (particularly on the
preferred orientation and on the grain size) (8-14) The physical and
mechanical properties of nickel deposited from a Watts bath are affected
by the electrodeposition parameters, such as current density, pH,
cathode material, electrolyte agitation, deposition time, and
electrolyte temperature, among others.15.16 Pure nickel is ductile and
tough because it possesses a face centered cubic crystalline structure
at temperatures below its melting point. Electrodeposition of Ni remains
the basis of most decorative plating processes, and it is used in
engineering applications and in electroforming processes. Ni
electrodeposition is usually performed at high temperatures
(50-70[degrees]C) and at high current densities. (17-20)
Electrodeposition of Ni has been reported both through direct current
(21-25) and pulse (26) methods. The literature reveals that in direct
current electrodeposition, an increase in current density leads to a
corresponding decrease of the grain size of the deposits, this behavior
being attributed to the evolution of more hydrogen at the cathode
interface at higher current densities. The presence of the hydrogen
modifies the growth interface on the cathode by influencing the surface
energy and growth mechanisms, which in the end will facilitate the
formation of larger grains in the deposited layer. (23), (27) Corrosion
resistance can be affected by the microstructure, such as grain size,
surface morphology, and texture, which are closely related to the
electrodeposition parameters. Electrodeposited nickel generally shows a
better corrosion resistance as a grain's crystal size decreases,
(18) but there are also other contradictory results reported in the
literature. (19) Nickel has good resistance to corrosion in the normal
atmosphere, in natural freshwaters, and in deaeration nano-oxidizing
acids, and it also has an excellent resistance to corrosion in alkaline
media. However, nickel deposits are strongly attacked by hydrochloric,
sulfuric, and nitric acid; chlorine; and sulfur compounds. Corrosion
performance in functional applications depends on the thickness of the
deposited Ni layer and on the quality of the materials to be plated, as
well as many other factors.
In this study. galvanostatic electrodeposition of nickel at
50[degrees]C was performed from a Watts bath, using different current
densities and deposition times. A systematic experimental study was made
to determine the role of the working parameters (current density and
deposition time) on the structure, hardness, and corrosion behavior of
electrodeposited layers.
Experimental procedure
The composition of the electrolyte used in the electrodeposition
experiments and the working parameters are shown in Table 1. The
electrodeposition was carried out using a potentiostat/galvanostat (EG
& G model 263A, USA) at current densities between 1 and 10 A
[dm.sup.-2.] The pH and the temperature of the electrolyte were kept
constant. The bath was vigorously stirred with a mechanical stirrer
(250-300 rpm). The temperature was controlled with a Haake thermostat
(model GD1, accuracy +1[degrees]C). A copper disc with an area of 2.26
[cm.sup.2] was used as a working electrode (WE), and it was introduced
vertically in the center of a double-walled electrochemical cell made
from glass. The WE was surrounded by a cylindrical nickel counter
electrode, and a saturated calomel electrode (SCE) was used as the
reference electrode (RE). The copper substrates(27) were mechanically
polished with different grades of emery papers (800, 1200, 2400, and
4000), then they were electrochemically degreased in an alkaline bath
(UNAR EL 63 solution from Schering, Germany) at 0.2 A[dm.sup.-2] for
20-30 s and finally activated at room temperature in HC1 (1 N) solution,
then rinsed with doubly distilled water, and dried with pressured air.
A thin Ni layer was initially potentiostatically deposited (10 min
at - 1 V) onto the Cu substrates, from an aqueous solution that
contained only 263 g L -1 NiS[O.sub.4].6[H.sub.2]0 (pH 2). This first Ni
layer was deposited in order to avoid the influence from Cu substrate on
the further growth of the Ni from the Watts bath. Moreover, cross
section SEM images of the deposits (not shown) proved the fact that
usually there are some voids between the Cu and the first Ni layer
deposited. However, better adhesion was observed between the two Ni
layers that were consecutively deposited.
The pH of the Watts bath was adjusted using [H.sub.2]S[O.sub.4]
(98%) and NaHC[O.sub.3] (99.5%) p.a. solutions.
After the electrodeposition process the samples were rinsed with
doubly distilled water and dried with pressured air.
The current efficiency (CE) was calculated based on the difference
in the mass of specimens before and after the plating process ([eta] =
[m.sub.exp][m.sub.theo]), where [m.sub.exp] is the mass of the deposit
obtained experimentally by gravimetry and mtheo is the theoretical mass
of the deposits that was determined according to Faraday's law.
The structure and surface morphology of the electrodeposits were
characterized by scanning electron microscopy (SEM; Zeiss DSM 982 from
Oberkochen, Germany).
X-ray diffraction analyses (XRD) were performed at room temperature
with a Siemens D 5000 instrument. The grain size and the texture of the
deposits were determined based on the XRD analysis The scan rate was
0.12[degrees][min.sup.-1]over a 20 range from 10 to 100[degrees]. The
Debye--Scherrer equation for the (200) and (111) reflections were used
for estimating the crystallite size of the nickel films. (28), (29) The
texture of the nickel films in the [hkl] crystallographic direction was
estimated using the relative texture coefficient, RTC[(hkl).sub.i]:
RTC[(hkl).sub.i] = [I.sub.hkl]/ [I.sub.hkl]
[I.sub.0]/1/5[[[SUMMATION OVER (TERM) ].sub.I=1].sup.5([I.sub.hkl]/
[I.sub.hkl] [I.sub.0]
where [I.sub.hkl]/[I.sub.hkl] [I.sub.0] is 5the relative intensity
of the (hkl) a reflection, and. 1/5[[[SUMMATION OVER (TERM) ].sub.I=1],
is the average value of all relative intensities for (111), (200),
(220), (311), and (222) crystalline directions. The superscript refers
to the intensities of a randomly oriented nickel powder sample (JCPDS
no. 4-850).
The hardness of the electrodeposits was determined with a Vickers
microhardness device (Fischer Scope HM 2000 S), as described in DIN EN
1S014577. The recorded values are averages of 10 measurements performed
on different locations on the surface of each sample. The thickness of
all coatings is more than five times the maximum indentation depth of 1
p.m in order to reduce effects of the substrates. The standard deviation
was typically between 17.1 and 47.1% (Table 2).
The electrochemical corrosion measurements were made using a
potentiostat/galvanostat/ZRA Gamry reference 600 with three electrodes,
namely: the WE was the Ni deposit, a platinum electrode was used as
auxiliary electrode (CE) and an SCE as RE (Era = 241.2 mV vs. NHE). The
test solution for the corrosion investigations was a 0.5 M
[Na.sub.2]S[O.sub.4] electrolyte (pH 2) at temperature (25 [+ or -]
1[degrees]C).
Potentiodynamic polarization curves were recorded after 30 min and
1 h from immersion of the Ni substrates in the test solution for
corrosion. The potential range for these measurements was fixed
from--600 mV from the open circuit value (OCP) in the cathodic regime,
to 100 mV from the OCP value in the anodic regime, at a scan rate of 1
mV[s sup.-1]. The corrosion current density (icon-) for the particular
specimens was determined by extrapolating the anodic and cathodic Tafel
slopes.
Electrochemical impedance spectroscopy (EIS) measurements were
performed from an initial frequency (IF) of [10.sup.5] Hz to a final
frequency (FF) of 0.1 Hz, with an AC sine wave (amplitude of 10 mV). Ten
points frequency were acquired in the EIS measurements, and the delay
before integration was fixed to 1 s. Impedance spectra were recorded
after different immersion times (10 min, 30 min, 1 h, 2 h, 4 h, 6 h, 24
h, and 30 h) of the Ni layers in the solution for the corrosion tests.
All EIS spectra were analyzed in the Nyquist representation.
[FIGURE 1 OMITTED]
Results and discussion
Current efficiency
Current efficiency for nickel electroplating varied between 86.5
and 99.9%, depending on the current density chosen for electrodeposition
and on the deposition time (Fig. 1).
The CE generally increased with increases in the deposition time
and the current density. Current efficiencies higher than 95% have been
achieved starting from a current density of 5 A [dm.sup.-2], and the
highest value was reached at 10 A [dm.sup.-2],. For 900 s, the yield is
relatively low (86.5%) when compared to that corresponding to the other
deposition times (1800, 3600, and 7200 s); but it increases with
increases in the current density (e.g., for 900 s of deposition time,
the CE was 86.5% at 1 A [dm.sup.-2], and 90% at 2 A [dm.sup.-2]).
Deposit morphology and crystallographic orientation
Figure 2 shows the SEM images obtained for nickel layers
electrodeposited at current densities of 1, 2, 5, and 10 A [dm.sup.-2].
The deposition time varied from 900 to 7200 s (Table 2). The SEM
images taken for all samples indicate significant structural changes in
morphology depending on current densities and deposition times used.
The shape and the size of the deposited layers are strongly
influenced by the current density chosen for electrode1osition. The
layers electrodeposited at 1 and 2 A[dm.sup.-2]present a distinct
separation of the crystalline grains formed. The shape of the grains
obtained at 5 and 10 A [dm.sup.-2] become less ordered, and the grain
separation was not as clear as that observed in the case of layers
deposited at 1 and 2 A [dm.sup.-2]. The general observation that one can
make based on the SEM images is that the grain size of the Ni layers
increases with the current density used in the electrodeposition
experiments. This result is contradictory to previously reported
results, (7-11) in which the grain size of the deposits was reported to
decrease with increasing the current density. This behavior can be
attributed to the increase of the nucleation and growth rates with
increases in the current density.(9), (27)
[FIGURE 2 OMITTED]
The XRD patterns of electrodeposited nickel are shown in Fig 3.
Generally the deposits consist of grains with a weak (200) plane and a
strong (111) plane orientation. Only the layers electrodeposited at 10 A
[dm.sup.-2 ] showed a strong reflection along the (200) plane and a weak
reflection along the (111) plane. The preferred orientation is
represented by the X-ray peak ratio of I(200) //I(111) Grain size was
assessed by the De bye-Scherrer equation. (30)
The values of the preferred orientation and the crystal sizes are
presented in Table 3. Increasing current density from 2 to 10 A
[dm.sup.-2] leads to an increase in grain size, which results from the
evolution of less hydrogen at the cathode interface. (11) A slight
decrease of the grain size was noticed when the current density
increased from 1 to 2 A [dm.sup.-2]. The modification of the growth
interface by the presence of hydrogen changes the surface energy and
growth mechanisms, and then facilitates the formation of a larger grain
size.
Microhardness measurements
It is well known that the microhardness of the electrodeposited
nickel layers can depend on many factors (e.g., electrolyte composition
or operating conditions). The average Vickers hardness (HV) values were
calculated from a set of at least 10 experiments for each deposited Ni
layer on the surface of the deposit. The HV values and the standard
deviation of the HV values are reported in Table 2. Average
microhardness values varied between 213 and 31.9.2 HV for the nickel
deposits. The microhardness of nickel layers vs. current density and
plating time are displayed in Fig. 4.
[FIGURE 3 OMITTED]
The nickel deposits tested indicated that the highest microhardness
values were reached when deposition time was just 900 s. An appreciable
difference for the deposits' microhardness depending on the current
density can be observed. The deposit hardness decreases when the average
current density increases from 1 to 5 A [dm.sup.-2].
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The FIV values seem to decrease with increases in the deposition
time for a given current density. At the same time, the microhardness
decreases with increases in the current density for the same deposition
time chosen. The only exception noticed for this behavior has been for
the electrodeposited layers at 5 A [dm .sup.-2] These layers proved to
be the most uniform and the grain size of the deposited layers was the
smallest compared with other current densities (Table 3).
Potentiodynamic polarization studies
Potentiodynamic polarization diagrams of Ni layers electrodeposited
on copper substrates (1 h deposition time, current densities between 1
and 10 A [dm .sup.-2]) were performed in a corrosive medium (0.5 M
Na2SO4 solution, pH 2), after 30 min and after 1 h from immersion (Fig.
5). The corrosion current density (icorr) and corrosion potential
(Ecorr) are calculated from the intercept of the Tafel slopes.
Polarization resistance ([R.sub.p]) and the corrosion rate (mm[
year.sup.-1]) are estimated from the polarization curves, and the
results are summarized in Table 4. Among all samples, those deposited at
1 and 2 A [dm.sup.-2] exhibit the lowest values of [R.sub.p].
The values for the [i.sub.corr] were not affected by the current
density at which the sample was prepared, or by the time the sample was
kept in the solution for testing its corrosion properties.
[R.sub.p] increases with increases in the immersion time and
current density used to prepare the electrodepos its. The
electrodeposition at 5 A [dm.sup.-2] exhibits the best corrosion
resistance at short times ([E.sub.corr ] = -245.1 mV), which is
attributed to the compact structure; while for the substrates kept long
enough in Na2SO4 solution, the corrosion potential shifts to more
cathodic values. The worst corrosion resistance was observed for the Ni
electrodeposited layers at 2 A dm 2 for 1 h immersion ([E.sub.corr ]=
-153.09 mv).
The polarization resistance calculated with the Stern--Geary
equation3[degrees] from potentiodynamic diagrams (Table 4) was generally
in good agreement with those obtained from impedance measurements (Table
5). However, a bad agreement of the data was seen in the case of the
deposits obtained at 2 and 5 A [dm.sup.-2]. Thus, from potentiodynamic
polarization curves obtained after 1 h of immersion time, the
polarization resistance is 14.7 k[ohm] [cm.sup.2] at 2 A [dm.sup.-2].
and, respectively, R1) is 63.03 k[ohm] [cm.sup.2] at 5 A [dm.sup.-2].
From impedance diagrams the polarization resistance is between 8.9
k[ohm] [cm.sup.2]to 2 A [dm.sup.-2] (after 1 h of immersion) and 53.7
k[ohm] [cm.sup.2] (after 1 h of immersion). We assume that the
differences obtained at 2 and 5 A [dm.sup.-2]. are due to statistical
errors produced by the equivalent circuit chosen for fitting the EIS
data and by the position where the anodic and cathodic Tafel slopes were
defined.
The corrosion rate, Corr., expressed in mm [year.sup.-1 ]was
calculated according to the following relation, taking into account the
corrosion current density ([i.sup.corr] ) jam, which has been determined
from the Tafel representation (Table 4), the density (D) of Ni, and the
mass of Ni lost through corrosion (M), and the valence (V) of the
electroactive species (31):
Corr = [i.sup.corr] (A[dm.sup.-2]) xM(g) /D(g[ cm.sup.-3]) x V
with 3270 = 0.01 x [1 year (in seconds) /96,485] and 96,485 = 1
faraday in coulombs mol1.
No significant differences in corrosion rate were observed for the
Ni layers obtained in this study (Table 4). The rate of corrosion is
somewhat higher for 5 and 10 A [dm.sup.-2]. The current density
increases with increasing potential in the active area for deposit of
nickel electrodeposited at 1 and 2 A [dm.sup.-2]. The curves display the
passive behavior between about--0.2 and 0 V for deposition at 5 and 10 A
[dm.sup.-2]. The critical current densities of passivation for the
nickel samples were approximately similar. It indicates that the
mechanism of passivity and passivation is similar in essence for nickel
electrodeposited and tested for 30 min and 1 h immersion time into 0.5 M
[Na.sub.2]S[O.sub.4].
Electrochemical impedance spectroscopy
The Nyquist plot representations of impedance spectra performed in
0.5 M [Na.sub.2]S[O.sub.4] solution (pH 2) after different immersion
times are shown in Fig. 6. The impedance plots exhibit depressed
semicircles, corresponding to a charge transfer resistance in parallel
with an equivalent capacitance, which can be better expressed in terms
of a constant phase element (CPE).
An equivalent electrical circuit was proposed to account for the
experimental impedance spectra. (31-33) The equivalent circuit, as shown
in Fig. 7, is used to fit the experimental data. A CPE is used as a
substitute for the capacitor to fit the impedance data of the
electrochemical double-layer capacitance, which is ascribed to the
imperfect surface of the electrode.34 In Fig. 7, the Re is the
electrolyte resistance.
An inductive loop can be seen in the low frequency range in the EIS
spectra of the samples that had the lowest thickness (e.g., layers
deposited at 1 A [dm.sup.-2]). This indicates that a mass transport
controlled process characterizes the corrosion of the thin Ni layers at
low frequencies, or that some electroactive intermediates are adsorbed
on the electrode. In the intermediary frequency range, one can see a
capacitive loop, which Indicates the charge-transfer processes in
parallel to the double-layer charging. As one single loop could be
observed, we assume that the corrosion of Ni layers is a process with
just one time constant. No clear tendency associating the diameter of
the capacitive loop with the immersion time, or with the current density
at which the layers were obtained, could be seen. For the layers
deposited at 5 and 10 A[dm.sup.-2], it seems that the corrosion
resistance of the Ni layers is bad in the beginning, increases (from 30
min up to 4 h), and then decreases dramatically up to 30 h. The layers
electro deposited at 2 A[dm.sup.-2.] display a linear decreasing of the
Rp value in time, as indicated by the continuous flattening of the
capacitive loop with increasing immersion time The layers deposited at 1
A [dm.sup.-2.] also presented the best corrosion protection at lower
immersion times, similar to the samples deposited at 2 A[dm.sup.-2.]
Then the Rp value decreases with increases in the immersion time, but
not linearly. In general, all samples, independent of the potential at
which they were obtained, corrode strongly over a longer time scale in
an acidic solution.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The fitted impedance spectra are in good agreement with the
measured impedance spectra (Fig. 6). The calculated values of circuit
elements are listed in Table 5. It can be found that all fitted
corrosion parameters of the electrodeposits vary with the change in
microstructure. The polarization resistance obtained for Ni at 2 A
[dm.sup.-2]is smaller than that of other deposits. No clear influence of
the current density on the Rp or CPE value at which the layer was
obtained was seen.
Conclusions
Nickel layers were electrodeposited from a Watts bath under
different deposition conditions: current densities between 1 and 10 A
[dm.sup.-2]and deposition times between 900 and 7200 s. Current
efficiencies close to 100% were obtained for current densities of 5 and
10 A [dm.sup.-2] and longer deposition times. Varying the deposition
parameters induced changes in the structure and properties of the
deposits. The microstructure of deposits changed with the current
density and deposition time. At a current density of 1 or 2 A
[dm.sup.-2]the nickel crystals were larger and better separated than at
higher current densities. The uniform deposits obtained at 5
A[dm.sup.-2] showed fine grains and better protection against corrosion.
Microhardness was lowest for the layers electrodeposited at 10
A[dm.sup.-2]. The deposit hardness for the nickel Watts bath was highest
for the layers electrodeposited at 1 A [dm.sup.-2]and diminished when
the current density was increased. The corrosion was dependent on the
electrodeposition parameters, and mostly on the current density used in
preparing the deposits. Nickel electrodeposited at 5 A[ dm.sup.-2 ]was
proved to present the best corrosion resistance.
Acknowledgment Financial support for this work was provided by
project SOP HRD--SIMBAD 6853, 1.5/S/ 15--01.10.2008 (D. E. Rusu Cosor)
and by project PNII-PCE-ID no. 2290.
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[c] ACA and OCCA 2011
D. E. Rusu, C. Gheorghies * G. Carac
Departments of Physics and Chemistry, "Dunarea de Jos"
University Galati, Domneasca Str. 47, 800008 Galati, Romania
e-mail: cgheorg@ugal.ro
A. Ispas, A. Bund
Technische Universitat Ilmenau Fakultat fiir Elektrotechnik and
Informationstechnik, Gustav-Kirchhoff-StraBe 6, Arrheniusbau, 98693
Ilmenau, Germany
DOI 10.1007/s11998-011-9343-0
Table 1: Composition of the electrolyte and working parameters
used for Ni electrodeposition
Bath composition Operating conditions
NiS[O.sub.4]6[H.sub.2]0 (g[L.sup.-1]) 236.7 pH
NiCI.sub.2] 6[H.sub.2]0 (g[L.sup.-1]) 50 Temperature([degrees]C)
[H.sub.3]B[O.sub.3] (g[L.sup.-1]) 30 Current density (A
[dm.sup.-2])
Sodium dodecyl sulfate (g[L.sup.-1]) 0.1 Time (min)
Bath composition Operating conditions
NiS[O.sub.4]6[H.sub.2]0 (g[L.sup.-1]) 4.0 [+ or -] 0.2
NiCI.sub.2] 6[H.sub.2]0 (g[L.sup.-1]) 50 [+ or -] 0.1
[H.sub.3]B[O.sub.3] (g[L.sup.-1]) 1; 2; 5; 10
Sodium dodecyl sulfate (g[L.sup.-1]) 15; 30; 60; 120Table 2: Evolution of current efficiency, deposits thickness,
and hardness of nickel layers electrodeposited from Watts bath
Deposition Current Thickness([mu]m)
conditions efficiency
(%)
Current Nr Time
density sample (s)
1 1 900 86.5 5.6
A[dm.sup.-2]
2 1800 95.1 9.2
3 3600 93.9 14.8
4 7200 98.2 27.5
2 A 5 900 90.1 8.6
[dm.sup.-2]
6 1800 95.4 15.0
7 3600 96.6 27.0
8 7200 97.8 51.3
5 A 9 900 94.7 18.0
[dm.sup.-2]
10 1800 97.8 33.5
11 3600 98.3 63.6
12 7200 99.8 127.0
10 A 13 900 99.1 34.4
[dm.sup.-2]
14 1800 98.6 63.8
15 3600 99.4 125.6
16 7200 99.9 250.0
HV Standard
deviation
for HV
(%)
Current
density
1 319.2 20.2
A[dm.sup.-2]
299.1 41.1
289.0 45.2
260.6 40.0
2 A 297.6 27.0
[dm.sup.-2]
272.2 39.3
221.3 30.9
241.0 43.8
5 A 290.8 39.4
[dm.sup.-2]
264.7 34.5
261.8 47.1
289.3 35.4
10 A 267.8 41.1
[dm.sup.-2]
237.2 31.8
256.0 17.1
213.0 39.8Table 3: Preferred orientation and crystal size along the (111) and
(200) crystallographic planes for electrode-posited nickel
Current [I.sub. Crystal size
density (200)]/ (nm)
[I.sub.
(111)]
(111) Standard (200) Standard
deviation (%) deviation (%)
1 0.59 128 9.6 78 11.5
A[dm.sup.-2]
2 A 0.27 97 10.5 56 13.4
[dm.sup.-2]
5 A 0.49 122 9.7 78 11.5
[dm.sup.-2]
10 1.93 187 8.7 194 8.7
A[dm.sup.-2]Table 4: Polarization resistance of nickel layers calculated from
polarization potentiodynamic diagrams obtained at 30 min and 1 h
after immersion in 0.5 M [Na.sub.2]S[0.sub.4], pH 2
Sample Time [E.sub.corr] [I.sub.corr]
conditions (mV) SCE ([micro]A [cm.sup.-2])
1 A [dm.sup.-2] 30 min 192.82 0.51
1 h 158.89 0.57
2 A [dm.sup.-2] 30 min 183.83 0.54
1 h 153.09 0.57
5 A [dm.sup.-2] 30 min 245.1 0.55
1 h 225.36 0.58
10 A[dm.sup.-2] 30 min 216.5 0.58
1 h 234.26 0.58
Sample [[beta].sub.a] [[beta].sub.c]
conditions (mV[dec.sup.-1]) (mV[dec.sup.-1])
1 A [dm.sup.-2] 9.27 -4.43
9.69 -6.61
2 A [dm.sup.-2] 10.06 -5.15
8.02 -4.92
5 A [dm.sup.-2] 8.05 -6.46
8.73 -9.74
10 A [dm.sup.-2] 7.64 -10.18
10.29 -9.53
Sample [R.sup.p] Corr
conditions (k[omega][cm.sup.-2]) ([mu]m [year.sup.-1])
1 A [dm.sup.-2] 7.22 5.49
15.84 6.14
2 A [dm.sup.-2] 9.82 5.81
14.7 6.14
5 A [dm.sup.-2] 25.81 5.92
63.03 6.25
10 A [dm.sup.-2] 22.92 6.25
96.59 6.25Table 5: Polarization resistance and capacitance values of corrosion
of nickel layers after different immersion times, calculated with
the equivalent circuit from Fig. 7
Current Data Immersion
density obtained time
from
impedance
spectra
10 30 1 h 2 h 4 h 6h 24 h
min min
1 A [R.sub.p] 7.2 8.6 13.5 13.7 8.6 14.7 18.0
[dm.sup.-2] (k [omega]
c[m.sup.
-2])
CPE ([mu]F 10.7 20.8 26.5 18.9 14.3 29.7 20.8
c[m.sup.
-2])
2 [R.sub.p] 9.8 10.9 8.9 15.8 13.6 15.5 12.1
A[dm.sup.-2] (k[omega]
c[m.sup.
-2])
CPE ([mu]F 8.9 12.0 15.3 16.4 16.0 10.8 18.5
c[m.sup.
-2])
5 A [R.sub.p] 30.3 33.7 53.7 33.3 52.0 32.2 14.4
[dm.sup.-2] (k[omega]
c[m.sup.
-2])
CPE ([mu]F 17.6 13.2 9.6 15.8 6.7 5.6 13.4
c[m.sup.
-2])
10 A [R.sub.p] 45.0 23.9 98.4 64.8 15.8 5.8 3.6
[dm.sup.-2] (k[omega]
c[m.sup.
-2])
CPE ([mu]F 24.0 17.9 18.5 11.7 10.5 16.9 52.3
c[m.sup.
-2])
Current Data Immersion
density obtained time
from
impedance
spectra
30 h
1 A [R.sub.p] 8.4
[dm.sup.-2] (k [omega]
c[m.sup.
-2])
CPE ([mu]F 14.7
c[m.sup.
-2])
2 [R.sub.p] 8.9
A[dm.sup.-2] (k[omega]
c[m.sup.
-2])
CPE ([mu]F 22.6
c[m.sup
-2])
5 A [R.sub.p] 14.9
[dm.sup.-2] (k[omega]
c[m.sup.
-2])
CPE ([mu]F 15.7
c[m.sup.
-2])
10 A [R.sub.p] 4.3
[dm.sup.-2] (k[omega]
c[m.sup.
-2])
CPE ([mu]F 51.4
c[m.sup.
-2])