Corrosion tests of nickel coatings prepared from a Watts-type bath.
Electrochemical reactions
Corrosion and anti-corrosives
Coatings industry
Rusu, D.E.
Ispas, A.
Bund, A.
Ghcorghies, C.
Carac, G.
Pub Date:
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Date: Jan, 2012 Source Volume: 9 Source Issue: 1
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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


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.


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)


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.


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].



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.



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.


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


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

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; 120

Table 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

                               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

                               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

                              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

                              14  1800        98.6              63.8

                              15  3600        99.4             125.6

                              16  7200        99.9             250.0

                HV     Standard
                       for HV


1             319.2       20.2

              299.1       41.1

              289.0       45.2

              260.6       40.0

2 A           297.6       27.0

              272.2       39.3

              221.3       30.9

              241.0       43.8

5 A           290.8       39.4

              264.7       34.5

              261.8       47.1

              289.3       35.4

10 A          267.8       41.1

              237.2       31.8

              256.0       17.1

              213.0       39.8

Table 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)

                      (111)    Standard      (200)     Standard
                             deviation (%)          deviation (%)

1               0.59    128            9.6     78          11.5

2 A             0.27     97           10.5     56          13.4

5 A             0.49    122            9.7     78          11.5

10              1.93    187            8.7    194           8.7

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                   6.25

Table 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

                           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]

              CPE ([mu]F  10.7  20.8  26.5       18.9  14.3  29.7  20.8

2             [R.sub.p]    9.8  10.9   8.9       15.8  13.6  15.5  12.1
A[dm.sup.-2]  (k[omega]

              CPE ([mu]F   8.9  12.0  15.3       16.4  16.0  10.8  18.5

5 A           [R.sub.p]   30.3  33.7  53.7       33.3  52.0  32.2  14.4
[dm.sup.-2]   (k[omega]

              CPE ([mu]F  17.6  13.2   9.6       15.8   6.7   5.6  13.4

10 A          [R.sub.p]   45.0  23.9  98.4       64.8  15.8   5.8   3.6
[dm.sup.-2]   (k[omega]

              CPE ([mu]F  24.0  17.9  18.5       11.7  10.5  16.9  52.3

Current       Data        Immersion
density       obtained    time

                          30 h

1 A           [R.sub.p]    8.4
[dm.sup.-2]   (k [omega]

              CPE ([mu]F  14.7

2             [R.sub.p]    8.9
A[dm.sup.-2]  (k[omega]

              CPE ([mu]F  22.6

5 A           [R.sub.p]   14.9
[dm.sup.-2]   (k[omega]

              CPE ([mu]F  15.7

10 A          [R.sub.p]    4.3
[dm.sup.-2]   (k[omega]

              CPE ([mu]F  51.4
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