The paper is devoted to the investigation of cathodic and anodic behavior of electrodeposited Ni-P alloys
of different composition in H2SO4 solutions. It was established that increase of phosphorous content
enhances anticorrosion ability of Ni-P coatings and decreases their catalytic activity toward to hydrogen
evolution reaction. The most probable mechanism of this reaction was suggested on the basis of analysis of
experimental data. It assumes that at low overpotentials hydrogen evolution proceeds by Volmer-
Heyrovsky route with limiting step of charge transfer. At higher potentials Volmer-Tafel mechanism with
slow chemical recombination step takes place.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3546

Dolgikh, O.V.

Sapronova, L.V.

Sotskaya, N.V.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 4,  04FCNF04 (4pp) (2012) 
 
 
2304-1862/2012/1(4)04FCNF04(4) 04FCNF04-1  2012 Sumy State University 
Electrochemical Behavior of Electrodeposited Ni-P Coatings in Acidic Solutions  
  
O.V. Dolgikh,* N.V. Sotskaya, L.V. Sapronova 
 
 Voronezh State University, 1, Universitetskaya Sq., 394006 Voronezh, Russia 
 
(Received 15 June 2012; published online 21 August 2012) 
 
The paper is devoted to the investigation of cathodic and anodic behavior of electrodeposited Ni-P al-
loys of different composition in H2SO4 solutions. It was established that increase of phosphorous content 
enhances anticorrosion ability of Ni-P coatings and decreases their catalytic activity toward to hydrogen 
evolution reaction. The most probable mechanism of this reaction was suggested on the basis of analysis of 
experimental data. It assumes that at low overpotentials hydrogen evolution proceeds by Volmer-
Heyrovsky route with limiting step of charge transfer. At higher potentials Volmer-Tafel mechanism with 
slow chemical recombination step takes place.   
 
Keywords: Ni-P Coatings, Anticorrosion Ability, Catalytic Activity, Hydrogen Evolution Reaction, Mech-
anism. 
 
  PACS numbers: 81.40.Np 
 
 
______________ 
* dov@chem.vsu.ru 
1. INTRODUCTION 
 
Electrodeposited Ni-P alloys are widely used in in-
dustry both as corrosion protective and catalytically 
active materials. It is generally accepted that Ni-P 
coatings exhibit higher corrosion resistance than pure 
nickel in acidic and neutral media, which is attributed 
to the suppression of their anodic dissolution [1-4]. At 
the same time conflicting reports were published in 
what concerns their catalytic activity to hydrogen evo-
lution reaction (HER). It follows from published results 
that some Ni-P electrodes are considerably active [5-7], 
while other electrodes even with comparable phospho-
rous content are significantly less active [8, 9]. The rea-
son of such uncertainties might be the differences of 
electrolytes compositions, deposition regimes, coatings 
thickness, etc. So, the aim of this work is to obtain Ni-P 
coatings in the wide range of P content and to investi-
gate their corrosion resistance and catalytic activity to 
HER in 0.05 M H2SO4 solutions.   
 
2. EXPERIMENTAL 
 
Prior to the experiments, the working nickel elec-
trode (Smet.Ni  0.22 cm2) was abraded with emery pa-
per; each successive paper was finer grained. Then, it 
was polished with magnesium oxide and soft chamois 
and degreased with ethyl alcohol. After every opera-
tion, the electrode was thoroughly washed with dis-
tilled water. The Ni–P coatings of various compositions 
were deposited from the electrolyte containing 0.08 M 
NiCl2, 0.20 M NH2CH2COOH, 0.12 M CH3COONa, and 
(0–1.00) M NaH2PO2 (all compounds were of reagent 
grade). The coatings were deposited under galvanostat-
ic conditions (i  –5 mA/cm2, τ  1000 s). Average depos-
it thickness was 1.19  0.14 m. 
Electrochemical behavior of Ni-P coatings was stud-
ied in 0.05 M H2SO4 deaerated solution. Quasi-steady 
state polarization curves were measured from open-
circuit potential at first in cathodic and then in anodic 
direction with scan rate v  1 mV/s. Potential-
relaxation transients were recorded after interruption 
of prior galvanostatic polarization (τ  400 s). Polariza-
tion currents were chosen individually for each alloy. 
Experiments were performed in a conventional 
three-electrode cell. A copper-sulfate electrode  
(Е  – 0.335 V) was used as the reference electrode; a 
platinum electrode with large surface area was used as 
the auxiliary electrode. The polarization was imposed 
with an IPC-compact potentiostat. In the paper, all 
potentials are given with respect to the s.h.e.  
 
3. RESULTS AND DISCUSSION 
 
The electrolytes used for coatings electrodeposition al-
low us to produce Ni-P alloys with phosphorous content 
from 0.1 to 12.6 wt. %. Structure and morphology of ob-
tained coatings were thoroughly described in our previous 
work [10]. Coatings with P content less than 7 wt. % were 
crystalline, while others were X-ray amorphous and con-
sisted of nanocrystalline nickel and its phosphides. Let us 
consider the effect of Ni-P composition on the main char-
acteristics of their electrochemical behavior. 
 
3.1 Corrosion potential and rates of anodic and 
cathodic reactions 
 
Quasi-steady state polarization curves obtained in 
0.05 M H2SO4 solution on studied Ni-P alloys are selec-
tively shown in Fig. 1. Main characteristics of Ni-P elec-
trochemical behavior such as corrosion potentials (Ecor) 
and rates of cathodic (ic) and anodic (ia) reactions were 
defined from polarization curves. The reaction rates were 
estimated by current densities measured at  
Ec  –0.40 V and Ea   0.15 V respectively. All these 
parameters are summarized in Table 1. 
Corrosion potential becomes more positive with in-
crease of phosphorous content in electrodeposited Ni-P 
coatings (see Table 1), indicating the improvement of 
their anticorrosion properties. Furthermore, rate of an-
odic reaction for phosphorous-rich Ni-P alloys three 
times lower than for pure nickel. This result also sup-
ports that the increase in phosphorous content enhances 
the anticorrosion ability of Ni-P coatings in acidic media. 
 
 
O.V. DOLGIKH, N.V. SOTSKAYA, L.V. SAPRONOVA PROC. NAP 1, 04FCNF04 (2012) 
 
 
04FCNF04-2 
 
 
Fig. 1 – E-log(i) polarization curves measured in 0.05 M 
H2SO4 solution on electrodeposited Ni-P coatings with P 
content: 1 – 0; 2 – 0.5; 3 – 1.4; 4 – 3.0; 5 – 6.5; 6 – 12.1 
wt. %. Inset: selection of linear plots on the cathodic polar-
ization curves  
 
In the contrast to the anodic process, the rate of 
cathodic reaction slightly decreases with rise of P 
content up to 6.5 wt. % and then the process in sub-
stantially accelerated. 
According to the general view, the rate of hydro-
gen evolution reaction is defined, on the one hand, by 
the catalytic properties of the material, which de-
pend on their electronic structure, and, on the other 
hand, by its real surface area, which is associated 
with roughness and porosity of the coatings. In order 
to eliminate the contribution of surface development 
in the rate of cathodic reaction we normalized ca-
thodic current densities ic to the corresponding rela-
tive roughness factor values fr  SNi-P / Smet.Ni, which 
were determined for Ni-P coatings of different com-
position earlier by potential-relaxation transients 
method (the values of fr were found to change from 
0.75 for pure Ni-coating to 34.5 for Ni-12.6P) [10]. 
The same normalization procedure was done for an-
odic current densities. The results are presented in 
Table 1. It can be seen that both anodic and cathodic 
reactions decelerate with increase of phosphorous 
content. Observed acceleration of HER by Ni-P coat-
ings with P content higher than 6.5 wt. % is the re-
sult of substantial surface development only. 
 
Table 1 – Corrosion potentials Ecor and rates of cathodic (ic) and anodic (ia) reactions (fr – relative roughness factor). 
 
[P], 
wt.% 
Ecor, 
V 
log (i) (mA/cm2) log (i / fr) (mA/cm2) 
cathodic anodic cathodic anodic 
0.0 -0.160 +0.820 +0.945 +0.945 +1.070 
0.5 -0.151 +0.825 +0.900 +0.939 +1.040 
1.4 -0.148 +0.659 +0.799 +0.772 +0.591 
3.0 -0.122 +1.207 +0.279 +0.974 +0.046 
6.5 -0.129 +1.303 +0.535 +0.304 -0.460 
10.1 -0.137 +1.209 +0.468 -0.380 -1.120 
12.6 -0.127 +1.171 +0.391 -0.365 -1.145 
  
3.2 Mechanism of hydrogen evolution reaction  
 
Despite the number of investigations devoted to the 
hydrogen evolution on Ni-P alloys, mechanism of the 
process still remains uncertain. In order to establish 
the mechanism of HER it is necessary to obtain and 
analyze some diagnostic criteria, namely tafel slopes of 
polarization curves, reaction order in hydrogen ions, 
slopes of E – log(dE/d ) dependencies, sign of dCtot/dE 
derivative [11-13]. 
As it can be seen from Fig. 1, cathodic polarization 
curves in E – log(i) coordinates are rather nonlinear. So, 
we selected two linear plots at low and high overpotentials 
(see inset in Fig. 1). Plot b1c1 with tafel slope b  0.08-
0.15 V can be found only in voltammograms for Ni-P al-
loys with phosphorous content up to 6.5 wt. %. Plot d1e1 
with higher tafel slopes (0.37-0.75 V) is typical for all in-
vestigated alloys. Both tafel slopes increase with phospho-
rous content in Ni-P coatings. Some additional infor-
mation can be obtained from reaction order in hydrogen 
ions. Its values virtually don’t depend on potential and are 
close to unity regardless of P content in Ni-P alloy.  
Tafel slopes bc and reaction order values RH are listed 
in Table 2. 
Table 2 – Tafel slopes bc and reaction orders in hydrogen 
ions RH for Ni-P alloys of different composition 
 
[P], 
wt.% 
bk = dE/dlog(i) 
RH = dlog(i)/dpH 
plot b1c1 plot d1e1 
Nimet. 0.133 – 0.91±0.09 
0.0 0.084 0.371 0.64±0.02 
0.2 0.090 0.417 0.87±0.05 
1.3 0.107 0.423 1.03±0.11 
3.0 0.107 0.465 0.87±0.07 
6.3 0.150 0.412 0.89±0.03 
8.2 – 0.659 0.85±0.05 
11.9 – 0.607 0.88±0.02 
12.6 – 0.594 0.86±0.04 
 
Potential-relaxation transients were recorded for 
metallic nickel electrode and some Ni-P alloys. They 
are shown in Fig. 2. Slope of these dependencies in 
E – log(dE/dτ) coordinates is also diagnostic criterion 
for elucidation of HER mechanism [14]: 
 
 / / / 0( ) lg lg( ) lg( )H
dE
E b b C b i
d
  
 
 ELECTROCHEMICAL BEHAVIOR OF ELECTRODEPOSITED… PROC. NAP 1, 04FCNF04 (2012) 
 
 
04FCNF04-3 
As in the case of polarization curves, E – log(dE/dτ) 
plots are nonlinear, therefore we selected two linear 
regions: b2c2 at low cathodic overpotentials, and d2e2 at 
high overpotentials (see inset in Fig. 2). Slopes of these 
plots are listed in Table 3. Their values correspond to 
tafel slopes for polarization curves.  
 
 
 
Fig. 2 – E-log(dE/dτ ) plots obtained from potential-
relaxation transients measured on metallic Ni (1) and 
electrodeposited Ni-P coatings with P content: 2 – 0; 3 – 
3.0; 4 – 6.5; 5 – 8.8; 6 – 12.6 wt.%. Inset: selection of 
linear sections 
 
Important information about HER mechanism can 
be obtained from dependencies of interfacial 
capacitance on potential. Total interfacial capacitance 
Ctot  CH  Cdl was calculated by combined processing of 
respective polarization curves and potential-relaxation 
transients. Derived dependencies of Ctot on potential for 
metallic nickel and some Ni-P coatings are shown in 
Fig. 3. They are characterized by relatively high (about 
100 μF/cm2) values of interfacial capacitance in the 
vicinity of equilibrium potential and sharp decrease 
with rise of cathodic potential. 
 
Table 3 – Slopes of E – log(dE/d ) dependencies for Ni-P 
alloys of different composition 
 
[P], wt.% 
dE/dlog(dE/dτ) 
plot b2c2 plot d2e2 
Nimet. 0.010 0.213 
0.0 0.011 0.138 
3.0 0.020 0.133 
6.3 0.021 0.137 
8.2 0.005 0.144 
12.6 0.011 0.161 
 
Unambiguous establishment of HER mechanism on 
electrodeposited Ni-P alloys is a problem, which cannot 
be solved on the basis of analysis of one criterion only. 
Even when we use the set of diagnostic criteria, some 
uncertainty of reaction stages nature remains because 
of the deviation of experimentally obtained parameters 
from theoretical ones due to participation of adsorbed 
intermediates in charge transfer step. Therefore, we 
can only talk about the most probable route of the 
process under study. 
 
 
 
Fig. 3 – Dependencies of interfacial capacitance Ctot on 
potential for metallic Ni (1) and electrodeposited Ni-P 
coatings with P content: 2 – 0; 3 – 3.0; 4 – 6.5; 5 – 8.8; 6 
– 12.6 wt.% 
 
The analysis was carried out separately for regions 
of low and high overpotentials. It was found that 
hydrogen evolution reaction on electrodeposited Ni-P 
alloys obeys general laws, regardless of the 
phosphorous content. At low overpotentials it proceeds 
by Volmer-Heyrovsky route with limiting stage of 
charge transfer. This is evidenced, mainly, by 
dependency of interfacial capacitance on potential 
(dCtot/De  0) [12]. At high cathodic potentials Volmer-
Tafel mechanism takes place with slow step of chemical 
recombination, which is testified by very high slopes of 
linear plots d1e1 and d2e2 on voltammograms and 
potential-relaxation transients respectively [12, 13].  
The influence of alloy composition appears in 
variation of tafel slopes both at low and high 
overpotentials, probably due to changes in charge 
transfer coefficients of Volmer stage α1 and surface 
coverage by adsorbed hydrogen H [11]. At low cathodic 
potentials main contribution to the change of tafel 
slope gives the reduction of α1 with increase of 
phosphorous content up to 6.5 wt. %. At high 
overpotentials variation of surface coverage plays a 
major role in increase of tafel slope.   
  
 O.V. DOLGIKH, N.V. SOTSKAYA, L.V. SAPRONOVA  PROC. NAP 1, 04FCNF04 (2012) 
 
 
 04FCNF04-4  
REFERENCES 
 
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K. Juttner, G. Kreysa, Int. Hydrogen Energ. 17, 9 (1992). 
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Electrochem. 46, 918 (2010). 
11. B.V. Tilak, C.-P. Chen, J. Appl. Electrochem. 23, 631 
(1993). 
12. V.V. Kuznetsov, G. Khaldeev V.I. Kichigin, Navodoro-
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(Mashinostroenie: 1993) (in Russian).  
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English

Electrochemical Behavior of Electrodeposited Ni-P Coatings in Acidic Solutions

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 4,  04FCNF04 (4pp) (2012)   2304-1862/2012/1(4)04FCNF04(4) 04FCNF04-1  2012 Sumy State University Electrochemical Behavior of Electrodeposited Ni-P Coatings in Acidic Solutions    O.V. Dolgikh,* N.V. Sotskaya, L.V. Sapronova   Voronezh State University, 1, Universitetskaya Sq., 394006 Voronezh, Russia  (Received 15 June 2012; published online 21 August 2012)  The paper is devoted to the investigation of cathodic and anodic behavior of electrodeposited Ni-P al-loys of different composition in H2SO4 solutions. It was established that increase of phosphorous content enhances anticorrosion ability of Ni-P coatings and decreases their catalytic activity toward to hydrogen evolution reaction. The most probable mechanism of this reaction was suggested on the basis of analysis of experimental data. It assumes that at low overpotentials hydrogen evolution proceeds by Volmer-Heyrovsky route with limiting step of charge transfer. At higher potentials Volmer-Tafel mechanism with slow chemical recombination step takes place.    Keywords: Ni-P Coatings, Anticorrosion Ability, Catalytic Activity, Hydrogen Evolution Reaction, Mech-anism.    PACS numbers: 81.40.Np   ______________ * dov@chem.vsu.ru 1. INTRODUCTION  Electrodeposited Ni-P alloys are widely used in in-dustry both as corrosion protective and catalytically active materials. It is generally accepted that Ni-P coatings exhibit higher corrosion resistance than pure nickel in acidic and neutral media, which is attributed to the suppression of their anodic dissolution [1-4]. At the same time conflicting reports were published in what concerns their catalytic activity to hydrogen evo-lution reaction (HER). It follows from published results that some Ni-P electrodes are considerably active [5-7], while other electrodes even with comparable phospho-rous content are significantly less active [8, 9]. The rea-son of such uncertainties might be the differences of electrolytes compositions, deposition regimes, coatings thickness, etc. So, the aim of this work is to obtain Ni-P coatings in the wide range of P content and to investi-gate their corrosion resistance and catalytic activity to HER in 0.05 M H2SO4 solutions.    2. EXPERIMENTAL  Prior to the experiments, the working nickel elec-trode (Smet.Ni  0.22 cm2) was abraded with emery pa-per; each successive paper was finer grained. Then, it was polished with magnesium oxide and soft chamois and degreased with ethyl alcohol. After every opera-tion, the electrode was thoroughly washed with dis-tilled water. The Ni–P coatings of various compositions were deposited from the electrolyte containing 0.08 M NiCl2, 0.20 M NH2CH2COOH, 0.12 M CH3COONa, and (0–1.00) M NaH2PO2 (all compounds were of reagent grade). The coatings were deposited under galvanostat-ic conditions (i  –5 mA/cm2, τ  1000 s). Average depos-it thickness was 1.19  0.14 m. Electrochemical behavior of Ni-P coatings was stud-ied in 0.05 M H2SO4 deaerated solution. Quasi-steady state polarization curves were measured from open-circuit potential at first in cathodic and then in anodic direction with scan rate v  1 mV/s. Potential-relaxation transients were recorded after interruption of prior galvanostatic polarization (τ  400 s). Polariza-tion currents were chosen individually for each alloy. Experiments were performed in a conventional three-electrode cell. A copper-sulfate electrode  (Е  – 0.335 V) was used as the reference electrode; a platinum electrode with large surface area was used as the auxiliary electrode. The polarization was imposed with an IPC-compact potentiostat. In the paper, all potentials are given with respect to the s.h.e.   3. RESULTS AND DISCUSSION  The electrolytes used for coatings electrodeposition al-low us to produce Ni-P alloys with phosphorous content from 0.1 to 12.6 wt. %. Structure and morphology of ob-tained coatings were thoroughly described in our previous work [10]. Coatings with P content less than 7 wt. % were crystalline, while others were X-ray amorphous and con-sisted of nanocrystalline nickel and its phosphides. Let us consider the effect of Ni-P composition on the main char-acteristics of their electrochemical behavior.  3.1 Corrosion potential and rates of anodic and cathodic reactions  Quasi-steady state polarization curves obtained in 0.05 M H2SO4 solution on studied Ni-P alloys are selec-tively shown in Fig. 1. Main characteristics of Ni-P elec-trochemical behavior such as corrosion potentials (Ecor) and rates of cathodic (ic) and anodic (ia) reactions were defined from polarization curves. The reaction rates were estimated by current densities measured at  Ec  –0.40 V and Ea   0.15 V respectively. All these parameters are summarized in Table 1. Corrosion potential becomes more positive with in-crease of phosphorous content in electrodeposited Ni-P coatings (see Table 1), indicating the improvement of their anticorrosion properties. Furthermore, rate of an-odic reaction for phosphorous-rich Ni-P alloys three times lower than for pure nickel. This result also sup-ports that the increase in phosphorous content enhances the anticorrosion ability of Ni-P coatings in acidic media.   O.V. DOLGIKH, N.V. SOTSKAYA, L.V. SAPRONOVA PROC. NAP 1, 04FCNF04 (2012)   04FCNF04-2   Fig. 1 – E-log(i) polarization curves measured in 0.05 M H2SO4 solution on electrodeposited Ni-P coatings with P content: 1 – 0; 2 – 0.5; 3 – 1.4; 4 – 3.0; 5 – 6.5; 6 – 12.1 wt. %. Inset: selection of linear plots on the cathodic polar-ization curves   In the contrast to the anodic process, the rate of cathodic reaction slightly decreases with rise of P content up to 6.5 wt. % and then the process in sub-stantially accelerated. According to the general view, the rate of hydro-gen evolution reaction is defined, on the one hand, by the catalytic properties of the material, which de-pend on their electronic structure, and, on the other hand, by its real surface area, which is associated with roughness and porosity of the coatings. In order to eliminate the contribution of surface development in the rate of cathodic reaction we normalized ca-thodic current densities ic to the corresponding rela-tive roughness factor values fr  SNi-P / Smet.Ni, which were determined for Ni-P coatings of different com-position earlier by potential-relaxation transients method (the values of fr were found to change from 0.75 for pure Ni-coating to 34.5 for Ni-12.6P) [10]. The same normalization procedure was done for an-odic current densities. The results are presented in Table 1. It can be seen that both anodic and cathodic reactions decelerate with increase of phosphorous content. Observed acceleration of HER by Ni-P coat-ings with P content higher than 6.5 wt. % is the re-sult of substantial surface development only.  Table 1 – Corrosion potentials Ecor and rates of cathodic (ic) and anodic (ia) reactions (fr – relative roughness factor).  [P], wt.% Ecor, V log (i) (mA/cm2) log (i / fr) (mA/cm2) cathodic anodic cathodic anodic 0.0 -0.160 +0.820 +0.945 +0.945 +1.070 0.5 -0.151 +0.825 +0.900 +0.939 +1.040 1.4 -0.148 +0.659 +0.799 +0.772 +0.591 3.0 -0.122 +1.207 +0.279 +0.974 +0.046 6.5 -0.129 +1.303 +0.535 +0.304 -0.460 10.1 -0.137 +1.209 +0.468 -0.380 -1.120 12.6 -0.127 +1.171 +0.391 -0.365 -1.145   3.2 Mechanism of hydrogen evolution reaction   Despite the number of investigations devoted to the hydrogen evolution on Ni-P alloys, mechanism of the process still remains uncertain. In order to establish the mechanism of HER it is necessary to obtain and analyze some diagnostic criteria, namely tafel slopes of polarization curves, reaction order in hydrogen ions, slopes of E – log(dE/d ) dependencies, sign of dCtot/dE derivative [11-13]. As it can be seen from Fig. 1, cathodic polarization curves in E – log(i) coordinates are rather nonlinear. So, we selected two linear plots at low and high overpotentials (see inset in Fig. 1). Plot b1c1 with tafel slope b  0.08-0.15 V can be found only in voltammograms for Ni-P al-loys with phosphorous content up to 6.5 wt. %. Plot d1e1 with higher tafel slopes (0.37-0.75 V) is typical for all in-vestigated alloys. Both tafel slopes increase with phospho-rous content in Ni-P coatings. Some additional infor-mation can be obtained from reaction order in hydrogen ions. Its values virtually don’t depend on potential and are close to unity regardless of P content in Ni-P alloy.  Tafel slopes bc and reaction order values RH are listed in Table 2. Table 2 – Tafel slopes bc and reaction orders in hydrogen ions RH for Ni-P alloys of different composition  [P], wt.% bk = dE/dlog(i) RH = dlog(i)/dpH plot b1c1 plot d1e1 Nimet. 0.133 – 0.91±0.09 0.0 0.084 0.371 0.64±0.02 0.2 0.090 0.417 0.87±0.05 1.3 0.107 0.423 1.03±0.11 3.0 0.107 0.465 0.87±0.07 6.3 0.150 0.412 0.89±0.03 8.2 – 0.659 0.85±0.05 11.9 – 0.607 0.88±0.02 12.6 – 0.594 0.86±0.04  Potential-relaxation transients were recorded for metallic nickel electrode and some Ni-P alloys. They are shown in Fig. 2. Slope of these dependencies in E – log(dE/dτ) coordinates is also diagnostic criterion for elucidation of HER mechanism [14]:   / / / 0( ) lg lg( ) lg( )HdEE b b C b id    ELECTROCHEMICAL BEHAVIOR OF ELECTRODEPOSITED… PROC. NAP 1, 04FCNF04 (2012)   04FCNF04-3 As in the case of polarization curves, E – log(dE/dτ) plots are nonlinear, therefore we selected two linear regions: b2c2 at low cathodic overpotentials, and d2e2 at high overpotentials (see inset in Fig. 2). Slopes of these plots are listed in Table 3. Their values correspond to tafel slopes for polarization curves.     Fig. 2 – E-log(dE/dτ ) plots obtained from potential-relaxation transients measured on metallic Ni (1) and electrodeposited Ni-P coatings with P content: 2 – 0; 3 – 3.0; 4 – 6.5; 5 – 8.8; 6 – 12.6 wt.%. Inset: selection of linear sections  Important information about HER mechanism can be obtained from dependencies of interfacial capacitance on potential. Total interfacial capacitance Ctot  CH  Cdl was calculated by combined processing of respective polarization curves and potential-relaxation transients. Derived dependencies of Ctot on potential for metallic nickel and some Ni-P coatings are shown in Fig. 3. They are characterized by relatively high (about 100 μF/cm2) values of interfacial capacitance in the vicinity of equilibrium potential and sharp decrease with rise of cathodic potential.  Table 3 – Slopes of E – log(dE/d ) dependencies for Ni-P alloys of different composition  [P], wt.% dE/dlog(dE/dτ) plot b2c2 plot d2e2 Nimet. 0.010 0.213 0.0 0.011 0.138 3.0 0.020 0.133 6.3 0.021 0.137 8.2 0.005 0.144 12.6 0.011 0.161  Unambiguous establishment of HER mechanism on electrodeposited Ni-P alloys is a problem, which cannot be solved on the basis of analysis of one criterion only. Even when we use the set of diagnostic criteria, some uncertainty of reaction stages nature remains because of the deviation of experimentally obtained parameters from theoretical ones due to participation of adsorbed intermediates in charge transfer step. Therefore, we can only talk about the most probable route of the process under study.    Fig. 3 – Dependencies of interfacial capacitance Ctot on potential for metallic Ni (1) and electrodeposited Ni-P coatings with P content: 2 – 0; 3 – 3.0; 4 – 6.5; 5 – 8.8; 6 – 12.6 wt.%  The analysis was carried out separately for regions of low and high overpotentials. It was found that hydrogen evolution reaction on electrodeposited Ni-P alloys obeys general laws, regardless of the phosphorous content. At low overpotentials it proceeds by Volmer-Heyrovsky route with limiting stage of charge transfer. This is evidenced, mainly, by dependency of interfacial capacitance on potential (dCtot/De  0) [12]. At high cathodic potentials Volmer-Tafel mechanism takes place with slow step of chemical recombination, which is testified by very high slopes of linear plots d1e1 and d2e2 on voltammograms and potential-relaxation transients respectively [12, 13].  The influence of alloy composition appears in variation of tafel slopes both at low and high overpotentials, probably due to changes in charge transfer coefficients of Volmer stage α1 and surface coverage by adsorbed hydrogen H [11]. At low cathodic potentials main contribution to the change of tafel slope gives the reduction of α1 with increase of phosphorous content up to 6.5 wt. %. At high overpotentials variation of surface coverage plays a major role in increase of tafel slope.      O.V. DOLGIKH, N.V. SOTSKAYA, L.V. SAPRONOVA  PROC. NAP 1, 04FCNF04 (2012)    04FCNF04-4  REFERENCES  1. R.B. Diegle, N.R. Sorensen, C.R. Clayton, M.A. Helfand, Y.C. Yu, J. Electrochem. Soc. 135, 1085 (1988) 2. J. Hajdu, Plat. Surf. Finish. 83, 29 (1996). 3. P. Peeters, G.v.d. Hoorn, T. Daenen, A. Kurowski, G. Staikov, Electrochim. Acta 47, 161 (2001). 4. A. Krolikowski, B. Karbownicka, O. Jaklewicz, Electro-chim. Acta. 51, 6120 (2006). 5. I. Paseka, J. Velicka, Electrochim. Acta 42, 237 (1997). 6. R.K. Sheverdami, A. Lasia, J. Electrochem. Soc. 144, 511 (1997). 7. T. Burchardt, Int. J. Hydrogen Energ. 25, 627 (2000). 8. J.J. Podesta, R.C.V. Piatti, A.J. Arvia, P. Ekundge, K. Juttner, G. Kreysa, Int. Hydrogen Energ. 17, 9 (1992). 9. A. Krolikowski, A. Wiecko, Electrochim. Acta 47, 2065 (2002). 10. O.V. Dolgikh, N.V. Sotskaya, Yu.G. Kravtsova, Russ. J. Electrochem. 46, 918 (2010). 11. B.V. Tilak, C.-P. Chen, J. Appl. Electrochem. 23, 631 (1993). 12. V.V. Kuznetsov, G. Khaldeev V.I. Kichigin, Navodoro-nance of metals in solutions of electrolytes (Mashinostroenie: 1993) (in Russian).  13. A.V. Vvedensky, I.A. Gutorov, N.B. Morozov, Cond. Envi-ronment and Interface Grains 12, 288 (2010) (in Russian).  14. B.E. Conway, L. Bai, D.F. Tessier, J. Electroanal. Chem. 161, 39 (1984).  

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Electrochemical Behavior of Electrodeposited Ni-P Coatings in Acidic Solutions

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