The influence of current density of deposition on Zn–Ni coatings properties
Zn–Ni coatings were deposited under galvanostatic conditions at current density in the range of 20 to 60 mA⋅cm⁻². The influence of current density of deposition on the surface morphology, chemical and phase composition and corrosion resistance was investigated. Structural investigations were perform...
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України
2011
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| Цитувати: | The influence of current density of deposition on Zn–Ni coatings properties / K. Wykpis, M. Popczyk, J. Niedbała, A. Budniok, E. Łągiewka // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 6. — С.107-114. — Бібліогр.: 12 назв. — англ. |
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irk-123456789-1383462018-06-19T03:05:01Z The influence of current density of deposition on Zn–Ni coatings properties Wykpis, K. Popczyk, M. Niedbała, J. Budniok, A. Łągiewka, E. Zn–Ni coatings were deposited under galvanostatic conditions at current density in the range of 20 to 60 mA⋅cm⁻². The influence of current density of deposition on the surface morphology, chemical and phase composition and corrosion resistance was investigated. Structural investigations were performed by the X-ray diffraction (XRD) method. The surface morphology and chemical composition of deposited coatings were studied using a scanning electron microscope JEOL JSM-6480. Studies of general electrochemical corrosion resistance were carried out in the 5% NaCl, using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. Local corrosion resistance was determined by Scanning Kelvin Probe (SKP) method. On the ground of the research, the possibility of deposition of Zn–Ni coatings containing 14…18 at.% Ni was shown. It was stated, that surface morphology, chemical and phase composition of these coatings to a small extent, depend on the current density of deposition. However, current density of deposition determines the quantity of zinc which is co-deposited with nickel and is bounded in the form of an intermetallic compound or a solid solution. Small differences in chemical composition and the uneven distribution of the Zn(Ni) and Ni₂Zn₁₁ phases on coatings surface may cause differences in the local Kelvin potential. As a result, the Zn–Ni coatings exhibit corrosion resistance that varies depending on the deposition current density. The optimal values of current density for the sake of corrosion resistance are found to be j = 30 and 40 mA⋅cm⁻². Покрытия на основе системы Zn–Ni гальваностатически осаждены при плотности тока j = 20…60 mA⋅cm⁻². Исследовано влияние плотности тока осаждения на морфологию поверхности, химический и фазовый состав, а также коррозионную прочность. Для структурных экспериментов использован рентгенодифракционный метод (XRD). Морфологию поверхности и химический состав осажденного покрытия изучали на сканирующем электронном микроскопе JEOL JSM-6480, а коррозионную прочность – в 5%-ом растворе NaCl, применяя методы потенциодинамической поляризации и электрохимической импедансной спектроскопии (EIS). Локальная коррозионная прочность определена с помощью метода сканирующего зонда Кельвина (SKP). Показана возможность осаждения Zn–Ni с 14…18 at.% Ni. Выявлено, что морфология поверхности, химический и фазовый состав этих покрытий слабо зависят от плотности тока осаждения, которая, тем не менее, определяет количество цинка, осажденного вместе с никелем и связанного в форме интерметаллического соединения или твердого раствора. Незначительные отличия в химическом составе и неравномерное распределение фаз Zn(Ni) и Ni₂Zn₁₁ на поверхности покрытия могут обусловить отличия в значениях локального потенциала Кельвина. В результате коррозионная прочность покрытий на основе системы Zn–Ni изменяется в зависимости от плотности тока осаждения. Оптимальные значения плотности тока, при которых коррозионная прочность покрытий наиболее высокая, составляют j = 30 и 40 mA⋅cm⁻². Покриви на основі системи Zn–Ni гальваностатично осаджено за густини струму j = 20…60 mA⋅cm⁻². Досліджено вплив густини струму осадження на морфологію поверхні, хімічний та фазовий склад, а також корозійну тривкість. Для структурних випроб використано рентгенодифракційний метод (XRD). Морфологію поверхні та хімічний склад осадженого покриву вивчали на сканівному електронному мікроскопі JEOL JSM-6480, а корозійну тривкість – у 5%-му розчині NaCl, застосовуючи методи потенціодинамічної поляризації та електрохімічної імпедансної спектроскопії (EIS). Локальну корозійну тривкість визначено за допомогою методу сканівного зонда Кельвіна (SKP). Показано можливість осадження Zn–Ni із 14…18 at.% Ni. Встановлено, що морфологія поверхні, хімічний та фазовий склад цих покривів слабо залежать від густини струму осадження, яка, однак, визначає кількість цинку, що співосаджується з нікелем і зв’язаний у формі інтерметалічної сполуки або твердого розчину. Незначні відмінності у хімічному складі і нерівномірний розподіл фаз Zn(Ni) та Ni₂Zn₁₁ на поверхні покриву можуть спричинити відмінності у значеннях локального потенціалу Кельвіна. В результаті корозійна тривкість покривів на основі системи Zn–Ni змінюється залежно від густини струму осадження. Оптимальні значення густини струму, за яких корозійна тривкість покривів найвища, становлять j = 30 і 40 mA⋅cm⁻². 2011 Article The influence of current density of deposition on Zn–Ni coatings properties / K. Wykpis, M. Popczyk, J. Niedbała, A. Budniok, E. Łągiewka // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 6. — С.107-114. — Бібліогр.: 12 назв. — англ. 0430-6252 http://dspace.nbuv.gov.ua/handle/123456789/138346 en Фізико-хімічна механіка матеріалів Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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English |
| description |
Zn–Ni coatings were deposited under galvanostatic conditions at current density in the range of 20 to 60 mA⋅cm⁻². The influence of current density of deposition on the surface morphology, chemical and phase composition and corrosion resistance was investigated. Structural investigations were performed by the X-ray diffraction (XRD) method. The surface morphology and chemical composition of deposited coatings were studied using a scanning electron microscope JEOL JSM-6480. Studies of general electrochemical corrosion resistance were carried out in the 5% NaCl, using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. Local corrosion resistance was determined by Scanning Kelvin Probe (SKP) method. On the ground of the research, the possibility of deposition of Zn–Ni coatings containing 14…18 at.% Ni was shown. It was stated, that surface morphology, chemical and phase composition of these coatings to a small extent, depend on the current density of deposition. However, current density of deposition determines the quantity of zinc which is co-deposited with nickel and is bounded in the form of an intermetallic compound or a solid solution. Small differences in chemical composition and the uneven distribution of the Zn(Ni) and Ni₂Zn₁₁ phases on coatings surface may cause differences in the local Kelvin potential. As a result, the Zn–Ni coatings exhibit corrosion resistance that varies depending on the deposition current density. The optimal values of current density for the sake of corrosion resistance are found to be j = 30 and 40 mA⋅cm⁻². |
| format |
Article |
| author |
Wykpis, K. Popczyk, M. Niedbała, J. Budniok, A. Łągiewka, E. |
| spellingShingle |
Wykpis, K. Popczyk, M. Niedbała, J. Budniok, A. Łągiewka, E. The influence of current density of deposition on Zn–Ni coatings properties Фізико-хімічна механіка матеріалів |
| author_facet |
Wykpis, K. Popczyk, M. Niedbała, J. Budniok, A. Łągiewka, E. |
| author_sort |
Wykpis, K. |
| title |
The influence of current density of deposition on Zn–Ni coatings properties |
| title_short |
The influence of current density of deposition on Zn–Ni coatings properties |
| title_full |
The influence of current density of deposition on Zn–Ni coatings properties |
| title_fullStr |
The influence of current density of deposition on Zn–Ni coatings properties |
| title_full_unstemmed |
The influence of current density of deposition on Zn–Ni coatings properties |
| title_sort |
influence of current density of deposition on zn–ni coatings properties |
| publisher |
Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
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2011 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/138346 |
| citation_txt |
The influence of current density of deposition on Zn–Ni coatings properties / K. Wykpis, M. Popczyk, J. Niedbała, A. Budniok, E. Łągiewka // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 6. — С.107-114. — Бібліогр.: 12 назв. — англ. |
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Фізико-хімічна механіка матеріалів |
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107
Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2011. – ¹ 6. – Physicochemical Mechanics of Materials
THE INFLUENCE OF CURRENT DENSITY OF DEPOSITION
ON PROPERTIES OF Zn–Ni COATINGS
K. WYKPIS, M. POPCZYK, J. NIEDBAŁA, A. BUDNIOK, E. ŁĄGIEWKA
University of Silesia, Institute of Materials Science, Katowice
Zn–Ni coatings were deposited under galvanostatic conditions at current density in the
range of 20 to 60 mA⋅cm–2. The influence of current density of deposition on the surface
morphology, chemical and phase composition and corrosion resistance was investigated.
Structural investigations were performed by the X-ray diffraction (XRD) method. The
surface morphology and chemical composition of deposited coatings were studied using a
scanning electron microscope JEOL JSM-6480. Studies of general electrochemical
corrosion resistance were carried out in the 5% NaCl, using potentiodynamic and electro-
chemical impedance spectroscopy (EIS) methods. Local corrosion resistance was determined
by Scanning Kelvin Probe (SKP) method. On the ground of the research, the possibility of
deposition of Zn–Ni coatings containing 14…18 at.% Ni was shown. It was stated, that
surface morphology, chemical and phase composition of these coatings to a small extent,
depend on the current density of deposition. However, current density of deposition deter-
mines the quantity of zinc which is co-deposited with nickel and is bounded in the form of
an intermetallic compound or a solid solution. Small differences in chemical composition
and the uneven distribution of the Zn(Ni) and Ni2Zn11 phases on coatings surface may cause
differences in the local Kelvin potential. As a result, the Zn–Ni coatings exhibit corrosion
resistance that varies depending on the deposition current density. The optimal values of
current density for the sake of corrosion resistance are found to be j = 30 and 40 mA⋅cm–2.
Keywords: electrodeposition, Zn–Ni coatings, corrosion resistance, SKP method.
The interest in zinc and their alloys [1–4] results from the good corrosive resis-
tance of these materials and search of a suitable alternative to toxic cadmium coatings.
It was proved, that corrosion resistance of Zn–Ni coatings depends on their chemi-
cal compositions, morphology of surface and structures [1, 5–8]. These properties are
formed depending on the electrodeposition conditions [6, 8].
Studies of Zn–Ni deposition in electrolytes containing ammonia ions have shown
that normal or anomalous codeposition takes place depending on the value of potential
or cathodic current. As a result Zn–Ni coatings may contain from ≥ 95 wt.% to
15 wt.% Ni [9].
Electrolytic Zn–Ni alloys are characterized by the occurrence of a wider range of
different phases. It was stated that nickel is dominant in a layer adjacent to the sub-
strate, independently on a kind of the substrate. Next, zinc in the nickel solid solution
(α-phase) is created, depending on the deposition current density [8]. Unlike an equi-
librium metallurgical alloy, α- and β-phases are not formed in electrolytic Zn–Ni alloy.
Using the potentiodynamic stripping method, potential values of current peaks were
attributed to the values of selective dissolution of particular phases [10]. In this way,
the γ-phase i.e. Ni5Zn21 and η-phase being 1% nickel in zinc solid solution, are formed.
On the basis of electrochemical investigations it was found that the γ-phase is very che-
mically active by the selective dissolution of zinc. The high corrosion resistance and
good mechanical properties of Zn–Ni layers are connected with the presence of the
γ-phase i.e. Ni5Zn21.
Corresponding author: K. WYKPIS, e-mail: katarzyna.wykpis@us.edu.pl
108
The aim of the present work was to obtain of electrolytic Zn–Ni coatings and to
determine of an influence of deposition current density on their chemical compositions,
structure and corrosion resistance.
Experimental. Electrolytic Zn–Ni coating was obtained from the ammonia bath
of composition (g⋅dm–3): NiSO4⋅7H2O – 50; ZnSO4⋅7H2O – 100; Na2SO4 – 75;
(NH4)2SO4 – 38; NH4OH – 250 cm3⋅dm–3. The temperature of the bath was 298 K; pH
was kept in the range of 9.6 to 10.4. The process of electrodeposition was carried out in
the galvanostatic conditions at current density in the range of 20 to 60 mA⋅cm–2.
The Zn–Ni coating was deposited on austenitic steel (OH18N9). The preparation
of substrate surface consisted of the following steps: cleaning with a detergent solution,
chemical treatment with HCl solution (1:1), rinsing in distilled water and degreasing.
Prior to deposition, the steel substrate was activated in HCl solution, using ca-
thode current density j = 5 mA⋅cm–2, for 2 min. The nickel underlayers, obtained from
the bath containing 350 g⋅dm–3 NiCl2⋅6H2O and 111 cm3⋅dm–3 HCl, were deposited
before obtaining of the Zn–Ni coatings in order to assure adhesion of the Zn–Ni
coating to the substrate.
The surface morphology and surface chemical composition of deposited coatings
were studied using a scanning electron microscope (JEOL JSM-6480) with EDS
attachment.
The XRD patterns were measured using the Philips X’Pert PW 3040/60 X-ray
diffractometer with copper radiation (λKα = 1.54056 Å). A graphite monochromator
was used to select the Kα radiation.
The electrochemical corrosion resistance of the prepared coatings was investiga-
ted in a three-electrode cell using the potentiodynamic and electrochemical impedance
spectroscopy (EIS) methods. These measurements were carried out in 5% NaCl solu-
tion, at a temperature of 293 K using AUTOLAB® electrochemical system. The auxi-
liary electrode was a platinum mesh and the reference electrode was the saturated
calomel electrode (SCE). The values of corrosion potential, corrosion current and po-
larization resistance were determined by the Stern method. The electrochemical impe-
dance measurements were performed at the corrosion potential. In these measurements
the amplitude of the ac signal 0.005 V. A frequency range from 10 kHz to 0.1 Hz was
covered with ten points per decade.
The SKP measurements were made using a Scanning Electrochemical Worksta-
tion Model 370 (Princeton Applied Research AMETEK). The SKP scans were made of
300 µm × 300 µm area surface. The vibrating amplitudes of the scanning probe were
adjusted to 30 µm. The Kelvin probe was placed above the surface of the Zn–Ni
coatings at a height of about 300 µm. The topography of Zn–Ni coatings surface was
characterized using Kelvin probe.
The corrosion behaviour of the Zn–Ni coatings was also examined using accelera-
ted corrosion test in a salt chamber (HK 400, KOHLER) according to PN-EN ISO 9227.
These coatings were subject to salt spray (5% NaCl) exposure at 308 K for 96 h.
Results and discussion. All obtained Zn–Ni coatings have a grey and smooth sur-
face. These coatings show good adhesion to the substrate and lack of cracks. Only on
the surface of the coating deposited at j = 60 mA⋅cm–2 small cracks are observed (Fig. 1).
The results of surface chemical composition analysis show, that the chemical
composition of coatings depends to a small degree on the deposition current density
and is in the range from 14.2±0.4 to 18.0±0.4 at.% Ni. The surface chemical composi-
tion initially decreases and then increases with an increase of deposition current den-
sity. The coating deposited at j = 30 mA⋅cm–2 is characterized by the least of the nickel
contents e.g. 14.2 at.%.
109
Fig. 1. Surface morphology
of Zn–Ni coatings deposited
at the current density j:
a – 20 mA⋅cm–2;
b – 30; c – 40; d – 50;
e – 60 mA⋅cm–2.
The X-ray phase analysis showed the differences in phase composition of the
Zn–Ni coatings depending on the deposition current density (Fig. 2). The presence of
reflexes coming from Zn phase shows only the XRD pattern of Zn–Ni coating deposi-
ted at j = 20 mA⋅cm–2. All X-ray diffraction patterns show the presence of reflexes cor-
responding to the Zn(Ni) solid solution and Ni2Zn11 intermetallic phases. These phases
are the result of co-deposition of zinc and nickel ions. The zinc content in nickel in the
case of the Ni(Zn) solid solution, determined on the basis of the Vegard law, increases
with an increase in deposition current density from about 23 to 30%. Thus, the current
density influences the amount of zinc connected with nickel in the solid solution or the
intermetallic compound. The differences in the phase composition and a non-uniform
distribution of the Ni(Zn) and Ni2Zn11 phases on the surface of obtained coatings, may
cause the differences in the coatings corrosion resistance.
Fig. 2. X-ray diffraction pattern of Zn–Ni coating deposited at current density j:
a – 20 mA⋅cm–2; b – 50 mA⋅cm–2. – Zn; – Ni2Zn11; – Ni(Zn).
Open circuit potentials of the
coatings were determined for 20 h.
A range of ±0.050 V was chosen
from the determined value and a po-
tentiodynamic curve was recorded
with a rate of v = 0.060 V⋅min–1.
On the ground of the obtained de-
pendences j = f (E), the values of
corrosion parameters were determi-
ned. It was found that for the Zn–Ni
coating obtained at j = 30 mA⋅cm–2,
Table 1. Corrosion parameters of Zn–Ni
coatings determined by the potentiodynamic
method depending on the current density
of deposition j
j, mA⋅cm–2 Ecor, V jcor, µA⋅cm–2 Rp, kΩ⋅cm2
20 –0.791 0.90 6.81
30 –0.755 0.14 37.41
40 –0.783 0.22 16.87
50 –0.807 0.81 8.45
60 –0.802 0.41 8.84
110
the value of corrosion current is lower and the value of polarization resistance is higher
compared with the other Zn–Ni coatings (Table 1). It is suggested, that the Zn–Ni
coating obtained at j = 30 mA⋅cm–2 is more corrosion resistant in 5% NaCl solution
than other Zn–Ni coatings.
Results of EIS investigations are submitted in the form of Nyquist and Bode
diagrams (Z″ = f (Z′), log |Z| = f (log ω) and Φ = f (log ω)) (Fig. 3–5).
Fig. 3. Fig. 4.
Fig. 3. Dependences of Z″ = f (Z′) for the Zn–Ni coatings obtained at the current densities:
j = 30 mA⋅cm–2 (1) and 50 mA⋅cm–2 (2) ( , – experimental points; — – approximation line).
Fig. 4. Dependences of log |Z| = f (log ω) for the Zn–Ni coatings obtained at the current
densities: – j = 20 mA⋅cm–2; – 30; – 40; – 50; – 60 mA⋅cm–2.
Fig. 5. Dependences of Φ = f (log ω)
for the Zn–Ni coatings obtained
at the current densities:
– j = 20 mA⋅cm–2; – 30; – 40;
– 50; – 60 mA⋅cm–2.
Impedance measurements display a depressed semicircle on the complex plane
plots. Examples of such plots for two coatings, which are characterized the extreme cor-
rosion resistance (obtained at j = 30 and 50 mA⋅cm–2) (Table 1) are presented in Fig. 3.
Using the complex nonlinear least-squares (CNLS) fitting program, the real (Z′) and
imaginary (Z″) components of the impedance, could be analysed. The coating obtained
at j = 30 mA⋅cm–2 exhibits the highest values of real and imaginary components. This
means that this coating is most corrosion resistant in the 5% NaCl solution compared
with the other obtained Zn–Ni coatings.
It has been found that the impedance of obtained coatings could be described by
the two-CPE electrode model, which represents the solution resistance, Rs, in series
with two parallel CPE – Rp elements (Fig. 6), explains the impedance behavior of the
coating containing pear-shape pores (Rp1, Rp2 [Ω⋅cm2] are the polarization resistances,
CPE1, CPE2 are the constant phase elements, where ZCPE = 1/[T(jω)φ] [11, 12]. The
Nyquist plots, presented in Fig. 3, show a good agreement between the experimental
points and approximations.
111
Fig. 6. Equivalent circuit
scheme for the two-CPE
electrode model.
As a result of approximation of the experimental data, the following parameters
could be obtained: Rp1, T1, φ1, Rp2, T2, φ2 and Rs, where T1, T2 are the capacity parame-
ters and φ1, φ2 are the CPE angles [10, 11]. Sum of Rp1 + Rp2 gives a total value of pola-
rization resistance, which is the highest for the Zn–Ni coating obtained at j = 30 mA⋅cm–2
(Table 2). The total values of Rp calculated by EIS method are approximately compa-
rable with values of Rp obtained using potentiodynamic method and therefore it also
could be a measure of corrosion resistance of coatings. The corrosion resistance of
obtained coatings initially increases and then decreases with an increase in deposition
current density. The maximum of corrosion resistance is observed for the coating depo-
sited at j = 30 mA⋅cm–2.
Table 2. Corrosion parameters of Zn–Ni coatings determined by EIS method
depending on current density of deposition j
j,
mA⋅cm–2
Rp1,
kΩ⋅cm2 T1 φ1
Rp2,
kΩ⋅cm2 T2 φ2
Rs,
Ω⋅cm2
20 0.78 0.000092 0.53 5.98 0.000172 0.72 0.65
30 1.60 0.000014 0.60 35.88 0.000031 0.81 0.64
40 1.19 0.000024 0.66 15.60 0.000051 0.82 0.68
50 0.85 0.000076 0.59 7.56 0.000126 0.79 0.69
60 0.93 0.000058 0.55 7.94 0.000103 0.78 0.63
Values of logarithm of impedance module for the obtained coatings are compar-
able in the range of high frequency. These values are the highest in the range of low
frequency for the Zn–Ni coating obtained at the current density of j = 30 mA⋅cm–2
(Fig. 4). Values of the phase angle for this coating, in the almost whole range of fre-
quency are the highest and show a wide range of independence of a logarithm of an-
gular frequency in comparison with the other coatings (Fig. 5). Moreover, Φ = f (logω)
dependences obtained for the coatings deposited at j = 20, 40, 50 and 60 mA⋅cm–2 are
characterized by the presence of two time-constants but for the coating deposited at
j = 30 mA⋅cm–2 only one time-constant is observed. Hence, it could be concluded that
in the case of coating obtained at the current density of j = 30 mA⋅cm–2 only one phase
corrodes and for the other coatings the corrosion process proceeds in two phases.
Based on these electrochemical investigations it was found that Zn–Ni coating
obtained at j = 30 mA⋅cm–2 is more corrosion resistant in 5% NaCl solution than the
other obtained Zn–Ni coatings.
Topography maps of deposited coatings were obtained using a Kelvin probe (Fig. 7).
It was stated that the coatings are characterized by a development area with an
average height of unevenness from about 5 to 8 mm. Coatings deposited at the current
density j = 30 and 40 mA⋅cm–2 are characterized by smaller development and greater
uniformity of the surface compared with the coatings deposited at j = 20, 50 and
60 mA⋅cm–2. Hence, the smallest number of corrosion centres can be expected for the
Zn–Ni coating deposited at j = 30 and 40 mA⋅cm–2.
112
Fig. 7. Topography maps
of Zn–Ni coatings deposited
at the current density j
obtained by a Kelvin probe:
a – 20 mA⋅cm–2; b – 30;
c – 40; d – 50;
e – 60 mA⋅cm–2.
The SKP maps obtained for Zn–Ni coatings deposited at current density in the
range of 20 to 60 mA⋅cm–2 show the differences in local potentials values (Fig. 8). It
is observed that values of Kelvin potential difference measured at the surface of depo-
sited coatings initially decrease and next increase with an increase in deposition current
density. Recorded local potentials are in the range from ∆E = 0.3 V to ∆E = 0.2 V. It
was found that the Zn–Ni coatings deposited at j = 30 and 40 mA⋅cm–2 are characterri-
zed by the smallest values of local potential and the most uniform their distribution on
the surface (Fig. 8b, c). This means that these coatings are more resistant to local cor-
rosion in comparison with coatings deposited at other current densities. This confirms
the good corrosion resistance of these coatings, shown in studies of the total corrosion.
Fig. 8. SKP potential
profile of Zn–Ni coatings
deposited at the current
density j: a – 20 mA⋅cm–2;
b – 30; c – 40; d – 50;
e – 60 mA⋅cm–2.
This correlates well with a small development of the surface of these coatings,
which can lead to a smaller number of favored places of corrosive attack, compared to
other Zn–Ni coatings.
113
Zn–Ni coatings obtained in the optimum range of current density were sub-
jected to visual observation before and after the test in a salt chamber. It was found
that the coatings are covered with little corrosion products. This demonstrates the
good corrosion resistance of these coatings to the salt spray (Fig. 9).
Fig. 9. Zn–Ni coating
deposited at j = 40 mA⋅cm–2
before (a) and after (b)
corrosion resistance
investigations
in a salt chamber.
CONCLUSIONS
On the basis of this research, the possibility of galvanostatic deposition of Zn–Ni
coatings containing 14…18 at.% Ni is shown. It is stated, that surface morphology,
chemical and phase composition of these coatings to a small degree depends on the
current density of deposition. However, the current density of deposition determines
the quantity of zinc which is co-deposited with nickel and is bound in the form of an
intermetallic compound or a solid solution.
Small differences in chemical composition and the non-uniform distribution of
Ni (Zn) and Ni2Zn11 phases on the surface of coatings, may cause local variations of
Kelvin potential. As a consequence corrosion resistance of Zn–Ni coatings is varied
depending on the current density. Coatings deposited at the current density j = 30 and
40 mA⋅cm–2 are characterized by the smallest values of local potential and the most
uniform distribution on the surface.
On the ground of electrochemical investigations it is found that the Zn–Ni coa-
tings obtained at the j = 30 and 40 mA⋅cm–2 are more corrosion resistant in 5% NaCl
solution than the others Zn–Ni coatings. The lower value of corrosion current and also
the higher value of polarization resistance results from above.
Moreover, it can be concluded that parameters calculated by EIS method can be a
measure of corrosion resistance of coatings and results are confirmed by the potentio-
dynamic method.
РЕЗЮМЕ. Покриви на основі системи Zn–Ni гальваностатично осаджено за густини
струму j = 20…60 mA⋅cm–2. Досліджено вплив густини струму осадження на морфологію
поверхні, хімічний та фазовий склад, а також корозійну тривкість. Для структурних ви-
проб використано рентгенодифракційний метод (XRD). Морфологію поверхні та хімічний
склад осадженого покриву вивчали на сканівному електронному мікроскопі JEOL JSM-6480,
а корозійну тривкість – у 5%-му розчині NaCl, застосовуючи методи потенціодинамічної
поляризації та електрохімічної імпедансної спектроскопії (EIS). Локальну корозійну трив-
кість визначено за допомогою методу сканівного зонда Кельвіна (SKP). Показано можли-
вість осадження Zn–Ni із 14…18 at.% Ni. Встановлено, що морфологія поверхні, хімічний
та фазовий склад цих покривів слабо залежать від густини струму осадження, яка, однак,
визначає кількість цинку, що співосаджується з нікелем і зв’язаний у формі інтерметаліч-
ної сполуки або твердого розчину. Незначні відмінності у хімічному складі і нерівномір-
ний розподіл фаз Zn(Ni) та Ni2Zn11 на поверхні покриву можуть спричинити відмінності у
значеннях локального потенціалу Кельвіна. В результаті корозійна тривкість покривів на
основі системи Zn–Ni змінюється залежно від густини струму осадження. Оптимальні
значення густини струму, за яких корозійна тривкість покривів найвища, становлять
j = 30 і 40 mA⋅cm–2.
114
РЕЗЮМЕ. Покрытия на основе системы Zn–Ni гальваностатически осаждены при
плотности тока j = 20…60 mA⋅cm–2. Исследовано влияние плотности тока осаждения на
морфологию поверхности, химический и фазовый состав, а также коррозионную проч-
ность. Для структурных экспериментов использован рентгенодифракционный метод
(XRD). Морфологию поверхности и химический состав осажденного покрытия изучали
на сканирующем электронном микроскопе JEOL JSM-6480, а коррозионную прочность –
в 5%-ом растворе NaCl, применяя методы потенциодинамической поляризации и электро-
химической импедансной спектроскопии (EIS). Локальная коррозионная прочность опре-
делена с помощью метода сканирующего зонда Кельвина (SKP). Показана возможность
осаждения Zn–Ni с 14…18 at.% Ni. Выявлено, что морфология поверхности, химический
и фазовый состав этих покрытий слабо зависят от плотности тока осаждения, которая, тем
не менее, определяет количество цинка, осажденного вместе с никелем и связанного в
форме интерметаллического соединения или твердого раствора. Незначительные отличия
в химическом составе и неравномерное распределение фаз Zn(Ni) и Ni2Zn11 на поверхнос-
ти покрытия могут обусловить отличия в значениях локального потенциала Кельвина. В
результате коррозионная прочность покрытий на основе системы Zn–Ni изменяется в зави-
симости от плотности тока осаждения. Оптимальные значения плотности тока, при кото-
рых коррозионная прочность покрытий наиболее высокая, составляют j = 30 и 40 mA⋅cm–2.
Acknowledgement. This research was financed with Project PBZ-MNiSW-
4/01/1/2007.
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Received 11.02.2010
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