study of the influence of the electrolysis parameters on ... · of steel bodies [8-18]. ......
TRANSCRIPT
J. Chem. Chem. Eng. 1 (2016) 13-27
doi: 10.17265/1934-7375/2016.01.003
Study of the Influence of the Electrolysis Parameters on
Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from
Electrolytes Containing Complexing Ligands
Gigla Tsurtsumia*, Nana Koiava, David Gogoli, Izolda Kakhniashvili, Tinatin Lejava, Nunu Jokhadze, Ermi
Kemoklidze
Rafael Agladze Institute of Inorganic Chemistry and Electrochemistry of Ivane Javakhishvili Tbilisi State University, 0186 Tbilisi,
Georgia
Abstract: The process of obtaining of high quality Mn-Zn, Mn-Cu and Mn-Cu-Zn alloy coatings from complexing ligands-citrate,
EDTA (ethylene diaminetetra acetic acid) and nitrilotriacetic acid solutions was studied. Factors affecting stability of solutions
containing ligand or ligands and influence of electrolysis parameters: electrolyte composition, pH, cathodic current density on
chemical composition of the obtained coatings, on their current efficiency, morphology and structure were investigated.
Key words: Mn-Zn, Mn-Cu, Mn-Cu-Zn, electrodeposition, complexing ligands, chemical composition, current density, current
efficiency, SEM, XRD.
1. Introduction
Obtaining of electrodeposited coating of alloys of
definite functional properties and development of
current methods is one of the priorities of
electrochemical deposition [1, 2]. As non-deficient
protecting material with high negative standard
potential, electrodeposited coatings of manganese and
its alloys, has been attracting attention of researchers
for a long time. Cu, Zn, Fe, Ni alloys are characterized
by high sacrificial protection and by their
technical-economic efficiency are as good as well
known protectors made on base of aluminum and
manganese [3]. Wide application of pure manganese
coatings is hindered by its relatively high chemical
activity and brittleness, which is conditioned by
changes of crystalline modification. Electrodeposition
yields plastic γ-Mn, with BCT (body central tetragonal)
structure, which is quickly transformed into BCC
*Corresponding auther: Gigla Tsurtsumia, doctor of
chemistry, research fields: electrochemistry of manganese and
its compounds, electrosynthesis, corrosion of metals, and
electrochemical methodsfor sewage treatment.
(body centered cubic) α-Mn of stable but brittle
properties [4]. To decrease chemical activity and
brittle form of manganese it is alloyed with metals of
lower negative standard potential, e. g. with Cu, Zn,
Co, Ni, Fe and others, which leads to stabilization of
γ-Mn form. For example, introduction of up to 3% of
copper into manganese contributes to preservation of
γ-Mn modification for a long period [5-7]. In contrast
with pure Zn electrodeposited coating
manganese-containing electrodeposited alloys (e.g.
Mn-Zn alloys) provide efficient corrosion protection
of steel bodies [8-18]. For electrodeposition of Mn-Zn
alloys sulfate solutions are mainly used. These
solutions often contain complexing ligands—citrate
and EDTA (ethylene diaminetetra acetic acid), as in
combination as well as separately, in forms of
additives. Irrespective of high quality of alloy coatings
and their acceptable current efficiency, the process is
complicated because of precipitation of manganese
citrate complex from the used electrolytes with the
lapse of time. Electrolytes containing fluoroborates [19]
and boric-sorbitol complex [20] are also known, but
D DAVID PUBLISHING
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
14
their toxicity, corrosiveness and high cost make
obstacles to their intense practical application.
The goal of the present research is obtaining of high
quality coating of manganese-containing alloys: Mn-Zn,
Mn-Cu and Mn-Cu-Zn from sulfate solutions with
complexing ligands-citrate, EDTA and nitrilotriacetic
acid; study of factors affecting stability of ligand or
ligand-containing solutions and determination of
influence of parameters of electrolysis-electrolyte
composition, pH, cathodic current density on chemical
composition morphology and structure of coatings and
on current efficiency, of the process.
2. Experiments
Electrodeposited coatings of manganese-containing
alloys were obtained using electrolytes of various
compositions. Electrolytes were prepared from
analytical grade chemicals: MnSO4·H2O, (NH4)2SO4,
ZnSO4·7H2O, CuSO4·5H2O, sodium citrate
(Na3C6H5O7·5.5H2O), EDTA (Na2H2C10H12O8N2·2H2O)
and nitrilotriacetic acid sodium salt
(Na3C6H6O6N·H2O). All electrolytes used for
obtaining electrodeposited coatings of
manganese-containing alloys contained ammonium
sulfate as a buffer additive. Initially, solutions
containing manganese and ammonium sulfates were
prepared. Ammonia was used to adjust solution to pH 7
in order to remove Fe2+
, Fe3+
, Co2+
, Ni2+
, Cu2+
ions.
Ammonium sulfide (5-8 g/L) was added to the heated
solution (70 °C) under intense stirring for 1 h. After
cooling, the solution was filtered through paper filters.
The obtained filtrate was boiled and cooled again at
ambient temperature and was filtered to remove fine
dispersed sulfur. Finally, the solution was purified by
preelectrolysis using graphite anodes and steel cathode
(ik = 2-4 A·dm-2
) until quality manganese plating was
obtained. Concentration of manganese ions was
determined in the purified manganese-ammonium
solution by Folgardt’s method, while composition of
ammonium sulfate was determined by formaldehyde
methods [21].
Electrolytes for obtaining electrodeposited coatings
of manganese containing alloys were prepared as
follows: Ligand—containing salt of definite quantity
was added to the calculated amount of zinc sulfate and
copper sulfate-solutions in a separate container after
complete dissolution of ligand—containing salt,
definite volume of purified manganese-ammonia
solution was added, pH of solution was adjusted by
concentrated sodium hydroxide and sulfuric acid
solutions.
Electrolysis was carried out in a rectangular bath
made of organic glass (polymethyl methacrylate),
separated by belting diaphragms (boiled in 1 mol/dm3
Na2SO4 water solution for 10 h) into three parts-two
anodic and middle-cathodic compartments. Catholyte
was circulated through cathodic chamber by
micro-pump and silicon tubes from the reservoir for
circulation of solution. Volume of circulated catholyte
was 500 mL, while volume of no stirring anolyte-300
mL. The tank for catholyte circulation was equipped
with electric heater placed in a quartz pipe for heating
the solution (30 ºC) and with the glass electrode for
pH control (МР5129, China).
Constant pH in catholyte during electrodeposition
was maintained by drop-wise addition of 45% H2SO4
into circulation tank. Electrodeposition was performed
in galvanostatic regime by means of constant current
source YK–AD6025 (China). Copper, steel or glassy
carbon electrodes of 4 cm2
area were used as cathodes.
To make chemical analysis of the coating, glassy
carbon electrodes appeared to be the best since in
distinct from other metallic electrodes, coating was
easily removed from its surface. The electrodes were
polished before each run with 1.00 μm and 0.25 μm
diamond powder and washed with distilled water.
TiO2 and RuO2 oxide modified titanium plates with
total area of 120 cm2 were used as anodes.
To study morphology and structure of alloy the
authors used, scanning electron microscope JSM-6510
series JEOL Ltd. (Japan) and diffractometer of
Russian origin (copper anode Кα-emission, λ =
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
15
1.54184 Å ), respectively. Chemical composition of
coating was determined by X-ray-fluorescence analysis
method (Delta-Analyzer < INNOV-X SYSTEMS >
USA) and by X-ray energy dispersion micro-spectral
analyzer (JSM 6510 LM, Japan). Adsorption spectra of
solutions for electrodeposition in visible region be
taken by colorimeter (КФК-2МП, Russia).
Current efficiency of alloy electrodeposition process
(Фalloy) was computed from the formula [22, 23]:
Фalloy = malloy/(I·τ·qalloy)·100%, (1)
where, malloy is the mass of alloy deposited on the
cathode, in grams, I—transmitted current in amperes,
τ—electrolysis time in hrs, q—electrochemical
equivalent of alloy.
qalloy = 1/(ω1/q1 + ω2/q2 +...), (g·A-1
)/h (2)
where, ω1, ω2, and q1, q2, are metal mass shares in
alloys and their electrochemical equivalents,
respectively.
Thickness (δ) of coating was calculated from the
formula:
δ = τ·icath·qalloy·Фalloy/dalloy, cm (3)
where, icath-cathodic current density, A·cm-2
,
dalloy-density of alloy, g/cm3.
3. Results and Discussions
3.1 Zn-Mn alloy Electrodeposition from Sodium
Citrate (Na3Cit) and Sodium Nitrilotriacetic
(Na3Y·H2O) Solutions
White-grayish Zn-Mn coating was obtained from
manganese-ammonium and zinc sulfates electrolyte,
which contained sodium citrate (Na3Cit) as an additive.
Electrolyte stability was achieved by selection its
composition and pH. Color of stable catholyte was
dark straw. Chemical composition and external view
of coating depends on concentration of salt, pH of
solution and cathodic current density (Table 1).
Table 1 shows that increase of cathodic current
density results in decrease of zinc concentration in
alloy and increase of relative mass of manganese, but
increase of rate of hydrogen evolution causes decrease
of current efficiency of alloy formation and worsens
external view of coating. SEM images (Fig. 1) show
grain form crystals, positioned practically in parallel
to each other Coating is nonporous (negative effect at
checking for porosity by the reagent K3[Fe(CN)6]).
X-ray diffraction pattern of coating (Fig. 2)
reveals two phases of HCP (hexagonal, closely
packed structure): η-Zn, (lattice constants: a = 2.67 Å
and c = 4.93 Å ) and ε-Zn (lattice constants: a = 2.77 Å
and c = 4.44 Å ).
Results from alloy electrodeposition do not differ
from the data available in literature [9, 10]. When
EDTA was used as a ligand instead of a citrate, the
authors failed to get zinc-manganese high quality
coating under the same conditions and from the same
composition of electrolytes. The authors think that it
should be conditioned by high values of stability
constants of complexes formed by interaction of zinc and
manganese ions with EDTA, compared with those of
citrate complexes (lgKZnEDTA
= 16.5; lgKMnEDTA
= 13.79;
lgKZnCit
= 4.5, lgKMnCit
= 3.67 [24]). Complication of
cathodic reduction due to possibility of stable
heteronuclear complex formation by EDTA with ions
of both metals in the solution is not excluded [25].
When the authors used nitrilotriacetic acid (Na3Y) as a
complexing agent, containing various concentrations
of zinc sulfate they obtained light, bright coatings
(Table 2).
Table 1 Influence of cathodic current density on chemical composition of Zn-Mn alloy, alloy current efficiency and
external view of coating. Catholyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.1 mol/dm3 Zn2+ + 0.2 mol/dm3 Na3Cit;
pHk 4.0; t = 30 °C; cathode-Cu plate, S = 4.0 cm2; catholyte circulation volume velocity 400 mL/min; anolyte: 0.5 mol/dm3
Na2SO4; pHa 2.5; τ = 20 min.
ik, (A·dm-2) ωZn, (wt.%) ωMn, (wt.%) Фalloy, (%) external view of coating
4 96.75 3.25 50 white-grayish
5 94.12 5.88 39.4 grayish
8 88. 62 11.38 32.3 dark-grayish
12 87.21 12.79 28.5 blackish plating
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
16
Fig. 1 SEM images of Zn-Mn alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.1 mol/dm3
Zn2+ + 0.2 mol/dm3 Na3Cit; pH 4; ik = 4 A·dm-2; t = 30 °C; τ = 20 min.
Fig. 2 XRD pattern of Mn-Zn alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2S04 + 0.1 mol/dm3
Zn2+ + 0.2 mol/dm3 Na3Cit; ik = 5 A·dm-2; pH 4; t = 30 °C; τ = 20 min.
Table 2 shows that increase of zinc concentration in
the catholyte results in the increase of current
efficiencies and amount of zinc in alloy. The latest can
be caused by increase of hydrogen evolution
overpotential and decrease of hydrogen evolution rate
due to increase of Zn amount in alloy. At the given
zinc concentration, increase of cathodic current
density results in increase of manganese concentration,
and respectively in decrease of zinc concentration and
current efficiencies of alloy formation. It should be
emphasized that, even when insoluble anodes are used,
alloy can be deposited from the nitrilotriacetic
sodium-containing electrolyte in a bath without a
diaphragm, whereas, in case of citrate-containing
electrolytes, citrate is oxidized on insoluble anodes in
a bath without diaphragm.
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
17
Table 2 Influence of concentration of Zn2+ ions and cathodic current density on the chemical composition of Zn-Mn alloy
coating; catholyte: 0.3 mol/dm3 Mn2+ + 0.5 mol/dm3 (NH4)2SO4 + 0.08 mol/dm3 Na3Y + X mol/dm3 Zn2+; pHk 4.0; t = 30 °C;
volume velocity of catholyte circulation: 400 mL/min; anolyte: 0.5 mol/dm3 Na2SO4; pHa 2.5; t = 30 °C; τ = 20 min.
CZn2+, (mol/dm3) ik, (A·dm-2) ωMn, (wt.%) ωZn,(wt.%) Фalloy, (%)
0.01
4 11.56 88.44 18.75
5 13.44 86.56 15.15
8 18.25 81.75 11.64
12 18.57 81.43 10.25
16 19.72 80.28 8.99
0.04
4 4.24 95.76 53.66
5 5.10 94.90 49.51
8 6.05 93.95 39.82
12 6.81 93.19 32.72
16 7.67 92.33 24.86
0.08
4 4.04 95.96 70.20
5 4.72 95.28 63.47
8 5.83 94.17 58.62
12 6.23 93.77 51.81
16 6.67 93.33 49.35
0.15
4 1.95 98.05 72.31
5 2.10 97.90 69.98
8 2.14 97.86 68.02
12 2.70 97.30 64.73
16 4.03 95.97 58.37
3.2 Mn-Cu Alloy Electrodeposition from Citrate and
EDTA-Containing Solutions
In distinct from Zn-Mn coating, silvery, fine
crystalline, solid and nonporous coatings of Mn-Cu
alloy are obtained only at high cathodic current densities
(ik ≥ 37.5 A·dm-2
) and within pH 6.5-7.5. At the low
current densities the authors used to get black spongy
coatings, easily removable from cathode surface. To
keep silvery hue of plating, an electrode, immediately
from the moment of its removal from the bath was
placed in 3% potassium bichromate, for 4-6 sec, flushed
by running water, washed by distilled water and dried
on air. It should be stated that just prepared catholyte
had dark bluish color because of copper citrate complex,
but this color, after addition of manganese-ammonium
sulfate light pink solution used to change in time, and
after 14 h, it acquired stable dark green-yellowish
coloring. Current efficiency of fine crystalline coating
is within 19%-22%. Chemical composition of coating
at various current densities is given in Table 3.
On a SEM micrograph (Fig. 3) of rather stable,
silvery surface of Mn-Cu coating circular form,
practically bare places of 5-10 μm diameter are
observed, probably formed due to surface screening
by intensively generated hydrogen bubbles, which
hinder the process of discharge. Coating is nonporous
(inspection by a reagent K3[Fe(CN)6] showed negative
results).
Current efficiency of silvery Mn-Cu alloy coating
from the same composition of electrolyte, but when
0.2 mol/dm3 EDTA was used as complexing agent,
was increased up to 42% (ik = 37.5 A·dm-2
, pH 6.5; t =
30 °C ); In the alloy, average content of manganese was
93%, copper 7%. But in distinct from citrate-containing
electrolyte, the obtained electrodeposited alloy turned
out porous which is well detected on SEM micrograph
(Fig. 4). X-ray diffraction pattern of the alloy (Fig. 5)
shows the BCT (body centered tetragonal) γ-Mn solid
solution phase with copper (lattice constants: a = 2.67 Å
and c = 3.58 Å ).
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
18
Table 3 Influence of cathodic current density on chemical composition of Mn-Cu alloy and current efficiency. Catholyte:0.3
mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.005 mol/dm3 Cu2+ + 0.2 mol/dm3 Na3Cit; pHk 6.5; t = 30 °C; catholyte circulation
volume velocity: 400 mL/min; anolyte: 0.5 mol/dm3 Na2SO4 ; pHa 2.5; τ = 20 min.
ik, (A·dm-2) ωMn, (wt.%) ωCu, (wt.%) Фalloy, (%) external view of coatings
< 37.5 - - - black, spongy
37.5 92.69 7.31 22.38 silvery
50 91.5 4 8.46 21.43 silvery
62.5 91.05 8.95 19.25 silvery
Fig. 3 SEM micrograph of Mn-Cu alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.005
mol/dm3 Cu2+ + 0.2 mol/dm3 Na3Cit; pH 6.5; ik = 37.5 A·dm-2; t = 30 °C ; τ = 20 min.
Fig. 4 SEM micrograph of Mn-Cu alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.005
mol/dm3 Cu2+ + 0.2 mol/dm3 EDTA; pH 6.5; ik = 37.5 A·dm-2; t = 30 °C; τ = 20 min.
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
19
Fig. 5 XRD pattern of Mn-Cu alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2S04 + 0.005
mol/dm3 Cu2+ + 0.2 mol/dm3 EDTA; ik = 37.5 A·dm-2; pH 6.5, t = 30 °C; τ = 20 min.
Average current efficiency of Mn-Cu alloy coating
from the electrolyte containing both complex forming
agent citrate and EDTA in equivalent quantities (each
in 0.1 mol/dm3), was to 30%, (ik = 37.5 A·dm
-2, pH 6.5;
t = 30 °C ), while manganese content equaled to
approximately 91.5%, and copper 8.5%. Coating was
nonporous and practically was silvery (Fig. 6), but at
the corners of cathode—it was light dark color. XRD
pattern of the obtained alloys are analogous to those
one presented above.
3.3 Mn-Cu-Zn Alloy Electrodeposition from Citrate
and EDTA-Containing Solutions
As it was stated above citrate containing solutions
prepared for electrodeposition use to change their
coloring according to their pH. After adjustment of pH
to 6.5-7.5 of manganese-ammonium, copper, zinc
sulfates and sodium citrate containing solutions the
electrolyte acquires greenish coloring, which gradually
passes to green-yellowish one and its pH practically
does not change. It was found that alteration of
coloring takes place at the final stage of electrolyte
preparation, when manganese—ammonium sulfate
solution was added to the zinc-copper sulfates and
sodium citrate-containing dark blue solution.
Absorption spectra of just prepared electrolyte
practically preserved their form, but their intensity
increased by time markedly within the frames of the
wave length λ = 420-570 nm (Fig. 7), which probably
refers to complex dynamic processes going on in
complex particles between a ligand and Mn2+
, Cu2+
and Zn2+
ions.
It is known that citrate-ion contains three
deprotonated carboxyl and one OH group [26], which
undergoes deprotonation only in alkali medium (pH ≥
10). At the initial stage of electrolyte preparation (see
practical part) in neutral medium, two carboxyl groups
of citrate-ions form stable blue color anionic complex
with 0.005 mol/dm3 Cu
2+ [27]:
Cu2+
+ Cit3-
↔ [CuCit]-
(stability constant K = 1.6·1014
)
At the excess of citrate-ions colorless complex of
analogous type-[ZnCit]-
might be formed with Zn2+
ions (0.1 mol/dm3 ZnS04), while at the final stage of
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
20
Fig. 6 SEM micrograph of Mn-Cu alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.005
mol/dm3 Cu2+ + 0.1 mol/dm3 Na3Cit + 0.1 mol/dm3 EDTA; pH 6.5; ik = 37.5 A·dm-2; t = 30 °C; τ = 20min.
Fig. 7 Changes in optical density of the electrolyte 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.1 mol/dm3 Zn2+ + 0.005
mol/dm3 Cu2+ + 0.2 mol/dm3 Na3Cit (pH 6.5; t = 20 °C), according to absorbed light wave length (cell thickness 1 cm) with the
time: 1: just prepared electrolyte; 2: after two days; 3: after four days; 4: after six days; 5: after seven days; 6: after two
weeks; 7: after one month.
electrolyte preparation, by adding
manganese-ammonium sulfate solution the authors
can receive brownish complex-[MnCit]-. In such type
of complexes citrate denticityes are not completely
used, therefore these places can be occupied by Mn2+
ions. In electrolyte Mn2+
are also connected with
ammonia formed via equilibrium reaction NH4+ +
H2O ↔ NH3 + H3O+ in the solution. (K = 5.8·10
-10).
Because of labile nature of ammonia and hydrated
complexes of manganese, manganese ions are able to
use maximum citrate denticity and form stable
heteronuclear citrate complexes. In literature the
authors found description of synthesis of
heteronuclear citrate complexes of the general formula:
M2I M
II Cit2·nH2O, where M
I -Zn, Co, Fe, Mn, Cu;
MII-Mn, Zn, Co, Cu [28]. Process of formation of such
0
0.5
1
1.5
2
2.5
3
300 400 500 600 700 800 900 1000
1 2
3
D
λ,nm
4 5
6 7
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
21
type of complexes occurs by participation of three
deprotonated carboxyl groups and, probably, in
addition by participation of a hydroxyl group. In the
solution, which is obtained by mixing of three metal
sulfate salts and sodium citrate, formation of such
mixed type complexes is quite possible. Alteration of
coloring with the time of electrolyte prepared for alloy
electrodeposition and alteration of absorption spectra
intensity with the time, should be associated with
formation of heteronuclear complexes, which are
characterized by complex dynamic equilibrium.
Chemical composition of silvery, solid, fine
dispersed coatings of triple Mn-Zn-Cu alloy with
corresponding current efficiencies according to
electrolysis conditions (current density and pH) is
given in Table 4.
Table 4 shows that chemical composition of
Mn-Zn-Cu alloy obtained at pH 6.5-7.0 and their
current efficiencies are practically identical and reveal
common regularity - increase of current density
decreases manganese concentration, increases copper
and zinc concentrations, and results in decrease of
alloy current efficiency. Coating is silvery and
nonporous (Fig. 8).
Increase or decrease of concentration of metal salts
in electrolytes yielded blackish coating. Increase of an
additive concentration-sodium citrate over 0.2 mol/dm3
practically had no effect on the electrodeposition
Table 4 Influence of cathodic current density and catholyte pH on chemical composition of Mn-Cu-Zn alloy coating and on
alloy formation current efficiency. Catholyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.1 mol/dm3 Zn2+ + 0.005
mol/dm3 Cu2+ + 0.2 mol/dm3 Na3Cit; t = 30 °C; catholyte circulation volume velocity 400 mL/min; anolyte: 0.5 mol/dm3
Na2SO4; pHa 2.5; τ = 20 min.
Catholyte, (pH) Ic, (A·dm-2) ωMn, (wt.%) ωCu, (wt.%) ωZn, (wt.%) Φ, (%)
6.5
35 82.33 6.17 11.50 36.01
45 79.49 7.62 12.89 35.67
65 78.66 7.99 13.35 34.82
7.0
35 83.38 5.16 11.46 35.72
45 79.38 6.96 13.66 34.51
65 74.14 7.71 18.15 32.69
7.5
35 71.88 7.78 20.34 33.33
45 64.96 10.75 24.29 32.72
65 58.86 11.15 29.99 25.43
Fig. 8 SEM micrograph of Mn-Cu-Zn alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.1
mol/dm3 Zn2+ + 0.005 mol/dm3 Cu2+ + 0.2 mol/dm3 Na3Cit; pH 6.5; ik = 37.5 A·dm-2; t = 30 °C; t = 20 min.
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
22
process and the external view of a coating, but after
increase of catholyte pH, in particular, over pH 8,
external view of coating suffered drastic worsening
and became black. XRD patterns of coating obtained
under controlled pH 6.5 and 7.0 (Fig. 9) showed only
BCT γ-Mn-solid phase solution with copper and zinc
(lattice constants a = 2.68 Å ; c = 3.59 Å ).
At the terms of application of EDTA instead of a
citrate, as a complexing agent in the process of
electrolyte preparation, the authors observed
formation of a precipitate in the solution, but when
both complex agents were taken in equal quantities
(each 0.1 mol/dm3) electrolyte turned out stable.
Electrodeposition yielded silvery, nonporous deposit
(Fig. 10). Mn-Zn-Cu alloy formation current
efficiency was increased up to 40%.
Fig. 9 XRD pattern of Mn-Cu-Zn alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2S04 + 0.1
mol/dm3 Zn2+ + 0.005 mol/dm3 Cu2+ + 0.2 mol/dm3 Na3Cit; ik = 37.5 A·dm-2; pH 6.5; t = 30 ºC; τ = 20 min.
Fig. 10 SEM micrograph of Mn-Cu-Zn alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.1
mol/dm3 Zn2+ + 0.005 mol/dm3 Cu2+ + 0.1 mol/dm3 Na3C6H5O7 + 0.1 mol/dm3 EDTA; pH 6.5; ik = 37.5 A·dm-2; t = 30 °C; τ =
20 min.
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
23
XRD pattern (Fig. 11) reveals only BCT γ-Mn solid
phase solution with copper and zinc (lattice constants:
a = 2.68 Å ; c = 3.59 Å ).
Comparison of the data of the Table 4 with those of
the Table 3 shows that current efficiencies of triple
alloy suffer significant increase. In particular, if in
double Mn-Cu alloy coating current efficiency is
within 21%, Mn-Zn-Cu triple alloy current efficiency
is increased up to 36%. Surprisingly when manganese
and zinc concentrations in electrolytes are unchanged,
in triple alloys, manganese, which is characterized by
more negative standard potential, is deposited
dominantly in contrast of double, Mn-Zn alloys
electrodeposition, where zinc content is in excess, as it
has relatively more positive standard potential
(E0Zn2+/Zn = -076 V; E
0Mn2+/Mn = -1.18 V). This
phenomenon is known as anomalous codeposition [29],
which is characteristic for electrodeposition process of
Fe group (Fe, Co, Ni) metal alloys and for Fe group
metal alloys with Zn and Cd [30, 31]. It was proved
that anomalous codeposition occurs only when metal
hydroxides are formed on the electrode surface. In our
experiments, in case of high cathodic current density,
formation of hydroxides on cathode surface is quite
feasible due to a parallel reaction of water reduction
with intense hydrogen evolution and release of
hydroxyl ions (Eq. (4)):
2 H2O + 2e → H2 + 2 OH-
(4)
Metal cations separated from the complex at the
impact of powerful field in the cathode adjacent layer
form adsorbed hydroxocomplexes on the electrode
surface:
Me2+
+ OH- → MeOH
+adc, (5)
which later participate in the process of reduction.
Formation of this type of hydroxide film in the alloy
electrodeposition process is described in the work by
Epelboin [32], in which, as a result of kinetic studies
participation of adsorbed hydroxo-forms was proved
Fig. 11 XRD pattern of Mn-Cu-Zn alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2S04 + 0.1
mol/dm3 Zn2+ +0.1 mol/dm3 Na3Cit + 0.1 mol/dm3 EDTA; ik = 37.5 A·dm-2; pH 6.5; t = 30 °C; τ = 20 min.
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
24
in cathodic process. Currently, thorough theoretical
explanation because of complexity of alloy
electrodeposition process is rather difficult, since it
requires accumulation of more experimental facts and
their analysis in order to determine and substantiate
the adequate mechanism. In our experiments uner
condition of intense hydrogen evolution at high
current densities it was impossible to carry out
voltammetric polarization measurement. According to
our observations obtaining of silvery coatings at high
cathodic current densities is preceded by the process
of formation of metal hydroxo-forms. This is
evidenced by the following experimental facts:
(1) Coatings obtained at low current densities (ik ≤
37.5 A·dm-2
) are black and spongy, and consist mainly
of oxide-hydroxides of all three metals and of their
metallic inclusions; coating is easily removed from the
cathode;
(2) The data obtained on the base of X-ray energy
dispersion microanalysis (Fig. 12) shows that silvery,
dense Mn-Cu-Zn coating, contains nonmetal
components - carbon and oxygen.
Chemical composition of triple alloys coating
obtained from solution containing various concentrations
of complex forming ligands is given in Table 5, which
shows that, nonmetals are in less quantity in coatings
where manganese concentration is high, zinc
concentration is low and ligand EDTA prevails in the
electrolyte. Coatings obtained under these conditions
are relatively fine crystalline (Fig. 13).
XRD pattern of the coatings are similar and only
BCT γ-Mn solid phase solution is revealed;
Fig. 12 SEM micrographand X-ray energy dispersion microanalysis of Mn-Cu-Zn alloy coating from the electrolyte: 0.3
mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.1 mol/dm3 Zn2+ + 0.005 mol/dm3 Cu2+ + 0.1 mol/dm3 Na3Cit + 0.1 mol/dm3 EDTA;
ik = 37.5 A·dm-2; pH 6.5; t = 30 °C; τ = 20 min.
Table 5 Influence of additives-citrate (Na3Cit) and EDTA on Mn-Cu-Zn alloy chemical composition; catholyte: 0.3 mol/dm3
Mn2+ + 0.6 mol/dm3 (NH4)2SO4+0.1 mol/dm3 Zn2+ + 0.005 mol/dm3 Cu2+ + X mol/dm3 Ligand; ik = 37.5 A·dm-2; pH 6.5; t =
30 °C; τ = 20 min.
X, (mol/dm3 ligand) ωMn, (wt. %) ωCu, (wt.%) ωZn, (wt.%) ωC, (wt.%) ωO, (wt.%)
0.2 mol/dm3 Cit 80.32 4.35 10.47 2.62 2.24
0.1 mol/dm3
Cit. + 0.1 mol/dm3 EDTA 77.59 5.42 11.95 2.66 2.38
0.05 mol/dm3
Cit.+ 0.15 mol/dm3 EDTA 84.54 4.41 7.35 2.44 1.25
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
25
Fig. 13 SEM micrograph of Mn-Cu-Zn alloy coating from the electrolyte: 0.3 mol/dm3 Mn2+ + 0.6 mol/dm3 (NH4)2SO4 + 0.1
mol/dm3 Zn2+ + 0.005 mol/dm3 Cu2+ + 0.05 mol/dm3 Na3Cit + 0.15 mol/dm3 EDTA; pH 6.5; ik = 37.5 A·dm-2; t = 30 °C; τ = 20
min.
(3) External view of triple alloy coating depends on
time of electrpdeposition process. In particular, after
25 min silvery coating is no more obtained; coating
surface is covered with metal blackish
oxides-hydroxides, probably because of creation of
increased quantity of metal hydroxo-forms;
(4) Negative effect of catholyte alkalization (pH ≥ 8)
on the external view of coating can be explained by
increased probability of formation of hydroxides.
Irrespective of intense evolution of hydrogen,
silvery coating of triple alloys at high cathodic current
density on copper or steel cathodes are characterized
by high adhesion and plasticity. Thickness of coatings
obtained at various conditions, calculated from Eq. (3)
varied within 50-60 nm. In future, the authors plan to
study tribological and corrosive properties of coatings.
4. Conclusions
White-grayish nonporous coatings of Mn-Zn double
alloy with 50% current efficiency and chemical
composition 96.75% of Zn and 3.25% of Mn were
obtained from citrate-containing electrolyte; XRD
pattern showed two phases of hexagonal closely
packed structure: η-Zn and ε-Zn. In case of using
EDTA instead of citrate as a ligand, zinc-manganese
high quality coating was not obtained. In case of
application of sodium nitrilotriacetic salt as a
complexing agent the authors obtained light color
bright coating with chemical composition 98% of Zn,
2% of Mn and current efficiency of 72%.
Mn-Cu alloy silvery, fine crystalline, solid and
nonporous coating was obtained only at high cathodic
current densities (ik ≥ 37.5 A·dm-2
) and within pH
6.5-7.5 limits. At low current densities black spongy
coatings was obtained which was easily removed from
cathode surface. Silvery, fine crystalline, nonporous
coating with current efficiency within 19%-22% was
obtained from citrate-containing electrolyte, while
current efficiency of silvery Mn-Cu alloy formation
using EDTA was increased up to 42% (ik = 37.5 A·dm-2
,
pH 6.5; t = 30 ºC), but the obtained coating was porous.
Manganese content in the alloy was 93%, and copper
7%. XRD pattern revealed BST (body centered
tetragonal) structure γ-Mn solid solution phase with
copper (lattice constants: a = 2.67 Å and c = 3.58 Å ).
Mn-Cu coating from the electrolyte, which contained
both additives as complexing agent, citrate and EDTA,
in equivalent quantities (each 0.1 mol/dm3), was
practically silvery, nonporous, with current efficiency
30% (ik = 37.5 A·dm-2
, pH 6.5; t = 30 ºC), while
Study of the Influence of the Electrolysis Parameters on Mn-Zn, Mn-Cu and Mn-Cu-Zn Alloys Coatings from Electrolytes Containing Complexing Ligands
26
manganese content was 91.5%, and that of copper
8.5%.
It was shown that citrate-containing solutions
prepared for alloy electrodeposition change their
coloring according to the solution pH. In particular, as
soon as pH of manganese-ammonium, copper, zinc
sulfates and sodium citrate containing solutions was
adjusted to 6.5-7.5 the electrolyte acquires greenish
color, which gradually, passed into greenish-yellowish
color, but its pH practically remained the same.
Alteration of coloring occurs at the final stage of
electrolyte preparation, when manganese-ammonium
sulfate solution was added to dark blue solution
containing zinc-copper sulfates and sodium citrate.
Absorption spectra of just prepared electrolyte
practically retained their form, but their intensity
gradually significantly increased within the limits of
wave length λ = 420-570 nm, which should probably
be connected to the formation of heteronuclear
complexes, which are characterized by complex
dynamic equilibrium.
Optimal pH of electrolyte for silvery, nonporous
coating of Mn-Cu-Zn triple alloy is within 6.5-7.5;
coating composition: 83%-71% Mn, 6%-7.8% Cu,
11.5%-20% Zn, current efficiency 36%-33% (ik =
37.5 A·dm-2
; t = 30 ºC); at pH 8 coating external view
suffers drastic worsening, it becomes black. XRD
patterns reveals BCT γ-Mn solid phase solution
(lattice constants a = 2.68 Å ; c = 3.59 Å ). Increase of
an additive concentration, sodium citrate above 0.2
mol/dm3 did not affect the electrodeposition process
and external view of coating. In case of application of
EDTA instead of citrate as a complexing agent the
authors observed formation of precipitate in the
solution, but if both complexing agents were used in
equal concentrations (each 0.1 mol/dm3) electrolyte
turned out stable. Electrodeposition yielded silvery,
nonporous deposit. Current efficiency of alloy
formation increased up to 40%. Solid silvery
Mn-Cu-Zn coating contained nonmetal components -
carbon and oxygen. Nonmetals were in lower
concentration in coatings where manganese content
was high, zinc content was low and ligand EDTA
prevailed in the electrolyte. Thickness of the coating
was within 50-60 nm. Silvery coating was not
obtained when electrodeposition time exceeded 25
min.
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