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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 =


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


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.


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 ).


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.


mol/dm3 Cu2+ + 0.2 mol/dm3 EDTA; pH 6.5; ik = 37.5 A·dm-2; t = 30 °C; τ = 20 min.


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


20


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


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.


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.


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.


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.


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


25


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


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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