Study of Gallium Plating of Metal Electrodes

doi:10.4236/msce.2013.15008

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Journal of Materials Science and Chemical Engineering, 2013, 1, 39-42

http://dx.doi.org/10.4236/msce.2013.15008 Published Online October 2013 (http://www.scirp.org/journal/msce)

Study of Gallium Plating of Metal Electrodes

S. V. Gladyshev, R. A. Abdulvaliev, K. O. Beisembekova, G. Sarsenbay

Center of the Earth Sciences, Metallurgy and Enrichment, JSC, Science and Education Ministry of the Republic

of Kazakhstan, Almaty, Kazakhstan

Email: mer-[email protected], [email protected]

Received August 2013

ABSTRACT

Conditions of gallium plating of metal electrodes were studied in the paper. It was found that stability of gallium cover

depends on the material and is increasing in the raw: stainless steel 08Х18Н12Т < Steel 1, Steel 2, Steel 3, Steel 45 < Ni

< Cd < Cu. Phase composition of the electrode surface layer obtained after gallium plating was studied. It was found

that gallium with nickel form Ga36Ni64(Ga Ni2) compound and gallium with copper form CuGa2 compound. Different

acids were used for electrode leaching: H2SO4; HNO3; H3PO4; HCI. It was shown that only hydrochloric acid is suit-

able for gallium plating of the electrodes.

Keywords: Gallium Plating; Metal Electrodes; Hydrochloric Acid; Gallium Coverin g; Stainless Steel 08х18н12т; Steel

1; Steel 2; Steel 3; Steel 45; Nickel; Cadmium; Copper

1. Introduction

Gallium is widely used in opto-electronics, telecommu-

nications, and airspace industry. Great share of gallium is

used for production of GaAs, integrated circuits for elec-

tronic industry. Importance of gallium for electronic in-

dustry of the world is great, whereas annual production

in tons is pretty small [1-4].

The analysis of the world market tendencies of non-

ferrous and rare metals indicates a rapid growth in de-

mand for gallium, up until 2020. For this reason there is a

need for substantial increase of its output. Currently, the

main source of gallium is aluminium raw materials proc-

essed by the aluminium oxide plants, which simultane-

ously extract gallium [5-9].

Use of gallium plated cathodes in electrolysis of alu-

minate-gallate solutions showed high efficiency of them

[10-12]. Therefore, determination of the conditions to get

stable and uniform gallium covering on metal electrodes

is an important practical task.

In electrolytic cells with solid revolving gallium plated

cathode the surface of the cathode is damping with liquid

gallium thus restoring the surface. The sites of the cath-

ode where gallium covering is poor could not be mois-

tening with liquid gallium.

When steel surface of the cathode in such sites in

alkaline solution becomes uncovered galvanic pair Me-

Ga is formed, where gallium is anode component. At

these sites gallium covering could be destroyed mechanic-

cally by evolved hydrogen gas. Vigorous bubble-forma-

tion at such sites during gallium electrolysis promotes

strong sludge formation. The sludge on the electrode

surface inhibits renovation of the sites during contact

with liquid gallium and promotes expansion of the sites

thus decreasing efficiency of gallium electrolysis on the

cathode.

When uniformity of gallium covering on cathode sur-

face is broken activity of the electrolysis cell is stopped,

electrolyte is removed and the cathode is covered with

gallium again. The process requires additional work time,

additional amount of gallium and the acid.

Stable and uniform covering of the electrodes with

gallium lets to increase efficiency of gallium electrolysis

and facilitate maintenance of the electrolysis cell during

exploitation.

Covering of electrode surfaces with liquid gallium or

its compounds i s des cribed i n pa pers [13-15].

2. Experimental Method

We studied dependency of gallium cover stability in al-

kaline solution from material of the electrode and com-

position of leaching solution using different inorganic

acids.

The time from the beginning of exposure to the time

when uniformity of gallium cover is broken and potential

of the gallium plated electrode in alkaline solution drops

sharply is consider ed as stability of gallium co ver. Metal

electrodes in the kind of 8 mm rods were used for the

experiments.

Before gallium covering surface of the electrode was

grinded, polished and degreased in the solution, g/dm3:

S. V. GLADYSHEV ET AL.

NaOH 20.0; Na2CO3 25.0; Na2SiO3 10.0. The treatment

was made for 10 ÷ 30 min at 70˚C ÷ 90˚C until complete

watering. The degreasing solution was washed out with

hot water. To exclude oxidizing of the electrode it was

put in 5% sol ution of hy drochl oric acid.

Metal electrodes were cover ed with gallium via multi-

ple submerging in liquid gallium with 20% hydrochloric

acid solution over it at 70˚C - 80˚C [13]. The duration of

gallium covering was 1 ÷ 2 min. Surplus of liquid gal-

lium was removed from the electrode surface with a filter

moistened with 5% hydrochloric acid solution.

Phase composition of solid-liquid interface was stud-

ied with X-ray diffractometer D8ADVANCE “Bruker”

using filtered Со-Кα radiation.

Stationary potential was recorded with PGSTAT12

potentiostate. All potential values were measured refer-

ring to standard hydrogen electrode. Concentration of

gallium in the solution was determined using chemical

methods.

To determine stability of the gallium cover electrodes

were put in the 100 ml Pyrex glass cell filled with alka-

line solution at 50˚C. Composition of the solution was

similar to that for commercial electrolytic cells, i.e.

Na2Ototal = 160 g/dm3. Each result was reproduced five

times.

3. Results and Discussion

Pretreatment of the electrode surface is very important to

get stable and uniform gallium cover.

Different inorganic acids, HCI, HNO3, H2SO4 and

H3PO4, with concentrations from 0.1% to 20%, were

used for the pretreatment but gallium covering is possible

only when hydrochl oric acid soluti o n wa s u s e d.

Leaching of metal electrodes (Steel 3) with HNO3,

H2SO4 and H3PO4, solutions cause vigorous hydrogen

bubble-formation on steel surface. The bubble-formation

is increased greatly when gallium is put in contact with

the surface.

Intensive gas formation on the surface of the electrode

excludes the possibility of damping the surface with gal-

lium. Besides, flow characteristics of gallium in solutions

of the acids are reduced.

The value of gallium potential in the acid solution (Ta-

ble 1) showed that in hydrochloric acid solution gallium

surface is activated. Potential of liqu id gallium is moving

to electronegative direction when concentration of hy-

drochloric acid in the solution is higher.

In the other acids potential of liquid gallium is be-

coming electropositive, i.e . the surface is passivated. The

passivation is most valuable in nitric acid solutions.

Stability of gallium covering was studied on the fol-

lowing electrodes: stainless steel 08Х18Н12Т, Steel 1,

Steel 2, Steel 3, Steel 45, nickel, cadmium and copper.

Thickness of gallium covering depends on gallium dif-

fusion into the metal, temperature of electrode and gal-

lium and the contact time. Chemical composition and

structure depend on the equilibrium diagram of gallium

and electrode material.

Initial compromise potentials of uncovered electrodes

in alkaline solution (160 g/dm3 Na2Ototal) at 50˚C are

shown in Table 2.

After submerging of steel gallium plated electrodes in

alkaline solution their stationary potential was 1,365 V

(Figu re 1). At the initial part А1В1 of the curve 1 one can

see small positive shift of the potential because of disso-

lution of gallium chloride and gallium oxide. Gallium

chloride is formed because of interaction of gallium with

hydrochloric acid solution whereas gallium oxide is

formed when gallium plated electrode is submerged in

alkaline solution [16].

Change of gallium diss olution rate dC/dt from the sur-

face of the metal electrode at this moment is reflected by

А3В3 part of the curve (Figure 2, curve 3). Sharp de-

crease of the rate of the process is related to decreasing

of gallium oxide amount on the surface of the gallium

plated electrode. The increase of gallium dissolution rate

at В3С3 part of the curve is related to de-passivation of

the gallium plated electrode surface due to disso lution of

the oxide film and the beginning of gallium layer uni-

formity breakage at point В3. After this potential of the

Table 1. Potential of gallium electrode in different acid so-

lutions at 80˚C.

Acid concentration, % Potential, V

HCI H2SO4 HNO3 H3PO4

0.1 −0,520 −0,56 −0,55 −0,530

0.5 −0,520 −0,56 −0,46 −0,525

1 −0,560 −0,54 −0,44 −0,522

3 −0,562 −0,52 −0,40 −0,520

5 −0,565 −0,51 −0,38 −0,500

10 −0,570 −0,50 −0,25 −0,485

20 −0,585 −0,49 −0,20 −0,450

Table 2. Initial compromise potentials in alkaline solution (160 g/dm3 Na2Ototal) at 50˚C.

Electrode material Stainless steel 08Х18Н12Т Steel 1 Steel 2 Steel 3 Steel 45 Ni Cu Cd Ga

φ, V −0,7 −0,8 −0,8 −0.8 −0.8 −0,55 −0,36 −0,85 −1,38

S. V. GLADYSHEV ET AL.

electrode is declining slightly, and curves 1 and 2 have

almost horizontal parts (part С1Д1 and С2Д2).

Reduction of gallium dissolution rate at С2Д2 part of

the curve can be explained by sharp increase of gallium

ion concentration at the electrode-solution interface. This

is confirmed by minor decrease of electrode potential

during the said time.

At electrode potential equal to – 1,295 V (point Д1)

surface of the electrode is covered with gallium mono-

layer, which is being dissolved in alkaline solution until

metal core is fully uncovered.

Galvanic pairs promote active dissolution of the gal-

lium. Potential of the electrode is moving fast to positive

direction beginning from – 1.15 ÷ - 1.17 V and is reach-

ing eventually compromise potential of passive steel

electrode in alkaline solution –0,126 V.

Study of phase composition of gallium plated steel

electrode surface layer showed that gallium and steel do

not interact and the layer is formed only because of

damping. This could be confirmed by the fact that poten-

tial curves of steel electrode and steel electrode with re-

moved gallium cover via dissolution in alkaline media

are coincide in time. The thickness of the gallium cover

is (1.5 ÷ 2) *10− 2 mm.

Stability of the gallium cover and potential curves of

gallium plated electrodes in time depending on the mate-

rial of the electrodes are shown in Table 3 and on Figure

3. It is seen from Figure 3 that stability of g allium cover

on the electrodes made of different steel (Steel 1, Steel 2,

Steel 3, Steel 45) is the same and is equal to 1.2 min after

2 minute gallium plating. The worst stability was found

for the stainless steel (curve 5), because of insufficient

activation of its surface in leaching solution before gal-

lium plating.

Stability of gallium cover on nickel core is 4 min

(curve 3), after gallium plating for 0.5 ÷ 1.0 min. Nickel

interacts with liquid gallium forming Ga36Ni64(Ga Ni2)

compound on the interface.

The best stability of gallium cover (14 - 15 min) was

found for copper core (Figure 3, curve 1), because of

good damping of copper with gallium. The surface layer

of the electrode is represented by Θ – phase of CuGa2

compound. The layer is thin because X-ray diffracto-

gram has intensive lines of pure copper.

Potential of gallium plated copper electrod e in alkaline

solution is decreased from – 1.43 V to – 1.05 V (potential

of copper covered with CuGa2) and not to – 0.36 V (po-

tential of copper).

Therefore, stability of gallium cover is mainly depends

on the core material in the following sequence: stainless

steel 08Х18Н12Т < Steel 1, Steel 2, Steel 3, Steel 45 <

Ni < Cd < Cu.

4. Conclusions

Conditions of gallium plating of metal electrodes were

studied. It is shown that stability of gallium cover de-

pends on the metal core and is increasing in the following

Figure 1. Dependency of gallium plated electrode potential

from time of exposition in alkaline solution.

Figure 2. Dependency of gallium concentration in the solu-

tion (2) and rate of gallium dissolution (3) from time of gal-

lium plated electrode exposition in alkaline solution.

Table 3. Stability of gallium cover depending on the electrode material.

Electrode Material of electrode

Stainless steel 08Х18Н12Т Steel 1 Steel 2 Steel 3 Steel 45 Ni Cd Cu

Stability of the cover, min 0,3 1,2 1,2 1,2 1,2 4 11 ÷ 12 14 ÷ 15

Time of gallium plating, min. 4 - 5 2 2 2 2 1 ÷ 0,5 0,5 0,2 ÷ 0,5

S. V. GLADYSHEV ET AL.

Figure 3. Dependencies of gallium plated electrode poten-

tials from time for different electrode cores. Materials of the

core: 1—copper; 2—Steel 1, Steel 2, Steel 3, Steel 45; 3—

nickel; 4 —cadmium; 5—stainl ess steel 08Х18Н12Т.

raw: stainless steel 08Х18Н12Т < Steel 1, Steel 2, Steel

3, Steel 45 < Ni < Cd < Cu. The best material for elec-

trode is copper.

Phase composition of electrode surface layer obtained

after gallium plating was studied. It was found that gal-

lium with nickel form Ga36Ni64(Ga Ni2) compound and

gallium with copper form CuGa2 compound.

Different inorganic acids were used for leaching of

electrode core surface before gallium plating. It was

shown that only hydrochloric acid activates damping of

the core with gallium

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