Electrodeposition of zinc alloys in chloride baths containing cobalt ions

Materials

Chemistry

and Physics,

OF ZINC

~~EcT~omPossrIo~

R. FRATESI

Received

( 1989) 529-540

IN

ALLOYS

529

CHLORIDE

CONTAINING

BATHS

IONS

COBALT

and G. ROVENTI

Dipartimento Universita

23

di Scienze di Ancona,

April

dei Materiali

Ancona

e della

Terra,

Facolta

di Ingegneria,

(Italy)

18, 1989; accepted

May 23, 1989

ABSTRACT The were

operating

studied

chemical

conditions

in order

for

to obtain

characteristics.

alloys

from

known

baths, the

which values

was

of

transition

to calculate

current

the

of the deposits

with literature

densities

and

and

on pure

iron

ranging

from

of cobalt

structural

Microhardness

deposited

concentrations

in the deposits

deposition.

shown

obtained.

densities

cobalt

of cobalt

have

baths

(10°C < T 5 30°C). An empirical

the percentage

anomalous-normal

analysis

in chloride

were performed

at current

temperatures

The percentage

of

diffractometrical slight,

permits

deduced.

codeposition

up to 0.8% Co with good physical

conditions,

5 to 30 mA cm-2 and at different relationship,

Co

The electrodepositions

under galvanostatic

substrates,

Zn and

in the

never

reaches

Morphological

modifications,

values

are in good

and though

agreement

data.

INTRODUCTION Many

steel,

replace

metal

and

as

a

elimination

of the

resistance

of the zn coatings in marine

For

this

0254-0584/89/$3.50

baths.

At

performed

requirements

new

Zn plating

due

last,

since

to improve

of Zn coatings

to the studies

they

are

of

is destined

toxicity to not

Of the

of the

improve SO

the

efficient

[Sl.

attentions

coatings

the

in decline

are desirable

atmospheres

purpose,

galvanic

to

been

resistance

the electrolytic

already

plating,

have

years,

consequence

especially

multilayer

recent

and the corrosion

[l-4]. Furthermore,

cadmium the

these

characteristics

especially

auto-industries to

during

studies,

the mechanical

[6-81,

were

focused

composite

on

processes

Zn films

[9-121

to

obtain

and coatings

0 ElsevierSequoia/Printedin The Netherlands

530

with electrodeposited [l, 131, Zn-Ni

In our previous content

up

to

composition

works

20%

baths

[21] in order

[221 pointed

properties

to the

with

chromatizing.

low

additives

alloys

less

current

obtained

obtained with

less

much

work

is

in chloride-based

than

1% Co. Verberne

is of the anomalous while

type and

the mechanical

very uniform stress

coatings

and

which

with good adherence furthermore

[241 have proposed

do

the effect

of Zn-Co alloys

and need

not

of different

in acidic

chloride

alloys with 0.1-l% Co.

work,

electrodeposition

than

out. The influence

composition,

the

of the alloys

auto-industries,

from 50% to lOO%,

on the electrodeposition

percentage

Ni

on

to those of pure Zn coatings.

and Verberne

present

with

of the alloys

electrodeposition containing

is improved

in order to obtain

carried

alloys

internal

Hadley

alloys

parameters

resistance

in European

of Zn-Co

[231 obtained

ductility,

such as Zn-Fe

was evaluated.

to obtain

are comparable

of Zn-Ni

process

characteristics

of the corrosion

study

resistance

Knaak a.

the

of

out that the Zn and Co codeposition

the corrosion

cobalt

influence

especially

however,

of the 8th group,

[lo, 17, 181.

electrodeposition

the

The improvement

dedicated

In

on the

to pure zinc coatings

Recently,

baths

and Zn-Co

[19-201,

being

organic

of Zn and metals

and the physico-chemical

was studied. respect

alloys

[2-4, 14-161,

of

1% in slightly

of various

density)

on

acidic

operative

the

Zn-Co

alloys

chloride

parameters

physico-chemical

containing baths

a

has

been

(temperature,

bath

properties

of

the

was studied.

EXPERIMENTAL Zn-Co 70 g dm-3

alloys

were

ZnCl2

surfactant

obtained

(0.51 M Zn2+),

(ethoxylate

solutions

were

Electrodepositions current

densities

for a sufficient to obtain the

was

used.

anodic anodic

were

H3803, 0.4-20

bath

time

with

carried

out

in accordance

with

a thickness were

solutions

220 g dm-3 KCl, g dm-3

The

central

products

to

pure

reagents

under

in bi-distilled

galvanostatic

with the density of 6 mm. During

mechanically

on the deposits

cathodic

cathodic

membrane

current

was

compartment

TO

used,

in order

joined

investigate

the

electrodeposition plexiglass

to the

prevented during

with

was conducted

the electrodeposition,

stirred.

which

water.

conditions

characteristics,

compartment

by a porous the

30 ml dm-3

. 6 H20 (1.7 x

CoC12

out at 10, 20, 25 and 3O'C. A three-compartment

compartments

composition:

pH = 5.5.

prepared

of temperature

were carried

acids),

the following

at 5, 10, 20 and 30 mA cm-2. The electrolysis

a deposit

cathodic

influence

26 g dm-3,

of fatty

10-3 - 8.4 x lo-2M Co2+), All

at 25'C using

two

cell

lateral

the diffusion

electrolysis.

of

zn-CO

531 electrodeposits

which

= 15 cm*), porous cm*.

were obtained

Before

emery

vertically

The

two pure

immersion,

paper,

followed

three minutes, air.

were

membranes.

by chemical

The polarization

with

respect

surface

cathode

was

volume

by means model).

with

the quantity

800 grade

water

for

with hot

was measured

in the vicinity

After

and

Sic wet

of the cathode

potential

placed

Capillary.

the

of 50

HCl solution and dried

of immersion

the cathodic

from area

the

of the

deposition,

the

alcohol

for

chemical

and

of Zn and Co, the deposits

were

stripped

in a

of 1:3 HCl solution

of Inductively By means

Coupled

of Faraday's

and their

percentage

in baths

% co = 100

mass Co total mass (Co + Ni)

The morphology

and the hydrochloric Plasma

law,

calculated

Microscopy

with

and ethanol

electrode

distilled

exposed

in an inhibited

water

of a Luggin

distances

a total

at the moment

reference

by means

equal

(exposed area

analysis.

To determine minimum

was applied

at had

iron discs

were ground

pickling,

the electrodeposition

washed

morphological

anodes

in distilled

to a calomel

cathodic

positioned

zinc

the iron surfaces

then washed

in the bath. During

on both sides of Armco

respective

X-ray

was observed

diffraction

analysis

was analysed

(ICP Perkin-Elmer

currents

curves

was calculated

of the deposits

(SEM).

the partial

polarization

and deposits

solution

Spectroscopy

were plotted.

according

was

The cobalt

to Brenner

by means

5500

of Zn and Co were

of Scanning

carried

out

[251:

Electron

(Philips

PW

1730, CuKCL = 0.15 nm) . The Knoop

microhardness

5 and 25 g loads the deposit

RESULTS

in order

thick, depth

was measured lower

than

using l/l0 of

AND DISCUSSION

density

and

up

uniform

to

homogeneous

30 mA

and dendritic

Figure the change

of total

of cobalt.

[171 using

sulfate

previously

possible

electrodeposits

cm-*.

Above

growth

was observed

1 shows the variation

percentage

used,

12 p

an indentation

thickness.

Coherent

AS

of the deposits, to obtain

to obtain

which

allows

electrolysis The trend

this

were

value,

the

deposits

with

current

appeared

less

along the edges of the samples.

of the content current

obtained

of cobalt

in the baths

is different

to that

in the alloys

containing

with

a different

found by Hygashi

ti.

baths. fo+nd

for

the

an empirical

electrodeposition relationship,

us to calculate

the cobalt

under

of

Zn-Ni

alloy,

the operating

content

in the

it was

conditions

deposits

from

532 the known

the current

value of

density

composition

and the cobalt

in

the

bath, cod = (2.86 i - 7) Cob x 10

where

Cod and Cob

are the cobalt

percentage

and i is the total electrolysis

respectively

-0

-3

in the deposit current

density

and in the bath in mA

cm-z.

0.60

s

20

(mAcm-*)

i Fig. 1. different T = 25Y.

Cobalt percentages in percentages of cobalt

Figure that

2 shows

in

the

zinc-cobalt in the baths

the dependence

baths

at

reference

[171, which

refers to electrodeposition

and

300

experimental of Zn-Co

cm-2,

curves

alloy

definition authors

mA

of

(C.R.L.)

are

also

with respect

in chloride Brenner

baths

[251.

which used different

Several anomalous

hypotheses codeposition

metals.

The ideas

Brenner

(25) studying

the anomalous

of

given

carried in

been

by

especially

on phenomena

the electrodeposition

2.

has

The

The

literature

data

those

of Zn-Ni

baths at

position

of

the

that the deposition

type, been

according

reported

to the

by

other

[17, 221.

researchers

occuring

with respect

from

indicates

conditions

advanced

alloys,

of nickel

taken

with

densities.

out in sulfate-based

is of the anomalous observation

in the deposits

current

Figure

to the C.R.L.

This

are focused

behaviour

and a curve

experimental

have

content

electrolysis

composition

50°C

line

alloys electrodeposited at v> various current densities.

of the cobalt

different

40

30

to

containing

explain iron

on the cathode alloys

the group

surface.

suggested

that:

to zinc could be attributed

to

an

adsorption

hydroxide

effect

which

of

'addition

an

is formed

on cathode

agent', surface

presumably

an

and hinders

oxide

or

the discharge

an of

nickel.

0.80 m s ”

0,60

-0

0.40

0.20

0.00 5

0

15

10 co

b

( %1

in electrodeposited zinc-cobalt alloys Fig. 2. Cobalt percentages percentages in the baths at the following current densities: Cl : 30 mA cm-2; l : 20 mA cm-2; n : 10 mA cm-2; A : 5 mA cm-2. X : Data from [17]. T = 25OC.

Dahms Fe-Ni

and

Croll

formation

of ferrous

Higashi

u.

codeposition inhibited to the because

who

that

hydroxide

studied the

alloy

by the formation

transport

cathodic

work

reactions.

ions.

On the

the nature

the

densities

of the two metals,

interface, coefficients.

their

respective

film which

exchange

of the

the anomalous

of the cobalt offers

Co(OHj2

kinetic

is

resistance

is not

formed,

of Zn-Ni alloy has parameters

on the equilibrium

at which two metals

the potential

by

for its precipitation.

of the

besides

hindered

to explain

electrodeposition

importance that,

is

The discharge

contrary

value

electrodeposition

nickel surface.

baths.

[19], anomalous

It is known

the ratio of the current

of

of a zinc hydroxide

of the Co2+

emphasising

anomalous

the same mechanism

in sulfate

the pH did not reach a critical

treated,

the

discharge

on the electrode

[17] proposed

of Zn-Co

In our previous been

1261,

concluded

alloy,

vs cobalt

are reduced

difference

currents

and

which on

of the

potential, depends

exists the

on

at the

transfer

534 In

Table I,

exchange

are

currents

reported

in more

nickel,

unlike

reflect

'electrochemical

zinc,

to the same author is that

are

that

thermodynamic

nobility

Table

I.

different

they

Exchange

can

and

current

by

'practical

densities

the

nickel

and

solutions.

low

exchange

out by Piontelli

and

currents

and

codeposition

of iron-group

significant

cobalt

Cobalt

[32]. According

for the anomalous

parameter

determine

zinc,

our

very

as pointed

reaction

of

than

likely explanation

cathodic

unfavourable

data

systems

characterised

inertia'

the most

the kinetic

literature

simple

metals

divergence

is so between

nobility'.

for

Zn 2+/Z*,

co2’/Co

and

Ni2'/Ni

in

solutions.

M

Solutions

Ref.

=o

co

0.85

1.4 x lo-'

[281

co Ni

0.1 - 2.0 N CoC12 2N KC1 + 0.01 N NiClZ

I3 x

10-7

1291

Ni Zn Zn

PR = 5.96 Sulfate Chloride Sulfate

1 2 3 3

10-s 10-a 10-S; 7 x 10-l 10-5

[301 I311 [311 [311

M

cosoq

x x x x

0,lO

0.00 0

20

40

30

i (mA cm Fig. 3. Effect of temperature on the cobalt percentages electrodeposited at various current densities. cob = 1.4%.

-2

)

in zinc-cobalt

alloys

535

The percentage Figure

3. Such

higher

the

cobalt

deposited.

it

they

is evident

prefixed bath.

current

4 and 5 show

efficiency

concentrations. attain COb

almost

increases

efficiency current constant

deposits,

even

which

efficiency

with

rising

is the

in

temperature

independently if the hydrogen

of

cobalt,

at

of the plating

potential

the

the

high

role

range,

cobalt

The

discharge

percentage

of

on cobalt

the

does

not

in the bath

currents

(97 - 98%).

and the

of different

concentrations

hydrogen

overpotential

currents

in baths

the zinc partial

to

is

Furthermore,

the important

discharge

The cobalt

always

is due

of

it

parameters.

deposition

remains

in The

percentage

deposits,

of deposited

of Zn and Co respectively

neither

the

concerning

the partial

conditions.

as shown density.

composition.

percentage

kinetic

alloy

current

higher

cobalt

assumption

respectively

1.4 - 13) influence

=

current 3% of

In the

limiting

the

the above

curves

the

the

temperature

on the current

far from the bath

3 that

by temperature-dependent

Figures

(%

density,

fact supports

of

with

also

density,

values

very

Figure

increases

depends

current the

remain

from

current

This

played

From

cobalt

however,

electrolysis

that

evident,

of reduced

increase,

nor the zinc remaining which

2 -

remains

cobalt

in

is less than

the

that on

zinc.

100 ol

N

I

‘E

E 0

a

2

-

=4

6

._

.I

25

0 -1050

-1100

- 1150

- 1200

-1250

VNHE Fig. 4. Partial polarization curves for zinc and baths containing the following cobalt percentages: om : COb = 1.4%; b A: COb = 13%, T = 25'C.

-1300

(ml!) cobalt

codeposited

from

536 c

100

’

10

CN

0 2

90

0-a

80

0.6

0.4

60

0.2

0

5

10

15

20

25 i

Fig. 5. Partial baths containing elm: CO,, = 1.4%;

30

35

(mAcm-2)

current efficiency for zinc and the following cobalt percentages: 6 + : CO), = 7%; A A: CO,, = 13%.

cobalt

codeposition

from

300 1

250

-

200

-

0.0

0.5

1.0

1.5

Fig. 6. Variation of microhardness versus the percentage of cobalt in the deposit. l : Present research; 0 : Data from [23]; n : Data from [33].

537

Figure

6 shows microhardness

obtained

at

20

Literature experimental the

mA

data

of

sufficient

to

microhardness

only

increase

the

-

a decrease

-

an increase

figure

in crystal the

%COd

of zinc-cobalt

alloys

the

is

Afterwards increase

obtained

the

in cobalt

in

different

[23, 331.

is generally

correlated

to:

or: imperfections. crystal

orientations

content

Zn

and

Zn-Co

analysis.

crystalloqraphlc -2 and T = 25OC at 20 mA cm

(Cod)

the

on

electrodeposited

percentages

of

of Cob.

(102)

(103)

(110)

(112)

9

68

100

I

10 11 10 8

61 100 100 100

100 64 47 46

8 4 4 4

18 25 9 9 17

13 14 15 17 15

3 4 5 6 5

value

100

is

increase

results deposit

attributed

crystal

content

in the deposit

(< O.l%),

highest

modifies

the results

and

have

preferential

probably, an

already

used,

results

increase

but mainly

show

0.02 and 0.08%.

directions

in

a

plane

in low percentage

a corresponding

shown that the morphology

density

The

to the crystal

in the range between

the

so

peak.

corresponding

of cobalt,

thus inducing,

on the current

the deposits.

the

show that the presence

imperfections

Furthermore,

to

in the peak intensity,

in the deposit

not depend

with the successive

that

deposits

(101)

(100) with cobalt

growths

different

it is evident in the

microhardness.

values,

in

(100)

remarkable

in the

figure, cobalt

of X-ray diffractometrical

various

concentrations.

obtained

(002)

0.02 0.08 0.33 0.69

Such

the

dislocations

cobalt

0.00

The

of

up to 0.7% Co,

size

of

from baths containing

From the

microhardness

preferential

by means

Influence

orientation

grain

cobalt

coatings

quantity

are reported

in crystal

obtained

II.

shown.

small

containing

different

Zn-Co

in microhardness,

Table II shows

Table

with

very slightly

same

of alloys

considerably

conditions,

The increase

baths

are also a very

increases

In

experimental

alloys,

in

electrolytic

conditions

presence

content.

cm-2

for

results

the

of crystal increase

in

microhardness.

of the deposits

on the cobalt

does

content

in

Figures 0.01,

0.1

I, 8, 9 show and

0.5%

Co.

structures The

of Zn-Co

observations

alloys

were

containing

made

bq

respectively

scanning

electron

microscopy. The alloy electrodeposit with

very

with 0.01% Co has a structure [191, while a cobalt content

uniform

size

and

with

smaller

very similar

to that of pure zinc

of 0.1% gives place dimensions

to structures

than alloys containing

0.01% co. Alloys with 0.5% show

a very uniform

structure

which appear

less deformed

even at higer magnification.

Fig. 7. Electrodeposited density: 5 mA cm-z.

Fig. 8. Electrodeposited 5 mA cm-z.

zinc-cobalt

zinc-cobalt

alloy

with

0.01%

alloy with 0.1% cobalt.

cobalt.

Current

Current

density:

539

Fig. 9. Electrodeposited 15 mA cm-2.

zinc-cobalt

alloy with 0.5% cobalt . Current

density:

CONCLUSIONS 1)

The

codeposition

of Zn-Co alloys in chloride

is of the

baths

anomalous

type.

2)

The percentage the current

density

3)

The current

efficiency

constant, 4)

of cobalt

deposited

small quantity

with

the bath

composition,

and the temperature. of the deposited

for all percentage

The microhardness

increases

of cobalt

of the alloy

alloy

is near1 .y 100% and almost

in the deposits. considerabl y by a relatively

is improved

of Co in the deposits.

ACKNOWLEDGEMENTS The research the National

was supported

Council

by the Scientific

of Research

Research

oji M.P.I.

(40%) and

of Italy.

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