Indian Journal of ChemicalTechnologyVol.2, May 1995,pp. 147-152

Electroplating of brass from citrate-based alloy baths

F H AssaF·, S S Abd El Rehimb, A S Mohamedc & A M Zaky"'Chemistry Department, Facultyof Science, Quena University, Quena, Egypt

bChemistry Department, Faculty of Science, Ain Shams University, Ain Shams, EgyptcChemistry Department, Faculty of Science, Sohag University, Sohag, Egypt

Received 13 June 1994;accepted 17 November 1994

The electrodeposition of copper, zinc and brass onto steel substrates from citrate-based alloy baths hasbeen studied for a variety of bath compositions, temperatures and current densities. The influences ofthesevariables on cathodic polarization, cathodic current efficiencies, quality ofthe deposits and composition ofthe alloys have been determined. The surface morphology of the deposits has been examined by scanningelectron microscopy and crystal structure by X-ray. Fine-grained and adherent deposits of brass have ~enplated in most cases.

Brass electroplating is one of the most importantindustrial processes that find extensive applicationsfor decorative purposes, protection of steel andpromotion of rubber adhesion to steel and othermetals1• The most important baths forelectrodeposition of brass are cyanide baths which areeasy to control and produce fine-grained and brightdeposits. The baths also have good throwing powerand are usually used to electroplate irregularly shapedarticals, but their high toxicity is a disadvantage.Hence, several studies have been carried out fornon-cyanide brass plating baths. These baths includepyrophosphate baths:, copper(II) glycerate-zincatebaths3.4, triethanolamine baths5 and alkalinetartarate baths6-9.

In the present work, systematic studies on theelectrodeposition of pure copper, zinc and Cu-Znalloys from alkaline citrate baths under differentexperimental conditions, have been carried out. Inthese alkaline solutions (pH > 13), Cu(II) presentspredominantly 10 as [CuC6Hs07J - while zinc exists aszincate [Zn(OH)4Y- ionsll.

Experimental ProcedureAll the plating baths used were freshly prepared

with doubly distilled water and analytical gradereagents. In order to facilitate, references symbolswere given to some baths to identify them and todesignate their composition (Table 1).

The experimental set-up employed has beendescribed earlier12• It consists of a rectangularperspex cell fitted with a steel cathode between twoparallel sheet anodes. For electrodeposition ofcopper, zinc and brass, the anodes were made of

specpure copper, zinc and 30% Zn-brass,respectively. The cathode and both anodes were ofequal area (2.3 cm x 2.0 cm). The distances betweenthe. cathode and the anodes remained constant

throughout the experiments. The electrodes weremechanically polished with 600 mesh emery paper,washed thoroughly with distilled water and rinsedwith ethanol. The experiments wen~ performed atrequired temperatures ±OSC with the help of an airthermostate. The plating duration was 15 min. Themetal contents in the deposited alloys weredetermined by the use of an energy dispersed X-raysystem type link EDS 800/500 which is compiled withscanning electron microscope lEOL SEM T200. Themicrostructure of the deposits were~xamined by thesame equipment.

The galvanostatic cathodic polarizationmeasurements were recorded by the use of an EG&Gpotentiostat model 273A. The measurements werecarried out in a three electrode cell, a steel cathode,copper, zinc or 30% Zn-brass anode and a referencesaturated calomed electrode SCE.

Results and DiscussionCathodic polarization~ The cathodic

potential-current density (£-i) curves of individualdeposition of copper and zinc under differentexperimental conditions were recorded and some ofthe results are given in Fig. 1.The data reveal that, thedischarge of copper and zinc from the test solutions isattcnd~d by large polarization. The cathodicpolarization of each metal increases with increasingcurrent density; .the concentration polarization ismost important in these complex solutions. The

j 'I j , ~H

148 INDIAN J.CHEM. TECHNOL., MAY 1995

i, mA cm-2

Fig. I-Polarization curves for electrodeposition from baths (a)Cu-2, (b) Cu-3, (c) Cu-I, (d) Zn-I, (e) Zn-3 and (0 Zn-2 at 25°C

20.8

b

5·2o

-1680

-1480

-1280

-280

f

,;...--,~~.~?;!~~(';:.~~',-;;:::-::':::-;' ~Q

I i,l : --,: .' ,I ;! I, :I I

I" I::1 I, f. .I. "

-10801-:/ l ".. I ,.: . I:/ I I.' " I" ,

-880~! i Ir' ,J ii,: i,'

- 680 ~ i,'II'/l,I'

" I.',I

:>E

10.4 15.6i.mA cm-2

Fig. 2-Polarization curves for electrodeposition from baths (a)Cu-2, (b) Zn-2, (~) Cu-Zn-3, (d) Cu-Zn-5, (e) Cu-Zn-4 and (0

Cu-Zn-2 at 25°C

20.815.610·45·2

.............. f

.~::.-, .._ ..._ e",...•.•. _.._..-d

i:;::;;<~~;:-:_·-:-~:·~:-=~;II' _,.".--0I: r'---- ...-:J i ,-,"S j I'• , IIi:/ i,': i "i '. ,

( ,-. ,, ,.• I/'.,1IIII,,

IIII,,,,

-1880

-1680

-1280

-880

- 280~ Io

-1480

-680,:II~

-480

-1080:>E

Table I-Composition of some baths used

Bath CuS04.5HzO..~

ZnS04·7HzONa3C6Hs07' 2HzONaOH% of Zn in

symbolg dm-3g dm-3g dm-3g dm-3each bath

Cu.1

10 18060CU.2

30 18060CU.3

30 32060Zn.1

10180'60Zn.2

3018060Zo.3

3018080Cu-Zn.1

10301806012.81Cu-Zn.2

30101806022.936Cu-Zo.3

30301806046.85Cu-ZnA

30303206046.85Cu-Zn.5

30301808046.85

polarization of each metal decreases with increasingits ion concentration in the bath. However, thecathodic polarization of copper increases withincreasing the concentration of citrate salt in the bath.Such effect is apparently due to the increase in thestability of copper-citrate complex with increasingconcentration of the com pIexing agent. Increasingthe concentration ofNaOH has no significant effect

on cathodic polarization of copper. Fig. I shows thatthe cathodic polarization of zinc increases withincreasing the concentration of caustic soda in thesolution. On the other hand, increasing theconcentration of citrate salt in the bath has no

significant influence on the features of the cathodicpolarization curves of zinc.

The cathodic potential-current density (£-i)

"" '" I!'I" '1" r' " I

ASSAF et al.: ELECTROPLATING OF BRASS 149

c

•

30 40 50 60T.mperCltur.,-C

-----100'00

• 80"- ..r/ I80;- ->-

-!U

.c:

•• 60.!! 60

>-uUc:- •- .~ 40UJ --

40III

20b 201IIIII

0010203040 020

-3C, ~dm CuS04.SHZO

Fig. 3--Effcct ofCu2 + ion concentration on the efficiencyof copperdeposition from bath containing 180-g"dm- 3 Na3C6H507-2HzOand 60 g dm - 3 NaOH at 25°C, (a) c.d. = 2 mA em - 2 and (b) c.d. = 5

mA cm-2

Fig. 5-Effect oftemperature on the efficiency of deposition frombaths (a) Cu-2, (b) Zn-2 and (c) Zn-4 at c.d:= 5 mA em-2

shifts the potential curve in the more negativedirection. Some of these results are given in Fig. 2. Thepotential dependence of the parent metals undersimilar conditions are also shown in the same figurefor comparison. It is found that both' the staticpotential and cathodic polarization of copper aremore positive than those of zinc indicating thatcopper behaves as a noble metal. The alloy curves laybetween the curves of the parent metals. This positionwhich is common in many alloy plating baths, infersthat the simultaneous deposition enables zinc (the lessnoble metal) to deposit at more positive potentialsand causes copper (the more noble metal) to deposit atmore negative potentials than in the individual cases.

Cathodic current efficiency-The cathodic currentefficiencies of copper and zinc were determined underdifferent conditions and some of the results are givenin Figs 3-5. The data display that the efficiency ofcopper deposition increases with increase in coppercontent in the bathup to about 27 g dIn -3 CuS04.5H20and thereafter remains almost constant at values

slightly higher than 100%. The high values of theefficiency may be due to the precipitation of basiccompounds on the cathode surface. The cathodicefficiency of copper deposition decreases slightly withincreasing current density. Increasing theconcentration ofNaOH or citrate salt or raising thebath temperature has no significant effect on thecathodic efficiency of copper deposition.

However, the cathodic efficiency of zinc depositionincreases considerably with increasing currentdensity and with increasing zinc content in the bath.Increasing the concentration ofNaOH decreases the

lOT ~b80

..!

• •..~ 60 .0c:

IGo

u:: 100UJ

ZO

oo 10 20 30 40

C, gdm-3.,ZnS04' 7HZ 0

Rig. 4-Effect of Zn2 + ion concentration on the efficiency of zincdeposition from bath containing 180 g dm - 3 Na3C6H507.2HzOand 60 gdm-3 NaOH at 25°C, (a)c.d.=2 mA em-2, (b)c.d. = 5 mA

cm-2

curves of brass were also recorded under differentexperimental conditions. The data show that anincrease in the total metal content in the bath reduces

the cathodic potential of the alloy. The reverse effect isobtained by increasing the concentration of eithercaustic soda or citrate salt in the bath, since increasingthe concentration of either of complexing agents

150 INDIAN 1. CHEM. TECHNOL., MAY 1995

100

80

a

100

80

o

d

b

c

62

20

.!.;., 60uc:•'u

;: 40IU

3 4 5

l,mA cm-2

Fig. 8-Effect of current density on the efficiency of alloydeposition (a) from the bath Cu-Zn-3, and on zinc per cent in alloyfrom bath, (b) Cu-Zn-3, (c) Cu-Zn-2 and (d) Cu-Zn-I at 25°C

o

20

~ 60c:•'u;: 40IU

oo 10 20 30

C, gdm-3 .•CuSO,. 5~O

Fig. 6-Effect ofCu2 + ion concentration on the efficiency of alloydeposition (a) and on zinc per cent in the alloy (b) from bathcontaining 10 g dm-3 ZnS04-7H20, 180 g dm-3Na3C6HsOT2H20 and 60gdm - 3 NaOH at c.d. of5 mA em - 2 and

at25"C - AB represents the Per cent of zinc in the bath

100

'y.

c

20o

10

.!.;; 60uc:•'u;: 40•-jiIIt 20

::JU

30 40 50 60r.mp.rotur •• ·C

Fig. 9--Effect of temperature on the efficiency ofaIIs>ydepositionfa) from the bath Cu-Zn-2, and on zinc per cent in the alloy, (b)from the bath Cu-Zn-2 and (c) from the bath Cu-Zn-3 at cd ofS

mA cm-2

80

then tends to level off at about 100%. However, in abath containing 30 g dm - 3 CuS04-5H20, changingthe content of zinc frorn 5-30 g dm - 3 ZnS04-7H20has no significant effect on the cathodic efficiency ofthe alloy deposition (Fig. 7). The effect of the currentdensity and temperature on the efficiency of alloydeposition from the bath Cu-Zn.3 is given in Figs 8and 9, respectively. The data show that an increase incurrent density enhances the efficiency while anelevation of temperature causes a decrease in theefficiency of the alloy deposition. These variations inthe efficiel}cy of brass deposition can be largelyattributed to the variation in the efficiency of zincdeposition as shown in the cathodic current efficiencyof individual deposition of zinc (Figs 4 and 5).

•• •--...0100~

80~

60~ >-u _8c: • ""u

40["

-~b

- IU~'

20

A"~

0'

,,I0

102030

C ,gdm-3ZnS04.7H20

Fig. 7-Effect of Zn2 + ion concentration on the efficiency of alloydeposition (a) and on zinc percent in the alloy (b) from bath 30 gdm-3 CuS04-SH20, 180 g dm-3 Na3C6~OT2H20 and 60 gdm - 3 NaOH at c.d. of SmA cm - 2 and at 2SoC AB represents the

percent of zinc in the bath

efficiency of zinc deposition apparently as a result ofincreasing the cathodic polarization. The same effectis produced by increasing the bath temperature.Increasing citrate ion concentration produces noremarkable influence on the efficiency of zincdeposition.

The cathodic current efficiency of brass depositionwere determined. Some of the results are given in Figs6-9. Data of Fig. 6 reveal that at a given concentrationof zinc content in the bath (10 gdm-3 ZnS04-7H20),the efficiency of brass deposition increases at first and

II I· ;j!

ASSAF et al.: ELECTROPLATING OF BRASS 151

Fig. Io-SEM photographs of electrodeposited copper from thebath Cu-2 (a) at2 mA cm-2 (b) at 5 mA cm-2, (c) zinc from the bathZn-2at25"Candc.d. of5mA cm-2 (d) Cu-Zn alloy from the bathCu-Zn-3 at 5 mA cm-2'and 25°C for 15 min. [(a) and (b)(magnification of2000 x), (c) and (d) (magnification of750 x)]

Composition of brass-The effect of the platingvariables studied on the percentage of zinc content inthe deposit are given in Figs 6-9. It is seen that thepercentage of the less noble metal (zinc) in the alloydeposits is always lies below its composition referenceline AB (the dotted line) that represents the metalpercentage of zinc in the bath. This indicates that themore noble metal (copper) deposits preferentially ase~pected from the polarization data and, therefore,

. the codeposition of copper and zinc from the testsolutions follows regular plating system!. Thepercentage of zinc in brass increases with increasingzinc content and with decreasing copper content in thebath. These results are consistent with Brenner'sobservation! that an increase in the metal percentageof a parent metal in the plating bath results in anincrease in the percentage of that metal in the alloy.The effect of current density on the percentage in thedeposits is shown in Fig.8. An increase in currentdensity increases the percentage of zinc in the depositsreflecting an increase in the rate of deposition of theless noble metal. Data of Fig. 9 gives the relationbetween the percentage of zine in the deposit andtemperature of the bath. An increase in temperature

decreases the percentage of zinc in the deposits. Thetrends of brass composition curves with respect t~current density and temperature are consistent withthe regular plating system. These trends could beinterpreted on the basis of simple diffusion theorysuggested earlier1. According to this theory, the rateof discharge of a given ion is controlled by its rate ofdiffusion through the cathodic diffusion layer.However, it is found that increasing the concentrationof caustic soda or sodium citrate i11the bath has nonoticeable influence on the composition of thedeposits.

Structure of the deposits-Visual observationsshow that the copper deposits obtained from the bathCu. 1 are bright red and adherent, while the colour ofthe copper deposits obtained from the baths CU.2 andCU.3 are darker. However, smooth andwell-adherent grey deposits of zinc were obtainedfrom the bath Zn-l and·Zn-2 at high current densities(>3 mA cm - 2). At low current densities ( < 3 mAcm - 2). the zinc deposits are dull and porous. On theother hand smooth, bright and adherent copper-zincalloys are obtained in most cases from the baths(Cu-Zn.l-Cu-Zn.5). The colour of the alloy depositsvaries with increasing content of zinc from red toyellow. Above 40°C, a loose dark film formed on thesurface of the deposits, under this film one finds ayellow, deuce of thin and bright deposits.

The influence of bath composition and currentdensity on the surface morphology of theas-deposited copper, zinc and brass were examinedby scanning electron microscope. Some of the SEMphotographs are given in Fig. 10. The microscopicexaminations reveal that the as-deposited copperconsists of spherical coarse grains covering thesubstrate to a high degree Fig. lOa. Increasing thecurrent density results in the formation offine-grained and leveled texture and better coveringof the cathode, Fig. lOb. This modification of thestructure may be due to the increase in cathodicpolarization which enhances the nucleation rate andfavours the formation of fine-grained deposits.However, the zinc deposits produced at low currentdensities ( < 3mA em - 2) and or low zinc content in thebath (< 15 g dm - 3) consists of fine grains sparselycovering the substrate and leaving some bare areas.This behaviour may be due to the low cathodicefficiency of zinc deposition and vigorous evolution ofhydrogen gas. Increasing the zinc content in the bathor ,increasing the current density leads to theformation of fine-grained, uniform arid leveleddeposits covering the whole surface of the substrate,Fig. lOco The copper-zinc as-deposited alloysgenerally consists of fine grains and leveled structure

152 INDIAN 1. CHEM. TECHNOL., MAY 1995

Table 2-X-ray diffraction data for the electrodeposited copper, zinc and Cu-Zn alloys deposited from selected baths at 5 mA em - 2 and25°C for 15min. Sample I deposited from bath Cu.2, sample II from bath Zn.2. Sample III from bath Cu-Zn-2 and contained 83.6% Cu.Sample IV from bath Cu-Zn.3 and containing 59.2% Cu

Sample

siIi9d.AIjr,%structurephaselatticeparameters. A

a

c

0.2006

3.84227feeCu3.3210.2313

3.333100feeCu0.2601

2.96426.7feeCu0.3592

2.14616.1feeCu0.4226

1.82411.5feeCu

II

0.20193.81847.5hepZn2.6594.9370.2130

3.61926.25hcpZn0.2316

3.329100hepZn0.2718

2.77215hcpZn0.3436

2.2448.75hepZn

III

0.20283.80144feecx(Cu-Zn)3.3190.2318

3.327100feecx(Cu-Zn)0.2630

2.93036feecx(Cu-Zn)0.3608

2.13620feecx(Cu-Zn)

IV

0.22833.3768.75feecx(Cu-Zn)3.3200.2355

3.272100feecx(Cu-Zn)0.2630

2.39822.22feecx(Cu-Zn)0.3923

1.96534.72beeI3(Cu-Zn)

as shown in Fig. IOd. The grain size in theas-deposited alloys decreases progressively withincreasing the percentage of zinc in the deposits.

X-ray diffraction analysis was carried out on somecopper, zinc and Cu-Zn alloy plates electrodepositedfrom selected baths at 5 mA cm - 2 and 25°C for 15min.The lattice parameters were calculated and some ofthese data are given in Table 2. The obtaineddiffraction patterns show that tinc is deposited in thecrystaUine hexagonal c1o~-packed (hcp) structurewhere copper is deposited in the crystallineface-centered cubic (fcc) structure. The data alsoshow that the alloy high in copper (> 65%) are singleCt-phase and has the face-centered cubic structurecharacteristic of copper. These copper rich alloys maybe considered to consist of a copper lattice in whichsome of the copper atoms have been replaced by zincatomsl. On the other hand, the results show the alloyhaving a lower percentage of copper (59.2%) iscomposed of a mixture of the face-centered cubicCt-phase and the body-centered cubic (bee) ~-phase.

Conclusion

Adherent and fine grain~d deposits of brass wereelectroplated onto steel substrates from

citrate-based alloy baths. The codeposition processbelongs to a regular alloy plating system wirh copperbeing the more noble metal. The crystal structure ofthe alloy deposit is controlled by the alloycomposition. An explanation has been offered fromthe various trend observed during the investigation inthe light of cathodic polarization.

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Nauchnr-Issled. Inst Khim Prom. 3 (1955) 46.6 Abd EI Rehim S S &. EI Ayashi M E, J Appl Electtochem. 8

(1978) 33.7 SalganWuertt, Zashch Mer. XVI (1980) 185;8 Tunkin S'M, Zashch Met. XX (1984) 942.9 De Filippo D, Rossi A & Atzei D, J Appl Electrochem. 22 (1992)

64.10 Sille'n L G & Martell A E, Stability Constants of Metal Ion

Complexes (Chemical Society, London), 1971.11 Fordyce J S & Baum R L, J Chern Phys. 43 (1965) 843.12 Abd EI Halim AM. Abd EI Wahaab S M, Abd EI Rehim S S &

Abd EI Meguid E A, J Appl Electrochem. 17 (1987) 49.

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II 1'·1 'I I I I IIHII'I·ill;.• I,