Indian Journal of Engineering & Materials Sciences Vo l. 10, August 2003, pp. 318-323

A new brightener for zinc plating from non-cyanide alkaline bath

Y Arthoba Naik & T V Venkatesha*

Department o f Studies in Chemi stry, Ku vempu University, Shankaraghatta 577 45 1. Ind ia

Received 17 lune 2002; accepted 20 May 2003

Z inc e lectrodeposition from non-cyanide a lkaline solution was carried out in presence o f sa licy laldehyde, a new brightener. The bath constituents and bath vari ables were optimi zed through standard Hull cell experi ments. Current e ffi ciency and throwing power o f the developed bath were measured. Polarization study re vealed the shifting o f potential towards more cathodic directi on in presence of addition agents. Corrosion resistance test o n zinc coated steel revealed good protection of the base metal by the zinc coating. SEM photomicrographs o f the deposits showed fine-grained crystal growth in the presence o f sa licylaldehyde. The consumption of brightener was 0.4 mLL·1 for 1000 A-h.

Electrodeposition of zinc on steel is carried out to protect steel from corrosion. The sacrificial protection afford by zinc is due to its position in electrochemical series with respect to iron . The reason for the pre­eminence of zinc in the world of electrodeposition can be attributed to its relative ease of deposition and better corrosion resistance. To get bright zinc deposition certain organic compounds are used in the bath solution l

.3

. The research on the development of new brighteners for non-cyanide alkaline solution is going on4

•10

. It was evident from the available literature that a single addition agent generally did not produce good deposit over a wide current density range. In order to get good deposit two or more addition agents were required 11-14 . The presence of many addition agents poses problems in determining their consumption during plating. Also some of the addition agents cause pollution problem and are a health hazard. In the present work, an attempt has been made to develop a non-cyanide alkaline bath solution containing a single brightener.

Experimental Procedure The chemicals used were of AR grade and the

solutions were prepared using distilled water. Zinc plate of 99.99% purity was used as an anode. Mild steel plates (AISI-I079) of standard Hull cell size were polished mechanically to get smooth surface using emery papers having different grit size (320-800) and degreased by dipping in boiling trichloroethylene after cleaning in soap solution. Finally, these were dipped in 10% Hel and followed

*For co rrespondence

by electrocleaning 15. The basic bath solution and operating conditions as given in the Table I were selected. The di fferent organic compounds such as amines, aldehydes and ketones were selected as addition agents. The standard Hull cell of 267 mL capacity was used to optimize the bath constituents and bath parameters 16. The zinc electroplated steel plates were subjected to water wash and given bright dip in 1 % nitric acid. The nature and appearance of electroplated zinc deposit was carefully studied and recorded through the Hull cell codes (Fig. I a).

The optimized solution through Hull cell study was taken in a rectangular methacrylate cell of 2.5 L capacity . Polished, degreased and electrocleaned cathodes of 3x4 cm2 were used for plating. These plated steel cathodes were used to test different metallurgical properties. Experiments were done in triplicate. Standard experimental procedures 15 were adopted for the measurement of metallurgical properties of the deposits such as ductility , hardness and adherence. In all the above studi es the average thickness of the deposit was 25 11m. The coating thickness was measured by us ing ~-ray back scattering gauge (Permascope ESD9, West Gut-ESD9 KB4, 220x50-60 Hz, German) and BNF jet methods.

Table I- Basic bath composition and operating conditions

Constituent Conc . Operating conditions

(gL·1)

ZnS04.7H20 30 Anode : Zinc Metal (99.99%)

NaOH 100 Cathode : Mild steel

CTAB 3 Temperature : 298 K

EDTA 10 Ce ll current : IA

NAIK & VENKATESHA: A NEW BRIGHTENER FOR ZINC PLATING FROM NON-CY ANIDE ALKALINE BATH 319

Key mLL- 1 gL-1 gL-1

Bright 0.4 2 ~ S :::

Semi Bright 0.8 4 10

Dull 1.2 S IS

Streab 1.6 6 20

Brittle 1.8 7 2S

Burnt 2.0 8 30

Uncoated 2.2

(a) (b) (c) (d)

gL-! gL-1 Temp(K) Current(A)

10 80 288

20 90 298 2

2S 100 303 3 ~ 30 110 303

35 120 318

40 130

(e) (f) (g) (b)

Fig. \ - Hull cell figures: (a) key, (b) effect of salicylaldehyde, (c) effect of CTAB, (d) effect of EDTA, (e) effect of ZnS04, (f) effect of NaOH, (g) effect of temperature, and (h) effect of cell current

For polarization studies, a three-compartment cell

was used . The zinc metal plate was used as an anode

and steel plate as cathode. The cathode potential was

measured , ga1vanostatically, with respect to a

saturated calomel electrode (SCE) at different current

densities . Current efficiency and throwing power

measurements were carried out using Haring and

Blum cell . The current distribution ratio between

anode and cathode was 1:5 for throwing power

measurement. SEM photomicrographs were taken to

know the nature of deposit in presence of addition

agents. For determining consumption of brightener a

rectangular 2.5 L methacrylate cell was used.

Results Hull cell studies

Effect of salicylaldehyde--Basic bath solution gave dull deposit in the current density range of 1.0 to 3.5 Adm-2. To improve the brightness of deposit salicylaldehyde was added. Salicylaldehyde concen­tration was varied from 0.1 to 2 roLL-). At low concentration of salicylaldehyde bright deposits were observed in the current density range of 1.5 to 3.0 Adm-2 . In the low current density region «1.5 Adm-2)

semi-bright and at high current density region burnt deposits were observed. With increase in the concen­tration of salicylaldehyde the current density range yielding bright deposit was extended towards both the

320 INDIAN 1. ENG. MATER. SCI, AUGUST 2003

regions and at a concentration of 1.6 mLL" of salicylaldehyde satisfactory bright deposit was observed . Further increase in the concentration of salicylaldehyde produced dull deposit in low current density region . Based on these observations the concentration of salicylaldehyde was fixed at 1.6 mLL,1 in the bath solution. The Hull cell patterns are shown in Fig. lb.

Effect of cetyl trimethyl ammonium bromide (CTAB) The concentration of CT AB was varied from I to

12 gL,I. The deposit was smooth-dull in the current density range of 0.1 to 3.5 Adm,2. Above 3.5 Adm,2, burnt deposit was observed. With increase in the concentration of CT AB, burnt deposit at high CUITent density region was found to decrease and at 6 gL,1 bright deposit over the entire current density region was obtained. Above this concentration no change in the nature of deposit was observed. Therefore, the concentration of CT AB was fixed at 6 gL" as optimum in the bath solution (Fig. Ic).

Effect of ethylene diamine tetraacetic acid (EDTA) The concentration of EDT A was varied from 5 to

50 gL" . At low concentration of EDTA «20 gL,I) the deposit was bright in the current density range of 0 .5 to 3.8 Adm,2. At a concentration of 20 gL" , bright deposit was obtained over the entire current density region (0.1-4.0 Adm,2). But above 20 gL" of EDTA, no improvement in the nature of deposit was observed. Therefore, the concentration of EDT A was fixed at 20 gL,1 as optimum. The effect of EDT A on Hull cell cathodes at I A cell current is shown in Fig. Id.

Effect of zinc sulphate The zinc content of the basic bath solution was

varied from 10 to 40 gL" . Low concentration of zinc in the bath solution resulted in bright deposit in the narrow current density region (0.1-2.5 Adm,2). The current density range resulting in the bright deposit increased with the concentration of zinc sulphate and attained a maximum when it reached 35 gL". Above this concentration dull deposit was observed in high current density region. The concentration of zinc sulphate was fixed at 35 gL,1 as optimum. Hull cell patterns, which show the effect of zinc sulphate, are given in Fig. Ie.

Effect of sodium hydroxide Hull cell experiments were conducted by varying

the concentration of sodium hydroxide. At low

concentration of sodium hydroxide « 100 gL,I) , the bath solution was turbid in nature. So this solution did not produced satisfactory deposit. At a concentration of 100 gL" the bath solution was clear and at a concentration of 120 gL I, the bath produced bright deposit over the entire Hull cell cathode at I A cell current. At still higher concentration of sodium hydroxide no improvement in the nature of deposit was observed. So, the concentration of sodium hydroxide was fixed at 120 gL, I. The effect of sod ium hydroxide on Hull cell panels is shown in Fig. If.

Effect of temperature The influence of temperature on the Hull cell

experiments under optimum composition of bath constituents is as shown in Fig. ] g. At temperature lower than 303 K the deposit was bright in the cun'ent density range 0.1-4.0 Adm,2. At higher temperature above 303 K, the deposit became du ll. The optimum temperature was fixed between 298 and 303 K.

Effect of cell currellt Hull cell experiments were conducted at different

cell currents (1-3 A) using the bath solution with optimum concentration of addition agents at 298 K. At I A cell CUITent, bright deposit was observed in the current density range of 0.1 to 4.0 Adm,2. At 2 A cell current, the bright range was observed in the current density range of 0.] to 5.0 Adm,2. Above 5.0 Adm,2 the deposit was cloudy in appearance. At 3 A cell current bright deposit was observed in the current density range 0.1-5.5 Adm,2. Black powdery deposit was observed above 5.5 Adm,2. Effect of Hull cell current on the deposit nature is shown in Fig. I h.

Current efficiency and throwing power The current efficiency and throwing power under

different plating conditions were evaluated. The effect of temperature, zinc and brightener concentration on current efficiency and throwing power was measured at a current density of 3 Adm,2 and the values are given in Table 2a. The current efficiency was found to vary from 54 to 68% and throwing power from 32 to 48%.

The current efficiency and throwing power were measured by taking optimum bath composition at different current densities. The current efficiency varied from 49 to 68% and throwing power varied from 44 to 48%. The variation of current efficiency and throwing power with current density is given 111

the Table 2b.

NAIK & VENKATESHA: A NEW BRIGHTENER FOR ZINC PLATING FROM NON-CYANIDE ALKALINE BATH 321

Table 2a-Current efficiency and throwing power at 3.0 Adm·2

current density

Bath Range Current Throwing consti tuentsl efficiency power Parameters (%) (%)

ZnS047H20 , gL" 20-40 54-68 32-48 NaOH, gL·1 100-150 58-68 36-48 CTAB, gL·1 2-10 62-68 42-48 EDTA, gL·1 10-40 60-68 36-48 Salicylaldehyde, mLL·1 0.8-2.0 66-68 44-48 Temperature, K 293-323 62-68 42-48

Table 2b--Current efficiency and throwing power at different current densities for the optimum bath composition

CUiTent density Current efficiency Throwing power

(Adm·2) (%) (%)

1.0 58 44 2.0 66 46 3.0 68 48 4.0 57 48 5.0 49 47

Table 3--Optimum bath composition and operating cond itions

Constituent Range Operating conditions

NaOH, gL" CTAB, gL·1

EDTA, gL·1

Salicylaldehyde, mLL·1

Polarization studies

35 Anode: Zinc Metal (99.99%)

120 Cathode: Mild steel 6 Temperature : 293-303 K 20 Bright current 1.6 density range :

0.1-5 .5 Adm·2

Agitation: Air

The potential of the steel cathode was measured, galvanostatically with respect to saturated calomel electrode, at different current densities. Cathodic polarization was measured using the bath solution with and without addition agents. The variation of cathode potential with current density is shown in Fig. 2. At any given current density, the cathode potential became more negative in presence of CTAB and EDT A. This shift was sti ll higher in presence of all the addition agents. The shift in the cathode potential is responsible for the fine-grained deposit and hence brightness to the deposit. This is a good indication of the usefulness of the brightening agent.

Metallurgical properties An important property of an electrodeposit is its

adhesion to the base metal. Usually, zinc deposits on mild steel have good adhesion. The plated specimens

12

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:; 6

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;; 4 !! ~

" U "l.

o

06 08 10 12 14

Catho de poten t ial, mVx 10'

Fig. 2-Effect of addition agents on cathodic potential

[(-) ZnS04 + NaOH (BB), ('\7) BB + EDTA,

(0) BB + CTAB. (0 ) BB + EDT A + CTAB,

(M BB + EDTA + CTAB + Salicylaldehyde]

16

from optimum bath were subjected to bend test through 90° and finally through 180°. Even after 180° bending no crack or peel off was observed in the deposit. This showed good adhesion of zinc deposit to the substrate. The more useful method for measuring microhardness involves making an indentation with an indenter of specified geometry under a specified load. The microhardness of zinc was found to be 135.

SEM photomicrographs of zinc deposit obtained from the basic bath solution with and without addition agents are shown in Fig. 3. These indicated that the basic bath produced only coarse-grained deposits . The grain size was refined further in presence of addition agents. Fine-grained smooth deposit was obtained from the bath solution containing an optimum concentration of all the addition agents (Table 3).

Corrosion resistance In presence of addition agents, it was found that the

deposits were pore-free above a thickness of 5 Ilm as indicated by ferroxyl test. In absence of addition agents, the coating was highly porous even at a thickness of 8 Ilm.

The corrosion resistance of zinc plated steel plates was tested by salt spray method (ASTM B-117). Mild steel plates (5 x 5 cm2

) were coated with zinc from an optimum bath solution. These plates were given bright dip in 1 % nitric acid followed by chromate passivation. Before subjecting to the salt spray test, the plates were kept in a clean and dry atmosphere for

322 INDIAN 1. ENG. MATER. SCI, AUGUST 2003

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4291 19KU M2 . eee""""ji'i; 11017

Fig. 3--SEM photomicrographs of the deposits obtained at 3.0 Adm-2 in the presence and absence of addition agents at 298 K. (A) Basic bath (BB), (B) BB + CTAB, (C) BB + EDTA, (D) BB + CTAB + EDTA, (E) Optimized bath, (F) Pass ivated deposit

24 h. Even after 120 h of salt spray test no white rust was observed on the specimens. This indicated good corrosion resistance of the zinc deposit when compared to the chromated zinc deposits obtained from the ac id baths6

.8

.

Consumption of brightener To test the consumption of brightener during the

plating experiment 2.5 L of bath solution was prepared and plating experiments were carried out continuously. After several experiments, dull deposits began to appear due to the decrease in brightener concentration. However, bright deposit was restored after the addition of adequate amount of salicylaldehyde. These results were again confirmed

by performing separate Hull cell experiments. The consumption of salicylaldehyde for 1000 A-h was 0.4 mLL· I

.

Conclusions

The alkaline bath developed was capable of producing bright deposit in the current density range of 0.1 to 5.5 Adm·2. Throwing power of the bath was 48%. The bath ingredients were cheaper and maintenance of the bath was easy. The addition agents were non-toxic and waste treatment was not required . Corrosion resistance of the deposit was good. These features of the bath make the process attractive for commercial use.

NAIK & VENKA TESHA: A NEW BRIGHTENER FOR ZINC PLATING FROM NON-CY ANIDE ALKALINE BATH 323

References I Bapu G N K, Rames, Devaraj G & Ayyapparaj J, J Solid

State Electrochem, 3( I) (1998) 48.

2 Tang Chun-hua, Diandu Yu Tushi, 12(4) (1993) 33.

3 Rosenberg WE & Holland F H, Plat Surf Finish, 78 (1991) 5l.

4 Blount E A, Eleclroplat Metal Finish, 23 (1970) 27.

5 Arthoba Naik Y, Venkatesha TV & Vasudeva Nayak P, Bull Electrochem, 16(11) (2000) 481.

6 . Arthoba Naik Y, Venkatesha T V & Vasudeva Nayak P, J Electrochem Soc India, 49-4 (2000) 170.

7 Arthoba Naik Y, Venkatesha T V & Vasudeva Nayak P, Indian J Chell! Technol , 8 (200 I) 390.

8

9

10

II 12

13 14 15

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Arthoba Naik y, Venkatesha T V & Vasudeva Nayak P, Turkish J Chem, 26 (2002) 725-733. Arthoba Naik Y, Venkatesha T V & Vasudeva Nayak P & Sherigara B S, Metal Fillish , USA (accepted). Arthoba Naik Y, Venkatesha T V & Vasudeva Nayak P, Plat Surf Fillish, USA (accepted). Rushmore, German Pat. 2,643,898 March (1077) . Suzuki Isamu, Susa Hideo, Jpn . Kokai Tokkyo Koho . JP 08209393. A2, August ( 1996). Hayashida, Jpn Pat,. 7.2 16,521 , May (1972). Hoyer, German Pat., 2,247,875, April (1973) . Graham A K, Electroplating Engineering Handbook, 3'd Ed.(Van Nostrand Reinhold Princeton, NJ), 1970. Parthasaradhy N V, Practical electroplating handbook, I" Ed., (Prentice-Hall , Inc., NJ), 1989, 283.