Indian Journal of Chemical Technology Vol. 8, September 2001. pp.390-395

Effect of condensation product on bright zinc electrodeposition from sulphate bath

Y Arthoba Naik, TV Yenkatesha* & P Vasudeva Nayak

Department of Studies in Chemistry, Kuvempu Universi ty, Shankaraghatta577 451, India

Received 18 August 2000; revised 19 March 2001; accepted 29 March 2001

Zinc electroplating is carried out in presence of condensation product formed between DL-Alanine and Glutaraldehyde. The bath constituents are optimized through Hull cell experiments. Operating parameters such as pH, temperature, current density are also opti mized. Current efficiency and throwing power are measured. Polarization study reveals high shift of potential towards negative direction in the presence of addition agents. Corrosion resistance test indicated good protection of steel by the coating. IR spectrum of the deposit shows the inclusion of the compound. SEM photomicrographs show fi ne grained deposit in the presence of addition agent. The consumption of brightener in the lab scale is 8 mL for 1000 amp-hour.

Zinc is used for coating the ferrous substrates due to its sacrificial protection. To get bright coating of zinc on steel certain organic compounds are added to the bath solution 1•4• Development of these brighteners for zi nc plating from non-cyanide aqueous solution is continuously taking place even though efficient baths are availables.s. It is evident from the available literature that single addition agent generally does not produce good deposit over a wide current density range. Presence of many addition agents in the bath poses problem in determining the consumption of brightener during electroplating and some addition agents also cause pollution and health hazard.

In the present work, efforts have been made to develop a bath solution containing a single addition agent. The addition agent is easily soluble in water. Generally, for good deposition, the addition agent must increase the deposition potential. In the present study various primary amines and aldehydes are subjected to condensation reaction9• Among these, the condensation product formed between DL-Alanine and glutaraldehyde is effective in getting quality deposit.

Experimental Procedure The chemicals used were of LR grade and easily

soluble in water. For the preparation of solutions distilled water was used. The standard Hull cell of 267 mL capacity was used to optimize the bath constituents. The Hull cell experiments with the bath solution (Table 1) were carried out without agitation.

*For correspondence (Fax: (08282) 37255; e-mail : root@ shikuv. kar.nic. in)

The pH of the bath solution was adjusted with 10% sulphuric acid or sodium carbonate solution. Zinc plate of 99.99% purity was used as anode. The anode was activated each time by immersing in 10% HCI followed by water wash. Mild steel plates (AISI-1079) of standard Hull cell size were mechanically polished to obtain a smooth surface and degreased by dipping in boi ling trichloroethylene. The scales and dust on the steel plates were remo ed by dipping in 10% HCI solution and were subjected to electrocleaning process. Then these steel plates were washed with water and used for the experiments as such. After the plating experiment the plates were subjected to bright dip in l% nitric acid for 2s followed by water wash. The nature and appearance of zinc plating was carefully studied and recorded through the Hull cell codes Fig. I (a).

All the experiments were conducted at 303± 1 K. The known amount of condensation product was added to the bath solution. The bath solution was stirred for 30 min and then used for Hull cell experiments.

The deposits were obtained at constant current density from the optimized solution taken in a rectangular methacrylate cell of 2.5 L capacity. Polished, degreased and e lectrocleaned cathodes of 3 x 4 cm2 were used for plating. These plated steel cathodes were used to test different metallurgical properties. Experiments were done in triplicate. Standard experimental procedures 10 were adopted for the measurement of metallurgical properties of the deposit such as ductility, hardness, adherence etc. In all the above studies the average thickness of the

NAIK et al.: EFFECT OF CONDENSATION PRODUCT ON BRIGHT ZINC ELECTRODEPOSITION 391

Table 1-Basic bath

Bath constituents I Operating range

ZnS04. ?H20. gL·'

Na2S04, gL·'

pH

Temperature,°K

Cell current, A

Plating Time, Minutes

Anode

Cathode

Concentrations/ Operating conditions

200

50

2.5

298 2

5

Zinc plate (99.99% pure)

Mild steel plate

deposit was 20 J..lm. The coating thickness was measured by using B-ray backscattering gauge (Permascope ESD9, West Gut-ESD9 KB4, 220x50 -60Hz, German) and BNF jet methods.

For corrosion resistance test the coated steel plates of 3 x 4 cm2 area were given bright dip followed by passivation in a solution containing 200 gL·' sodium dichromate and 2 mLL' 1 of sulphuric acid at 303 K for 5s. These passivated samples were dried for 24 h in a clean atmosphere and subjected to neutral salt spray test in accordance with ASTM standard method B-117 using 5% neutral sodium chloride solution at 303 K.

Polarization studies were carried out by using a three compartment cell. The area of zinc anode was 2 cm2. Mild steel was used as cathode with exposed area of 2 cm2• The cathode potential was recorded, galvanostatically, with respect to saturated calomel electrode, at different current densities. Haring and Blum cell was used to measure current efficiency and throwing power of the bath solution. For throwing power measurement the current distribution ratio between anode and cathodes was 1:5. For determining consumption of brightener, again a rectangular cell of 2.5 L capacity was used.

IR spectra of the condensation product and scratched deposit were taken to know the inclusion of addition agent. SEM photomicrographs were taken to know the nature of the deposit in the presence of addition agents.

Results

Effect of condensation product Basic bath solution gave coarse dull deposit

between the current density range of 1.0 to 7.0 Adm·2

at 2 A cell current. To improve the nature of deposit, condensation product formed between DL-alanine and glutaraldehyde, was added to the bath solution.

Key mLL·'

BRIGHT 8

SEMIBRtGHT 12

DULL 16 150

STREAKS 20 200

BURNT 24 250

UNCOATED 28

32

(a) (b) (c)

Temp. Cell CuiTI!IIt gL·' pH

~~ (A)

20 ;§ 30 3.0 : 40

50 323 (g)

60 (f)

80

(d) (e)

Fig.l-Hull cell figures: (a) Key, (b) Effect of Condensation product, (c) Effect of ZnS04, (d) Effect of Na2S04, (e) Effect of pH, (f) Effect of temperature, and (g) Effect of cell current

Condensation product was prepared by the procedure described elsewhere9 . The known amount of this solution was added to the bath solution. Various compositions of DL-alanine and glutaraldehyde were used to prepare the condensation product. Among these, the composition containing 1.4 mLL·' glutaraldehyde (25% aqueous solution) and 1.2 gL· ' DL-alanine gave satisfactory deposits. To optimize the concentration, condensation product was prepared with 23.2 g of DL-alanine and 5 mL of glutaraldehyde and made up to 100 mL. This aqueous product was added to the basic bath solution in small volumes and stirred for 30 min before plating. At lower concentrations of the product the deposit was good semibright between the current density range of 1.0 to 7.0 Adm·2. At lower current density dull and at higher current density burnt deposits were obtained. With increase in the concentration, the nature of the deposition was improved and at a concentration of 28 mLL·' of the condensation product, the Hull cell panels were bright between the current density range of 0.5 to 8.0 Adm·2• Further increase in the concentration of the condensation product the nature

392 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

Table 2-Current efficiency and throwing power at different current densities

Current density (Adm.2)

2 4 6 8

Current efficiency (%)

94 97 96 94

Throwing power (%) 25.0 26.6 26.5 26.7

of the deposition became brittle at higher current density region. Therefore, on the basis of the above observations the concentration of the condensation product was kept at 28 rnLL-1 as optimum. The Hull cell patterns are shown in Fig. l b.

Effect of zinc sulphate To find out the effect of zinc metal IOn

concentration, the zinc sulphate concentration was varied from 50 to 300 gL- 1• At lower concentrations, the bright deposit was observed in the current density range between 2.0 to 6.0 Adm-2• At lower current density region, uncoated and at higher current density region burnt deposits were obtained. With increase in the concentration of zinc sulphate, the brightness range was extended to higher and lower current density regions. At a concentration of 250 gL-1 the satisfactory bright deposit was obtained in the current density range of 1.0 to 8.0 Adm-2 at 2 A cell current. With further increase in the concentration of zinc sulphate no improvement in the deposit nature was observed. The concentration of zinc sulphate was fixed at 250 gL· 1 as optimum and Hull cell patterns are as shown in Fig.lc.

Effect of sodium sulphate

Sodium sulphate was added to increase the conductance of the bath solution. The concentration of sodium sulphate was varied from 20 to 80 gL-1• At lower concentrations the Hull cell panels suffer burnt deposit at high current density region and uncoated at lower current density region. The burnt and uncoated regions were found to be reduced and at 60 gL-1 of sodium sulphate, in the bath solution, the deposit was bright over the current density range of 1.0 to 8.0 Adm·2• Further increase in the concentration did not introduce any effect on the nature of deposition and also it was found that the voltage of the bath solution remains constant after 60 gL-1 of sodium sulphate in the bath solution. So, the concentration of sodium sulphate was fixed at 60 gL-1 in the bath solution. The

12

l 1 0

Cathodic polenlia l , mVXIOl, w.r.t.SC E

Fig.2- Effect of addition agents on cathodic potential, 0 :

without addition agents (Basic bath solution), • : with DL­

Alanine, t:J. : with condensation product and 0 : with

Glutaraldehyde.

0.60 .---------------·---,-.....,...,

0.55

~ 0.50 c "' l:' E "' E 0.45

"' .= 0.40

o.J5 1.......-........ - .......... -......J......._~..._~.....,...~.......,-.........J 4000 3500 3000 2500 2000 1500 1000 500

Wavenumber cm·1

Fig.3--IR spectra of scratched deposit at 4.0 Adm·2 from optimum bath

Hull cell patterns showing the effect of sodium sulphate are as shown in Fig.l d.

Effect of pH and temperature

To know the effect of pH, the pH of the bath solution was varied from 1.5 to 3.0. In higher pH (between 2.2 to 4.0) the Hull cell panels showed burnt deposit at high current density region. At pH 2.2 satisfactory bright deposit was obtained. At lower pH ( <2.2) the specimens had uncoated area at low current density region. From the above observations the pH of

NAIK eta/.: EFFECT OF CONDENSATION PRODUCT ON BRIGHT ZINC ELECTRODEPOSITION 393

---€ 3 4 ~ ~ 0 f 1 1 : , 2 , (n:n) 1(1 ~'111 IW 3 9

Fig.4--SEM photomicrographs of deposits obtained at 4.0 Adm·2 presence and absence of addition agents at 30°C, (a) Basic bath, (b) Basic bath+ DL-Alanine, (c) Basic bath+ Condensation product, (d) Passivated deposit

the bath solution was kept at 2.2 as optimum. The Hull cell patterns are as shown in Fig.le.

To study the effect of temperature on Hull cell experiments, the plating experiments were carried out in a thermostat. The temperature of the thermostat was varied from 293 to 323 K. At lower temperatures ( <303 K), the deposition was bright in the current density range between 1.0 to 8.0 Adm·2 at 2 A cell current. Above 308 K the deposition was dull in the low current density range. So the optimum temperature range was 293 to 298 K. The Hull cell panels showing the effect of temperature are shown in Fig. If.

Effect of cell current

The Hull cell experiments were carried out at different cell current (lA to 3A) for 5min using optimum bath solution. It was found that at a cell current of I A the deposit was bright in the current density range of 1.5 to 4.0 Adm·2, and thin at 0 to 1.5

Adm·2. At a cell current of 2A, the deposit was bright in the current density range of 1.0 to 8.0 Adm-2. At a cell current of 3A the deposition was bright over the current density range between 1.0 to 11.0 Adm·2.

These observations revealed that the bath gave the bright deposit in the current density range of 1.0 to 11.0 Adm·2• The Hull cell patterns are as shown in Fig. lg.

Current efficiency and throwing power

Current efficiency and throwing power were measured at different current densities by using optimized bath solution. The current efficiency was measured by taking a rectangular methacrylate cell. At lower current density (2.0 Adm.2) the current efficiency was found to be 94%. At a current density of 4.0 Adm·2 the efficiency was increased to 97%. With increase in the current density above 4.0 Adm·2,

the current efficiency was found to be decreased and at 8.0 Adm·2 it was 94.5%.

394 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

Table J.-Optimum bath composition and conditions\

Bath constituents/

Optimum range

ZnS04 .7H20, gL·1

Na2S04, gL·1

Condensation Product, mLL-1

pH

Temperature, °K

Bright current density Range, Adm-2

Optimum concentrations/

Operating conditions

250 60 28 2.2 298-303

2- 11

Throwing power was measured by using Haring and Blum cell at different current densities. At lower current densities the throwing power was 25% with increase in the current density it increased to 26.7% (Table 2).

Polarization studies The potential of the steel cathode was measured,

galvanostatically, with respect to saturated calomel electrode at different current densities. The variation of potential in the presence of different bath constituents is as shown in Fig.2. The shift in cathodic potential towards negative direction was observed in presence of addition agents.

IR spectrum of the scratched deposit shows the inclusion of the condensation product in the deposit (Fig. 3).

Corrosion resistance For corrosion resistance study, the steel cathodes

were given deposit of varying thickness from 5 to 15 ~lm. The specimens after plating were subjected to bright dip in I% nitric acid and followed by passivation. The porosity of the deposit was tested with potassium ferricyanide paper. The soaked paper with potassium ferricyanide was placed on the deposit and no blue spots were observed. This test indicated pore free deposit. Further corrosion resistance test is carried out in a salt spray chamber. The deposited plates, after passivation, were subjected to continuous spray of neutral 5% sodium chloride solution. The deposit not showed any rust even after 96 h of testing. This study shows good resistance of the deposit.

SEM photomicrographs showed fine-grained deposit produced in the presence of addition agents Fig. 4.

Metallurgical properties Standard bend test was used to measure both

adherence and ductility of zinc deposits. Mild steel

panels of 1 mm thick ( 1 x 10 cm2 area) were electroplated with zinc to different thickness (5 - 20 )lm). The samples were subjected to bending test through 180°. No crack or peel off in the deposit was noticed even after 180° bending of the specimen. This indicates the good adherence and ductility of zinc deposit on steel. The more useful method for measuring microhardness involves making an indentation with an indenter of specified geometry under a specified load. The length of indentation is measured and microhardness is calculated (Vicker Hardness Tester, British Make). The values are expressed in vickers hardness number (VHN). Zinc was electroplated on mild steel panels up to a thickness of 20 )lm and a load of 50 g was employed. Jhe microhardness of zinc deposit was found to be 130.

Consumption of brightener In electroplating, the addition agents play an

important role in producing lustrous deposits. The addition agents are consumed during plating and thus their concentration decreases. When this concentration goes below the optimum value the deposit becomes dull in appearance. To know the amount of addition agents consumed in the present bath, 2.5 L of bath solution were taken and plating was carried out at different current densities. The total number of coulumbs passed to lhe bath solution were recorded at the time when the bath just started to give semibright deposit. The used bath solution was Hull cell tested by adding different amount of condensation product. The concentration of condensation product, at which once again bright deposit was obtained, was determined. The amount of condensation product consumed for 1000 amps-hour was 8 mLL-1•

Conclusion

The developed bath produces good deposit over wide current density range. The optimized bath composition is shown in Table 3. The constituents of the bath are only condensation product and conducting salts. This is an advantage over other baths containing various ingredients. The throwing power is reasonably good. The addition agents are non-toxic. The deposit is pore free and corrosion resistant.

Acknowledgement

The authors are grateful to the authorities of Kuvempu University for providing necessary facilities.

NAIK eta/.: EFFECT OF CONDENSATION PRODUCT ON BRIGHT ZINC ELECTRODEPOSITION 395

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