For chromium plating, along with chromic acid a small quantity of sulphate ion is very much essential. In the absence of sulphate ion the chrome bath will not produce any bright deposit but will produce only brown film. The sulphate ion is nonually introduced as sulphuric acid at a ratio of 100:1 to chromic acid. In this paper, in order to study the effect of other sulphate ions, it is aimed to study the effect of cations associated with sulphates. The SUlphate to the chrome bath was introduced in the form of sulphuric acid, sodium sulphate, ammonium sulphate, potassium sulphate and chromium sulphate. Hull Cell experiments were carried out to study the nature of the deposit. Plating expe:;ments were also carried out to study the current efficiency, rate of buildup, porosity, hardness, and nature of deposit and the results are presented

Kennedy, C.J.

Krishnan, R.M.

Mary, J.G.

Sekar, R.

Sobha , J.

Sriveeraraghavan, S.

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Bulletin ofElectrochemistry 15 (7-8) luly-August IQ9Q, pp 233-237 0256-1654/99/$ 3-50 © IQQQ CECRI EFFECT OF VARIOUS SULPHATES ON CHROMIUM PLATING RM KRISHNAN. S SRIY ERARAGIIAYAN, SOBHA JAYAKRISIJNAN C JOSEPH KENNEDY·, R SmCAR AND JACINlllA GNANA MARY· Central Electrochemical Research Institute, Karaikudi 630 006. INDIA * Gt"ep Industrial Syndicate Limited, Allahabad 211 001. INDIA For chromium plating, along with chromic acid a small quantity of sulphate ion is very much essential. In the absence of sulphate ion the chrome bath will not produce any bright deposit but will produce only brown film. The sulphate ion is nonually introduced as sulphuric acid at a ratio of 100:1 to chromic acid. In this paper, in order to study the effect of other sulphate ions, it is aimed to study the effect of cations associated with sulphates. The SUlphate to the chrome bath was introduced in the form of sulphuric acid, sodium sulphate, ammonium sulphate, potassium sulphate and chromium sulphate. Hull Cell experiments were carried out to study the nature of the deposit. Plating expe:;ments were also carried out to study the current efficiency, rate of buildup, porosity, hardness, and nature of deposit and the results are presented. Keywords: Chromium plating, sulphales, Hull Cell experiments and the effect of sulphate on decorative nickel-chromium electrode.position were also studied and reported [4,5). INTRODUCTION In ord r to study the effect of various cations associated with Chromium plating, an industrially importallt oating, is at sulphate, the SUlphate to the chromic acid was introduced as present mostly produced from conventional chrome bath sulphamic acid, sodium sulphate, potassium sulphate, consisting of chromic acid and sulphuric acid at a ammonium sulphate and chromium sulphate and Hull Cell concentration ratio of 100:1. It is well known that for and plating experiments were carried out to study the nature hromiu1l1 plating along with chromic acid a small quantity of the deposit, current efficiency, rate of build up, porosity of sulphate ion is very much essential. Tn the absence of and microhardness. The results are presented ill this paper. sulphate ion the chrome bath will not produce any bright chromium deposit but will produce only brown film. Thus EXPERIMENTAL sulphate ion is one of the important constituent in chromium For the chromium plating experiments mild steel specimensplating and few workers have carried out studies on the effect were used. The surface was prepared by mechanical,of sulphate concentration of chromium plating and the effect chemical as well as electrochem ical methods as given inof chromic acid to sulphate ratio on chromium plating. Table I. Technical grade chromic acid along with other A microscopic study of the fonnation of cathodic films on laboratory chemicals were used for this study. A three phase steel during electrolysis of chromium plating solutions at rectifier with A.C. ripple less than 5% and calibrated various chromic acid to sulphate ratios have been reported ammeters were used. The temperature of the electrolyte was [11. The cbaractcristic properties of c1ectrodeposited maintained at 323 :t; 1 K with the help of electrical glass chromium from solutions with varying sulphate chromic acid immersion heaters. concentrations have been studit"d [2]. Influence of sulphat s HulJ Cell experiments and silicofluorides on the properties of the cathod ic film fomlC'd during electrodeposition of chromium was studird Hull Cell experiments were carried out in order to observe 13]. Role of SUlphate ion in the chrome, plating electrolyte the nature of tbe deposit over a range of current dl:nsitics. 233 KRISHNAN, SRIVEERARAGHAVAN, SOBHA, JOSEPH, SEKAR, JACINTIlA-Effect of various sulphates on chromium plating TABLE I: Results on cathode efficiency and rate of build up at different current densities from chromium plating bath containing-chromic acid 250 gil, oxalic acid 4 gil and ulphuric acid at different concentrations at 323 K for 1 hour duration ConCD Current Current Rale or Poro- Hard- Nature of 804 z..- deDslt~ emcl- build slty ness of (HzS04) (A/dm ency up (%) (Kg/mm2}deposlt (gil) (%) (~m/h(") 1.25 15.5 14.93 10.39 20 980 A 31.0 11.96 16.66 10 970 B 2.50 15.5 9.40 6.54 25 980 A 31.0 16.28 22.66 10 980 A 5.00 15.5 9.30 6.47 30 950 A 31.0 13.46 18.75 25 960 C A = Bright C, deposit; B = Bright Cr with pores C = Semi bright Cr deposit Polished and cleaned mild steel cathodes of 100 x 60 x 0.25 mm size were used with a cell current of 7 A and for a duration of 5 minutes. A 267 ml Hull Cell of standard dimensions and 93:7 lead tin alloy anode was used for this study. Current efficiency experiments Mechanically polished and cleaned mild sted specimens of 30 x 30 mm size were used as cathodes in an de trodeposition assembly consisting of two 93:7 lead tin alloy anodes on either side of the cathode. The plating bath was agitated occasionally during use to maintain the temperature ± 1 K. The cathodes were weighed before and --­no -D Fig. 1: Codes used for the various nature of the deposits 1.25F ,'---- _ I .I 44 21 2 Fi~. 2: Effect of varying concentration of S04- (HZ-';04) in conventional chrome bath after the experimenL The current efficiency and the rate of buildup was found out gravimetrically in each case. The values of microhardness of eleClrodeposited chromium obtained from different chromium plating electrolytes at different conditions of current density and temperature were determined by using Russian made PMT-3 microscope at a load of 50 g. The Vickers microhardness of the deposit in kg/mm 2 was detennined by using Ibe appropriate formula. The porosity of chromium electrodeposits obtain under different condilions was evaluated by means of ferroxyl test. RESULTS AND DlSCl/ SION The results of the various studies carried oul are presented in Figs. 1-6 and Tables I-V. Hull Cell studies Figs. 1-6 present the Hull Cell patterns obtained under different conditions. Fig. 1 indicates the different codes employed to describe tbe nature of the chromium deposits obtained under various conditions. Fig. 2 presents the Hull 1.25 2.511 5.00IUiiiiiiludl1 _ 44 21 10 2 2Ahttn Fi§:- 3: Effect of varying concentration of so4 (Na;SO 4) in conventional chrome bath 234 KRISHNAN, SRIVEERARAGHAVAN, SOBHA. JOSEPH, SEKAR, JAONTIlA-Effect of various sulphates on chromium plating !~~--­ 1.25 r~~D,--_ 5.00ffi~~__~ ... 21 10 2 2N*nFig. 4: Effect of varying concentration of SO~-(NH4)z504 in conventional chrome bath Cell paltems obtained with varying concentrations of sulphuric acid added to the 250 gil chromic acid and when operated at 323 K with a cell current of 7 A for 5 minutes duration. From the figure it may be seen that when the sulphuric acid concentration is 1.25 gil in 250 gil cbromic acid concentration bright chromium deposit is obt<lin d only at the high current density area but when sulphuric acid concentration is increased to 2.5 gil the bright deposit region extends to a wider level. Further increase in concentration of sulphuric acid once again the bright rr.gion decreases thereby indicating that 2.5 g/I sulphuric acid concentration alongwith 250 gil chromic acid is ideal to produce bright chromium deposit over a wide current density range. Fig. 3 presents the effect of addition of sodium sulphate instead of the conventionally added sulphuric acid to the 250 g/I chromic acid when operatrd at 323 K and at 7 A. cell current for 5 minutes duration. From the figure it may be seen that 1.25 g/l sodium ulphatc along with 250 g/1 chromic acid produces bright chromium deposits over a wide current density range. Further increase in concentration of coeec::amtAT1ON OF K I 904 gil UI 5.00_iiii o 21 10 2 A/dm l Fig,. 5: Effect of varying concentration of So;-(Kz504} in conventional chrome bath 1.25 ~.""""""r.%:;;;-<~If:......---.l ... : : :. : : •: :: Mri:~ 2.50 5.00 ... 21	 2 Fig. 6: Effect of varying concentration of S~- (Cr (S04)3) in onventional chrome bath 2 sodium sulphate does not produce any improvement in the nature of the deposit. This indicates that even a small concentration of sodium sulphate namely 1.25 gil is sufficient to produce bright chrome deposit. Fig. 4 indicates the effect of addition of varying concentration of ammonium sulphate to the 250 gil cbromic acid when operated at 323 K and at 7 A cell current for 5 minutes duration. This figure indicates tbat when the concentration of ammonium sulphate is 2.5 g/l, the bath produces bright chromium over a wide current density range. This indicates that 2.5 gil ammonium sulphate along with 250 gil chromic acid is more suitable to produce quality deposits at 32..1 K. TABLE U: Result.. on current efficiency and rate of build up at different current densities from chromium plating bath containing chromic acid 250 gil, oxalic acid 4 gil and sodium sulphate at different concentrations at 323 K for I hour duration Conen Current Current Rate or Poro- Hard- Nature of S04z.- density emel- build slty ness of (NIl2SO f ) (A/dm2) eney up (%) (~/mm2)dcposlt (gil) (%) (Ilmlhr) 1.25	 15.5 5.IQ 3.61 50 940 A 31.0	 4.03 5.61 50 920 B 2.50 15.5 Q.8Q 6.88 40 Q50 C 31.0	 15.40 21.44 30 950 0 5.00	 155 9.03 6.28 50 QOO E 310 IQ.02 26.47 10 890 F A = Bright rough Cr d<'posit B = Matte white rough pits Cr deposit C = Brighl Cr deposit; 0 = Milky white Cr deposit E = Malle white C .. deposit 235 KRISHNAN, SRIVEERARAGHAVAN, SOBHA, JOSEPH, SEKAR, JACINTHA-Effect of various sulphates on chromium plating TABLE ill: Results on current efficiency and rate of build up at different current densities from cbromium plating batb containing cbromic acid 250 Wi, oxalic acid 4 gil and ammonium sulpbate at different concentrations at 323 K for 1 bour duration Conen of CurrentCurrent Rate or Poro- Hard- Nature 2­804 densit~ efficl- build slty ness of ((NH.JzS04) (Aldm) ency up (%) (Kg/mm2)leposlt (WI) (%) (....m/hr) 1.25 15.5 12.52 8.71 30 950 A 31.0 12.82 17.84 30 960 B 2.50 15.5 11.78 8.20 40 970 A 31.0 \6.62 23.14 20 980 C 5.00 15.5 6.37 4.43 50 920 C 31.0 13.\7 18.34 40 900 C A = Semibright Cr deposit B = Semi bright Cr with pils at the edge C = Bright Cr deposit Fig. 5 presents the effect of addition of different concentrations of potassium sulphate in 250 gil chromic acid, when operated at 323 K and 7 A cell current for a duration of 5 minutes. From the figure it may be seen tbat when the concentration of potassium sulphate is increased to 2.5 gil bright chromium deposit is obtained more or less on the entire area of the Hull Cell plate. Thus it can be seen tbat tbe addition of 2.5 gil potassium sulphate to 250 gil chromic acid is more beneficial to produce bright deposit over a wide range of current density. Fig. 6 presents the Hull Cell patterns obtained for different concentrations of chromium sulphate in 250 gil chromic acid and when operated at 323 K with a cell current of 7 A for 5 minutes duration. From the study it may be concluded that the chromium sulphate addition at all concentrations studied does not produce brigbt deposit but produces- mostly dull white deposit. From these tudies it is confinlled that along with chromic acid the presence of a sulphate anion is necessary to produce bright chromium deposit. It may also be pointed out from these studies that the cations associated with sulphate anion also plays an important role in modifying the deposit. Thus from the Hull Cell patterns obtained it may be concluded that the potassium cation associated with sulphate anion is highly beneficial in producing bright chrome deposit over a wide current density range. Current efficiency studies The results of the current efficiency experiments carried out are given in Tables I-V. Table I presents the results when varying concentrations of sulphuric acid is added to 250 gil chromic acid and operated at 323 K and at 15.5 A/dm2 and 31 A/dm2. It may be seen from the table that the cathode efficiency increases with increase in CUrrent density. But on the other hand the cathode· efficiency decreases with increase in concentration of sulphuric acid. It may be concluded from the table that 2.5 gil sulphuric acid is more suitable to be added to 250 gil chromic .acid electrolyte to obtain· maximum current efficiency. Table II presents the results on the current e(ficiency experiments carried out when varying concentrations of sodium sulphate added to 250 gil chromic acid and operated at 323 K and at 15.5 A/dm2 and 31.0 A/dm2. From the results it may be seen that chromium plating electrolyte containing 250 gil chromic acid and 5 gil sodium sulphate exhibits high cathode efficiency than the other conditions stUdied. From Table III it can be noted that 2.5 gil ammonium sulphate may be added to 250 gil chromic acid in order to obtain high current efficiency under normal working conditions. From Table IV, which indicates the effect of potassium sulphate in chrome bath operated at 323 K and at 15.5 A/dm2 and 31 A/dm2, it can be seen that the current efficiency increases marginally with increase in concentration of potassium sulphate. From Table V it can be TABLE IV: Results on catbode efficiency and rate of build up at different current densities from cbromlum plating bath containing chromic acid 250 Wi, oxalic acid 4 gil and potassium sulpbate at different concentrations at 323 K for 1 bour duration Conen C:urrent Current Rate of Poro- Hard- Nature or 8042- denslt~ emcl- build slty ness of (HzS04) (A/dmj ency up (%) (~/mmldeposlt (WI) (%) (....m/br) 1.25 15.5 11.22 7.8\ 25 950 A 31.0 13.36 18.60 20 960 B 2.50 15.5 9.99 6.95 30 990 A 31.0 \5.32 21.33 20 980 A 5.00 15.5 9.27 6.54 50 940 A 31.0 16.45 23.03 20 960 A 236 KRISHNAN, SRIVEERARAGHAVAN, SOBRA, JOSEPH, SEKAR, JACINTHA-Effect of various suJphates on chromium plating TABLE V: Results on cathode efficiency and rate of build up at different current densities from chromium plating bath containin~ chromic acid 250 gil and chromium sulphate at different concentrations at 323 K for I hour duration Conen of CurrentC'urrent Rate of Poro- Hard- Nature SO;- denslt~ effici· . build slty ness of (\r2 (S04h) (A/dm) eney up (lib) (~/mm2)icposlt (gil) (%) (Ilm / hr) 1.25 15.5 12.55 8.70 20 920 A 31.0 17.35 24.15 10 900 B 2.50 15.5 6.61 4.60 30 940 B 31.0 13.14 18.29 30 930 B 500 15.5 1.57 1.09 50 910 C 31.0 8.01 J1.15 40 900 B A = Bright Cr deposit; B = Semi bright Cr deposit C = Light grey Cr dr.posit seen that the current efficiency fa lis down with increase in concentration of chromium sulphate. Nature of the deposit The results of the studies on the nature of the deposit obta ined after plating are produced in Tables I-V. From these results it may be seen that the addition of either H2S04 or ~S04 at 1% concentration to chromic acid is more favourable to produce bright acceptable deposits. Rate of buildup The results of the experiments carrie.d out on the rate of build up of chrome deposits under different conditions studied are given in Tables I-V. From the tables it may generally be seen that the rate of build up of chrome deposits increases with incre.ase in current density. Microhardness of deposits The results of measurements of the microhardness values for deposits obta ined from different rlcctrolytes under different conditions are given in Tables I-V. From the results it may be seen that 2.5 gil sulpburic acid addition to 250 gil cbromic acid produces more hard deposits. In a similar manner the addition of potassium sulphate to the 250 gil chromic acid also produces more hard deposits under all operating conditions studied when compared to other sulphates containing hrome baths. Porosity From tbe results as given in Tables I-V it may be seen that in general the porosity of chrome deposits decreases with increase in thickness. From the experiments carried out it Illay be seen tbat the deposits produce.d under different operating c.onditions from electrolytes containing 250 gil cbromic acid along witb 2.5 gil of H2S04 or K2S04 exbibit<; less porous chromium depo its. CONCLUSION From the limited experiments carried out it may be concluded tbat the sulphate anion is necessarily to be added to tbe chromic acid to produce cbrome deposit and the ratio of chromic acid to sulpbate may be preferably 100:1. Thl~ following chrom ium plating bath composition is recommr.nded to produce brigbt chrome deposit with high current efficiency, bigh microhardness and with less pore density. Chromic acid: 250 gil; Potassium sulphate.: 2.5 gil; Temperature: 323 K; Current density: 15.5-31.0 A/dm2 REFERENCES 1.	 J C Saiddington and G R Hoey, J Electrocltem Soc, 117 (1970) 1011 2.	 M Me omic and S J Do~on, C'uniushefield, Dept Electronic & Electrical Eng., heffield, SI (31 D) Trans Inst Metal Finish 6S (l Q87) 24 3.	 Z A Solovane and L N Solodkova, Electrokhimiya, 12 (1976) 47 (Russ) 4.	 CD Tok, M A Shluger and I H Barabanova, ZaclL~h Me/all 22 (1986) 273 5.	 R Nishinusa, H Fukushima, T Akiyama and R lIigashi, Metal Finish, 8S (1987) 45 237 

Effect of various sulphates on chromium plating

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