Reduction of Chromium (VI) When Solar Selective Black Chromium Is Deposited in the Presence of Organic Additive Branko N

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1992 Reduction of Chromium (VI) when Solar Selective Black Chromium is Deposited in the Presence of Organic Additive Branko N. Popov University of South Carolina - Columbia, [email protected]

Ralph E. White University of South Carolina - Columbia, [email protected]

D. Slavkov University Kiril and Metodij, Skopje, Yugoslavia

Z. Koneska University Kiril and Metodij, Skopje, Yugoslavia

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Publication Info Journal of the Electrochemical Society, 1992, pages 91-98. © The Electrochemical Society, Inc. 1992. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The ra chival version of this work was published in the Journal of the Electrochemical Society. http://www.electrochem.org/ DOI: 10.1149/1.2069206 http://dx.doi.org/10.1149/1.2069206

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14. L. T. Romankiw, S. Krongelb, E.E. Castellani, A.T. stracts, Vol. 88-2, Chicago, IL, Oct. 9-14, 1988. Pfeiffer, B. J. Stoeber, and J. D. Olsen, IEEE Trans. 24. T. Kessler and R. C. Alkire, Plat. Surf. Finish., 63, 22 Mag., MAG-10, 828 (1974). (1980). 15. D. A. Rudy, Abstract 264, p. 658, The Electrochemical 25. S. Mehdizadeh, J. Dukovic, P.C. Andricacos, L.T. Society Extended Abstracts, Vol. 76-1, Washington, Romankiw, and H. Y. Cheh, This Journal, 137, 110 DC, May 2~7, 1976. (1990). 16. D. A. Rudy, Abstract 266, p. 685, The Electrochemical 26. A. T. Kuhn, "Techniques in Electrochemistry, Corro- Society Extended Abstracts, Vol. 76-2, Las Vegas, sion and Metal Finishing--A Handbook," pp. 95-116, NV, Oct. 17-22, 1976. John Wiley & Sons, Inc., New York (1987). 17. B, Littwin, IEEE Trans. Mag., MAG-14, 123 (1978). 27. N. Ibl, "Comprehensive Treatise of Electrochemistry," 18. J. Horkans and L. T. Romankiw, This Journal, 124, Vol. 6, Plenum Publ. Corp., New York (1982). 1499 (1977). 28. C. A. Brebbia and S. Walker, "Boundary Element 19. J. Dukovic, IBMJ. Res. Develop., 34, 693 (1990). Techniques in Engineering," Newnes-Butterworths, 20. R. C. Alkire, T. Bergh, and R.L. Sani, This Journal, Boston (1980). 1~5, 1981 (1978). 29. J. Dukovic and C.W. Tobias, This Journal, 134, 331 21. R. C. Alkire and D. B. Reiser, Electrochim. Acta, 28, (1987). 1309 (1983). 30. L. Lapidus, "Digital Computation For Chemical Engi- 22. E.C. Hume, III, W.M. Deen, and R.A. Brown, This neers," p. 227, McGraw Hill, Inc., New York (1962). Journal, 131, 1251 (1984). 31. M. Matlosz, C. Creton, C. Clerc, and D. Landolt, This 23. U. Landau, R.T. Galasco, and J. Tang, Abstract 330, Journal, 134, 3015 (1987). p. 484, The Electrochemical Society Extended Ab- 32. M. Datta and L. T. Romankiw, ibid., 136, 285C (1989).

Reduction of Chromium (VI) when Solar Selective Black Chromium is Deposited in the Presence of Organic Additive

B. N. Popov* and R. E. White* Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122

D. Slavkov and Z. Koneska Faculty of Technology and Metallurgy, University "Kiril and Metodij," Skopje, 91000, Yugoslavia

ABSTRACT Electrochemical dc methods were used to study the reduction of chromium (VI) in the presence of organic additives. It is shown that F ~, thiourea, and citric acid are essential to enable deposition of black chrome from concentrated chromic acid solutions. A new formulation of the plating bath is defined. The optimum operating conditions under which spectrally selective surface of black chrome is deposited have been determined.

The classical process of chromium deposition begins si- The deposition of black chromium is generally done multaneously with formation of the oxide layer which rap- with conventional chroming bath free from sulfates, to idly deteriorates permitting the access of Cr(VI) ions to the which catalyst such as acetic acid, sodium nitrate, ammo- cathode and their reduction to metallic chromium. Ac- nium methavanadate or fluorine compounds are added cording to the polarization studies (1-10), the cathodic (15-21). The exact mechanism by which the catalyst ions oxide film is formed only if a polychromate ion exists in enable chromium reduction which leads to formation of the electrolyte. Polychromates are formed in strong solu- black deposits is still open to conjecture. tions of chromic acid at low pH. The sulfate ion present in One of the most common commercial solutions for black conventional chroming bath exists in strong acids as chrome deposition is ChromOnyx plating solution from HSO~ and favors the elimination of the cathodic oxide film the Harshaw Chemical Co., which contains acetic acid as (11). A phenomenological model for the deposition of Cr catalyst. As reported in the literature (17), use of this solu- from hexavalent baths was proposed by Hoare et aI. (7-10). tion may lead to production of black chrome surfaces with According to these authors the polychromates decompose different optical properties as a result of the degradation to chromous hydroxide and a dichromate which can po- process of the catalyst. The purpose of the present work is lymerize back to a trichromate by condensation with other to study the electrochemical reduction of chromic acid in chromates in the solution. The HSO~, exerts its catalytic the presence of organic additives and to determine the activity by forming a chromousoxybisulfate complex electroplating conditions for obtaining a selective absorb- through hydrogen bonding with chromous oxide. With the ing surface. An attempt has been made to define a new for- successive transfer of two electrons to the specifically ab- mulation of plating bath containing NaF, citric acid, and sorbed chromousoxybisulfate complex on the cathode thiourea. The preliminary polarization studies have shown surface, Cr(I) and finally metallic Cr along with the regen- that these compounds are essential to enable deposition of erated HSO 4 ion are then obtained. black chromium. According to Saiddington and Hoey (12), reduction of pure chromic acid in the absence of sulfate ions results in Experimental formation of an amorphous oxide film which grows on the In order to establish the influence of the constituents of cathode. This film prevents the metal reduction processes the plating on the reduction of chromic acid, measure- from taking place on the cathode surface, while allowing ments of the rest potential at open circuit and c.v. meas- the reduction of hydrogen only. The existence of such urements were made. All electrochemical measurements films has been proved by double layer capacity measure- were carried out in a special Teflon cell because of the ments (13) and optical microscope studies (14). presence of fluorine ions. Working Pt electrodes were used, mounted into a Teflon holder with a geometric area * Electrochemical Society Active Member. of 6.2 x 10-2 cm ~. The electrodes were mechanically pol-

Downloaded 16 Jun 2011 to 129.252.106.20. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp 92 J. Electrochem. Soc., Vol. 139, No. 1, January 1992 The Electrochemical Society, Inc.

ished, ultrasonically cleaned in an alcohol and rinsed with Table I. Open-circuit potential values with respect to (SCE) obtained distilled water prior to each measurement. A standard cal- on platinum electrodes in chromic acid and in solutions catalyzed by omel electrode (SCE) was used as a reference electrode. F- ions, thiourea, and citric acid. Black chrome was electrodeposited from a solution containing 4M CrO3 and 0.05M BaCO3 in presence of fluorine Solution Concentration (M) E, (V), (SCE) ions and organic additives. The additives were added in the cathode compartment during the course of the run. [Cr30,0]- 2.0 0.701 All electrochemical measurements were carried out 2.5 0.74 using a PAR Model 273 potentiostat-galvanostat con- 3.0 0.74 4.0 0.91 nected via Model 273 interface, to an IBM computer. Ex- 4M [CrsO10]= + citric acid 0.15 0.92 perimental data were stored and plotted using a 7470 Hew- 0.45 0.95 lett-Packard plotter. 0.80 0.96 An ideal selective surface is totally reflecting, or white to 0.12 0.98 the infrared radiation emitted by a black body at the tem- 4M [Cr3010]= + NaF 0.024 1.043 0.078 1.025 perature of operation of the collector. However, an ideal 0.261 1.010 selective surface does not exist in practice, and therefore 0.357 1.015 one can conceive a real surface whose spectral reflection 4M [Cr30,0]- + thiourea 0.25 1.025 coefficient varies with wavelength in a manner close to 0.48 1.015 0.60 1.040 that of an ideal black body. Such a surface can be charac- 1.1 1.035 terized by its hemispherical absorptance, as, and its normal thermal emittance, eT. Normal reflection spectra of the samples were measured obtained using a scan rate of 1 mV/s by scanning from on the Perkin-Elmer Model 180 spectrophotometer + 1.3 V vs. SCE to - 1.2 V vs. SCE. There is no evidence for equipped with reflection cells. A gold mirror was used as any differences in the shape of the polarization curves ob- reference for reflection in order to measure the reflectivity tained for different waiting times indicating that concen- of the samples. The measured spectral range was from 2.5 trated chromic acid solution is highly oxidizing and that to 50 ~m. The reflection spectra of the samples in the range the passive film formation on the electrode surface is a between 0.4 and 2.5 ~m have been measured with an very fast process. The passive film actually is formed in the Opton II instrument equipped with an integrating sphere first contact of the electrode surface with chromic acid so- for measuring the hemispherical reflectivity. A computer lution. The dichromate ion present in the concentrated program was used for calculation of the normal thermal chromic acid solution (23-26) is reduced on the surface, ox- emittance (ew) and the hemispherical absorptance (as) from idizing Pt to PtO. The curves in Fig. 1 also show a decay of specular normal and hemispherical reflectivities accord- the current at + 1.0 V Vs. SCE which indicates a passivation ing to the equations process, while the fine structure of the polarization curves is connected with the passivation process and the changes fl ~ e(X, T)M~ T)dX of the oxidation states of Pt and Cr present in the composi- .3 ~m tion of the passive film. A low hydrogen overvoltage is eT -- [1] present on the electrode. According to the observations foi: ~m M~ T)dk during the run of the experiment as well as from the curves in Fig. 1, hydrogen evolution starts at -0.25 V vs. SCE. Thus, the entire cathodic current corresponds to and Fin combination of hydrogen reduction and reduction of Cr f s0 ~(k, T)ES(k)dk ions which occurs at approximately -0.9 V vs. SCE (a c broad peak in Fig. 1 is observed at this potential). a s -- [2] ]0 ~in ES(MdX Figure 2 show potentiodynamic polarization curves taken at sweep rate of 1 mV/s in the presence of 0.2M NaF (curve 1), in the presence of 0.35M thiourea (curve 2), and in where a(~, T) and e(X, T) are, the hemispherical spectral ab- the presence of 0.5M citric acid (curve 3). It was found that sorptance and emittance, respectively, ES(~,) the spectral in the presence of the additives the hydrogen evolution solar irradiance, and M~ ,T) the spectral emittance of the drops drastically and two more cathodic peaks appear at black body at temperature T. For opaque materials (trans- mittance = 0) to which black chrome deposits belong, spectral absorptance for directional incidence and the di- 1300 "'l'"l ' "l""l ' "1'~ '' 'I~,~T./"I"~ "l""l '+ 'r"1 ' "1~1 + "1" 1 rectional emittance can be calculated from the spectral reflectance or directional irradiation as: as = e = 1 - r, where r is the spectral reflectance. Results and Discussion Open-circuit potential studies.--Open-circuit potentials observed on Pt cathodes in pure chromic acid and in chro- /, 300 mic acid in which NaF and organic compounds were added are listed in Table I. High anodic values of the observed open-circuit potentials indicate that platinum passivates spontaneously in oxygen free chromic acid solu- -200 tions. The rest potentials are probably mixed potentials of the reduction of dichromate ion and oxidation of Pt to PtO (22-29). The passive film is reducing when the cathode is

polarized at more negative potentials. -700 Potentiodynamic measurements.--Potentiodynamic polarization experiments were carried out in order to deter- ] mine the shape of the cathodic and anodic curves and the existence of the passivation currents. The electroactive Cr -1200 .,,hP-' ,,.t,.,I ,,,f.J ,,a,.l ,,,h.,,I ,,.h..I ,,,I,.J ,,.t..J ~l,,h, species were reduced in a nitrogen atmosphere using Pt 1~3 162 ~61 +o~ lO' 1o2 lO3 lo" lOs ',o6 working electrodes. I [J~A/cm 2 ) Typical cathodic potentiodynamic polarization curves for electroreduction of pure chromic acid obtained Fig. 1. Typical potentiodynamic polarization curves for electrore- starting at open-circuit potential for different aging times duction of 4M chromic acid obtained using a scan rate of 1 mV/s and (0.5, 30, 60, and 90 min) are shown in Fig. 1. All curves were for different aging times: 1, 0.5; 2, 30; 3, 60; and 4, 90 min.

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1300 '"P'"I '"l'tll '"1'"~ '"1''1 ',,~wJ~-~ '"l" t"l"ql ,.pu

- (mA) 7.6

8OO r,.) \~, 6.8

[Cr301o] = = 4M E 300 ~ I 6.0 CNal~ = 0.36M ,j 2 5.2 -200 1%

4.4

-70t 1. CNar=0.2 M 2. CThlourea=0-35M 3.6 3. Ccit~r Acid--0.5M

2.8 I ~ , I I -'120[ ,..hi J ,t,l,,~ t.hm~ ,Hlu~ ~.1,.I ,,,INmt ,,d.ml ,.ha J I. E (V), SCE i I l'-A/cm 2 ) Fig. 4. Cyclic voltammogram for reduction of 4M chromic acid in the Fig. 2. Potentiodynamic polarization curves for electroreduction of presence of 0.36M fluorine ions, v = 0.02 V/s. 4M chromic acid obtained in the presence of NaF, thiourea, and citric acid taken at sweep rate of 1 mV/s. creases with increasing the sweep rate. The peak current was not found to be a linear function of the square root of -0.3 V and at -0.5 V vs. SCE which were studied in more the sweep rate, suggesting that the process is not diffusion details using cyclic voltammetry. controlled. Figure 4 shows the cyclic voltammograms for the elec- Cyclic voltammetry.--The curves were obtained using different scan rates in the range from 0.01 V/s up to 0.1 V/s trochemical reduction of chromic acid in the presence of 0.36M fluorine ions. In the presence of the catalyst ions two and scanning from - 0.2 V vs. SCE to - 1.2 V vs. SCE where the reduction of water becomes a predominant reaction. more cathodic peaks appear at -0.3 V and at-0.5 V vs. Figure 3 shows typical c.v. curves for reduction of pure SCE. The dependence of the magnitude of the first peak on chromic acid (4M) in which only 0.05M BaCO~ was added the concentration of fluorine ions (from 0.33M to 0.76M) is in order to precipitate SO~- ions. As can be seen from shown in Fig. 5. The curves were obtained using a scan rate of 0.01 V/s and scanning from 02 V to -0.4 V vs. SCE. Fig. 3, the curve shows a cathodic maximum at -0.95 V vs. SCE which is formed during cathodic course of the run, It was found that the magnitude of the peak at -0.3 V does and this can be attributed to the reduction of Cr ions. not depend on NaF concentration. This peak appears only There is no evidence of a corresponding anodic peak dur- at concentrations of the additive higher than 0.31M at ing the anodic course of the run, indicating that the prod- which the evolution of hydrogen drops drastically. uct formed on the cathode cannot be dissolved when the The peak height of the reduction process at -0.5 V vs. electrode is polarized anodically. Repeated cycles (curve 2) SCE depends on the concentration of NaF, Fig. 6. The de- on the same figure do not show the cathodic peak at pendence of the peak currents on the sweep rates (0.005 to -0.95 V, only hydrogen evolution, which indicates that the 0.050 V/s) for the reduction processes at -0.5 V and at electrode becomes completely passive during the anodic -0.95 V vs. SCE is shown in Fig. 7. The curves were ob- course of the previous cycle. Neither fluorine, thiourea, tained in the presence of 0.276M NaF and scanning from nor citric acid have an influence on the magnitude of the 0.0 V to -1.2 V vs. SCE. The dependence of the peak cur- reduction peak at -0.95 V, and the peak current only in-

I-fmA/

- (mA)

8.8

[Crt01oJ= 4M 3.6 4 v= 0.05 V/s = . 2.0 4.0 2.0 L CN#--02~M v--'0.0lV/s 2. C~---028M

2.4 ~.20~ .~6M P

C / ' b o.~ ~ "~'~ -o.~ -0.3 0.4 -o.5 E (V), (SCE) !+{mA) 0.8 f 4.2 4.4 JJ.6 -0.8 -L0 -1.2 1 E (V), SCE Fig. 5. Cyclic voltammograms for electrochemical reduction of 4M Fig. 3. Cyclic voltammograrms for reduction of 4M chromic acid, chromic acid at -0.3 V vs. SCE, obtained in the presence of different v = 0.0S0 V/s; 1, first cycle, 2, second cycle. concentrations of NaF, and v = 0.01 V/s.

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I-(mA)

s.2II-(mA)

&o 4A J.s

2,6

1. CThlo=r~=0.23M 2. CT~aur~=0.26M z~ 3. CThiourea=0.28M 4. CThloure~=0.38M 1.8 v= 0.02V/s

I i :> -o.6 -o.~. -i.o E (V), {SCE} 1. C~--0.142M 2. CNtr=0.166M 3. C~.r--O,190M I§

Fig. 8. Cyclic voltammogram for electrochemical reduction of 4M chromic acid at -0.3 V vs. SCE, obtained in the presence of different - o.2 04 -o.~ o.a - l.o E (V), (SCE) 0,2 concentrations of thiourea, and v = 0.02 V/s.

Fig. 6. Cyclic voltammogroms for electrochemical reduction of 4M thiourea concentration. This peak only appears at concen- chromic acid at -0.5 V vs. SCE, obtained in the presence of different trations higher than 0.23M. concentrations of NaF, and v = 0.01 V/s. The peak current at -0.5 V depends on the concentration of thiourea, as shown in Fig. 9, but the dependence was not found to be a linear function of the concentration rent at -0.5 V vs. SCE on the square root of the sweep rate of the additive. The dependence of the peak currents of the is not linear precluding the possibility that the process is reduction processes at -0.5 V and at -0.95 V vs. SCE on diffusion controlled. The reduction, rather, is a surface re- the sweep rates (0.005 to 0.050) is shown in Fig. 10. The action catalyzed in the presence of F- ions. curves were obtained in the presence of 0.2M thiourea and The results obtained for the reduction of chromic acid in scanning from 0.0 V to -1.2 V vs. SCE. The peak current at the presence of thiourea are similar to those obtained -0.5 V vs. SCE was not found to increase with increasing when the reduction was studied in the presence of NaF. the sweep rate. This suggests that the reduction at -0.5 V The dependence of the magnitude of the peak at -0.3 V on in the presence of thiourea is not diffusion controlled. the concentration of thiourea (from 0.23 to 0.38M) is shown In Table II are shown cathodic charges (C/cm2) calcu- in Fig. 8. The curves were obtained using a scan rate of lated by integrating the surfaces under the cathodic peaks 0.02 V/s and scanning from 0.0 V to -0.4 V vs. SCE. It was at -0.5 V in the presence of fluorine ions and thiourea. In found that the magnitude of the peak does not depend on the case of NaF additions the charges increase from 10.07 • 10 -3 to 95.22 x 10 -3 C/cm 2. It is obvious from Table II that both additives have an accentuated influence on the magnitude of the reduction peak at -0.5 V vs. SCE, ' Z-fm.4,t decreasing remarkably the hydrogen evolution. The potentiodynamic and c.v. measurements as well as the measurements carried out at open circuit indicate that

4 Z-{reAl C~aF :0.276M /-%

v= 0.005 V/s / ~ v= o.olo v/s / \ ~. o:o2o v/, / \ 7.2 2,2

5.6

4.0 t.4

L CThleu~= 0.031M 1.0 2. CThlour~= 0.047M 2A 3. CTlllour~= 0.071M 4. CThiourea= 0.095M O~ 5. CThiour~= 0.U9M v= 0.02V/s

OoZ -o.~ -o., % -o.~, -,~o "~ -o~ ~ -oi E (V), (SEE) o~ 7.o E (V), (SCE) - 0.,~ o,2

I*(mA]

Fig. 9. Cyclic voltammograms for electrochemical reduction of 4M Fig. 7. Cyclic voltammograms for reduction of 4M chromic acid taken chromic acid at -0.5 V vs. SCE, obtained in the presence of different for different sweep rates in the presence of 0.276M NaF. concentrations of NaF, and v = 0.02 V/s.

-oJ +-. - . , .... , .... , . . . ~ .... , .... I-[mA]

CThI~=O.~M

2 v=O.OlO VIs 3. v=O.020 V/s ~-o.|

+q2 q4 -o~ -oas -1.o q~ E {V), (SEE) I+~A)

-I.o .... ' .... , - - - , i , , . , .... , .... -s,o -ILI~ -~.o -l.l~ -I,O -o& o.o ~o- (,).(v/.) Fig. 10. Cyclic voltammograms for reduction of 4M chromic acid in Fig. 11. The shift of the peak potential as a function of log (v). the presence of 0.2M thiourea taken for different sweep rates.

Cr(VI) to Cr(III) is eliminated as a possibility occurring in the passive film formed in concentrated chromic acid has one reduction step, rather the reduction occurs through an important role in the reduction mechanism because it Cr(V) and Cr(IV) intermediaries, where the coordination shifts the deposition of metallic chromium and chromium changes are smaller. According to our studies the peak oxide to very high overpotentials. Platinum oxides and which occurs at -0.5 V vs. SCE can be attributed to reduc- chromium oxides present in the composition in the film tion of Cr(VI) to Cr(IV). The peak height at -0.5 V vs. SCE are reduced when the cathode is polarized at -0.35 V vs. depends upon the concentration of NaF and thiourea, and, SCE. As is shown in our polarization studies this process is as will be shown later, are essential to enable deposition of clearly seen only in the presence of the additives, when the black chromium. Cr(VI) ions in concentrated chromic acid evolution of hydrogen drops drastically. The magnitude of solutions essentially are in the form of Cr20~ and in its de- the peak does not depend upon chromic acid concentra- rivative complexes (7-10, 25-29). The additives used in this tion or the concentration of the additives indicating that study also form complexes with Cr ions. These complexes the reduction is not controlled by the ions from the elec- move toward the cathode through the cathodic film and trolyte. are reduced at the surface which results in the dependence At further cathodic polarization, a cathodic film is of the peak height of the reduction process at -0.5 V vs. formed, composed from absorbed Cr(VI), Cr(III), and spe- SCE upon the concentration of the additives. Thus, the cies with lower oxidation states of chromium (30-34). complexes formed in the cathode region catalyze the For any cation which forms complexes in the solution, in transient stages in the electrodeposition process. order to be reduced or oxidized, it is necessary that its co- The important features of the diagrams in Fig. 7 and ordination number be changed (22, 24). This fact, in con- Fig. 10 are the increase of the peak current and the shift of nection with Frank-Condon principle, prevents three-elec- the peak-potential with an increase of the sweep rate. This tron reduction from Cr(VI) to Cr(III), because their shift of the peak potential with the increased sweep rate is coordination numbers are 4 and 6, respectively. The reduc- plotted in Fig. 11 as a function of log v as suggested by tion of Cr(III) to Cr(II) is also limited, despite that both spe- Srinivasan and Gileadi for the case when the product of cies have the same coordination number. This limitation the electrochemical reaction are absorbed species which results from the structural configuration of the Cr(III) block the surface for further reaction. As seen, a linear de- complexes, which are octahedral, while Cr(II) complexes pendence is obtained. According to Srinivasan and Gileadi have strong Jahn-TeUer distortion of octahedral configu- (35), the peak potential and sweep rate Iv, V/s], should be ration. Thus, the slow change in the coordination structure related by equation compared with the fast electron transfer limits certain electron exchange reactions. Accordingly, the reduction of RT kF RT Ep = -- In-- + -- In v [3] ~F klRT ~F

Table II. Cathodic charges calculated in integrating the surfaces where ~ is the transfer coefficient, kl is the reaction rate under the peaks at -0.5 V (SCE) as a function of the concentration constant, and k is the amount of charge required to form a of NaF and thiourea. monolayer of absorbed product. The linearity obtained in Fig. 11 which follows the prediction of Eq. [3] indicates Solution Q (C/cm2) • 10 -3 that the peak at -0.95 V probably results from Cr(IV) to Cr(III) reduction on the surface of the electrode. The prod- 4M [Cr3Om]+ + NaF (M) uct of the electrochemical reduction at this potential are 0.024 10.07 0.048 15.89 Cr(III) species which form stable electrochemically nonac- 0.072 23.12 tive complexes with F- and very stable [Cr(H20)8] 3+, also an 0.096 40.15 electrochemically nonactive complex in water solutions. 0.110 47.69 These complexes are absorbed at the electrode blocking 0.144 70.35 the surface for further reduction. From the slope in Fig. 11 0.168 88.93 0.192 95.22 one can calculate a = 0.6. At more cathodic potential, re- 4M [Cr3010]+ + thiourea (M) duction to metallic chromium also occurs with heavy hy- 0.25 15.40 drogen evolution as a result of water reduction at high 0.48 20.35 overvoltages. 0.55 35.68 0.85 47.12 The amount of liberated hydrogen during the electrodep- 1.10 59.65 osition of black chromium from chromic acid solution in

Downloaded 16 Jun 2011 to 129.252.106.20. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp 96 J. Electrochem. Soc., Vol. 139, No. 1, January 1992 The Electrochemical Society, Inc.

Table III. Hydrogen evolution as a function of the applied potential, Table IV. Variation of the normal thermal emittance (eT), the Apt = 0.6 cm2. hemispherical absorptance (as) and selectivity (as/eT) of black chromium deposited for 1 min (curve 1) and for 2 min (curve 2). H. E. in H.E. in catalyzed Hydrogen catalyzed 4M chromic acid Optical properties Deposited for 1 min Deposited for 2 rain evolution (H.E.) chromic acid solution with in 4M citric acid with 0.238M F- 0.48Mthiourea -E (V) (SCE) (ml) (ml) (ml) as 0.916 0.928 eT (40~ 0.039 0.100 as/eT (40~ 23.490 9.280 0.293 ------eT (200~ 0. I00 0.167 0.416 4.0 1.4 1.8 as/eT 9.160 5.650 0.509 8.15 3.05 5.6 0.866 32.75 4.15 6.4 0.932 40.60 5.00 6.9 1.028 58.40 6.30 8.2 1.040 75.35 7.80 9.9 1) and for 120 s (curve 2). Variation of the optical properties 1.203 over 100 8.30 14.8 of these two black chrome specimens are given in

1.360 -- 13.7 -- Table IV. The data indicate that specimen 1 has better optical characteristics, which can be explained by the influence of the thickness on the optical properties of the de- the absence and in the presence of the additives was meas- posit. ured quantitatively and the results are given in Table III. On the basis of these preliminary measurements we The experiments were carried out in a special hermetically have formulated our own composition of chromate bath, sealed cell with an outlet connected to a device where hy- which was used for electrodeposition of black chrome. drogen gas was collected and its volume measured during The best results concerning black chrome deposition were the run of the experiment. As seen from Table III the hy- obtained with a bath, whose composition is given in drogen evolution drops drastically in the presence of the Table V. The tests were carried out on steel samples previ- additives. ously nickel-plated by electrolysis in a Watts bath and on copper samples previously also coated with nickel. The Electrodeposition of black chromium.--Optical micro- thickness of the nickel coating was about 20 ~m in the case scope investigations were carried out in order to analyze when steel samples were used and about 10 ~n when cop- the reduction products at -0.3, -0.5, and -0.95 V vs. SCE. per samples were used. The optimum thermo-optical prop- No evidence was found that any type of deposit is formed erties were obtained within 60 s. The optimum plating pa- on the Pt surface when chromic acid was potentiostati- rameters proved to be current density of 0.35 A/cm2 and a cally electrolyzed at -0.3 and at -0.5 V. Evidence of black solution temperature of 18~ Using the above we obtained chrome deposits on the Pt surface was found only in the black chromium coatings with "a" values of 0.94 and "e" case when the electrode was polarized at potentials more values of 0.07-0.12. Both thermo-optical properties of the negative than -0.95 V and in the presence of NaF concen- black chromium coating were found to depend upon the trations higher than 0.3M. The amount of the deposited deposition time, but they were independent of the type of material increases with the increase of the concentration nickel undercoating used. of the catalyst in the electrolyte up to 0.5M, when it levels AES measurements revealed Cr and O and some F- to be off. Further increase in the fluorine ion concentration in- the main constituents of black chrome deposits, while hibits the deposition process. In the absence of fluorine some S and C were found only as impurities. After ion ions in the electrolyte, only dark grayish deposits were ob- bombardment the peaks of these elements disappeared tained, indicating that catalyst ions are essential for the from the spectra. This is in agreement with literature data process which leads to a formation of black chrome deposits. (6, 7, 11) that black chrome deposits consist of metallic chromium particles dispersed in a matrix of chromium In order to determine the optimum operating conditions oxides. under which a spectrally selective surface of black chrome In order to check the thermal stability of black chrome appears, spectral reflectance of black chrome deposited on coatings, two different types of thermal treatment have nickel-plated copper has been studied as a function of dep- been used. In the first one black chrome specimens were osition time. The following standard experimental condi- heated up to 450~ for 10 h in electric furnace in an air at- tions were used in preparing the samples for reflectance mosphere. In the second series, heating was done in a vac- measurements: agitation of the electrolyte with nitrogen, uum. From the AES spectra of thermally treated speci- temperature 295~ current density 10-12 A/dm 2, cathode mens, as well as from SEM investigations it has been to anode area ratio 1:1. A computer program was used to concluded that our black chrome deposits are stable up to calculate the normal thermal emittance and the hemis- 350~ Prolonged heating over this temperature especially pherical absorptance from specular normal and hemis- in an air atmosphere will result in oxidation of the metallic pherical refleetivities in order to determine the selective chromium particles. After 5 h heating at 500~ no metallic properties of the electroplated black chrome coatings. Fig- chromium was found in the black chrome deposit. The ure 12 shows reflection spectra of two black chrome de- heating also resulted in degradation of the selective optical posits in a "as prepared" state electrolyzed for 60 s (curve response. Figure 13A and B show the surface morphologies of black chrome deposit as defined by scanning electron mi-

.8 Table V. Composition and experimental operating conditions for .6 black chromium plating bath. L 60s

Compound Concentration (g/l)

.2 Chromic acid 420 Barium carbonate 10 Reactive 85 (NaF-citric 25 i acid-thiourea) 0.4 i 8 3 4 5 I0 2O A, (#m) Experimental operating conditions Black chromium plating bath Fig. 12. Spectral reflection spectra of two black chrome deposits pre- Temperature of the bath 20-25~ Material of the anode lead with 5% Sb pared onto nickel plated copper at 0.5 A/cm 2 for 1 min (curve 1) and Distance anode-cathode (cm) 4 for 2 min (curve 2). Ratio of the area anode/cathode 1

Fig. 13. SEM micrographs of black chrome deposits in "as plated" Fig. 14. SEM micrographs of black chrame deposits after thermal state with: A, magnification 1000 and B, magnification 10,000 times. treatment: A, magnification 100 and B, magnification 10,000 times.

croscopy (SEM). SEM micrographs of the black chrome prevent further increase of the film thickness. At more ca- deposits were obtained in an "as plated" state using: A, thodic potentials, reduction to metallic chromium occurs magnification of 1000 and B, 10,000 times. One observes a with a hydrogen evolution as a result of water reduction at dendritic growth of the deposit. This is a rather different high overvoltages. structure from the structure of black chromium coatings The electrolyte given in Table V was found to be most deposited from acetic acid baths reported in literature suitable for a black chrome plating process for so]ar ab- (6-9). Figure 14A, magnification 100, and Fig. 14B magnifi- sorber tubes. The optimum plating parameters proved to cation 10,000, represent the structure of thermally treated be a current density of 0.35 A/cm2 and a solution tempera- specimens. According to Fig. 14A and B thermal treatment ture of 18~ Using the above parameters a black chro- at lower temperatures (400~ leads to formation of micro- mium coating has been obtained with "a" value of 0.94 and cracks. Higher temperature thermal treatment results in "e" values of 0.07-0.12. Both thermo-optical properties of complete structure degradation of black chrome deposits. the black chromium coating were found to depend upon the deposition time but were independent of the type of Conclusions the nickel undercoating used. High anodic open-circuit potentials observed on Pt cathodes in pure concentrated chromic acid and in chromic Acknowledgments acid in which NaF, and organic compounds are added in- We thank the United Nations and MZT Skopje Yugosla- dicate that platinum passivates spontaneously. The pas- via for financial support of this research. The authors sive film formed has an important role in the reduction would like to express their appreciation to Dr. Carl M. mechanism because it shifts the deposition of metallic Lampert from the University of Berkeley and to Dr. Derek chromium and chromium oxide to very high overvoltages. Lovejoy from the United Nations for their helpful discus- A low hydrogen overvoltage (-0.25 V vs. SCE) is present sions. at the working electrode. The measured current for the en- Manuscript submitted March 18, 1991; revised manu- tire cathodic region corresponds to hydrogen reduction script received Aug. 12, 1991. and reduction of Cr electroactive species. In the presence of NaF, thiourea, and citric acid, hydrogen evolution drops Texas A&M University assisted in meeting the publica- drastically. Besides the reduction peak which occurs at tion costs of this article. -0.95 V in the absence of the additives, two new peaks at -0.3 V and at -0.5 V vs. SCE appear in their presence. The REFERENCES magnitude of the peak at -0.3 V vs. SCE does not depend 1. C. A. Knorr, G. Munster, and H. Feigl, Z. Elektrochem., upon chromic acid concentration, and neither peak de- 60, 1089 (1956). pends upon the concentration of the additives, indicating 2. R. L. Sass and S. L. Eisler, Plating, 41, 497 (1955). that the reduction is not controlled by the ions from the 3. E. L. King and J. A. Neptune, J. Am. Chem. Soc., 77, electrolyte. It has been assumed that at -0.3 V oxides 3186 (1955). which have been formed at open-circuit potential are re- 4. D. N. Usachev, Zhur. Fiz. Khim., 36, 711 (1962). duced. The peak which occurs at -0.5 V vs. SCE can be at- 5. Z. A. Sololeva and A. T. Vagramyan, ibid., 36, 752 (1962). tributed to reduction of Cr(VI) to Cr(IV). The Cr(IV) spe- 6. D. Rimdzhyute, M. Mitskus, and Yu Matulis, Lietuvos cies formed at this potential are further reduced to Cr(III) T. S. R. Mokslu Akad. Darbai, Ser. B, 4, 91 (1959); at -0.95 V vs. SCE. Chem. Abstr., 54, 1922d (1960). An increase of the peak current and the shift of the peak 7. J. P. Hoare, This Journal, 130, 1475 (1983). potential with an increase of the sweep rate is observed for 8. J. P. Hoare, A. H. Holden, and M. A. LaBoda, Plating the peak at -0.95 V indicating that Cr(III) species formed Surf. Fin., 67, 42 (March 1980). at the interface block the surface for further reduction and 9. J. P. Hoare, This Journal, 130, 1475 (1983).

Downloaded 16 Jun 2011 to 129.252.106.20. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp 98 J. Electrochem. Soc., Vol. 139, No. 1, January 1992 The Electrochemical Society, Inc.

10. J. P. Hoare, in "Electrodeposition Technology, Theory 23. J. P. Hoar, J. Phys, Chem., 79, 2175 (1975). and Practice," L.T. Romankiw and D.R. Turner, 24. P. M. Driver, R. W. Johnes, C. L. Riddeford, and R. J. Editors, PV 87-17, p. 233, The Electrochemical Soci- Simpson, So~ar Energy, 19, 30i (1977). ety Softbound Proceedings Series, Pennington, NJ 25. C. Wyon and J. Valignat, Solar Energy Mat., 11, 435 (1987). (1985). 11. G. Dubpernell, Plating, 47, 35 (1960). 26. A. F. Diaz and D. Schermer, This Journal, 132, 2571 12. J. C. Saiddington and G. R. Hoey, This Journal, 117, (1985). 1001 (1970). 27. D.-T. Chin and H. Zhang, Electrochim. Acta, 31, 299 13. J. A. Sotoleva and A. T. Vagramyan, Zh. Fiz. Khim., 36, (1986). 711 (1962). 28. G. B. Smith, A. Ignatiev, and G. Zajac, J. Appt. Phys., 14. J. C. Saiddington and G. R. Hoey, This Journal, 12@, 51, 8 (1980). 1475 (1973). 29. B. Popov and J. V. Ivsin, Kern. Ind., 36, 103 (1987). 15. H. Tabor, in "Transitions of Conference on the Use of 30. A. V. Pamfilov and A. I. Lapushauskaya, Ukr. Khim. J., Solar Energy, 1955," University of Arizona Press, 26, 46 (1960); Chem. Abstr., 55, 8916 (1961). Tucson, AZ (1958). 31. A. T. Vagramyan and D. N. Usachev, Zh. Priklad. 16. C. M. Lampert and J. Washburn, Solar Energy Mat., Khim. Leningrad, 32, 1900 (1958). 1-1, 82 (1979). 32. A. T. Vagramyan and D. N. Usachev, Chem. Abstr., 52, 17. G. B. Smith and A. Ignatiev, ibid., 2, 461 (1980). 7905h (1958). 18. C.M. Lampert, Thin Solid Films, 7, 75 (1980). 33. D. Rimdzhyute, M. Miskus, and Yu. Matulis, Liet TSR 19. D. Bacon and A. Ignatiev, Solar Energy Mat., 9, 3 Akad., Darb., Set. B, 4, 91 (1959); Chem. Abstr., 54, (1983). 1922d (1960). 20. G. B. Smith, G. Zajac, and A. Ignatiev, ibid., 29, 279 34. A. Ignatiev, P. O'Neill, and G. Zajac, Solar Energy (1982). Mat., 1, 69 (1979). 21. J. Spitz, T. van Danh, and A. Aubert, ibid., 1, 189 (1979). 35. S. Srinivasan and E. Gileadi, Electrochim. Acta, 29, 21 22. P.M. Driver, ibid., 4, 179 (1981). (1984).

Hydride Effect on the Kinetics of the Hydrogen Evolution Reaction on Nickel Cathodes in Alkaline Media

D. M. Soares, O. Teschke,* and I. Torriani Instituto de Fisica, UNICAMP, 13081 Campinas, SP, Brazil

ABSTRACT The time-dependent hydrogen evolution reaction (HER) on Ni electrodes shows a large increase in electrode overpotential with time. This is ascribed to hydride formation at active Ni cathode surfaces. Hydride formation was detected by x-ray diffraction, morphological changes at the electrode surfaces, and resulting changes in secondary electron emis- sivities. Nickel electrodes annealed for 2 h in an argon atmosphere at 1000~ after HER did not show x-ray lines assigned to hydride. On the other hand, hard nickel electrodes (180 HV5/30) show high overvoltages as well as hydride x-ray diffraction lines after HER. By taking the variation of the nickel electronic density of state following hydrogen sorption into ac- count, we are able to satisfactorily explain the increase in nickel overpotential after a few hours of HER.

Nickel cathodes show marked overvoltage increase with hydride is unstable, a few physical properties have been time in alkaline electrolytes (i). The challenge is to develop reported. X-ray diffraction studies showed that the face- methods of electrode activation which give stable per- centered cubic structure was retained with a lattice spac- formance for prolonged operating periods; therefore we ing about 5.5% larger than that of pure nickel (16). Nickel need a detailed understanding of the factors Contributing containing hydrogen can exist in one of two different hy- to electrode overvoltage. In particular, time variation ef- dride phases. The a-hydride is simply a solid solution of fects occurring on the metal must be recognized and un- hydrogen atoms in the octahedral interstices of nickel up derstood. Four possible mechanisms have been postulated to a H]Ni atomic ratio of 0.03 (ii, 17). Above a H/Ni atomic by previous investigators as being responsible for the in- ratio of about 0.6, only the G-hydride phases coexist. Fur- crease in cathodic overpotential, ~c, in time. (i) Increases in ther, Szklarska-Smialowska (i I) and Smialowski (18) have ~Ic are caused by a loss of electrocatalytically active mate- shown that the ~-nickel hydride phase acts as a diffusion rial from the cathode (2, 3). (ii) Increases in ~c are caused barrier to continued hydrogen diffusion. This supports the by a slow reduction of nickel hydroxide to nickel, which assertion of a complicated nickel-hydrogen interaction. may be a poorer evolver of hydrogen. (iii) Increases in ~c The hydrogen evolution reaction in alkaline media is as- are caused by deposition of impurities from the electrolyte sumed to proceed via well-known steps (19). Hydrogen into the cathode (4). (iv) Increases in ~c are caused by the sorption with charge transfer absorption of atomic hydrogen into the lattice of the nickel cathode (5). Time effects are recoverable, and an investiga- H20 + e- M m HM + OH- [i] tion of the recovery mechanism has yielded evidence that eounterindicates mechanisms (i), (ii), and (iii), and sup- Hydrogen desorption with charge transfer ports the hydrogen absorption mechanism (6, 7). HM + H20 + e- M ~ H2M + OH- [2] It has been conclusively shown by volumetric measurement of desorbed hydrogen that nickel does absorb atomic Hydrogen desorption by recombination without charge hydrogen during electrolytic evolution of hydrogen gas transfer (8-15). Nickel saturated with hydrogen from a cathodic charging has been studied previously by x-ray diffraction HM + HM ~ H2M + M [3] techniques (12). The studies show that cathodic evolution of hydrogen on nickel can produce effects similar to those The palladium electrode, when cathodically polarized at a caused by local lattice strains. Even though the resulting constant current density, i, exhibits after current interrup- tion a characteristic overpotential ~ transient which de- * Electrochemical Society Active Member. cays with time t. The overall ~I vs. t transient curve is usu-

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