behavio~r of anodic oxide film on antimony in alcoholic aqueous solutions

Transcription

1 Indian Journal of Chemical Technology VoL, July 1995, pp lectrochemical behavio~r of anodic oxide film on antimony in alcoholic aqueous solutions Department AS Mogoda, W A Badawy & M M Ibrahim of Chemistry, Faculty of Science, Cairo University, Giza, gypt Received 5 April 1994;accepted 3 January 1995 Dissolution behaviour of the anodic oxide film on antimony in alcoholic aqueous solutions has been studied using impedance technique. The dissolution rate of the oxide film found to decrease with increase of alcohol concentration in the dissolution medium. Dissolution of the oxide film in some practically important redox systems has also been examined. It is found that the dissolution rate increases with increase in the redox potential of the redox system. The dissolution process of the duplex-nature antimony oxide follows a zero order mechanism in the test solutions. Antimony oxides were found to exhibit some promising properties as thin film capacitors 1,. Presently, antimony oxide is considered an attractive photoconductor3 because of its low cost, although its durability is low due to corrosion. Several studies have already been made on the stability of the anodic oxide films on antimony in aqueous solutions4-7 The corrosion rate of aluminium in HCl was found to decrease with increase of the alcohol concentration in the test solution8 This was attributed to the structural properties of the water-alcohol mixtures. Acetonitrile was shown to behave as an interface inhibitor in the anodic dissolution of iron in acidic solutions9 The purpose of the present work is to study the stability of the anodic oxide film on antimony in HS04 containing some aicohols like methanol and ethanol. Also the effects of WO~- ions and some practically important redox systems example I /1 -,Fe(CN)~- /4 and Ce4+/3+ on the dissolution behaviour of the oxide film was taken into consideration. The study was conducted by impedance measurement. xperimental Procedure The electrode was made from 99.99% antimony rod (British Drug Houses Ltd., ngland). The electrode was fitted into a glass tube with Araldite resin leaving surface area of 0.46 C1llin contact with the test solution. lectrolytic solutions were prepared from analytical grade reagents and triply distilled water. Solutions were used as naturally aerated at 5 ±0.1 C. The electrode surtace was polished with finer. grades of emergy papers down to 4/0 until a mirror bright surface was attained, then rubbed with a soft cloth, rinsed with distilled water and immersed in the anodizing solution. The anodic oxide film was formed at a constant current density of 3.54 x 10-3 A cm- in 0.01 N HS04 until 40 V. The current was then interrupted and the electrode was removed from the polarization cell, washed with distilled water, dried by the tip ofa filter paper and immersed in the dissolution medium. The electrode impedance, viz., capacitance Cm and resistance Rm was measured using a high precision standardized bridge of the Wien type. The bridge and electrolytic cell were essentially the same as those described elsewherelo,ll. The input ac voltage to the bridge was always 10 mv and the frequency used was 1000 Hz. At this frequency the measured capacitance is inversely proportional to the oxide thickness 1. Results and Discussion ffect of alcohols on the dissolution of the anodic oxide filmon antimony ffect of methano/- Dissolution of the anodic oxide film on antimony prepared as described in the experiment, was examined in 0.1 N HS04 containing different concentrations of methanol. The capacitance Cmand resistance Rm were followed with time till steady state values were attained, i.e., complete dissolution of the oxide film had occurred. Investigation of the oxide surface at different imrtlersion times revealed that the surface always appears homogeneous. The dissolution was uniform and the interference colours changed gradually during the dissolution in a manner the reverse of thatof the oxide formation.

2 18 INDIAN 1. CHM. TCHNOL., JULY u 610 LL I N..... u ::L ft 4 Q: 100 c: ~ - a The reciprocal capacitance 11em, which is proportional to the oxide thickness J3, decreases with time owing to dissolution of the oxide film as shown in Fig, la, Also the resistance Rm decreases giving further evidence for the oxide dissolution (Fig. 1b). The validity of the zero order mechanism for dissolution of the oxide at different ratios of methanol is clear from Fig. la and can be describedoby the equation d(1icm)= K - dt... (1) Fig. I-Variation of (a) liem and (b) Rm with time during the dissolution of the oxide film in 0.1 N HS04 containing different ratios of methanol [(D) 10 wt %, (&) 0 wt %, (0) 30 wt % and (.) 40 wt %J Table I-Dissolution rate constants of the oxide film in 0,1 N HS04 containing different ratios of alcohols Dissolution medium Dissolution rates wt% (cm,uf-1 min-i) ,054 0, ,61 0,19 0,083 KK 0,01 0,014 0, Dl8 0, ,080 0,85 0, KI where Cm is the measured capacit~nce at time t and K is a constant. Previously, it was found that the dissolution of the anodic oxide films on antimony5 and zirconium14 in HN03 is a zero order reaction. Figs 1a and 1b show breaks in the dissolution curves which can be attributed to the duple nature of the oxide film. The rate of dissolution of the outer layer, K l' is higher than that of the inner one, K (Table 1). Beside the possible difference in the chemical composition between the two layers15 they may have some physical difference, i.e., in the type and concentration of defects within each layer. The outer layer seems to contain more defects in the structurel6,17 than the inner one, since the former dissolution. is more susceptible to ffect of ethanol"- The dissolution behaviour of the anodic oxide film on antimony was examined in 0.1 N HS04 containing different ratios by weight of ethanol. The decrease in 11Cm with time (Fig. ) follows the zero order q. (1) as in case of methanol. Also, Rm was observed to decrease with time in accordance with dissolution of the oxide. The major part of the anodic oxide film on antimony is composed of Sb0318 In acid solution, the oxide film dissolves as follows 19 Sb03 + H+ = SbO+ + H0... () The average rate constant for dissolution of the entire oxide film K, was calculated on the basis of the length of each segment of the dissolution curve in Figs la and. From Table 1, it is clear that the values of the dissolution rate constants decrease with increase of concentration of methanol or ethanol. This may be attributed to the increase in the viscosity of the dissolution medium with increase of alcohol concentration, which leads to a decrease in the coefficient for diffusion of the dissolution products of the oxide film from the oxide surface to the bulk solution. ii' I; "~lilll 'I I II

3 MOGODA et at.: ANODIC OXID FILM ON ANTIMONY IN ALCOHOUC AQUOUS SOLUTIONS 19 1 T II.. N <. u ~ - - J U II.. u N <. f 46 Fig. - Variation of 1/ Cm with tivj.e during the dissolution of the oxide film in 0.1 N HzS04 containing different ratios of ethanol [(D) 10 wt %, (.) 0 wt %, (0) 30 wt % and (.) 40 wt%] 0.40 Fig. 4- Variation of 11Cm with time during the dissolution of the oxide film in different concentrations of NaZW04 [(D) 0.01 N, (0) 0.10 N and (e) 0.50 N]., 0.30., lj <. N "' IX: NNazW04 NNazW04 (pure) IOwt%methanol % o Table -Dissolution rate constants of the oxide film in 0.1 N NazWO 4 containing different ratios of methanol Dissolution medium Dissolution rates cmz,uf- ) min - I Kz K K) Concention of alcohols,wt,. Fig. 3-Dependence of the aveffige rate constant of dissolution, K, on alcohols concentration in 0.1 N HzS04 [(e) methanol and fo) ethanol] It has been reportedo that the corrosion of alumininm in NaOHsolutions is inhibited by alcohols as a result of the increase in the viscosity of the medium which leads to a decrease in the coefficient for diffusion of the corrosion products of aluminium. This type of behaviour was observed for dissolution of the anodic oxide film on bismuth in oxalicacid1 The results showed that the dissolution rate decreases with increasing the acid concentration as a result of the slower diffusion of the dissolution products from the oxide/ electrolyte interface. Fig. 3 shows that the average rate constant of dissolution, K, decreases as the alcohol concentration increases. This can be also noticed from Table 1, where the values of the dissolution rate constants in ethanol are lower than those in methanol. This may be related to the increase in the viscosity of the medium as the chain length of the alcohol molecule increases. Previously, it was found that the dissolution rate of zinc in trichloroacetic acid is less in presence of ethanol than in methanol containing solutions ffect of methanol on the dissolution of the oxide film in sodium tungstate The dissolution rate of the anodic oxide film was found to decrease with increasing tungstate ions concentration as shown in Fig. 4. The decrease in the dissolution rate may be attributed to the adsorption of WO~- ions on the oxide/electrolyte interface since the adsorption process (primary physical interaction) is fast3 4 enough to occur before the dissolution process (essentially chemical interaction). It was found that the variation of 11em with time during the dissolution of the oxide film in tungstate solution containing different concentrations of methanol follows q. (1). The values of the dissolution rate constants in

4 0 INDIAN J. CHM. TCHNOL., JULY u o o a b 10 find that the dissolution rate constants are running parallel to the values of the redox potentials and the dissolution rate increases as the redox potential increases. The dissolution behaviour of the oxide film is independent of the redox system itself, i.e., the ionic species of the system, but on its redox potential. The presence of methanol in any of the investigated redox systems results in a decrease in the dissolution rate (d. Table 3 and Fig. 5b). As already pointed out, the addition of alcohol to the dissolution medium will inhibit the dissolution of the oxide film by increasing the viscosity of the medium which causes a decrease in the coefficient for diffusion of the dissolution products of the oxide film. Fig. 5 - Variation of 11em with time during the dissolution of the oxidefllm in different redox systems (a) containing no alcohol and (b) containing 10 wt % methanol He) 0.01 MI/O.1 M KI, (0) 0.01 M Ce(S04)/0.01 M Ce(S04h/0.5 M HS04, and (0) 0;05 M K3[Fe(CN)6VO.05 M K4[Fe(CN)6Y 0.5 MKN03] Table 3-Dissolution KI KK O.OlO K 0.16 in absence Dissolution rate ofin Dissolution methanol constants rates presence (lowt%) of rates ofmethanol the oxide film in different redox cm Ce4+/Ce3+ /J.F-I min-i systems case of pure tungstate solution are higher than those in case of the same solution containing different ratios of methanol (Table ). This indicates that the presence of methanol in the tungstate solution makes the oxide filmmore stable. Dissolution behaviour of the anodic oxide in different redox systems In this part, the dissolution behaviour of the anodic oxide film on antimony in some redox systems of practical interest, e.g., 1/1-, Fe(CN)~-/4 and Ce4+/3+ was studied. The study also includes the effect of alcohol on such behaviour. In absence of alcohol, the dissolution rate of the oxide film decreases in the order Ce4+/3+> Fe(CN)g-/4- > 1/1- (cf. Table 3 and Fig. 5a). Considering the redox potentials of the in~stigated redox couples V for 1/1-, V for Fe(CN)~-/4- and V for Ce4+/3+, one may Conclusions I The impedance results indicate that the dissolution rate of the anodic oxide film decreases as the alcohol concentration increases in the dissolution medium. This can be attributed to the slower diffusion of the dissolution products of the oxide film from the oxide/electrolyte interface. Dissolution rate of the oxide film decreases with increase of WO~- ions concentration as a result of adsorption of such ions on the oxide surface. 3 Increase of the redox potential of the redox couple causes the oxide dissolution to increase. References 1 Patel S M & George V, IndionJ Phys, 5 (1978) 7. Kumar J S, Naragana G, Chandra M, Subba Rao U V & Babu V H, Phys Status Solidi A, 78 (1983) Metikos-Hukovic M & Lovrecek B, lectrochim Acta, 5 (1980) I-Basiouny M S, Hefny M M & Magoda A S, Ann Chim, 74(1984)79. 5 I-Basiouny M S, Hefny M M & Mogoda A S, Corrosion, 41 (1985) Hefny M M, Badawy W A, Mogoda A S & I-Basiouny M S, lecrrochimactll, 30 (1985) Badawy W A, Mogoda A S & Ibrahim M M, lectrochim Acta, 33 (1988) Abd-I-Nabey B A, Khalil N & Khamis, Corros Sc~ 5 (1985) 5. 9 Ahlberg & Friel M, lectrochimactll, 34 (1989) 187. lo I-Basiouny M S, I-Kot A M & Hefny M M, Corrosion, 36 (1980) I-Basiouny M S & Bekheet A M, Br Corros 1, 15 (1980) Srinivasan R & Bose C S C, J Appl lectrochem, 1 (198) Diggle J W, Downie T C & Goulding C W, lectrochim Acta, 15 (1970) L4 I-Taib HeakaI F, Mogoda AS, Ameer M A & Ghoneim A A, Hung J Ind Ohem, (1994) 9. 11'11 I "~'I I "III "'!'I'I" "1'1 "I"f' 111'" 'I " "II "I "11"'1'" I~ "I' II "I'll II I 'I!',

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