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Corrosion of Sn–Co alloy in alkaline media and the effect of Cl− and Br− ions
Applied Surface Science 147 Ž1999. 67–76
Corrosion of Sn–Co alloy in alkaline media and the effect of Cly and Bry ions S.A.M. Refaey
)
Chemistry Department, Faculty of Science, Minia UniÕersity, 61519 Minia, Egypt Received 17 August 1998; accepted 20 December 1998
Abstract Sn–Co electrodeposits alloy of approximate composition 80% Sn–20% Co Žwt%. can be obtained from a gluconate bath as single phase CoSn 2 , which is similar in appearance to decorative chromium. The potentiodynamic and cyclic voltammogram techniques were used to study the corrosion behaviour of CoSn 2 in sodium borate solutions ŽNa 2 B 4O 7 . at pH s 9.6. The effect of different factors such as concentration of borate ions, pH, potential scan rate, successive cyclic voltammetry, and progressive addition of halide ions ŽCly and Bry. on the electrochemical behaviour of CoSn 2 alloys are discussed. The observed corrosion resistance of electrodeposited CoSn 2 alloy is due to the formation of a thin passive film, which is examined by X-ray spectroscopy and believed to be mainly tin and cobalt oxides. The voltammograms involve four anodic peaks, the first and second of which correspond to the formation of SnO and SnO 2 and the third and fourth related to the formation of cobalt oxides. SEM examination confirms that pitting corrosion takes place in presence of borax and is increased by adding halide ions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Tin–cobalt alloy; Borax; Passivation; Pitting corrosion
1. Introduction A plated CoSn 2 alloy that looks like chromium but has none of the environmental or safety hazards associated with chromium is attracting interest from metal finishers. Decorative chromium plating adds an attractive, durable finish to manufactured goods. However, chromic acid is a hazardous material that requires great care in its handling, use, containment, and disposal. Chromium plating also has serious
)
Fax: q20-086-332601; E-mail: [email protected]
limitations in that concentrated, toxic materials are used and the corrosive electrolyte has poor throwing power w1x. Electrodeposition of the CoSn 2 alloy yields a film whose hardness and wear resistance is sufficient for most indoor decorative applications w1x, and the electrolyte is non-toxic, non-corrosive, has excellent throwing power, and the process is highly energy efficient. The CoSn 2 alloy has found extensive applications as a convenient and economic way of providing an attractive finish for lock and door hardware, plumbing fixtures, appliance components, office equipment, tools, computer electrical components, tubular furniture, store fixtures, jewellery, hinges, kitchen utensils, tubular furniture and automobile interior trim and fittings w2x.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 8 0 - X
68
S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
Several authors w2–10x described the deposition of CoSn 2 alloy from different baths. Davidson w1x reported that the CoSn 2 alloy has a chromium appearance within the range 17–23% Co, where the deposit is mat grey. At greater than 23% Co the appearance darkens. In our previous work w10x electroplated bright tin–cobalt alloy was obtained from a bath containing stannous and cobalt sulphate, sodium gluconate as a chelating agent, and sodium sulphate as supporting electrolyte. X-ray diffraction studies of the deposit w10x revealed that the phase structure Ž88% tin alloy. consists mainly of a tetragonal SnCo 2 phase with a minor quantity of b-Sn. The alloy deposits were generally adherent, bright, and similar to chromium in appearance. The surface morphology of the asplated SnCo 2 alloy was examined w10x by SEM, which showed that the deposit is compact. The deposit consists of regularly oriented fine grains. Nemoshkalenko et. al. w11x determined the mechanism of interatomic interactions of tin cobalt compounds, and found that the Co–Sn bonds can be regarded as donor–acceptor bonds due to the transfer of electron density from the Sn to the Co atoms, where the binding energy of the electrons become stabilised toward SnCo 2 . These interactions are accompanied by a dative Co ™ Sn interaction mediated by the valence p-electrons of the Co metal. The present study was undertaken with the following objectives in mind: Ž1. To determine the
Fig. 1. Current Ž at scan rate 50 mV sy1 .
effect of the presence of a small amount Ž20%. of Co in the composition of the CoSn 2 alloy on the passivation and pitting corrosion. Ž2. To investigation the corrosion behaviour of the tin–cobalt alloy in an alkaline medium, both in the absence and presence of halide ions. Ž3. To examine the chemical nature and stability of the passive film formed on the CoSn 2 alloy in borate solution under different conditions using potentiodynamic and cyclic voltammetry techniques complemented by X-ray, EDX, and SEM studies.
2. Experimental CoSn 2 alloy was used as a working electrode. The alloy was deposited on a plan copper cathode with circular area 1 cm2 from a gluconate bath under conditions similar to those described elsewhere w10,12x. Experiments were carried out in solutions containing 5 g dmy3 SnSO4 , 8 g dmy3 CoSO4 P 7H 2 O, 32 g dmy3 C 6 H 11O 7 Na and 20 g dmy3 KSO4 , c.d. 30 mA cmy2 , pH 5.5 and 408C. The plating duration was 30 min, after which the cathode was withdrawn, washed with distilled water and dried. All solutions used were freshly prepared with doubly distilled water and analytical grade chemicals. The required pH was adjusted by dropwise addition of standard boric acid or sodium hydroxide solutions. A pH meter was used to measure the pH
. and charge Ž- - -. variations versus potential of CoSn 2 alloy in 0.5 mol dmy3 borax solution during one cycle,
S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
values. The measurements were performed in a three-electrode cell provided with an electrodeposited CoSn 2 alloy as a working electrode, a Pt wire as a counter electrode and a AgrAgCl as a reference electrode. Experiments were carried out in various concentrations of borate solutions in absence and presence of NaCl or NaBr. Before each run the
69
CoSn 2 alloy electrode was washed with distilled water and rinsed in acetone. All solutions were used under purified nitrogen. All experiments were carried out at room temperature Ž258C.. The potentiodynamic and cyclic voltammograms curves were run by a potentiostat ŽAMEL model 2049., which was controlled by a PC. X-ray diffraction analysis was
Fig. 2. Scanning electron micrograph of the surface CoSn 2 anode, Ža. before and Žb. after scanning, at 50 mV sy1 from y1000 to 1100 mV in 0.5 mol dmy3 borax.
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S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
carried out using an X-ray diffractometer ŽSiemens D500r501.. The composition of the deposits before and after examination was determined by an EDX-ray spectrometer ŽCamScan Cambridge Scanning.. The morphology of the deposits was examined by scanning electron microscopy ŽCamScan Cambridge Scanning..
3. Results and discussion The cyclic voltammogram is given in Fig. 1, curve a for a stationary electrodeposited CoSn 2 alloy in 0.5 mol dmy3 Na 2 B 4 O 7 solutions ŽpH s 9.6. where the potential was swept from y1000 mV to 1100 mV Žall potentials are given vs. AgrAgCl electrode. at a scan rate n s 50 mV sy1 . The anodic curve exhibits four dissolution peaks ŽPeaks A I ™ A IV .. The anodic peak A I is similar to that observed previously for Sn in alkaline medium. Shah and Davies w13x and Refaey w14x suggested that the Sn metal in Na 2 B 4O 7 solution produces stannous hydroxide wSnŽOH. 2 x which dehydrated to stannous oxide ŽSnO. according to:
Fig. 4. Anodic potentiodynamic curves for CoSn 2 anode in 0.5 mol dmy3 sodium borate solutions at various pH values: Ž1. 1.5, Ž2. 6.8, Ž3. 8, Ž4. 9.6, at scan rate 50 mV sy1 .
x can On the other hand the stannite ion wHSnOy 2 form directly by dissolution of Sn w14x Sn q 2OHy™ HSnOy 2 q H 2 O q 2e. The potential of shoulder peak A II may be correlated to the formation of SnŽOH.4 according to the following equations: Sn Ž OH . 2 q 2OHys Sn Ž OH . 4 q 2e
Sn q 2OHy™ Sn Ž OH . 2 q 2e
and
with a further reaction involving dehydration of stannous oxide:
SnO q 2OHyq H 2 O s Sn Ž OH . 4 q 2e
Sn Ž OH . 2 ™ SnO q H 2 O.
which dehydrates to SnO 2 Sn Ž OH . 4 ™ SnO 2 q 2H 2 O. Both SnO and SnO 2 were dissolved in alkaline media to yield stannite and stannate, respectively
Fig. 3. Anodic potentiodynamic polarisation of CoSn 2 anode in solutions of various concentrations of borax at scan rate 50 mV sy1 Concentrations: Ž1. 0.05, Ž2. 0.1, Ž3. 0.5, Ž4. 0.8 and Ž5. 1.0 mol dmy3 .
Fig. 5. Successive cyclic voltammograms for CoSn 2 anode in 0.5 mol dmy3 sodium borate solutions at scan rate 50 mV sy1 , the numbers in figure indicate the cycle number.
S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
Fig. 6. Anodic and cathodic charge changes as a function of potential scan number for CoSn 2 anode in 0.5 mol dmy3 borate solutions, at scan rate 50 mV sy1 .
w13,15–17x. So the stannic oxide may be present as stannate ion: Sn q 5OHy™ HSnOy 3 q 2H 2 O q 4e. Fig. 1, curve Ža. shows a current peak A III , which may be corresponding to the dissolution of cobalt present in the alloy to CoO and CoOOH. This result is in agreement with the previous Mossbauer studies ¨ w18x, which show that the cobalt was shown to be essentially free of a corrosion film during cathodic polarisation at y1000 mV, but at y100 mV an anodic film is formed which has two oxidation states Co 2q and Co 3q. In the passive region of the polarisation curve Žfrom q100 mV to q500 mV., the passive film contained the primarily Co 3q oxidation state. The Pourbaix diagram of cobalt w19x indicates
Fig. 7. Anodic potentiodynamic curves of CoSn 2 anode in 0.5 mol dmy3 sodium borate solutions ŽpH s9.6. at various scan rates: Ž1. 10, Ž2. 30, Ž3. 60, Ž4. 200, Ž5. 300 mV sy1 .
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Fig. 8. The dependence of anodic current peaks A I , A III and A IV on square root of scan rate for CoSn 2 alloy in 0.5 mol dmy3 sodium borate solutions.
that the equilibrium potential between CoO 2 and CoOOH is 715 mV ŽSCE. at pH 8.5. Formation of Co 4q in the present study was found at potentials as low as 635 mV Žpeak A IV .. These results suggest that the decomposition of CoO 2 occur according to the following reaction w20x: 3CoO 2 ™ Co 3 O4 q O 2 . This result is explained later in the pH effect section. The anodic film formed at transpassive potentials q850 mV was found to consist of Co 3q and Co 4q ions w18x. At potential higher than q900 mV, the passive film consists of both CoOOH and CoO 2 . Apparently the CoOOH in contact with alloy is not completely oxidised to tetravalent CoO 2 at anodic
Fig. 9. Anodic potentiodynamic polarisation of CoSn 2 anode in 0.5 mol dmy3 borax solution at scan rate 50 mV sy1 in the presence of various concentration of NaBr. wNaBrx: Ž1. 0, Ž2. 0.4. Ž3. 0.6, Ž4. 0.8 mol dmy3 .
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potentials up to q900 mV. The composition of the passive films formed on the surface of the electrodeposited CoSn 2 alloy after complete potentiodynamic polarisation Žfrom y1000 to 1100 mV. treatment with a scan rate of 50 mV sy1 in 0.5 mol dmy3 borax ŽpH s 9.6. was examined by X-ray diffraction.
The X-ray diffraction data reveal the composite nature of the passive films and confirmed the presence of SnO, SnO 2 , CoO, CoO 2 . CoOOH, Co 2 O 3 and Co 3 O4 in the investigated samples. When the polarisation reaches a certain critical potential, the pitting potential, Epit , the passive current suddenly rises
Fig. 10. Scanning electron micrograph of the surface CoSn 2 anode: Ža,b. in 0.5 mol dmy3 borax solution containing 0.8 mol dmy3 NaCl and Žc,d. in 0.5 mol dmy3 borax solution containing 0.8 mol dmy3 NaBr after scanning at 50 mV sy1 from y1000 to 1100 mV.
S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
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Fig. 10 Žcontinued..
steeply without any sign for oxygen evaluation denoting the breakdown of the passive layer and initiation of visible pitting corrosion. The cathodic part of the cyclic voltammogram ŽFig. 1, curve a. shows two weak cathodic peaks ŽC I and C II .. C I may be correlated to the electroreduction of cobalt oxides to cobalt metal, and C II represents the electroreduction of tin oxides formed to tin
metal. Fig. 1 curve b shows the correlation between the potential and the charge consumed during the cyclic voltammogram. The data indicate that the total charge consumed during the first half of the cycle increases with increasing potential in the positive direction. The charge quantity starts to increase Žat y780 mV. when the passive film starts to form in the potential region of active dissolution. This may
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S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
be due to the breakdown of the alloy to its simple metals and then these metals ŽSn and Co. oxidised to their oxides SnO, SnO 2 , CoO, CoO 2 , CoOOH, Co 2 O 3 and Co 3 O4 as discussed above. Fig. 1 curve b indicates that the charge consumed during the cathodic part Žreduction of oxides, i.e., formation of metals. is very small compared to the charge consumed in the anodic part Želectrooxidation of metals, i.e., formation of metal oxides.. This result may be explained by the incomplete reduction of the oxides to the corresponding parent metal. This may be due to a sluggish discharge process caused by the increase in the resistance of the oxide film against the completion of the discharge Žreduction. process. This means that some of the passive film remains unreduced. SEM examination of electrodeposited CoSn 2 surface potentiodynamics polarised from y1000 mV to 1100 mV at a scan rate 50 mV sy1 in 0.5 mol dmy3 borax solution was performed. The results, shown in Fig. 2, indicate that the surface of the electrodeposited CoSn 2 alloy was destroyed. This may be clearer when comparing SEM photographs of the alloy before ŽFig. 2a. and after ŽFig. 2b. the alloy surface was polarised from y1000 to 1100 mV at scan rate 50 mV sy1 in 0.5 mol dmy3 borax solution. The data also show that in the presence of borate ions the surface was covered with some pits and the passive layer was destroyed. Fig. 3 shows potentiodynamic polarisation curves of CoSn 2 alloy in various concentrations of sodium borate solutions in potential ranging from y1000 to 1100 mV at scan rate 50 mV sy1 . The data reveal that increasing the borax content causes an increase in the current density values of peaks ŽA I ™ A III ., at the same time these peaks shift towards more negative potential values. It is probable that the formation of CoŽIV. species is inhibited by increasing the concentration of borax as manifested by the disappearance of peak A IV at higher concentration of borax. In contrast, the formation of SnŽIV. species is inhibited by decreasing concentration of borax as manifested by the disappearance of peak A IV at lower concentration of borax. The effect of pH on the anodic polarisation features for electrodeposited CoSn 2 alloy in 0.5 mol dmy3 borax at 50 mV sy1 is illustrated in Fig. 4. It is noted the anodic current peak A I at low pH Žcurve
1, pH s 1.5. is very high, this result is in agreement with the previous literature w21x, which indicates that the corrosion rate of tin is very high at pH - 3. The peak A I appeared around 0.0 V characterises the dissolution of Sn; this is also in agreement with the previous studies on Sn in citric acid w22x. At the same time, the anodic peak A II related to the formation of CoSn 2 disappears at pH 1.5 Žcurve 1.. It is probable that the formation of SnŽIV. species is inhibited by lowering the pH w23x. The height of the anodic current peaks A I and A II are decreased with increasing pH value to 6.8. ŽCurve 2., where the corrosion rate of Sn in near neutral medium is very low related to the acidic medium w23x. In curve 1, there is an anodic current peak A III , which is related to the formation of Co 2q, and an anodic peak A IV , which corresponds to the formation of Co 3q and Co 4q ions. The heights of the anodic current peaks A III and A IV correspond to the formation of cobalt oxides, and decreased with increasing pH values. This result is in agreement with previous work w24x. At high pH, i.e., pH ) 7.5 Žcurves 3 and 4., the anodic current peaks A I and A II , which correspond to the dissolution of Sn to tin oxides, again increase in height. This result can be explained by increasing the corrosion rate at pH ) 7.5 w21x. This effect is presumably due to the dissolving power of the OHy ion, with both SnO and SnO 2 formed on the surface dissolving in alkaline media to form stannite and stannate respectively w14x. Fig. 5 shows cyclic voltammograms of CoSn 2 alloy in 0.5 mol dmy3 borax at a scan rate 50 mV sy1 , repeated in the potential range from y1000 to 1100 mV for 10 cycles. The data show that the current passing through the whole range decreases with increasing number of cycles. Fig. 6 shows the change of charge amount consumed in the anodic and cathodic part of cyclic voltammograms. It is found that the charge consumed during anodic dissolution Ž Qa . of the alloy increases as the number of cycles increase. At the same time, the amount of charge consumed Ž Qc . in the cathodic part Ždischarge of the metal ions. is nearly constant with increasing cycle number. This may be due to a sluggish discharge process caused by an increase in the resistance of the oxide film against the completion of the discharge Žreduction. process. This means that passivation increases with increasing cycle num-
S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
ber. Consequently the passive film thickness is increased. This result is elucidated by the EDX results, which showed that the composition of the passive film formed by repeated cycling does not have the same composition as the film before potentiodynamic treatment. In particular, the metal percentage decreased, with the decrease in Co% content greater than that of the Sn% content. This result may be explained as the Co reduction process being slower than that of Sn. Fig. 7 shows potentiodynamic profiles recorded for CoSn 2 alloy in 0.5 mol dmy3 borate solution in the potential range from y1000 to 1100 mV at different scan rates Ž n .. The results show that the increase of the scan rate leads to increasing the current density through the whole potential range and the potential of different peaks ŽA I ™ A IV . shifts to more positive potential. A linear relationship is obtained ŽFig. 8. between the current density peaks ŽA I , A III and A IV . and the square root of the scan rate; the three lines pass near the origin. This indicates that the peaks are due to a dissolution process controlled by diffusion. Fig. 6 shows that the ratio of the anodic to cathodic charge Ž QarQc . is greater than unity at low potential sweep rates. This fact and the shape of the cyclic voltammogram, ŽFig. 1., suggest an active dissolution–passivation process occurs w25x. Therefore, the anodic current peaks are indicative of two processes, the dissolution of the metallic component of the alloy and the formation of the passive film from tin and cobalt oxides. The effect of increasing amounts of Cly and Bry on the passivation and pitting corrosion of CoSn 2 alloy in 0.5 mol dmy3 borax ŽpH s 9.6. solutions was recorded potentiodynamically between y1000 and 1100 mV, 50 mV sy1 . Fig. 9 is an example. The data show that the addition of halide ions has a significant effect on the corrosion potential Ž Ecorr . of the alloy: Ecorr is shifted towards positive potential. This means that the addition of the halide ions increases the general corrosion of the alloy. The increase of halide concentration leads to an increase of the passivation current and the heights of the anodic current peaks related to formation of tin and cobalt oxides. This result can be explained by the direct participation of the halide ions in the dissolution of the metals component of the alloy. SEM examination of the alloy surfaces potentiodynamics
75
polarised from y1000 to 1100 mV at scan rate 50 mV sy1 in 0.5 mol dmy3 borax solution containing 0.8 mol dmy3 NaCl or 0.8 mol dmy3 NaBr was carried out. In the presence of chloride ions ŽFig. 10, as an example., the data show that the surface is covered with a high density of pits. At the same time increasing the concentration of the halide ion leads to increase in pitting corrosion, i.e., shift the Epitt towards the negative potential. The aggressiveness of the halide ions as pitting agents may be attributed w26x to their competitive adsorption with the passivating species as OHy and H 2 O on the surface of the passive film. When the applied potential reaches the pitting potential, the aggressive ions penetrate the passive layer and form pits. Then after reaching the pitting potential value, the halide ions were adsorbed and the passive layer broken down. This result means that by increasing the halide ion concentration an increasing of general and pitting corrosion results.
4. Conclusions The corrosion behaviour of CoSn 2 alloy was investigated in alkaline medium by electrochemical techniques, the data indicated the following. The addition of a small amount of Co metal Ž20%. to the tin metal to form CoSn 2 alloy Ž80% Sn–20% Co by wt%. results in a film with a higher corrosion resistance compared to the tin metal alone w14x. The effect of aggressive anions such as borate or halide ions on the general and pitting corrosion may be negligible compared to that in tin alone in alkaline medium w14x. The observed corrosion resistance of CoSn 2 alloy is due to the formation of a thin passive film. The corrosion resistance of this alloy suggests that it can be used as a decorative coating in alkaline medium. The passive layer formed on the surface of the alloy in alkaline medium shows negligible corrosion rates in aqueous solution ŽpH 3 to 7.. The passivity of CoSn 2 alloy is far superior to that of both tin and cobalt. Despite of the low effectiveness of the passive layer against aggressive anions in alkaline medium, the inhibition of the general and pitting corrosion of this alloy by different organic and inorganic inhibitors may be possible and further work is in progress.
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References w1x T. Davidson, Met. Finish. 88 Ž12. Ž1990. 49. w2x J.D.C. Hemsley, M.E. Roper, Trans. Inst. Met. Finish. 57 Ž1979. 77. w3x Brit. Pat. 661154, US Pat. 2658866, Ger. Pat. 830859. w4x J.W. Cuthbertson, J. Electrodep. Tech. Soc. 27 Ž1951. 13. w5x V. Sree, T.L. Rama Char, J. Electrochem. Soc. 9, 13 Ž1960. ŽBull India Sect... w6x M. Clarke, R.G. Elbourne, C.A. MacKay, Trans. IMF 50 Ž1972. 160. w7x US Pat. 3881919 and 3966564. w8x US Pat. 4035249 Žother patents pending.. w9x Ger. Pat. 670403. w10x S.S. Abd El Rehim, S.A.M. Refaey, G. Schwitzgebel, F. Taha, M.B. Saleh, J. Appl. Electrochem. 26 Ž1996. 413. w11x V.V. Nemoshkalenko, V.N. Uvarov, E.G. Litvin, V.Ya. Nagornyi, Phys. Metals 4 Ž1. Ž1982. 56. w12x S.A.M. Refaey, PhD Thesis, Chemistry Department, Faculty of Science, Minia University, 61519 Minia, Egypt, 1994. w13x S.N. Shah, D.E. Davies, Electrochem. Acta 8 Ž1963. 663. w14x S.A.M. Refaey, Electrochim. Acta 41 Ž16. Ž1996. 2545. w15x A.B. Garraet, R.E. Heiks, J. Amer. Chem. Soc. 63 Ž1937. 562.
w16x R. Kerr, D.J. MacNaughten, J. Electrodep. Tech. Soc. 12 Ž1937. 19. w17x M. Pugh, L.M. Warner, D.R. Gabe, Corros. Sci. 7 Ž1967. 807. w18x G.W. Simones, E. Kellerman, H. Leidheiser Jr., J. Electrochem. Soc. 123 Ž9. Ž1976. 1276. w19x M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon, Oxford, 1966, pp. 322–329. w20x G.W. Simmons, A. Vertes, M.L. Varsanyi, H. Leidheiser Jr., J. Electrochem. Soc. 126 Ž2. Ž1979. 187. w21x E. Deltombe, N. de Zoubov, M. Pourbaix, Rappt. Tech. 25 Ž1955. 24. w22x B.F. Giannetti, P.T. Sumodjo, T. Rabockai, J. Appl. Electrochem. 20 Ž1990. 672. w23x S.S. Abd El-Rehim, F. Taha, M.B. Saleh, S.A.M. Refaey, Corros. Sci. 33 Ž11. Ž1992. 1789. w24x N. Sato, T. Ohtsuka, J. Electrochem. Soc. 125 Ž11. Ž1978. 1735. w25x A.J. Calandra, N.R. de Tacconi, R. Pereiro, A.J. Arvia, Electrochim. Acta 19 Ž1974. 901. w26x F.A. Cotton, G. Wilkinson ŽEd.., Advanced Inorganic Chemistry, Wiley, 3rd edn., London, 1972, p. 475.
Corrosion of Sn–Co alloy in alkaline media and the effect of Cly and Bry ions S.A.M. Refaey
)
Chemistry Department, Faculty of Science, Minia UniÕersity, 61519 Minia, Egypt Received 17 August 1998; accepted 20 December 1998
Abstract Sn–Co electrodeposits alloy of approximate composition 80% Sn–20% Co Žwt%. can be obtained from a gluconate bath as single phase CoSn 2 , which is similar in appearance to decorative chromium. The potentiodynamic and cyclic voltammogram techniques were used to study the corrosion behaviour of CoSn 2 in sodium borate solutions ŽNa 2 B 4O 7 . at pH s 9.6. The effect of different factors such as concentration of borate ions, pH, potential scan rate, successive cyclic voltammetry, and progressive addition of halide ions ŽCly and Bry. on the electrochemical behaviour of CoSn 2 alloys are discussed. The observed corrosion resistance of electrodeposited CoSn 2 alloy is due to the formation of a thin passive film, which is examined by X-ray spectroscopy and believed to be mainly tin and cobalt oxides. The voltammograms involve four anodic peaks, the first and second of which correspond to the formation of SnO and SnO 2 and the third and fourth related to the formation of cobalt oxides. SEM examination confirms that pitting corrosion takes place in presence of borax and is increased by adding halide ions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Tin–cobalt alloy; Borax; Passivation; Pitting corrosion
1. Introduction A plated CoSn 2 alloy that looks like chromium but has none of the environmental or safety hazards associated with chromium is attracting interest from metal finishers. Decorative chromium plating adds an attractive, durable finish to manufactured goods. However, chromic acid is a hazardous material that requires great care in its handling, use, containment, and disposal. Chromium plating also has serious
)
Fax: q20-086-332601; E-mail: [email protected]
limitations in that concentrated, toxic materials are used and the corrosive electrolyte has poor throwing power w1x. Electrodeposition of the CoSn 2 alloy yields a film whose hardness and wear resistance is sufficient for most indoor decorative applications w1x, and the electrolyte is non-toxic, non-corrosive, has excellent throwing power, and the process is highly energy efficient. The CoSn 2 alloy has found extensive applications as a convenient and economic way of providing an attractive finish for lock and door hardware, plumbing fixtures, appliance components, office equipment, tools, computer electrical components, tubular furniture, store fixtures, jewellery, hinges, kitchen utensils, tubular furniture and automobile interior trim and fittings w2x.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 8 0 - X
68
S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
Several authors w2–10x described the deposition of CoSn 2 alloy from different baths. Davidson w1x reported that the CoSn 2 alloy has a chromium appearance within the range 17–23% Co, where the deposit is mat grey. At greater than 23% Co the appearance darkens. In our previous work w10x electroplated bright tin–cobalt alloy was obtained from a bath containing stannous and cobalt sulphate, sodium gluconate as a chelating agent, and sodium sulphate as supporting electrolyte. X-ray diffraction studies of the deposit w10x revealed that the phase structure Ž88% tin alloy. consists mainly of a tetragonal SnCo 2 phase with a minor quantity of b-Sn. The alloy deposits were generally adherent, bright, and similar to chromium in appearance. The surface morphology of the asplated SnCo 2 alloy was examined w10x by SEM, which showed that the deposit is compact. The deposit consists of regularly oriented fine grains. Nemoshkalenko et. al. w11x determined the mechanism of interatomic interactions of tin cobalt compounds, and found that the Co–Sn bonds can be regarded as donor–acceptor bonds due to the transfer of electron density from the Sn to the Co atoms, where the binding energy of the electrons become stabilised toward SnCo 2 . These interactions are accompanied by a dative Co ™ Sn interaction mediated by the valence p-electrons of the Co metal. The present study was undertaken with the following objectives in mind: Ž1. To determine the
Fig. 1. Current Ž at scan rate 50 mV sy1 .
effect of the presence of a small amount Ž20%. of Co in the composition of the CoSn 2 alloy on the passivation and pitting corrosion. Ž2. To investigation the corrosion behaviour of the tin–cobalt alloy in an alkaline medium, both in the absence and presence of halide ions. Ž3. To examine the chemical nature and stability of the passive film formed on the CoSn 2 alloy in borate solution under different conditions using potentiodynamic and cyclic voltammetry techniques complemented by X-ray, EDX, and SEM studies.
2. Experimental CoSn 2 alloy was used as a working electrode. The alloy was deposited on a plan copper cathode with circular area 1 cm2 from a gluconate bath under conditions similar to those described elsewhere w10,12x. Experiments were carried out in solutions containing 5 g dmy3 SnSO4 , 8 g dmy3 CoSO4 P 7H 2 O, 32 g dmy3 C 6 H 11O 7 Na and 20 g dmy3 KSO4 , c.d. 30 mA cmy2 , pH 5.5 and 408C. The plating duration was 30 min, after which the cathode was withdrawn, washed with distilled water and dried. All solutions used were freshly prepared with doubly distilled water and analytical grade chemicals. The required pH was adjusted by dropwise addition of standard boric acid or sodium hydroxide solutions. A pH meter was used to measure the pH
. and charge Ž- - -. variations versus potential of CoSn 2 alloy in 0.5 mol dmy3 borax solution during one cycle,
S.A.M. Refaey r Applied Surface Science 147 (1999) 67–76
values. The measurements were performed in a three-electrode cell provided with an electrodeposited CoSn 2 alloy as a working electrode, a Pt wire as a counter electrode and a AgrAgCl as a reference electrode. Experiments were carried out in various concentrations of borate solutions in absence and presence of NaCl or NaBr. Before each run the
69
CoSn 2 alloy electrode was washed with distilled water and rinsed in acetone. All solutions were used under purified nitrogen. All experiments were carried out at room temperature Ž258C.. The potentiodynamic and cyclic voltammograms curves were run by a potentiostat ŽAMEL model 2049., which was controlled by a PC. X-ray diffraction analysis was
Fig. 2. Scanning electron micrograph of the surface CoSn 2 anode, Ža. before and Žb. after scanning, at 50 mV sy1 from y1000 to 1100 mV in 0.5 mol dmy3 borax.
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carried out using an X-ray diffractometer ŽSiemens D500r501.. The composition of the deposits before and after examination was determined by an EDX-ray spectrometer ŽCamScan Cambridge Scanning.. The morphology of the deposits was examined by scanning electron microscopy ŽCamScan Cambridge Scanning..
3. Results and discussion The cyclic voltammogram is given in Fig. 1, curve a for a stationary electrodeposited CoSn 2 alloy in 0.5 mol dmy3 Na 2 B 4 O 7 solutions ŽpH s 9.6. where the potential was swept from y1000 mV to 1100 mV Žall potentials are given vs. AgrAgCl electrode. at a scan rate n s 50 mV sy1 . The anodic curve exhibits four dissolution peaks ŽPeaks A I ™ A IV .. The anodic peak A I is similar to that observed previously for Sn in alkaline medium. Shah and Davies w13x and Refaey w14x suggested that the Sn metal in Na 2 B 4O 7 solution produces stannous hydroxide wSnŽOH. 2 x which dehydrated to stannous oxide ŽSnO. according to:
Fig. 4. Anodic potentiodynamic curves for CoSn 2 anode in 0.5 mol dmy3 sodium borate solutions at various pH values: Ž1. 1.5, Ž2. 6.8, Ž3. 8, Ž4. 9.6, at scan rate 50 mV sy1 .
x can On the other hand the stannite ion wHSnOy 2 form directly by dissolution of Sn w14x Sn q 2OHy™ HSnOy 2 q H 2 O q 2e. The potential of shoulder peak A II may be correlated to the formation of SnŽOH.4 according to the following equations: Sn Ž OH . 2 q 2OHys Sn Ž OH . 4 q 2e
Sn q 2OHy™ Sn Ž OH . 2 q 2e
and
with a further reaction involving dehydration of stannous oxide:
SnO q 2OHyq H 2 O s Sn Ž OH . 4 q 2e
Sn Ž OH . 2 ™ SnO q H 2 O.
which dehydrates to SnO 2 Sn Ž OH . 4 ™ SnO 2 q 2H 2 O. Both SnO and SnO 2 were dissolved in alkaline media to yield stannite and stannate, respectively
Fig. 3. Anodic potentiodynamic polarisation of CoSn 2 anode in solutions of various concentrations of borax at scan rate 50 mV sy1 Concentrations: Ž1. 0.05, Ž2. 0.1, Ž3. 0.5, Ž4. 0.8 and Ž5. 1.0 mol dmy3 .
Fig. 5. Successive cyclic voltammograms for CoSn 2 anode in 0.5 mol dmy3 sodium borate solutions at scan rate 50 mV sy1 , the numbers in figure indicate the cycle number.
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Fig. 6. Anodic and cathodic charge changes as a function of potential scan number for CoSn 2 anode in 0.5 mol dmy3 borate solutions, at scan rate 50 mV sy1 .
w13,15–17x. So the stannic oxide may be present as stannate ion: Sn q 5OHy™ HSnOy 3 q 2H 2 O q 4e. Fig. 1, curve Ža. shows a current peak A III , which may be corresponding to the dissolution of cobalt present in the alloy to CoO and CoOOH. This result is in agreement with the previous Mossbauer studies ¨ w18x, which show that the cobalt was shown to be essentially free of a corrosion film during cathodic polarisation at y1000 mV, but at y100 mV an anodic film is formed which has two oxidation states Co 2q and Co 3q. In the passive region of the polarisation curve Žfrom q100 mV to q500 mV., the passive film contained the primarily Co 3q oxidation state. The Pourbaix diagram of cobalt w19x indicates
Fig. 7. Anodic potentiodynamic curves of CoSn 2 anode in 0.5 mol dmy3 sodium borate solutions ŽpH s9.6. at various scan rates: Ž1. 10, Ž2. 30, Ž3. 60, Ž4. 200, Ž5. 300 mV sy1 .
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Fig. 8. The dependence of anodic current peaks A I , A III and A IV on square root of scan rate for CoSn 2 alloy in 0.5 mol dmy3 sodium borate solutions.
that the equilibrium potential between CoO 2 and CoOOH is 715 mV ŽSCE. at pH 8.5. Formation of Co 4q in the present study was found at potentials as low as 635 mV Žpeak A IV .. These results suggest that the decomposition of CoO 2 occur according to the following reaction w20x: 3CoO 2 ™ Co 3 O4 q O 2 . This result is explained later in the pH effect section. The anodic film formed at transpassive potentials q850 mV was found to consist of Co 3q and Co 4q ions w18x. At potential higher than q900 mV, the passive film consists of both CoOOH and CoO 2 . Apparently the CoOOH in contact with alloy is not completely oxidised to tetravalent CoO 2 at anodic
Fig. 9. Anodic potentiodynamic polarisation of CoSn 2 anode in 0.5 mol dmy3 borax solution at scan rate 50 mV sy1 in the presence of various concentration of NaBr. wNaBrx: Ž1. 0, Ž2. 0.4. Ž3. 0.6, Ž4. 0.8 mol dmy3 .
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potentials up to q900 mV. The composition of the passive films formed on the surface of the electrodeposited CoSn 2 alloy after complete potentiodynamic polarisation Žfrom y1000 to 1100 mV. treatment with a scan rate of 50 mV sy1 in 0.5 mol dmy3 borax ŽpH s 9.6. was examined by X-ray diffraction.
The X-ray diffraction data reveal the composite nature of the passive films and confirmed the presence of SnO, SnO 2 , CoO, CoO 2 . CoOOH, Co 2 O 3 and Co 3 O4 in the investigated samples. When the polarisation reaches a certain critical potential, the pitting potential, Epit , the passive current suddenly rises
Fig. 10. Scanning electron micrograph of the surface CoSn 2 anode: Ža,b. in 0.5 mol dmy3 borax solution containing 0.8 mol dmy3 NaCl and Žc,d. in 0.5 mol dmy3 borax solution containing 0.8 mol dmy3 NaBr after scanning at 50 mV sy1 from y1000 to 1100 mV.
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73
Fig. 10 Žcontinued..
steeply without any sign for oxygen evaluation denoting the breakdown of the passive layer and initiation of visible pitting corrosion. The cathodic part of the cyclic voltammogram ŽFig. 1, curve a. shows two weak cathodic peaks ŽC I and C II .. C I may be correlated to the electroreduction of cobalt oxides to cobalt metal, and C II represents the electroreduction of tin oxides formed to tin
metal. Fig. 1 curve b shows the correlation between the potential and the charge consumed during the cyclic voltammogram. The data indicate that the total charge consumed during the first half of the cycle increases with increasing potential in the positive direction. The charge quantity starts to increase Žat y780 mV. when the passive film starts to form in the potential region of active dissolution. This may
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be due to the breakdown of the alloy to its simple metals and then these metals ŽSn and Co. oxidised to their oxides SnO, SnO 2 , CoO, CoO 2 , CoOOH, Co 2 O 3 and Co 3 O4 as discussed above. Fig. 1 curve b indicates that the charge consumed during the cathodic part Žreduction of oxides, i.e., formation of metals. is very small compared to the charge consumed in the anodic part Želectrooxidation of metals, i.e., formation of metal oxides.. This result may be explained by the incomplete reduction of the oxides to the corresponding parent metal. This may be due to a sluggish discharge process caused by the increase in the resistance of the oxide film against the completion of the discharge Žreduction. process. This means that some of the passive film remains unreduced. SEM examination of electrodeposited CoSn 2 surface potentiodynamics polarised from y1000 mV to 1100 mV at a scan rate 50 mV sy1 in 0.5 mol dmy3 borax solution was performed. The results, shown in Fig. 2, indicate that the surface of the electrodeposited CoSn 2 alloy was destroyed. This may be clearer when comparing SEM photographs of the alloy before ŽFig. 2a. and after ŽFig. 2b. the alloy surface was polarised from y1000 to 1100 mV at scan rate 50 mV sy1 in 0.5 mol dmy3 borax solution. The data also show that in the presence of borate ions the surface was covered with some pits and the passive layer was destroyed. Fig. 3 shows potentiodynamic polarisation curves of CoSn 2 alloy in various concentrations of sodium borate solutions in potential ranging from y1000 to 1100 mV at scan rate 50 mV sy1 . The data reveal that increasing the borax content causes an increase in the current density values of peaks ŽA I ™ A III ., at the same time these peaks shift towards more negative potential values. It is probable that the formation of CoŽIV. species is inhibited by increasing the concentration of borax as manifested by the disappearance of peak A IV at higher concentration of borax. In contrast, the formation of SnŽIV. species is inhibited by decreasing concentration of borax as manifested by the disappearance of peak A IV at lower concentration of borax. The effect of pH on the anodic polarisation features for electrodeposited CoSn 2 alloy in 0.5 mol dmy3 borax at 50 mV sy1 is illustrated in Fig. 4. It is noted the anodic current peak A I at low pH Žcurve
1, pH s 1.5. is very high, this result is in agreement with the previous literature w21x, which indicates that the corrosion rate of tin is very high at pH - 3. The peak A I appeared around 0.0 V characterises the dissolution of Sn; this is also in agreement with the previous studies on Sn in citric acid w22x. At the same time, the anodic peak A II related to the formation of CoSn 2 disappears at pH 1.5 Žcurve 1.. It is probable that the formation of SnŽIV. species is inhibited by lowering the pH w23x. The height of the anodic current peaks A I and A II are decreased with increasing pH value to 6.8. ŽCurve 2., where the corrosion rate of Sn in near neutral medium is very low related to the acidic medium w23x. In curve 1, there is an anodic current peak A III , which is related to the formation of Co 2q, and an anodic peak A IV , which corresponds to the formation of Co 3q and Co 4q ions. The heights of the anodic current peaks A III and A IV correspond to the formation of cobalt oxides, and decreased with increasing pH values. This result is in agreement with previous work w24x. At high pH, i.e., pH ) 7.5 Žcurves 3 and 4., the anodic current peaks A I and A II , which correspond to the dissolution of Sn to tin oxides, again increase in height. This result can be explained by increasing the corrosion rate at pH ) 7.5 w21x. This effect is presumably due to the dissolving power of the OHy ion, with both SnO and SnO 2 formed on the surface dissolving in alkaline media to form stannite and stannate respectively w14x. Fig. 5 shows cyclic voltammograms of CoSn 2 alloy in 0.5 mol dmy3 borax at a scan rate 50 mV sy1 , repeated in the potential range from y1000 to 1100 mV for 10 cycles. The data show that the current passing through the whole range decreases with increasing number of cycles. Fig. 6 shows the change of charge amount consumed in the anodic and cathodic part of cyclic voltammograms. It is found that the charge consumed during anodic dissolution Ž Qa . of the alloy increases as the number of cycles increase. At the same time, the amount of charge consumed Ž Qc . in the cathodic part Ždischarge of the metal ions. is nearly constant with increasing cycle number. This may be due to a sluggish discharge process caused by an increase in the resistance of the oxide film against the completion of the discharge Žreduction. process. This means that passivation increases with increasing cycle num-
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ber. Consequently the passive film thickness is increased. This result is elucidated by the EDX results, which showed that the composition of the passive film formed by repeated cycling does not have the same composition as the film before potentiodynamic treatment. In particular, the metal percentage decreased, with the decrease in Co% content greater than that of the Sn% content. This result may be explained as the Co reduction process being slower than that of Sn. Fig. 7 shows potentiodynamic profiles recorded for CoSn 2 alloy in 0.5 mol dmy3 borate solution in the potential range from y1000 to 1100 mV at different scan rates Ž n .. The results show that the increase of the scan rate leads to increasing the current density through the whole potential range and the potential of different peaks ŽA I ™ A IV . shifts to more positive potential. A linear relationship is obtained ŽFig. 8. between the current density peaks ŽA I , A III and A IV . and the square root of the scan rate; the three lines pass near the origin. This indicates that the peaks are due to a dissolution process controlled by diffusion. Fig. 6 shows that the ratio of the anodic to cathodic charge Ž QarQc . is greater than unity at low potential sweep rates. This fact and the shape of the cyclic voltammogram, ŽFig. 1., suggest an active dissolution–passivation process occurs w25x. Therefore, the anodic current peaks are indicative of two processes, the dissolution of the metallic component of the alloy and the formation of the passive film from tin and cobalt oxides. The effect of increasing amounts of Cly and Bry on the passivation and pitting corrosion of CoSn 2 alloy in 0.5 mol dmy3 borax ŽpH s 9.6. solutions was recorded potentiodynamically between y1000 and 1100 mV, 50 mV sy1 . Fig. 9 is an example. The data show that the addition of halide ions has a significant effect on the corrosion potential Ž Ecorr . of the alloy: Ecorr is shifted towards positive potential. This means that the addition of the halide ions increases the general corrosion of the alloy. The increase of halide concentration leads to an increase of the passivation current and the heights of the anodic current peaks related to formation of tin and cobalt oxides. This result can be explained by the direct participation of the halide ions in the dissolution of the metals component of the alloy. SEM examination of the alloy surfaces potentiodynamics
75
polarised from y1000 to 1100 mV at scan rate 50 mV sy1 in 0.5 mol dmy3 borax solution containing 0.8 mol dmy3 NaCl or 0.8 mol dmy3 NaBr was carried out. In the presence of chloride ions ŽFig. 10, as an example., the data show that the surface is covered with a high density of pits. At the same time increasing the concentration of the halide ion leads to increase in pitting corrosion, i.e., shift the Epitt towards the negative potential. The aggressiveness of the halide ions as pitting agents may be attributed w26x to their competitive adsorption with the passivating species as OHy and H 2 O on the surface of the passive film. When the applied potential reaches the pitting potential, the aggressive ions penetrate the passive layer and form pits. Then after reaching the pitting potential value, the halide ions were adsorbed and the passive layer broken down. This result means that by increasing the halide ion concentration an increasing of general and pitting corrosion results.
4. Conclusions The corrosion behaviour of CoSn 2 alloy was investigated in alkaline medium by electrochemical techniques, the data indicated the following. The addition of a small amount of Co metal Ž20%. to the tin metal to form CoSn 2 alloy Ž80% Sn–20% Co by wt%. results in a film with a higher corrosion resistance compared to the tin metal alone w14x. The effect of aggressive anions such as borate or halide ions on the general and pitting corrosion may be negligible compared to that in tin alone in alkaline medium w14x. The observed corrosion resistance of CoSn 2 alloy is due to the formation of a thin passive film. The corrosion resistance of this alloy suggests that it can be used as a decorative coating in alkaline medium. The passive layer formed on the surface of the alloy in alkaline medium shows negligible corrosion rates in aqueous solution ŽpH 3 to 7.. The passivity of CoSn 2 alloy is far superior to that of both tin and cobalt. Despite of the low effectiveness of the passive layer against aggressive anions in alkaline medium, the inhibition of the general and pitting corrosion of this alloy by different organic and inorganic inhibitors may be possible and further work is in progress.
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