Electrochemical dissolution of nichrome in sulphuric acid

Hydrometallurgy 59 Ž2001. 45–54 www.elsevier.nlrlocaterhydromet

Electrochemical dissolution of nichrome in sulphuric acid M. Chakravortty a , R.K. Paramguru b,) , P.K. Jena c a

c

Non-Ferrous Process DiÕision, National Metallurgical Laboratory, Jamshedpur 831 007, India b Regional Research Laboratory, Bhubsaneswar, India Department of Material Science and Metallurgy, Catholic UniÕersity of Rio De Janeiro, Rio De Janeiro, Brazil Received 26 March 2000; accepted 22 July 2000

Abstract The paper reports polarisation studies on the dissolution of nichrome in different concentrations of H 2 SO4 at room temperature. The cathodic current density Ž i c . and anodic current density Ž i a . at fixed over-potential, together with the corrosion current density Ž i corr ., are found to be proportional to H 2 SO4 concentration, whereas they decrease progressively at higher Na 2 SO4 concentration, the H 2 SO4 concentration being maintained at 1.0 M. Anodic polarisation studies indicate that passivity becomes more difficult at increased acid concentrations, whereas an increase in the sodium sulphate concentration decreases not only the open-circuit corrosion potential, but also the passive current Ž i p .. The Langmuir adsorption isotherm was used to establish the mechanism of the process. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical dissolution; Nichrome; Sulphuric acid

1. Introduction Nichrome Ž80% nickel and 20% chromium. is widely used in the manufacture of heating elements for industrial and laboratory furnaces. Spent nichrome heating elements are usually treated as waste material on account of their nominal scrap value. Due to a worldwide shortage of various non-ferrous metals, such as nickel, cobalt, chromium, cadmium, etc., considerable efforts have been directed towards the recovery of these metals from scraps and other in-

)

Corresponding author. E-mail address: drrkparam guru@ yahoo.com Paramguru..

Ž R.K.

dustrial wastes w1–3x. Considering the high nickel content of nichrome scrap, efforts have also been made to recover nickel from spent nichrome scraps by using hydrometallurgical techniques w4–6x. Sulphuric and hydrochloric acids, the common leaching media for metals and alloys, as well as their ores and minerals, have been used to leach nichrome w5,6x. It has been observed that the dissolution of nickel and chromium, which is proportional to their content in nichrome, is electrochemical in nature and the rate is influenced by the hydrogen ion activity and temperature; the activation energy values have been determined to be 68.9 and 58.6 kJrmol in sulphuric w5x and hydrochloric acid w6x, respectively. The dissolution process could be explained by applying corrosion principles with the anodic half-process of metal

0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 1 4 2 - 0

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M. ChakraÕortty et al.r Hydrometallurgy 59 (2001) 45–54

dissolution being coupled with hydrogen evolution as the cathodic half-process. Thus, the study of corrosion behaviour of nichrome assumes significance, specifically in understanding the leaching behaviour. Further, there exists considerable scope for using nichrome-heating elements directly to heat lixiviants, solutions, electrolytes, etc., used in various hydrometallurgical operations. Knowledge of corrosion behaviour of nichrome is useful in such circumstances. A number of studies on corrosion behaviour and passivity of alloys containing nickel and chromium in acidic solutions have been carried out w7–17x. In the case of Fe–Cr, the dissolution pattern of the alloy is similar to that of iron up to a Cr content of 5%, so it follows that of chromium beyond 12% Cr and the 7–12% Cr range can be termed as the transition region w7–9x. The iron–nickel alloys show similar behaviour with the 40–60% nickel alloy representing the bridge between ‘iron-like’ and ‘nickellike’ behaviour w12x. Chromium is the passivating agent in Fe–Cr alloy and nickel in the Fe–Ni alloy. Several studies on passivity of stainless steel, which contains both nickel and chromium have also been made w16–19x. Passive films formed on the austenitic stainless steel are complex and potential dependent. In acidic media Ž0.5 M H 2 SO4 . and at not too high potentials Ž E - 1.1 V SHE., such films can be characterised by Cr-enrichment. In this background, the behaviour of Ni–Cr alloy should be interesting. Over a long period, it was supposed that the electrons are the only particles taking part in this type of electrochemical reaction. However, it was later established that anions of the electrolyte also participate in reactions of metal dissolution. Such dissolution was initially attributed to hydroxyl ions and, subsequently, to HSO4yrSO42y or Cly ions w14,15,20,21x. Mechanisms involving formation of surface adsorbed charge transfer complexes and competitive adsorption of anions were discussed w20–25x. Hodge and Wilde w26x studied the influence of chloride ions on the anodic dissolution of a series of Ni–Cr binary alloys and reported that chloride ions affected the cathodic process over a wide range Ž20–100%. of Cr. Increase in chromium content in the Cr–Ni alloy increased the stability of the passive state with a progressive shift in the corrosion potential of Ni in the noble direction. Recently, the present

authors w27x studied the corrosion behaviour of nichrome in hydrochloric acid media. Though the dissolution rate of nichrome depended directly on HCl concentration, it was also dependent on Cly concentration at a particular HCl level. When HCl was low, the rate was independent of both Hq and Cly concentrations. These results could be explained in the light of the Langmuir adsorption isotherm. The present paper examines the electrochemical behaviour of nichrome in H 2 SO4 with and without Na 2 SO4 , and aims at understanding the dissolution mechanism.

2. Experimental details The electrochemical polarization measurements were carried out using a conventional three-electrode cell. A 500-ml Corning beaker served as the cell. A nichrome specimen containing 80% nickel and 20% chromium and of 25-cm length and 0.2-cm diameter in the AUB form was used as the working electrode. The top portion of the specimen, as well as the edges, were coated with paraffin so that the projected area of the specimen was 12 cm2 . A platinum foil of 20 cm2 and a saturated standard calomel electrode ŽSCE. were used as the auxiliary and reference electrodes, respectively. Polarization curves were obtained using an EG & G Princeton Applied Research Potentiostat ŽModel 362. coupled with a series 2000 Omnigraphic X-Y recorder Model RE 0092. Before each experiment, the working electrode specimen was polished with emery paper to a mirror-like finish, washed with distilled water, followed by acetone, and finally, dried. It was then placed in the cell along with the auxiliary electrode with their faces vertically parallel and 2 cm apart. A Luggin capillary placed close to the working electrode surface was connected to the reference electrode. Two hundred fifty milliliters of electrolyte, H 2 SO4 –Na 2 SO4 or H 2 SO4 solution, was admitted into the cell. The working and auxiliary electrodes were immersed in the cell for half an hour before recording the open-circuit potential. The potentiostatic polarization plots were obtained with a scan rate of 20 mVrs. After the experimental run, the specimen was rinsed with distilled water followed by acetone and preserved.

M. ChakraÕortty et al.r Hydrometallurgy 59 (2001) 45–54

Sulphuric acid and sodium sulphate used in this study were of analytical grade. Potentials reported in the subsequent sections are against SCE and are at room temperature Ž258C..

47

the passivating region. The critical current density, i crit , Flade potential, EF , and the passive current density, i p , are also marked on the figure. 3.1. Effect of acid concentration

3. Results and discussion Fig. 1 presents anodic, as well as cathodic, polarization plots for nichrome in 0.25 M H 2 SO4 electrolyte. Starting from a rest potential of y220 mV, the anodic plot shows active, passive and trans-passive regions, whereas the cathodic plot has been terminated in the active region. The corrosion parameters have been marked in the figure. The meeting point of the extended Tafel lines of both the anodic and cathodic curves in the rest potential line gives the corrosion current density, i corr . In this figure, the cathodic Tafel line is clear, whereas the Tafel line on the anodic plot is not so clear because of the onset of

Cathodic, as well as anodic, polarization curves for nichrome at different concentrations Ž0.05–0.5 M. of H 2 SO4 were obtained. The anodic plots are similar to that of Fig. 1 with active, passive and trans-passive regions. The open-circuit potential Ž Ecorr ., corrosion current density Ž i corr ., critical current density Ž i crit ., Flade potential Ž EF ., passive current density Ž i p . and potential Ž Ep ., anodic Ž i a . as well as cathodic Ž i c . current density at a fixed overpotential are presented in Table 1. Fig. 2a presents plots i vs. acid concentration. The current densities are found to increase with increase in sulphuric acid concentration and a plot of log i corr vs.

Fig. 1. Potentiostatic anodic and cathodic polarisation plots of nichrome in 0.05 M sulphuric acid at room temperature.

M. ChakraÕortty et al.r Hydrometallurgy 59 (2001) 45–54

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Table 1 Corrosion parameters of nichrome at different acid concentrations H 2 SO4 ŽM.

Ecorr ŽmV.

i corr ŽmA cmy2 .

ia ŽmA cmy2 . a

ic ŽmA cmy2 . b

i crit ŽmA cmy2 .

EPP rmV

iP ŽmA cm2 .

EF ŽmV.

0.05 0.25 0.375 0.5

y280 y220 y240 y242

0.011 0.06 0.07 0.1

0.0704 0.222 0.47 0.72

0.0951 0.28 0.428 0.65

0.6 1.0 7.1 6.5

q5 y10 y70 y80

0.0035 0.015 0.0086 0.018

40 60 80 100

a

‘i a’ at constant overpotential of 50 mV. ‘i c ’ at constant overpotential of y100 mV.

b

logwH 2 SO4 x results in a slope of ; 1. Table 1 also indicates that other parameters like Ecorr , E F , i crit

and i p also increases, whereas Ep decreases, with increase in acid concentration.

Fig. 2. Current density vs. Ža. sulphuric acidrŽb. sodium sulphate concentration plots for nichrome at room temperature.

M. ChakraÕortty et al.r Hydrometallurgy 59 (2001) 45–54

These results are examined with respect to a Langmuir adsorption isotherm. The dissociation of H 2 SO4 is described by Eqs. Ž1. and Ž2..

™ H q HSO HSO ™ H q SO H 2 SO4 y 4

q

q

y 4

2y 4

Ž 1. Ž 2.

It may be assumed that the first dissociation of H 2 SO4 is complete and the second dissociation is negligible, as K for this reaction is only 1.2 = 10y2 . The Hq and HSO4y ions are adsorbed on the surface of the nichrome specimen and these are responsible for the dissolution of nichrome. The activation polarization increases with increase in current density in accordance with the Tafel equation.

h s "b log iri o

Ž 3.

Here, h , b , i and i o are overpotential, Tafel constant Ž2.3 RTra nF ., applied and exchange Žin this case, corrosion. current densities, respectively; R, T, a , n and F being universal gas constant, absolute temperature, symmetry coefficient, number of electrons and Faraday number, respectively.

49

Eq. Ž3. can be reorganised as i s i o e " h r b . However, i o Ž i corr here. is directly proportional to u , the fraction of surface covered by adsorbed species responsible for the dissolution. Then i o s k u , where k is a constant, and therefore, i s ku e " h r b

Ž 4.

Let u be the fraction of surface covered by Hq and u X by the HSO4y ions. Then, the bare fraction of the surface is Ž1 y u y u X .. The rate of adsorption by Hq is given by V1 s k 1 C H q Ž1 y u y u X .. The rate of desorption of Hq is Vy1 s ky1 u . At the equilibrium, the rate of adsorption and desorption are equal. Hence, the following equation:

ur Ž 1 y u y u X . s Ž k 1rky1 . C H qs KC H q

Ž 5.

Here, ‘K ’ is equilibrium constant for adsorption of Hq. Similarly, the equilibrium adsorption of HSO4y becomes:

u Xr Ž 1 y u y u X . s K X C HSO y4

Ž 6.

Fig. 3. Potentiostatic anodic and cathodic polarisation plots of nichrome in a mixture of 1.0 M sulphuric acid and 0.125 M sodium sulphate at room temperature.

M. ChakraÕortty et al.r Hydrometallurgy 59 (2001) 45–54

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Here, K X is equilibrium constant for the adsorption of HSO4y. From Eqs. Ž5. and Ž6., the fractions covered by Hq and HSO4y are found to be: X

compared to HSO4y, i.e. C HSO y4 < C H q. Based on this assumption, Eq. Ž7. becomes:

u s KC H qr Ž 1 q KC H q .

u s KC H qr Ž 1 q KC H qq K C HSO y4 .

Ž 7.

u X s K X C HSO y4 r Ž 1 q KC H qq K X C HSO y4 .

Ž 8.

Ž 9.

resulting in the following rate equations:

In the case of only sulphuric acid, the hydrogen ion is likely to be preferentially adsorbed on the surface

i o Ž s i corr . s kKC H qr Ž 1 q KC H q .

Ž 10a.

i s kKC H qe " h r br Ž 1 q KC H q .

Ž 10b.

Fig. 4. Inverse of current density vs. sulphuric acid concentration plots for nichrome at room temperature: Ža. linear scale, Žb. logarithmic scale.

M. ChakraÕortty et al.r Hydrometallurgy 59 (2001) 45–54

By taking the reciprocal of the rate from Eq. Ž10a., one gets the following relation:

Ž 1ri o . s Ž 1rkK . Ž 1rC H q . q Ž 1rk .

Ž 11 .

A similar relation can be obtained from Eq. Ž10b. with different constants because of incorporating e " h r b , which is a constant at fixed h. Fig. 3 shows the relationship of the reciprocal of i with the reciprocal of the concentration of H 2 SO4 , indicating the validity of Eq. Ž11.. Since the first dissociation ŽEq. Ž1.. is complete, the experimental C H SO may be 2 4 taken as equal to C H qs C HSO y4 . The value of Ž kK . calculated from the i corr plot in Fig. 3a is 0.22 mA cm2 sy1 moly1 . Fig. 3a also presents plots of the applied current density obtained from the activation region of the anodic plot at an overpotential of 50 mV and of the cathodic plot at an overpotential of y100 mV. These two plots also give straight lines.

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Na 2 SO4 concentration. Log i corr vs. logwtotal sulphatex gives a slope of ; y1. Table 2 also indicates that Ecorr and i P decrease, while i crit and E F increase, with an increase in Na 2 SO4 concentration. These results are also analyzed by applying a Langmuir adsorption isotherm. From Eq. Ž2., it can be seen that with an increase in SO42y concentration in the solution, the concentration of HSO4y increases and HSO4y would be adsorbed more strongly. As a result, u becomes equal to Ž KC H q .rŽ1 q K X C HSO y4 ., and therefore, the rate equation becomes: i o Ž s i corr . s Ž k 2 KC H qr Ž 1 q K X C HSO y4 .

Ž 12a.

and i s Ž k 2 KC H qe " h r br Ž 1 q K X C HSO y4 .

Ž 12b.

By taking the reciprocal ŽEq. Ž12a.. of the rate, the following equation emerges:

Ž 1ri o . s Ž 1rk 2 K . Ž 1rC Hq .

3.2. Effect of sodium sulfate concentration

q Ž K X C HSO y4 rk 2 KC H q . . A set of experiments have been carried out on the cathodic, as well as anodic, polarization of nichrome in a number of solutions containing 1.0 M H 2 SO4 and varying concentrations Ž0.125–0.75 M. of Na 2 SO4 . Fig. 4 presents the plots for a concentration of 0.125 M Na 2 SO4 and the features active, passive and trans-passive regions in the anodic plot are identical at each concentration. Table 2 shows the electrochemical parameters Ecorr , i corr , EF , i P , EPP and i a and i c at specific overpotentials. Fig. 2b shows the plots of current densities against Na 2 SO4 concentrations. It is seen that the dissolution current densities decrease progressively with an increase in

Ž 13 .

As the concentration of H 2 SO4 is kept constant, C H q may be considered constant and Hq might be adsorbed weakly compared to HSO4y. Then Eq. Ž13. becomes:

Ž 1ri o . s C q K Y C HSO y4

Ž 14 .

H ere, C s Ž 1r k 2 K .Ž 1r C H q . and K 0 s Ž K Xrk 2 KC H q .. Thus, on plotting the reciprocal of corrosion current density Ž1ri o . against Na 2 SO4 concentration, a straight-line relationship is obtained ŽFig. 5. with k 2 K value of y0.036 mA cmy2 sy1 moly1 . A similar relationship with different slopes

Table 2 Corrosion parameters of nichrome at different concentrations of Na 2 SO4 H 2 SO4 concentration is 1.0 M Na 2 SO4 ŽM.

Ecorr ŽmV.

i corr ŽmA cmy2 .

ia ŽmA cmy2 . a

ic ŽmA cmy2 . b

i crit ŽmA cmy2 .

E PP ŽmV.

iP ŽmA cm2 .

EF ŽmV.

0.125 0.25 0.5 0.75

y310 y320 y330 y340

0.035 0.030 0.027 0.024

0.324 0.264 0.215 0.195

0.285 0.290 0.285 0.278

11.0 12.0 18.0 170

y45 y50 y60 y70

0.0034 0.0030 0.0021 0.0028

20 40 140 300

a

‘i a’ at constant overpotential of 50 mV. ‘i c ’ at constant overpotential of y100 mV.

b

52

M. ChakraÕortty et al.r Hydrometallurgy 59 (2001) 45–54

Fig. 5. Inverse of current density vs. sodium sulphate concentration plots for nichrome at room temperature.

and intercept is obtained when i a and i c at fixed h is plotted. The critical current density, i crit , and passive current density, i p , for different concentrations of H 2 SO4 with and without Na 2 SO4 presented in Tables 1 and 2 are significant. It is found that with the addition of Na 2 SO4 , the rate of dissolution decreases. The addition of Na 2 SO4 to H 2 SO4 decreases the open-circuit corrosion potential, as well as i p and increases i crit . This increase in i crit , along with the movement of Flade potential towards more positive values, is significant. This means that the passive film formation is rather helped by sodium sulphate addition by increasing these to more positive values. In only sulphuric acid, with an increase in concentration, the passivity becomes difficult to attain due to movement of the pre-passive potential, EPP , towards less positive values. Possible links between adsorption phenomena and the structure and chemical composition of passive layers are important but not dealt with here. Though

such information on nichrome is not reported in literature, some available data on stainless steel containing 17–19% Cr and 9–11% Ni are relevant and may be mentioned w19x. The data suggest that formation of Cr`O bonds leads to a stronger passive film on this alloy and the sorption behaviour of Cly and HSO4yrSO42y ions is different due to differing sorption mechanisms. Since the adsorbed chloride is weakly bonded to the passive layer, adsorption of Cly ions, as well as formation of chloro-complexes, occur on the steel surfaces to initiate pit formation. Thus, the Cly ion was found to have a positive influence on the dissolution of nichrome in HCl w6,27x. In sulphuric acid medium, the SO42y anion bipolarizes the passive chromium oxide barrier layer and enhances the deprotonation process w18x. This leads to further growth of the above barrier layer. This probably explains the observed increase in passivation in the presence of higher wNa 2 SO4 x in this study. However, further studies are needed to support this view.

M. ChakraÕortty et al.r Hydrometallurgy 59 (2001) 45–54

4. Conclusions The anodic and cathodic polarization studies of nichrome in sulphuric acid in the presence and absence of sodium sulphate leads to the following conclusions: 1. The corrosion, as well as the applied current density, is proportional to the sulphuric acid concentration. 2. In 1.0 M H 2 SO4 , these current densities decrease progressively with increase in the sodium sulphate concentration. 3. The results could be explained in the light of the Langmuir adsorption isotherm. 4. It is seen that sodium sulphate not only decreases open-circuit corrosion potential, but also decreases the passive current Ž i p . and increases the critical current Ž i crit .. The passive film formation is helped by sodium sulphate addition by both increasing i crit and shifting Flade potential to more positive values. 5. With an increase in the concentration of H 2 SO4 Žand no Na 2 SO4 ., the passivity is less attainable due to the movement of EPP towards less positive values.

w3x w4x w5x

w6x

w7x w8x

w9x

w10x

w11x w12x

w13x

w14x

Acknowledgements w15x

The authors acknowledge valuable discussions with the late Prof. S.C. Sircar, Department of Metallurgical Engineering, IIT, Kharagpur, India. One of the authors expresses her sincere thanks to Prof. B.K. Dhinda, Head, Department of Metallurgical Engineering, IIT, Kharagpur, India, for providing the facilities to carry out the experimental work. The author also expresses her gratitude to the Council of Scientific and Industrial Research for awarding fellowship during this work.

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