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Electrochemical behaviour of chromium, molybdenum and manganese in liquid ammonia
Ekcmchlmica
Pergamoa
Acra Vol. 39, No. 18.9p. 2795-2797.1994 Copyright Q 1994 Elsevier Scimce Ltd. Printed in Britain. All righta rcrcrved 0013~4686p4 s7.00 + 0.00
Grut
SHORT COMMUNICATION
ELECTROCHEMICAL BEHAVIOUR OF CHROMIUM, MOLYBDENUM AND MANGANESE IN LIQUID AMMONIA D. HUEFITA, AI ZHOUPING,PING
Luo and K. E. HEUSLER*
Abteilung Korrosion and Korrosionsschutx, Institut fur Metallkunde und Metallphysik, Technische Universitiit Clausthal, D-38678 Clausthal-Z, Germany (Received
16 March 1994)
Abstract-Steady state polarization of Cr, Mn and MO were measured in liquid ammonia with NH,NO, and NH,CI at 0°C and 20°C. Cr and MO can corrode in the active and passive states. Mn remains active, but a porous film is deposited after prolonged dissolution. From the Tafel slopes it is concluded that the metals are oxidized to divalent ions during active dissolution.
Key words: liquid ammonia, electrochemical kinetics, corrosion, chromium, manganese, molybdenum. INTRODUCTION
In early studies, the electrochemistry of transition metals in liquid ammonia[1,2] usually was investigated only in terms of current efficiencies for the cathodic deposition or the anodic dissolution, but electrode potentials were not measured. Sometimes the results were contradictory, probably because concentrations of water in the liquid ammonia electrolytes were unknown or the possibility of passivation was neglected. The current efficiency for electrodeposition of manganese was found to be high, but low for electrodeposition of chromium[3]. Later[4,5], no electrodeposition of chromium could complex except for the observed be [Cr(NH,),(H,0)]2’. It was agreed[3,4] that mofybdenum is not electrodenosited. There was evidence of anodic corrosion of mdlybdenum[3], manganese and chromium[4], but details were not reported. Studies of the electrode kinetics in liquid ammonia free of water have shown that iron[6,7] can be passivated, while nickel[8] only dissolves in the active state. Both metals are electrodeposited readily. Apart from nickel, the metals chromium, molybdenum and manganese are commonly used as alloying elements in steels and may influence their corrosion behaviour in liquid ammonia[9]. Therefore, the electrochemial behaviour of these metals was investigated. EXPERIMENTAL
A stainless steel autoclave[8] or a glass cell were used for the experiments. Measurements with the autoclave were performed at 293 K and with the glass cell at 273 K to keep the pressure below 4 bar. * Author to whom correspondence should be addressed.
Working electrodes of chromium were electrodeposited on Pt wires after dissolution of the metal into liquid ammonia from a block of pure chromium. The chromium block was also used directly, although its area was difficult to estimate. The nickel electrode with 99.9% Ni was a wire of 0.56mm diameter, the molybdenum electrode with 99.95% MO a wire of 0.7 mm diameter. A sheet of electrolytic manganese 0.5mm thick was the manganese working electrode. It was impossible to establish contact with platinum by spot welding. Therefore, the electrode was fastened to Pt wire through a hole with 1 mm diameter which was made using a YAGlaser. The contact areas between platinum and the metal electrode were insulated by Parafilm (American Can Company) being resistant to ammonia. The pressure-tight seals of the electrodes were made as described earlier[lO]. Tl/TlCl in 0.1 M NH,Cl or Ni/NiBr, in 0.1 M NH,Br were used as reference electrodes. For experiments of long duration the Tl/TlCl electrode is not recommended because of the danger to contaminate the electrolyte with the slightly soluble TlCl from which Tl can be deposited. Therefore, a new reference electrode was employed, the Ni/NiBr, electrode. The solubility of NiBr, in liquid ammonia is much lower than the solubility of TlCI. The potential of the Ni/NiBr, electrode vs. the Tl/TlCl-electrode is -0.4V. With the formal standard potential E’(Ni/ Ni’+) = 6mV[8] follows a solubility product c(Ni’+)c(Br-)’ z lo-r6 M3. The reference electrodes were placed in separate glass cells of 3cm3 volume connected to the main cell by a Haber-Luggin capillary. The counter electrode was a platinum sheet of 0.25 cm2 area. The salts used as the electrolytes were dried for 24h at temperatures between 150 and 180°C and transferred immediately into the cell after cooling and weighing the desired amounts. The cell was
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to about 4Pa for 2 h. The remaining oxygen was displaced by admitting pure nitrogen (99.99% N,) to the cell and evacuating it again. Chromium salt was introduced into the electrolyte containing 1 M ammonium nitrate by anodic dissolution. Polarization curves were measured with a potentiostat (Jaissle 1001 TNC). The electrode potential was controlled by a programmable generator (Prodis l/14). Transients were recorded with a digital oscilloscope (N&let 3091) or a digital signal analyzer (Iwatsu SM 2100B). Charges were obtained by integration of current transients. evacuated
Manganese The quasi steady state polarization curve in Fig. 2 for manganese in liquid ammonia with 0.1 M ammonium nitrate at 293 K was measured changing the electrode potential by 20mV every minute starting from the free corrosion potential. The manganese dissolved only in the active state. At the free corrosion potential E x -0.83 V vs. the Ni/NiBr, electrode, manganese corroded at a rate of z 1 ~Acm-*. An anodic Tafel slope of x 6OmV dec- ’ was observed as expected for transfer of Mn(I1) in one step. During anodic polarization a porous insulating layer grew on the electrode resulting in an increase of resistance polarization with time.
RESULTS AND DISCUSSION Molybdenum
Chromium
Figure 1 shows a steady-state polarization curve of chromium in 1 M ammonium nitrate and 20.01 M chromium nitrate. Active chromium dissolved close to O.OV vs. the Tl/llCl. In the vicinity of the rest potential the currents were mainly determined by dissolution and deposition of chromium. The anodic Tafel slope was a40mV dec- ‘, the cathodic slope x 150mVdec- ’ after correction for the limiting current density x0.6mAcmW2 of chromium deposition. These results are compatible with the dissolution and deposition of Cr(I1) via a consecutive mechanism with Cr(I) as the intermediate. At negative potentials the cathodic current is mainly due to hydrogen evolution with a Tafel slope of the polarization curve. The inter=90mVdec-’ section of the Tafel lines for hydrogen evolution and chromium dissolution yielded a corrosion current density of the order 0.1 PA cm-*. The critical passivation current density x 1 mAcm_’ was attained at E z 0.1 V. The corrosion current density in the passive state decreased to a few ~Acmat potentials 0.1 < E/V c 0.3 and rose to a constant value x lOOpAcm_* at 0.4 < EfV < 0.8. At potentials >0.8V, there was evolution of nitrogen in addition to corrosion of chromium. Assuming the corrosion rate to remain independent of potential, a Tafel slope of z 190 mV dec- ’ was obtained for the evolution of nitrogen. In an electrolyte containing ammonium chloride, chromium dissolved only in the passive state.
Figure 3 shows the steady state polarization curve for molybdenum in liquid ammonia containing 1 M ammonium chloride. At potentials positive to the free corrosion potential E z 0.4V vs. Ni/NiBr, in 0.1 M NH,Br molybdenum dissolved in the active state the Tafel slope being 65 mVdec_ ’ probably due to formation of Mo(I1) in one step. The corrosion rate was determined by the presence of traces of oxidants. The cathodic Tafel line for the deposition of hydrogen had a slope of about 1OOmVdec- ‘. The intersection with the Tafel lines for the active dissolution of molybdenum yielded a corrosion rate of about 0.25 nAcn-*. Beyond a critical passivation current density 0.2 mA cm-* at E z 0.6V, molyb-
-2 -1.8-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.2 0 0.2 EIV
Fig. 2. Current densitiesj at manganese in liquid ammonia with 0.1 M NH,NO, at 293 K as a function of the electrode potential E vs. Ni/NiBr, in 0.1 M NH,Br.
M 0 a
-5
-5 -6
-0.6 -0.4 -0.2 0
0.2 0.4 0.6 0.8 1.0
-71
’
1 ’
-0.6-0.4-0.2 0 EN
Fig. 1. Steady-state current densities j at chromium in liquid ammonia with 1 M NH,NO, and 20.01 M chromium nitrate at 273 K as a function of the electrode potential E vs. Tl/TICI in 0.1 M NH&I.
ITI
11
11
0.2 0.4 0.6 0.8 1.0 1.2 1.4
EN Fig. 3. Steady-state current densities j at molybdenum in liquid ammonia with 1 M NH&I at 293 K as a function of the electrode potential E vs. Ni/NiBr, in 0.1 M NH,Br.
Short communication
denum became passive and the current density dropped to ~30~Acm-~. At very positive potentials in the region of nitrogen evolution the Tafel slope approached z 120mVdec-‘. Acknowledgement-This work was supported by the German corrosion research program FEKKs under FKZ llA3095.
REFERENCES 1. 0. R. Brown, in Electrochemistry, Vol. 4, p. 55. The Chemical Society, London (1974). 2. K. E. Heusler, in Korrosion und Korrosionsschutz, (Edited by E. Kunze). Walter de Gruyter & Co., in print.
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3. H. S. Booth and M. Merlub-Sobel, J. phys. Chem. 35, 3303 (1931). 4. G. W. Watt and J. W. Vaughn, J. electrochem. Sot. 110, 723 (1963). 5. J. B. Gill, M. Hall and D. Macintosh, Electrochim. Acta 32, 589 (1987). 6. M. Ahrens, W. Schmitt and K. E. Heusler, Werkstoffe Kerr. 32, 167 (1981). 7. M. Ahrens and K. E. Heusler, Electrochim. Acta 27,239 (1982). 8. S. Kutzmutz and K. E. Heusler, J. electroanal. Chem. 235,93 (1990). 9. D. Huerta and K. E. Heusler, Werkstoffe Kerr., in print. 10. M. Ahrens, Korrosion des Eisens injiissigem Ammoniak, thesis, Clausthal (1979).
Pergamoa
Acra Vol. 39, No. 18.9p. 2795-2797.1994 Copyright Q 1994 Elsevier Scimce Ltd. Printed in Britain. All righta rcrcrved 0013~4686p4 s7.00 + 0.00
Grut
SHORT COMMUNICATION
ELECTROCHEMICAL BEHAVIOUR OF CHROMIUM, MOLYBDENUM AND MANGANESE IN LIQUID AMMONIA D. HUEFITA, AI ZHOUPING,PING
Luo and K. E. HEUSLER*
Abteilung Korrosion and Korrosionsschutx, Institut fur Metallkunde und Metallphysik, Technische Universitiit Clausthal, D-38678 Clausthal-Z, Germany (Received
16 March 1994)
Abstract-Steady state polarization of Cr, Mn and MO were measured in liquid ammonia with NH,NO, and NH,CI at 0°C and 20°C. Cr and MO can corrode in the active and passive states. Mn remains active, but a porous film is deposited after prolonged dissolution. From the Tafel slopes it is concluded that the metals are oxidized to divalent ions during active dissolution.
Key words: liquid ammonia, electrochemical kinetics, corrosion, chromium, manganese, molybdenum. INTRODUCTION
In early studies, the electrochemistry of transition metals in liquid ammonia[1,2] usually was investigated only in terms of current efficiencies for the cathodic deposition or the anodic dissolution, but electrode potentials were not measured. Sometimes the results were contradictory, probably because concentrations of water in the liquid ammonia electrolytes were unknown or the possibility of passivation was neglected. The current efficiency for electrodeposition of manganese was found to be high, but low for electrodeposition of chromium[3]. Later[4,5], no electrodeposition of chromium could complex except for the observed be [Cr(NH,),(H,0)]2’. It was agreed[3,4] that mofybdenum is not electrodenosited. There was evidence of anodic corrosion of mdlybdenum[3], manganese and chromium[4], but details were not reported. Studies of the electrode kinetics in liquid ammonia free of water have shown that iron[6,7] can be passivated, while nickel[8] only dissolves in the active state. Both metals are electrodeposited readily. Apart from nickel, the metals chromium, molybdenum and manganese are commonly used as alloying elements in steels and may influence their corrosion behaviour in liquid ammonia[9]. Therefore, the electrochemial behaviour of these metals was investigated. EXPERIMENTAL
A stainless steel autoclave[8] or a glass cell were used for the experiments. Measurements with the autoclave were performed at 293 K and with the glass cell at 273 K to keep the pressure below 4 bar. * Author to whom correspondence should be addressed.
Working electrodes of chromium were electrodeposited on Pt wires after dissolution of the metal into liquid ammonia from a block of pure chromium. The chromium block was also used directly, although its area was difficult to estimate. The nickel electrode with 99.9% Ni was a wire of 0.56mm diameter, the molybdenum electrode with 99.95% MO a wire of 0.7 mm diameter. A sheet of electrolytic manganese 0.5mm thick was the manganese working electrode. It was impossible to establish contact with platinum by spot welding. Therefore, the electrode was fastened to Pt wire through a hole with 1 mm diameter which was made using a YAGlaser. The contact areas between platinum and the metal electrode were insulated by Parafilm (American Can Company) being resistant to ammonia. The pressure-tight seals of the electrodes were made as described earlier[lO]. Tl/TlCl in 0.1 M NH,Cl or Ni/NiBr, in 0.1 M NH,Br were used as reference electrodes. For experiments of long duration the Tl/TlCl electrode is not recommended because of the danger to contaminate the electrolyte with the slightly soluble TlCl from which Tl can be deposited. Therefore, a new reference electrode was employed, the Ni/NiBr, electrode. The solubility of NiBr, in liquid ammonia is much lower than the solubility of TlCI. The potential of the Ni/NiBr, electrode vs. the Tl/TlCl-electrode is -0.4V. With the formal standard potential E’(Ni/ Ni’+) = 6mV[8] follows a solubility product c(Ni’+)c(Br-)’ z lo-r6 M3. The reference electrodes were placed in separate glass cells of 3cm3 volume connected to the main cell by a Haber-Luggin capillary. The counter electrode was a platinum sheet of 0.25 cm2 area. The salts used as the electrolytes were dried for 24h at temperatures between 150 and 180°C and transferred immediately into the cell after cooling and weighing the desired amounts. The cell was
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to about 4Pa for 2 h. The remaining oxygen was displaced by admitting pure nitrogen (99.99% N,) to the cell and evacuating it again. Chromium salt was introduced into the electrolyte containing 1 M ammonium nitrate by anodic dissolution. Polarization curves were measured with a potentiostat (Jaissle 1001 TNC). The electrode potential was controlled by a programmable generator (Prodis l/14). Transients were recorded with a digital oscilloscope (N&let 3091) or a digital signal analyzer (Iwatsu SM 2100B). Charges were obtained by integration of current transients. evacuated
Manganese The quasi steady state polarization curve in Fig. 2 for manganese in liquid ammonia with 0.1 M ammonium nitrate at 293 K was measured changing the electrode potential by 20mV every minute starting from the free corrosion potential. The manganese dissolved only in the active state. At the free corrosion potential E x -0.83 V vs. the Ni/NiBr, electrode, manganese corroded at a rate of z 1 ~Acm-*. An anodic Tafel slope of x 6OmV dec- ’ was observed as expected for transfer of Mn(I1) in one step. During anodic polarization a porous insulating layer grew on the electrode resulting in an increase of resistance polarization with time.
RESULTS AND DISCUSSION Molybdenum
Chromium
Figure 1 shows a steady-state polarization curve of chromium in 1 M ammonium nitrate and 20.01 M chromium nitrate. Active chromium dissolved close to O.OV vs. the Tl/llCl. In the vicinity of the rest potential the currents were mainly determined by dissolution and deposition of chromium. The anodic Tafel slope was a40mV dec- ‘, the cathodic slope x 150mVdec- ’ after correction for the limiting current density x0.6mAcmW2 of chromium deposition. These results are compatible with the dissolution and deposition of Cr(I1) via a consecutive mechanism with Cr(I) as the intermediate. At negative potentials the cathodic current is mainly due to hydrogen evolution with a Tafel slope of the polarization curve. The inter=90mVdec-’ section of the Tafel lines for hydrogen evolution and chromium dissolution yielded a corrosion current density of the order 0.1 PA cm-*. The critical passivation current density x 1 mAcm_’ was attained at E z 0.1 V. The corrosion current density in the passive state decreased to a few ~Acmat potentials 0.1 < E/V c 0.3 and rose to a constant value x lOOpAcm_* at 0.4 < EfV < 0.8. At potentials >0.8V, there was evolution of nitrogen in addition to corrosion of chromium. Assuming the corrosion rate to remain independent of potential, a Tafel slope of z 190 mV dec- ’ was obtained for the evolution of nitrogen. In an electrolyte containing ammonium chloride, chromium dissolved only in the passive state.
Figure 3 shows the steady state polarization curve for molybdenum in liquid ammonia containing 1 M ammonium chloride. At potentials positive to the free corrosion potential E z 0.4V vs. Ni/NiBr, in 0.1 M NH,Br molybdenum dissolved in the active state the Tafel slope being 65 mVdec_ ’ probably due to formation of Mo(I1) in one step. The corrosion rate was determined by the presence of traces of oxidants. The cathodic Tafel line for the deposition of hydrogen had a slope of about 1OOmVdec- ‘. The intersection with the Tafel lines for the active dissolution of molybdenum yielded a corrosion rate of about 0.25 nAcn-*. Beyond a critical passivation current density 0.2 mA cm-* at E z 0.6V, molyb-
-2 -1.8-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.2 0 0.2 EIV
Fig. 2. Current densitiesj at manganese in liquid ammonia with 0.1 M NH,NO, at 293 K as a function of the electrode potential E vs. Ni/NiBr, in 0.1 M NH,Br.
M 0 a
-5
-5 -6
-0.6 -0.4 -0.2 0
0.2 0.4 0.6 0.8 1.0
-71
’
1 ’
-0.6-0.4-0.2 0 EN
Fig. 1. Steady-state current densities j at chromium in liquid ammonia with 1 M NH,NO, and 20.01 M chromium nitrate at 273 K as a function of the electrode potential E vs. Tl/TICI in 0.1 M NH&I.
ITI
11
11
0.2 0.4 0.6 0.8 1.0 1.2 1.4
EN Fig. 3. Steady-state current densities j at molybdenum in liquid ammonia with 1 M NH&I at 293 K as a function of the electrode potential E vs. Ni/NiBr, in 0.1 M NH,Br.
Short communication
denum became passive and the current density dropped to ~30~Acm-~. At very positive potentials in the region of nitrogen evolution the Tafel slope approached z 120mVdec-‘. Acknowledgement-This work was supported by the German corrosion research program FEKKs under FKZ llA3095.
REFERENCES 1. 0. R. Brown, in Electrochemistry, Vol. 4, p. 55. The Chemical Society, London (1974). 2. K. E. Heusler, in Korrosion und Korrosionsschutz, (Edited by E. Kunze). Walter de Gruyter & Co., in print.
2197
3. H. S. Booth and M. Merlub-Sobel, J. phys. Chem. 35, 3303 (1931). 4. G. W. Watt and J. W. Vaughn, J. electrochem. Sot. 110, 723 (1963). 5. J. B. Gill, M. Hall and D. Macintosh, Electrochim. Acta 32, 589 (1987). 6. M. Ahrens, W. Schmitt and K. E. Heusler, Werkstoffe Kerr. 32, 167 (1981). 7. M. Ahrens and K. E. Heusler, Electrochim. Acta 27,239 (1982). 8. S. Kutzmutz and K. E. Heusler, J. electroanal. Chem. 235,93 (1990). 9. D. Huerta and K. E. Heusler, Werkstoffe Kerr., in print. 10. M. Ahrens, Korrosion des Eisens injiissigem Ammoniak, thesis, Clausthal (1979).