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Electrodeposition of Zn–Ni protective coatings from sulfate–acetate baths
Surface and Coatings Technology 151 – 152 (2002) 444–448
Electrodeposition of Zn–Ni protective coatings from sulfate–acetate baths E. Beltowska-Lehmana,*, P. Ozgaa, Z. Swiateka, C. Lupib a
Polish Academy of Sciences, Institute of Metallurgy and Materials Science, 25 Reymonta Str., 30-059 Cracow, Poland b Department of Chemical Engineering, University ‘La Sapienza’, via Eudossiana 18, 00184 Rome, Italy
Abstract The electrodeposition conditions for Zn – Ni alloys from sulfate – acetate electrolytes have been studied with the view of preparing protective coatings. The influence of electrolyte composition (different wNi(II)x y wZn(II)x ratios, pH, buffer), cathode current density, cathode potential and hydrodynamic conditions on the composition of coatings, cathode current efficiency and corrosion resistance were determined. For all of the conditions examined, strong inhibition of nickel reduction with simultaneous increase in the rate of zinc discharge characteristic of an anomalous system, have been observed. The Zn(II) discharge becomes diffusion-controlled at more negative cathode potentials, whereas the partial nickel current densities are independent of electrode rotation speed. Consequently, nickel content and current efficiency are reduced with decreasing thickness of the diffusive layer. An increase in pH above 3.3 causes a significant catalysis of Zn – Ni deposition with a simultaneous decrease of the nickel in coatings. This effect may be related to the formation and increasing concentration of Zn(II) and Ni(II) acetate complexes in this condition. The Zn – Ni coatings obtained (5 – 18% Ni) characterise improved corrosion resistance in comparison to Zn layers deposited under the same conditions. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrodeposition; Zn – Ni coatings; Sulfate – acetate electrolytes
1. Introduction Electrodeposited coatings of zinc alloys, in particular Zn–Ni, are currently the subject of many studies. They mainly concern sulfate or chloride baths w1–5x. These coatings are regarded as replacements for electrodeposited Cd in anticorrosion applications. Zn–Ni layers provide good protection for steel, and the maximum protective ability is reached with a Ni content up to 14%, which depends on bath composition and operating conditions. In the present work, the effect of the Zn–Ni sulfate–acetate electrolyte parameters on the deposition ratios and partial polarisation behaviours were investigated under potentiostatic conditions using an rotating disk electrode (RDE) system. The relationship between the Zn–Ni electrodeposited microstructure and corrosion behaviour in an aqueous chloride solution has also been determined. * Corresponding author. Tel.: q48-12-6374200; fax: q48-126372192. E-mail address: [email protected] (E. BeltowskaLehman).
2. Experimental procedure The chemical composition of the electrolytes used is given in Table 1. The wNix y wZnx concentration ratio was investigated in the range of 0.11–4.0 for a constant wNixqwZnxs0.5 M. The bath pH was varied between 2 and 5.2. The electrolysis was carried out in a 500-ml cell with a RDE, supplied by a PARA 273A potentiostat. The cathode was a low carbon steel disk (0.071 and 2.63 cm2) rotating at 11–68 radys. A Pt sheet (5 cm2) was used as the anode. The cathode potentials were referred to a saturated calomel electrode (SCE) and were corrected for ohmic drop (CI method). The chemical composition and mass of the deposits were determined by electron dispersive spectroscopy (EDS) analysis using a LINK-ISIS apparatus (Oxford Instruments). The surface morphology of the deposits was evaluated by scanning microscopy on a SEM Philips XL-30. X-Ray diffractometry, using the CoKa line (diffractometer Philips PW 1710), was used to identify the phase composition in the deposits. The corrosion behaviour of the coatings (Icor — corrosion current, Rp — polarisation resistance, Ecor — corrosion potential) was evaluated in 5% sodium chloride solutions at pH 3,
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 6 1 4 - 0
E. Beltowska-Lehman et al. / Surface and Coatings Technology 151 – 152 (2002) 444–448
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Table 1 Chemical composition of the electrolytes No.
Electrolyte compositionq0.2 M CH3COONaq0.1 M (NH4)2SO4
wNix y wZnx
pH
1 2 3 4 5 6 7 8 9 10 11 12
0.05 M NiSO4, 0.45 M ZnSO4 0.1 M NiSO4, 0.4 M ZnSO4 0.2 M NiSO4, 0.3 M ZnSO4 0.225 M NiSO4, 0.275 M ZnSO4 0.25 M NiSO4, 0.25 M ZnSO4 0.275 M NiSO4, 0.225 M ZnSO4 0.3 M NiSO4, 0.2 M ZnSO4 0.325 M NiSO4, 0.175 M ZnSO4 0.35 M NiSO4, 0.15 M ZnSO4 0.4 M NiSO4, 0.1 M ZnSO4 0.2 M ZnSO4, 0.3 M Na2SO4 0.3 M NiSO4, 0.2 M Na2SO4
0.11 0.25 0.67 0.82 1.0 1.22 1.5 1.86 2.33 4.0 – –
4.3 4.3 4.3 4.3 4.3 4.3 2.0 – 5.2 4.3 4.3 4.3 4.3 4.3
using the polarisation resistance technique (perturbation "30 mV around an open-circuit potential) w6x. 3. Results and discussion Fig. 1 shows potentiodynamic polarisation curves recorded for different composition electrolytes. It can be observed that an increase in the wNi(II)x y wZn(II)x concentration ratio causes progressive inhibition of alloy deposition. Simultaneously the nickel content in the deposits increases from 3 to 16.8% at increasing wNi(II)x y wZn(II)x concentration ratios from 0.11 to 4.0, for a current density (d.c.) 7 Aydm2, which also results in significant changes in deposit morphology (Fig. 8). Fig. 2 presents steady-state partial polarisation curves for Zn(II) and Ni(II) separate discharges and for their co-deposition. As can be seen over the whole polarisa-
Fig. 1. Potentiodynamic polarisation curves recorded in electrolytes of different compositions (electrolytes: 1, 3, 5, 8, 9, 10).
Fig. 2. Steady-state partial polarisation curves for nickel, zinc and hydrogen in separate Ni(II) and Zn(II) discharges and for their codeposition (electrolytes: 7, 11, 12).
tion range, nickel deposition is strongly inhibited compared to its separate discharge, whereas zinc deposition is slightly stimulated at the initial stage of the alloy electrodeposition and then undergoes inhibition. Under the whole of the conditions examined, Zn–Ni codeposition shows an anomalous behaviour with zinc being the preferentially deposited metal. As it is clearly seen from the dependencies in Fig. 3, the Zn(II) diffusion-controlled discharge is obtained at
Fig. 3. Partial current densities as a function of the square root of RDE speed.
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E. Beltowska-Lehman et al. / Surface and Coatings Technology 151 – 152 (2002) 444–448
Fig. 4. Effect of RDE rotation speed on nickel content of Zn – Ni coatings and on current efficiency. Fig. 5. Potentiodynamic polarisation curves recorded in electrolyte no. 7, at different pH and constant ratio wNi(II)xywZn(II)xs1.5.
higher cathodic polarisation where a linear dependence of the partial current as a function of the square root of the electrode rotation speed is observed. On the contrary, activation-controlled discharges of Ni(II) and H2 are obtained in the same conditions. Consequently, nickel content and current efficiency are reduced with decreasing thickness of the diffusive layer (Fig. 4). A strong influence of pH on Zn–Ni electrodeposition kinetics has been observed. Fig. 5 shows the potentiodynamic polarisation curves recorded in electrolytes of different pH. Fig. 6 presents the partial current density dependence on the pH. It can be seen that an increase in pH (especially above 3.3) causes a significant catalysis of Zn–Ni coating deposition, in particular the Zn(II) discharge. The pH effect on the deposition of Zn–Ni is related to the nature and concentration of the complexed species. In the range of pH above 3.3 the acetate complexes of Zn(II) and Ni(II) are formed and their concentration increases with further alkalinity of the solution w7x. This results on the deposit composition as clearly shown in Fig. 7. Moreover, the nickel concentration in the deposit decreases with increasing cathodic polarisation and its minimum occurs at y1.1 to y1.12 VySCE. Further increase of the cathodic polarisation does not influence the Ni content in practice, due to the polarisation behaviour (see Fig. 2). The deposits (7 Aydm2, 68 radys) with low nickel percentage (-8%, Fig. 8a) present platelet hexagonal morphology. For the deposits with more nickel (12– 16%), the morphology of the grains becomes polyhedral (Fig. 8c). In the intermediate nickel content range (8– 12%) elongated plates have been observed (Fig. 8b). The phase composition of Zn–Ni alloys depends mainly on the nickel content. The coatings deposited
under the conditions corresponding to point a of the curve in Fig. 9 consist of a mixture of two phases: the h-phase (a solid solution of nickel in zinc at low Ni content) and the d-phase (Ni3Zn22 intermetallic compound), whereas the d-phase predominates in deposits according to point b. The deposits with the highest Ni content (point c) consist mainly of the g-phase with composition Ni5Zn21. The corrosion potential increases progressively (in the range from approx. y1070 mV up to y1030 mVy SCE) with the Ni content increasing up to approximately
Fig. 6. Dependence of the partial current densities of nickel, zinc and hydrogen on the pH (electrolyte no. 7, wNi(II)ywZn(II)xs1.5).
E. Beltowska-Lehman et al. / Surface and Coatings Technology 151 – 152 (2002) 444–448
447
Unlike the corrosion potential changes observed for alloys deposited from chloride solutions w3x, the Ecor determined for coatings electrodeposited from sulfate– acetate solutions shift towards a higher Ni content. The effect can be explained by the presence of a d-phase in deposits obtained from sulfate–acetate baths. This phase was also identified in coatings deposited from other sulfate solutions w1,5x. 4. Conclusions
Fig. 7. Ni content in coatings deposited from electrolyte no. 7 with different pH as a function of the cathode potential.
13%, while a considerable effect has been observed above that value (Fig. 9). A similar behaviour was reported in Bories and co-workers w3,4x where the authors explained it by a transition of the deposits with Zn in a hexagonal phase going into a cubic g-phase.
It has been observed that the addition of acetate sufficiently stabilises sulfate electrolytes for Zn–Ni electrodeposition in the pH 2–5.2 range. Zn–Ni codeposition shows an anomalous behaviour, i.e. the less noble zinc is deposited preferentially with strong inhibition of Ni(II) discharge. An increase of the wNi(II)y wZn(II)x concentration ratio results in a progressive inhibition of the Zn–Ni deposition process and in an increased nickel content in the deposits. On the contrary, a pH increase has an opposite effect due to formation of Zn(II) and Ni(II) acetate complexes. The corrosion rate of the deposits diminishes as the Ni content increases. A shift of the range of the Ecor towards higher Ni content might be connected to the presence of a d-phase in the deposits. An abrupt change of the corrosion
Fig. 8. SEM images of Zn – Ni coatings with different compositions (a, 5% Ni; b, 12% Ni; c, 15% Ni).
Fig. 9. Effect of nickel content in Zn – Ni coatings on corrosion potential and corrosion current.
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E. Beltowska-Lehman et al. / Surface and Coatings Technology 151 – 152 (2002) 444–448
potential for Zn–Ni deposits with Ni content above 13% might be explained by the cubic g-phase prevailing in these conditions. References w1x F.J. Fabri Miranda, O.E. Barcia, O.R. Mattos, R. Wiart, J. Electrochem. Soc. 144 (1997) 3441–3457. w2x Y.S. Jin, T.Y. Kim, K.Y. Kim, Surf. Coat. Technol. 106 (1998) 220–227.
w3x C. Bories, J.P. Bonino, A. Rousset, J. Appl. Electrochem. 29 (1999) 1045–1051. w4x K.R. Baldwin, M.J. Robinson, C.J.E. Smith, Corros. Sci. 36 (1994) 1515–1531. w5x K. Nagaraja Rao, M.I.A. Siddigi, C.V. Suryanarayana, Electrochem. Acta 10 (1965) 557–563. w6x F. Mansfeld, in: M.G. Fontana, R.W. Staehle ŽEds.., Advances in Corrosion Sciences and Technology, vol. 6, Plenum Press, New York, 1976, pp. 163–262. w7x L.G. Sillen, A.E. Martel, in: The Chemical Society (Eds.), Stability Constants of Metal-Ion Complexes, London, 1964, pp. 250 – 254.
Electrodeposition of Zn–Ni protective coatings from sulfate–acetate baths E. Beltowska-Lehmana,*, P. Ozgaa, Z. Swiateka, C. Lupib a
Polish Academy of Sciences, Institute of Metallurgy and Materials Science, 25 Reymonta Str., 30-059 Cracow, Poland b Department of Chemical Engineering, University ‘La Sapienza’, via Eudossiana 18, 00184 Rome, Italy
Abstract The electrodeposition conditions for Zn – Ni alloys from sulfate – acetate electrolytes have been studied with the view of preparing protective coatings. The influence of electrolyte composition (different wNi(II)x y wZn(II)x ratios, pH, buffer), cathode current density, cathode potential and hydrodynamic conditions on the composition of coatings, cathode current efficiency and corrosion resistance were determined. For all of the conditions examined, strong inhibition of nickel reduction with simultaneous increase in the rate of zinc discharge characteristic of an anomalous system, have been observed. The Zn(II) discharge becomes diffusion-controlled at more negative cathode potentials, whereas the partial nickel current densities are independent of electrode rotation speed. Consequently, nickel content and current efficiency are reduced with decreasing thickness of the diffusive layer. An increase in pH above 3.3 causes a significant catalysis of Zn – Ni deposition with a simultaneous decrease of the nickel in coatings. This effect may be related to the formation and increasing concentration of Zn(II) and Ni(II) acetate complexes in this condition. The Zn – Ni coatings obtained (5 – 18% Ni) characterise improved corrosion resistance in comparison to Zn layers deposited under the same conditions. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrodeposition; Zn – Ni coatings; Sulfate – acetate electrolytes
1. Introduction Electrodeposited coatings of zinc alloys, in particular Zn–Ni, are currently the subject of many studies. They mainly concern sulfate or chloride baths w1–5x. These coatings are regarded as replacements for electrodeposited Cd in anticorrosion applications. Zn–Ni layers provide good protection for steel, and the maximum protective ability is reached with a Ni content up to 14%, which depends on bath composition and operating conditions. In the present work, the effect of the Zn–Ni sulfate–acetate electrolyte parameters on the deposition ratios and partial polarisation behaviours were investigated under potentiostatic conditions using an rotating disk electrode (RDE) system. The relationship between the Zn–Ni electrodeposited microstructure and corrosion behaviour in an aqueous chloride solution has also been determined. * Corresponding author. Tel.: q48-12-6374200; fax: q48-126372192. E-mail address: [email protected] (E. BeltowskaLehman).
2. Experimental procedure The chemical composition of the electrolytes used is given in Table 1. The wNix y wZnx concentration ratio was investigated in the range of 0.11–4.0 for a constant wNixqwZnxs0.5 M. The bath pH was varied between 2 and 5.2. The electrolysis was carried out in a 500-ml cell with a RDE, supplied by a PARA 273A potentiostat. The cathode was a low carbon steel disk (0.071 and 2.63 cm2) rotating at 11–68 radys. A Pt sheet (5 cm2) was used as the anode. The cathode potentials were referred to a saturated calomel electrode (SCE) and were corrected for ohmic drop (CI method). The chemical composition and mass of the deposits were determined by electron dispersive spectroscopy (EDS) analysis using a LINK-ISIS apparatus (Oxford Instruments). The surface morphology of the deposits was evaluated by scanning microscopy on a SEM Philips XL-30. X-Ray diffractometry, using the CoKa line (diffractometer Philips PW 1710), was used to identify the phase composition in the deposits. The corrosion behaviour of the coatings (Icor — corrosion current, Rp — polarisation resistance, Ecor — corrosion potential) was evaluated in 5% sodium chloride solutions at pH 3,
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 6 1 4 - 0
E. Beltowska-Lehman et al. / Surface and Coatings Technology 151 – 152 (2002) 444–448
445
Table 1 Chemical composition of the electrolytes No.
Electrolyte compositionq0.2 M CH3COONaq0.1 M (NH4)2SO4
wNix y wZnx
pH
1 2 3 4 5 6 7 8 9 10 11 12
0.05 M NiSO4, 0.45 M ZnSO4 0.1 M NiSO4, 0.4 M ZnSO4 0.2 M NiSO4, 0.3 M ZnSO4 0.225 M NiSO4, 0.275 M ZnSO4 0.25 M NiSO4, 0.25 M ZnSO4 0.275 M NiSO4, 0.225 M ZnSO4 0.3 M NiSO4, 0.2 M ZnSO4 0.325 M NiSO4, 0.175 M ZnSO4 0.35 M NiSO4, 0.15 M ZnSO4 0.4 M NiSO4, 0.1 M ZnSO4 0.2 M ZnSO4, 0.3 M Na2SO4 0.3 M NiSO4, 0.2 M Na2SO4
0.11 0.25 0.67 0.82 1.0 1.22 1.5 1.86 2.33 4.0 – –
4.3 4.3 4.3 4.3 4.3 4.3 2.0 – 5.2 4.3 4.3 4.3 4.3 4.3
using the polarisation resistance technique (perturbation "30 mV around an open-circuit potential) w6x. 3. Results and discussion Fig. 1 shows potentiodynamic polarisation curves recorded for different composition electrolytes. It can be observed that an increase in the wNi(II)x y wZn(II)x concentration ratio causes progressive inhibition of alloy deposition. Simultaneously the nickel content in the deposits increases from 3 to 16.8% at increasing wNi(II)x y wZn(II)x concentration ratios from 0.11 to 4.0, for a current density (d.c.) 7 Aydm2, which also results in significant changes in deposit morphology (Fig. 8). Fig. 2 presents steady-state partial polarisation curves for Zn(II) and Ni(II) separate discharges and for their co-deposition. As can be seen over the whole polarisa-
Fig. 1. Potentiodynamic polarisation curves recorded in electrolytes of different compositions (electrolytes: 1, 3, 5, 8, 9, 10).
Fig. 2. Steady-state partial polarisation curves for nickel, zinc and hydrogen in separate Ni(II) and Zn(II) discharges and for their codeposition (electrolytes: 7, 11, 12).
tion range, nickel deposition is strongly inhibited compared to its separate discharge, whereas zinc deposition is slightly stimulated at the initial stage of the alloy electrodeposition and then undergoes inhibition. Under the whole of the conditions examined, Zn–Ni codeposition shows an anomalous behaviour with zinc being the preferentially deposited metal. As it is clearly seen from the dependencies in Fig. 3, the Zn(II) diffusion-controlled discharge is obtained at
Fig. 3. Partial current densities as a function of the square root of RDE speed.
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Fig. 4. Effect of RDE rotation speed on nickel content of Zn – Ni coatings and on current efficiency. Fig. 5. Potentiodynamic polarisation curves recorded in electrolyte no. 7, at different pH and constant ratio wNi(II)xywZn(II)xs1.5.
higher cathodic polarisation where a linear dependence of the partial current as a function of the square root of the electrode rotation speed is observed. On the contrary, activation-controlled discharges of Ni(II) and H2 are obtained in the same conditions. Consequently, nickel content and current efficiency are reduced with decreasing thickness of the diffusive layer (Fig. 4). A strong influence of pH on Zn–Ni electrodeposition kinetics has been observed. Fig. 5 shows the potentiodynamic polarisation curves recorded in electrolytes of different pH. Fig. 6 presents the partial current density dependence on the pH. It can be seen that an increase in pH (especially above 3.3) causes a significant catalysis of Zn–Ni coating deposition, in particular the Zn(II) discharge. The pH effect on the deposition of Zn–Ni is related to the nature and concentration of the complexed species. In the range of pH above 3.3 the acetate complexes of Zn(II) and Ni(II) are formed and their concentration increases with further alkalinity of the solution w7x. This results on the deposit composition as clearly shown in Fig. 7. Moreover, the nickel concentration in the deposit decreases with increasing cathodic polarisation and its minimum occurs at y1.1 to y1.12 VySCE. Further increase of the cathodic polarisation does not influence the Ni content in practice, due to the polarisation behaviour (see Fig. 2). The deposits (7 Aydm2, 68 radys) with low nickel percentage (-8%, Fig. 8a) present platelet hexagonal morphology. For the deposits with more nickel (12– 16%), the morphology of the grains becomes polyhedral (Fig. 8c). In the intermediate nickel content range (8– 12%) elongated plates have been observed (Fig. 8b). The phase composition of Zn–Ni alloys depends mainly on the nickel content. The coatings deposited
under the conditions corresponding to point a of the curve in Fig. 9 consist of a mixture of two phases: the h-phase (a solid solution of nickel in zinc at low Ni content) and the d-phase (Ni3Zn22 intermetallic compound), whereas the d-phase predominates in deposits according to point b. The deposits with the highest Ni content (point c) consist mainly of the g-phase with composition Ni5Zn21. The corrosion potential increases progressively (in the range from approx. y1070 mV up to y1030 mVy SCE) with the Ni content increasing up to approximately
Fig. 6. Dependence of the partial current densities of nickel, zinc and hydrogen on the pH (electrolyte no. 7, wNi(II)ywZn(II)xs1.5).
E. Beltowska-Lehman et al. / Surface and Coatings Technology 151 – 152 (2002) 444–448
447
Unlike the corrosion potential changes observed for alloys deposited from chloride solutions w3x, the Ecor determined for coatings electrodeposited from sulfate– acetate solutions shift towards a higher Ni content. The effect can be explained by the presence of a d-phase in deposits obtained from sulfate–acetate baths. This phase was also identified in coatings deposited from other sulfate solutions w1,5x. 4. Conclusions
Fig. 7. Ni content in coatings deposited from electrolyte no. 7 with different pH as a function of the cathode potential.
13%, while a considerable effect has been observed above that value (Fig. 9). A similar behaviour was reported in Bories and co-workers w3,4x where the authors explained it by a transition of the deposits with Zn in a hexagonal phase going into a cubic g-phase.
It has been observed that the addition of acetate sufficiently stabilises sulfate electrolytes for Zn–Ni electrodeposition in the pH 2–5.2 range. Zn–Ni codeposition shows an anomalous behaviour, i.e. the less noble zinc is deposited preferentially with strong inhibition of Ni(II) discharge. An increase of the wNi(II)y wZn(II)x concentration ratio results in a progressive inhibition of the Zn–Ni deposition process and in an increased nickel content in the deposits. On the contrary, a pH increase has an opposite effect due to formation of Zn(II) and Ni(II) acetate complexes. The corrosion rate of the deposits diminishes as the Ni content increases. A shift of the range of the Ecor towards higher Ni content might be connected to the presence of a d-phase in the deposits. An abrupt change of the corrosion
Fig. 8. SEM images of Zn – Ni coatings with different compositions (a, 5% Ni; b, 12% Ni; c, 15% Ni).
Fig. 9. Effect of nickel content in Zn – Ni coatings on corrosion potential and corrosion current.
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E. Beltowska-Lehman et al. / Surface and Coatings Technology 151 – 152 (2002) 444–448
potential for Zn–Ni deposits with Ni content above 13% might be explained by the cubic g-phase prevailing in these conditions. References w1x F.J. Fabri Miranda, O.E. Barcia, O.R. Mattos, R. Wiart, J. Electrochem. Soc. 144 (1997) 3441–3457. w2x Y.S. Jin, T.Y. Kim, K.Y. Kim, Surf. Coat. Technol. 106 (1998) 220–227.
w3x C. Bories, J.P. Bonino, A. Rousset, J. Appl. Electrochem. 29 (1999) 1045–1051. w4x K.R. Baldwin, M.J. Robinson, C.J.E. Smith, Corros. Sci. 36 (1994) 1515–1531. w5x K. Nagaraja Rao, M.I.A. Siddigi, C.V. Suryanarayana, Electrochem. Acta 10 (1965) 557–563. w6x F. Mansfeld, in: M.G. Fontana, R.W. Staehle ŽEds.., Advances in Corrosion Sciences and Technology, vol. 6, Plenum Press, New York, 1976, pp. 163–262. w7x L.G. Sillen, A.E. Martel, in: The Chemical Society (Eds.), Stability Constants of Metal-Ion Complexes, London, 1964, pp. 250 – 254.