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Anodic behavior of NiP alloys studied by impedance spectroscopy
Ekctrochimica Actu, Vol. 38. No. 14, pp. 1979-1983, Printed in Great Britain.
ANODIC
1993
oow46686/93 56.00 + 0.00 e: 1993. Pergamon Prtrs Ltd.
BEHAVIOR OF Ni-P ALLOYS STUDIED IMPEDANCE SPECTROSCOPY A.
KR~LIKOWSKI
BY
and P. Bu~~tnwtcz
Institute of Solid State Technology, Department of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3,00-664 Warsaw, Poland (Received 10 March 1993) A&met-Anodic behavior of electrodeposited Ni-P alloys with P content ranging from 6 to 28 at% was investigated in chloride and sulphate neutral solutions. Measurements were performed by potentiodynamic polarization and impedance spectroscopy methods. It was found that the tested alloys exhibit comparable corrosion resistance under open circuit conditions. However, anodic polarization rises significantly the differentiation in dissolution mechanism of Ni-P. Crystalline flow P) samples show active dissolution accompanied by the formation of a greyish-black nonprotective film on the alloy surface. On the contrary, amorphous (P rich) alloys exhibit passivation. Passive properties of these alloys are quite insensitive to the presence of chloride ions.
Key words: anodic dissolution, passivation, Ni-P, structure, impedance spectroscopy, polarization curve.
INTRODUCTION
Ni-P alloys as corrosion protective coatings constitute the earliest industrial application of amorphous metals[l]. These alloys are widely used for corrosion protection in many corrosive environments: food industry, oil, gas, mining, chemical manufacturing, aerospace, etc.[2-61. However, it is surprising how inconsistent the published data are about the corrosion behavior of this material. There is still no consent as to the essential corrosion characteristics of Ni-P: the nature of its anodic dissolution, ability to passivation or susceptibility to pitting. Cpvr+.l inv~t;o.tirmr $h& _I.“* .mnmhn,n. -.1x_ . . . . ~“.a~U”“a~” chnurr.A “.a”““.+ pa” YY Ni-P does not passivate in acidic media[7-91. Kawashima et al.[lO] reported active dissolution of this alloy, followed by the formation of a porous greyish black surface layer composed mainly of nickel phosphate. Habazaki et al. suggested that active dissolution of Ni-P leads to accumulation of elemental P on the alloy surface[ll]. On the contrary, other authors found passivation of Ni-P in mprlirrri7-i4i‘TJ. Y.“P’ nip-i- I. Dt Y..L’7, nrri4i nFfinn.d n .sAA;r ..UY_ ..IWY...L’_ Y’VyvYWUY concept of chemical passivity involving the absorption of hypophosphite ions on the alloy surface. The common opinion is that Ni-P alloys with high P content exhibit a slower anodic dissolution than low P alloys[15-181. An increase in P content is, however, accompanied by the transition from the crystalline to amorphous structure of this alloy. This transition occurs over a phosphorus content range of 10-15 at%. It remains unclear as to whether the suppression of anodic dissolution of Ni-P is the result of the chemical action of alloying P itself or the unique glassy structure of P-rich alloys. To date a great deal of effort has been expended to reveal the role of the structure and content of phosphorus in comparing the anodic behavior of Ni-P with that of pure policrystalline NiC13, 14, 19, 203. However, such an
approach does not provide a clear distinction between the structural and compositional effects. This work aims to clarify the anodic dissolution behavior of Ni-P alloys in relation to their structural state and elemental composition. It was executed by means of comparative studies of Ni-P alloys with varying P content. The dissolution of electrodeposited Ni-P lilms with P content in the range corresponding to the occurrence of both crystalline and amorphous structure, was examined.
EXPERIMENTAL Series of Ni-P alloys with P content ranging from 6 to 28 at% were electrodeposited galvanostatically from sulphatehypophosphite solution[21] onto a copper foil. The 6lms were 30-4Opm in thickness. The structure of the alloys was examined by X-ray diffraction. Results of these investigations revealed that electrodeposits with less than 10% P are crystalline (fee), whereas samples with P content above 15% -vh;l.;;t nmrrmhr.... “.xl.a”.& ol*.u.yr.vuu
ntr.,m+,...n Cc.1&L U”I”lti. 1 “L
th.. ,ur tmnn:+;rr..al LIauUILL”.Idu
range between 10 and 15 at% of P, alloys of the same composition were either crystalline or amorphous. Samples showing different structural states were subjected to electrochemical measurements. Anodic behavior of these samples was studied by potentiodynamic polarization and impedance spectroscopy methods. A computer controlled Electrochemical Measurement System Atlas was used. Samples preparation and design of the electrolytic cell were similar to those described elsewhereC22-j. The electrode area exposed to the solution was O.Scm’. Measurements were performed mainly in 0.1 N NaCl solution at 20 f 2°C; 0.1 N Na,SO, was also used for comparison. Prior to and during the experiments, the solutions were deaerated by bubbling with nitrogen. The reference electrode was a
1979
1980
A. KR~LIKOWSKIand P. BUTKIEWICZ
saturated calomel electrode and all values of potential are given vs. this electrode. Anodic potentiodynamic curves were measured at potential scan of 1 mVs_‘. Before the measurements were started the samples were prepolarized cathoditally (-0.7 V) to reduce possible air-formed oxide layers. Impedance studies were employed at the corrosion potential and selected anodic potentials. The frequency range was 10 kHz to 50 mHz. Before starting measurements the samples were exposed to the solution for 0.5 h to reach a stationary state. Computer software[23] was used to fit the experimental impedance data to model and determine characteristic parameters of the system studied.
RESULTS
AND
DISCUSSION
Anodic polarization curves of Ni-P alloys in NaCl solution are shown in Fig. 1. Evidently different performances of crystalline and amorphous samples are observed. Alloys containing up to 11% P behave similarly. They are active throughout the anodic region, show high dissolution currents and no tendency to passivate. The formation of a greyish black film on the alloy surface was observed during anodic polarization. On the contrary, P rich alloys exhibit current arrest, suggesting passivation. These samples show much the same anodic behavior, irrespective of P content. The surface appearance of these alloys remained intact (lustrous) after completing the test. -___ ..__ no __ vlsiole ..1-?LI- evlaence -..:I- .__ 01 -l. pimng. _.I..~._ Au .I. samples ~~ -* I nere was dissolving actively are crystalline, whereas all samples showing the current arrest are amorphous. These findings indicate that the dissolution mechanism of Ni-P is deeply modified by the structural transition. The structural state of this alloy is of primary importance in respect to the ability to passivation. This standpoint agrees with findings of Immel[ 173. This conclusion is additionally supported by the polarization behavior of alloy with 14% P, that is from the range of composition where structural transition occurs. The individual Ni-14P samples exhibit the anodic curve midway between loo
NI-P QIN NaCl/N, ImV9
Fig. I . Anodic polarization curves of Ni-P alloys in 0.1 N NaCl solution. The numbers denote P content in the alloy in at”/.
those for crystalline and amorphous alloys. As the matter of fact, it is not an intermediate behavior but rather the result of formal averaging. These samples exhibit very scattered polarization data. They show courses of polarization curves not far from those typical of either crystalline or amorphous alloys, according to the structural state of the individual samples. It is worth noting that ihe above anodic behavior of Ni-P alloys is reasonably similar to that observed in sulphate solution. Dissolution currents and extent of the current arrest are comparable. Thus, the presence of chloride ions does not degrade the passive behavior of amorphous alloys. Similar results were found for Ni-19P in acidic media[l4]. Further information about the anodic behavior of Ni-P was drawn from impedance data. Impedance measurements were performed at the corrosion potential, and at selected anodic potentials corresponding to the current arrest on polarization curves for amorphous Ni-P. Before starting impedance measurements the samples were exposed to the solution to reach a stationary state (typically 0.5 h). It is worth noting that amorphous samples showed suppression of dissolution current, when exposed to anodic polarization. The stationary anodic currents were substantially lower than those resulting from potentiodynamic polarization curves (Fig. 1). No presence of any black film was observed for amorphous samples. They remained unattacked after test. coinc;des with nhwrv.tinn This r. .I.. cx VYYIl . %...“I_ ef Koevecses[24], who found that only low P alloys show the formation of a black film during the atmospheric corrosion test. The shape of the impedance spectra observed in anodic region depends markedly on the structure of Ni-P alloy. Results representing the amorphous alloys are demonstrated in Fig. 2. Spectra show only one resistive-capacitanc arc. Values of impedance are quite high-tens of kohm. They decrease when samples are exposed to increasing anodic polarization. Thus, amorphous alloys show quite similar impedance data taken under different conditions, suggesting similar nature of the dissolution process. A fairly diverse relation was found for crystalline alloys (Fig. 3). The spectrum taken at the corrosion potential (Fig. 3a) does not differ much from spectra for amorphous samples. It consists of one resistivecapacitance arc and the impedance values are comparable to those observed for amorphous samples. All crystalline samples are semigloss under open circuit conditions. However, when exposed to anodic polarization, the formation of the greyish black film was observed. It was accompanied by a sharp increase in dissolution current indicating nonprotective nature of this film. After a few minutes, when the surface turned completely black, the currents became stabilized at a very high level (in excess of corresponding values from potentiodynamic polarization curves). Impedance spectra measured under these conditions differ significantly from that taken at the corrosion potential. Much smaller impedance values are observed-several hundred ohms. Moreover, an additional low frequency response appears. It is poorly marked at the potential -0.15 V (Fig. 3b), but at higher anodic poten-
Anodic behavior of Ni-P alloys (a)
1981
(a)
xO.l5
5
20
60
40 2:
Z,’ kohm
kahm
(b)
(b)
-10
I
I _^
.^
p 0
I _^
I
I
5 2:
IO kahm
Fig. 2. Impedance spectra typical of amorphous alloys (Ni24P) in 0.1 N NaCl at various potentials: (a) corrosion potential, (b) - 0.15 V and (c) 0.1 V. Several values of frequeneies (in Hz) are indicated.
Fig. 3. Impedance spectra typical of erystalhne alloys (NMP) in 0.1 N Nail at various potentials: (a) corrosion potential, (b) -0.15V and (c) 0.1 V. Several values of frequencies (in Hz) are indicated.
tials it is more distinct as a second loop (Fig 3c). This phenomenon is most likely associated with the covering of the surface by the greyish black film. Very high capacitance and low resistance can be approximated for this loop, suggesting large porosity of the surface film, in agreement with its nonprotective nature. The evident change in impedance data of crystalline alloys indicates a modification in dissolution mechanism due to anodic polarization. The differentiation in anodic behaviour of crystalline and amorphous alloys is evident upon results of quantitative analysis of impedance data. Figure 4 shows a relation between charge transfer resistance and composition of the Ni-P alloy. At the corrosion potential all samples exhibit quite comparable values of charge transfer resistance, regardless of the alloy structure and composition. This observation correlates well with similar corrosion rates of crystalline and amorphous Ni-P alloys measured in neutral media[25]. However, anodic polarization leads to reduction of the charge transfer resistance. This effect
is much more pronounced for crystalline samples indicating their much faster dissolution. Changes of double layer capacitance are shown in Fig. 5. More or less similar values of capacitance are
loo
5
ECORR
/ X-\
t-J--~
IO
15
x-
20
-0.15v
25
30
%P
Fig 4. Charge transfer resistance of Ni-P in 0.1 N NaCl (at various potentials) as a function of the alloy composition.
A. KR~UKOWSKI and P. BUTKIEWICZ
observed for alloys from the transitional region where samples of crystalline and amorphous state coexist. This fact confirms again the crucial role of structural factor.
CONCLUSION
I
I
I
I
I
I
5
IO
I5
20
25
30
%P Fig. 5. Double layer capacitance of Ni-P in 0.1 N NaCl (at
various potentials) as a function of the alloy composition. observed at the corrosion potential. Amorphous alloys exhibit similar values at anodic potentials too. Under the same conditions (at anodic potentials) crystalline samples show markedly higher capacitance. A likely explanation is the coverage of the surface by the greyish black film and/or the surface roughening during intense dissolution (after removal of this film the surface was found severely roughened). Anodic polarization of crystalline alloys gives rise to an evident change in the time constant for the charge transfer process (Fig. 6), suggesting a modification of dissolution mechanism (much faster process). The plots for anodically polarized samples in Figs 4-6 show a dramatic change at the composition range corresponding to the transition from crystalline to amorphous structure of Ni-P. The impedance characteristics of amorphous Ni-P are scarcely a&cted by P content. These findings point to the primary role of the alloy structure in governing the .x...-...,:..UIJD”L”U”ll Xnn,.l..&... ..C NT:LD n1L.m Dnnh ..A,.+ A. ,Xn. UlIWIb “1 1.r1 au”,U. LI_LI p”nL u1 1 a&u 4-6 represents averaged value calculated for at least three samples. Values of impedance parameters for amorphous alloys showed standard deviations less than 25%. Somewhat bigger deviations were found for crystalline samples (especially for capacitance values). However, definitely the highest scatter was
“---=ECORR -_+-O.ISV I.0
3 c
0.1
0.01
5
IO
20
15
25
50
%P Fig. 6. Time constant for charge transfer process of NCP in 0.1 N NaCl (at various potentials) as a function of the alloy composition.
The results of this work show clearly that, under the open circuit conditions, the corrosion resistance of Ni-P alloys in neutral solutions is comparable, regardless of their structure and composition. Anodic polarization, however, gives rise to evident differentiation in the dissolution mechanism. When exposed to anodic polarization, amorphous alloys exhibit suppression of dissolution, like passivation, but features of this process are similar to those observed at the corrosion potential. Under the same conditions, anodic process of crystalline alloys is modified, and active, intense dissolution occurs. Thus, anodic behavior of Ni-P is very strongly dependent on the structural state of this alloy. Variation in P content has a slight effect, although the contribution of alloying P to the dissolution process is self-evident.
REFERENCES 1. C. H. de Minjer and A. Brenner, Plating 44, 1297
(1957). 2. R. N. Duncan, Mater. Perform.2518 (1983). 3. M. Bayes, B. Jackson and G. Shawhan, Trans. Insst. Metal Finish. 62, 145 (1986). 4. W. Riedel, Ind.-Anz. lk 1; (1986). 5. A. P. Van Gool. P. J. Boden and S. J. Harris. Trans. Inst. Metal Finish. 65, 108 (1987). 6. B. Jackson, R. Macary and G. Shawhan, Trans. Insst. Metal Finish. 68,75 (1990).
7. N. R. Sorense&F. i. Huhkeler and R. M. Latanision, Corrosion 40,619 (1984). 8; A: Ks_washima+K; As& and K, Ha&&ncto+J. NonCryst. Solids 70.69 (1985). 9. P. V. Nagarkar, P. C. Searson and R. M. Latanision, Proc. Symp. Corrosion, Electrochemistry and Catalysis of Metallic Glasses (Edited by R. B. Diegle and K. Hashimoto), p. 118. Proc. Electrochem. Sot. 88-l (1988). 10. A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci. 24,807 (1984). 11. H. Habazaki, S.-Q. Ding, A. Kawashima, K. Asami, K. Hashimoto. A. ____________, _.Inoue ____ __ and T; Masumoto: Corr. Sci, 29: 1319 (1989). 12. R. B. Diegle, N. R. Sorensen and G. C. Nelson. J. electrochem. $0~. 133,1769 (1986). 13. G. Salvano and G. Fumanalli. Met. Finish. 85. 31 _ (1987). 14. R. B. Diegle, N. R. Sorensen, C. R. Clayton, M. A. Helfand and Y. C. Yu, J. electrochem. Sot. 135, 1085 (1988). 15. J. Fils and D. J. Duquette, Corrosion41,700 (1985). 16. A. Kawashima, Y. P. Lu, H. Habazaki, K. Asami and K. Hashimoto, Corr. Engng %,643 (1989). 17. W. Immel, Galuanotechnik81,448 (1990). 18. H. Krcye; H.-G. Schenzel, j. C. Puip&, J. VanhumBeck and G. Ide. Metalloberflaeche 44.63 (19901. 19. M. Da Cunha .Belo, B. Rondot ahd k. kavarro, Metaux Cowos. Ind. 55,210 (1980). 20. J. L. Carbajal and R. E. White, J. electrochem. Sot. 135, ?952 (1988).
Anodic behavior of Ni-P alloys 21. R. S. Vakhidov, Elektrokhimiya 8,70 (1972). 22. A. Krblikowski, Proc. 10th Int. Gong. Metallic Corrosion, M&as 1987, Vol. II, 1169 (1987). 23. A. Kr6likowski and W. Stokarski, Abstr. Symp. Automatization
in
Corrosion
and
Electrochemical
1983
Research, $widno 1989, Inst. Phys. Chemistry PAN, Warsaw 1989, p. 1 (1989). 24. J. Kocvuxes, Metalloberjkzeche 31,7 (1977). 25. A. Kr6likowski and B. Pokrywa, Metabqia Odlewnictwo 16 111 (1990).
ANODIC
1993
oow46686/93 56.00 + 0.00 e: 1993. Pergamon Prtrs Ltd.
BEHAVIOR OF Ni-P ALLOYS STUDIED IMPEDANCE SPECTROSCOPY A.
KR~LIKOWSKI
BY
and P. Bu~~tnwtcz
Institute of Solid State Technology, Department of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3,00-664 Warsaw, Poland (Received 10 March 1993) A&met-Anodic behavior of electrodeposited Ni-P alloys with P content ranging from 6 to 28 at% was investigated in chloride and sulphate neutral solutions. Measurements were performed by potentiodynamic polarization and impedance spectroscopy methods. It was found that the tested alloys exhibit comparable corrosion resistance under open circuit conditions. However, anodic polarization rises significantly the differentiation in dissolution mechanism of Ni-P. Crystalline flow P) samples show active dissolution accompanied by the formation of a greyish-black nonprotective film on the alloy surface. On the contrary, amorphous (P rich) alloys exhibit passivation. Passive properties of these alloys are quite insensitive to the presence of chloride ions.
Key words: anodic dissolution, passivation, Ni-P, structure, impedance spectroscopy, polarization curve.
INTRODUCTION
Ni-P alloys as corrosion protective coatings constitute the earliest industrial application of amorphous metals[l]. These alloys are widely used for corrosion protection in many corrosive environments: food industry, oil, gas, mining, chemical manufacturing, aerospace, etc.[2-61. However, it is surprising how inconsistent the published data are about the corrosion behavior of this material. There is still no consent as to the essential corrosion characteristics of Ni-P: the nature of its anodic dissolution, ability to passivation or susceptibility to pitting. Cpvr+.l inv~t;o.tirmr $h& _I.“* .mnmhn,n. -.1x_ . . . . ~“.a~U”“a~” chnurr.A “.a”““.+ pa” YY Ni-P does not passivate in acidic media[7-91. Kawashima et al.[lO] reported active dissolution of this alloy, followed by the formation of a porous greyish black surface layer composed mainly of nickel phosphate. Habazaki et al. suggested that active dissolution of Ni-P leads to accumulation of elemental P on the alloy surface[ll]. On the contrary, other authors found passivation of Ni-P in mprlirrri7-i4i‘TJ. Y.“P’ nip-i- I. Dt Y..L’7, nrri4i nFfinn.d n .sAA;r ..UY_ ..IWY...L’_ Y’VyvYWUY concept of chemical passivity involving the absorption of hypophosphite ions on the alloy surface. The common opinion is that Ni-P alloys with high P content exhibit a slower anodic dissolution than low P alloys[15-181. An increase in P content is, however, accompanied by the transition from the crystalline to amorphous structure of this alloy. This transition occurs over a phosphorus content range of 10-15 at%. It remains unclear as to whether the suppression of anodic dissolution of Ni-P is the result of the chemical action of alloying P itself or the unique glassy structure of P-rich alloys. To date a great deal of effort has been expended to reveal the role of the structure and content of phosphorus in comparing the anodic behavior of Ni-P with that of pure policrystalline NiC13, 14, 19, 203. However, such an
approach does not provide a clear distinction between the structural and compositional effects. This work aims to clarify the anodic dissolution behavior of Ni-P alloys in relation to their structural state and elemental composition. It was executed by means of comparative studies of Ni-P alloys with varying P content. The dissolution of electrodeposited Ni-P lilms with P content in the range corresponding to the occurrence of both crystalline and amorphous structure, was examined.
EXPERIMENTAL Series of Ni-P alloys with P content ranging from 6 to 28 at% were electrodeposited galvanostatically from sulphatehypophosphite solution[21] onto a copper foil. The 6lms were 30-4Opm in thickness. The structure of the alloys was examined by X-ray diffraction. Results of these investigations revealed that electrodeposits with less than 10% P are crystalline (fee), whereas samples with P content above 15% -vh;l.;;t nmrrmhr.... “.xl.a”.& ol*.u.yr.vuu
ntr.,m+,...n Cc.1&L U”I”lti. 1 “L
th.. ,ur tmnn:+;rr..al LIauUILL”.Idu
range between 10 and 15 at% of P, alloys of the same composition were either crystalline or amorphous. Samples showing different structural states were subjected to electrochemical measurements. Anodic behavior of these samples was studied by potentiodynamic polarization and impedance spectroscopy methods. A computer controlled Electrochemical Measurement System Atlas was used. Samples preparation and design of the electrolytic cell were similar to those described elsewhereC22-j. The electrode area exposed to the solution was O.Scm’. Measurements were performed mainly in 0.1 N NaCl solution at 20 f 2°C; 0.1 N Na,SO, was also used for comparison. Prior to and during the experiments, the solutions were deaerated by bubbling with nitrogen. The reference electrode was a
1979
1980
A. KR~LIKOWSKIand P. BUTKIEWICZ
saturated calomel electrode and all values of potential are given vs. this electrode. Anodic potentiodynamic curves were measured at potential scan of 1 mVs_‘. Before the measurements were started the samples were prepolarized cathoditally (-0.7 V) to reduce possible air-formed oxide layers. Impedance studies were employed at the corrosion potential and selected anodic potentials. The frequency range was 10 kHz to 50 mHz. Before starting measurements the samples were exposed to the solution for 0.5 h to reach a stationary state. Computer software[23] was used to fit the experimental impedance data to model and determine characteristic parameters of the system studied.
RESULTS
AND
DISCUSSION
Anodic polarization curves of Ni-P alloys in NaCl solution are shown in Fig. 1. Evidently different performances of crystalline and amorphous samples are observed. Alloys containing up to 11% P behave similarly. They are active throughout the anodic region, show high dissolution currents and no tendency to passivate. The formation of a greyish black film on the alloy surface was observed during anodic polarization. On the contrary, P rich alloys exhibit current arrest, suggesting passivation. These samples show much the same anodic behavior, irrespective of P content. The surface appearance of these alloys remained intact (lustrous) after completing the test. -___ ..__ no __ vlsiole ..1-?LI- evlaence -..:I- .__ 01 -l. pimng. _.I..~._ Au .I. samples ~~ -* I nere was dissolving actively are crystalline, whereas all samples showing the current arrest are amorphous. These findings indicate that the dissolution mechanism of Ni-P is deeply modified by the structural transition. The structural state of this alloy is of primary importance in respect to the ability to passivation. This standpoint agrees with findings of Immel[ 173. This conclusion is additionally supported by the polarization behavior of alloy with 14% P, that is from the range of composition where structural transition occurs. The individual Ni-14P samples exhibit the anodic curve midway between loo
NI-P QIN NaCl/N, ImV9
Fig. I . Anodic polarization curves of Ni-P alloys in 0.1 N NaCl solution. The numbers denote P content in the alloy in at”/.
those for crystalline and amorphous alloys. As the matter of fact, it is not an intermediate behavior but rather the result of formal averaging. These samples exhibit very scattered polarization data. They show courses of polarization curves not far from those typical of either crystalline or amorphous alloys, according to the structural state of the individual samples. It is worth noting that ihe above anodic behavior of Ni-P alloys is reasonably similar to that observed in sulphate solution. Dissolution currents and extent of the current arrest are comparable. Thus, the presence of chloride ions does not degrade the passive behavior of amorphous alloys. Similar results were found for Ni-19P in acidic media[l4]. Further information about the anodic behavior of Ni-P was drawn from impedance data. Impedance measurements were performed at the corrosion potential, and at selected anodic potentials corresponding to the current arrest on polarization curves for amorphous Ni-P. Before starting impedance measurements the samples were exposed to the solution to reach a stationary state (typically 0.5 h). It is worth noting that amorphous samples showed suppression of dissolution current, when exposed to anodic polarization. The stationary anodic currents were substantially lower than those resulting from potentiodynamic polarization curves (Fig. 1). No presence of any black film was observed for amorphous samples. They remained unattacked after test. coinc;des with nhwrv.tinn This r. .I.. cx VYYIl . %...“I_ ef Koevecses[24], who found that only low P alloys show the formation of a black film during the atmospheric corrosion test. The shape of the impedance spectra observed in anodic region depends markedly on the structure of Ni-P alloy. Results representing the amorphous alloys are demonstrated in Fig. 2. Spectra show only one resistive-capacitanc arc. Values of impedance are quite high-tens of kohm. They decrease when samples are exposed to increasing anodic polarization. Thus, amorphous alloys show quite similar impedance data taken under different conditions, suggesting similar nature of the dissolution process. A fairly diverse relation was found for crystalline alloys (Fig. 3). The spectrum taken at the corrosion potential (Fig. 3a) does not differ much from spectra for amorphous samples. It consists of one resistivecapacitance arc and the impedance values are comparable to those observed for amorphous samples. All crystalline samples are semigloss under open circuit conditions. However, when exposed to anodic polarization, the formation of the greyish black film was observed. It was accompanied by a sharp increase in dissolution current indicating nonprotective nature of this film. After a few minutes, when the surface turned completely black, the currents became stabilized at a very high level (in excess of corresponding values from potentiodynamic polarization curves). Impedance spectra measured under these conditions differ significantly from that taken at the corrosion potential. Much smaller impedance values are observed-several hundred ohms. Moreover, an additional low frequency response appears. It is poorly marked at the potential -0.15 V (Fig. 3b), but at higher anodic poten-
Anodic behavior of Ni-P alloys (a)
1981
(a)
xO.l5
5
20
60
40 2:
Z,’ kohm
kahm
(b)
(b)
-10
I
I _^
.^
p 0
I _^
I
I
5 2:
IO kahm
Fig. 2. Impedance spectra typical of amorphous alloys (Ni24P) in 0.1 N NaCl at various potentials: (a) corrosion potential, (b) - 0.15 V and (c) 0.1 V. Several values of frequeneies (in Hz) are indicated.
Fig. 3. Impedance spectra typical of erystalhne alloys (NMP) in 0.1 N Nail at various potentials: (a) corrosion potential, (b) -0.15V and (c) 0.1 V. Several values of frequencies (in Hz) are indicated.
tials it is more distinct as a second loop (Fig 3c). This phenomenon is most likely associated with the covering of the surface by the greyish black film. Very high capacitance and low resistance can be approximated for this loop, suggesting large porosity of the surface film, in agreement with its nonprotective nature. The evident change in impedance data of crystalline alloys indicates a modification in dissolution mechanism due to anodic polarization. The differentiation in anodic behaviour of crystalline and amorphous alloys is evident upon results of quantitative analysis of impedance data. Figure 4 shows a relation between charge transfer resistance and composition of the Ni-P alloy. At the corrosion potential all samples exhibit quite comparable values of charge transfer resistance, regardless of the alloy structure and composition. This observation correlates well with similar corrosion rates of crystalline and amorphous Ni-P alloys measured in neutral media[25]. However, anodic polarization leads to reduction of the charge transfer resistance. This effect
is much more pronounced for crystalline samples indicating their much faster dissolution. Changes of double layer capacitance are shown in Fig. 5. More or less similar values of capacitance are
loo
5
ECORR
/ X-\
t-J--~
IO
15
x-
20
-0.15v
25
30
%P
Fig 4. Charge transfer resistance of Ni-P in 0.1 N NaCl (at various potentials) as a function of the alloy composition.
A. KR~UKOWSKI and P. BUTKIEWICZ
observed for alloys from the transitional region where samples of crystalline and amorphous state coexist. This fact confirms again the crucial role of structural factor.
CONCLUSION
I
I
I
I
I
I
5
IO
I5
20
25
30
%P Fig. 5. Double layer capacitance of Ni-P in 0.1 N NaCl (at
various potentials) as a function of the alloy composition. observed at the corrosion potential. Amorphous alloys exhibit similar values at anodic potentials too. Under the same conditions (at anodic potentials) crystalline samples show markedly higher capacitance. A likely explanation is the coverage of the surface by the greyish black film and/or the surface roughening during intense dissolution (after removal of this film the surface was found severely roughened). Anodic polarization of crystalline alloys gives rise to an evident change in the time constant for the charge transfer process (Fig. 6), suggesting a modification of dissolution mechanism (much faster process). The plots for anodically polarized samples in Figs 4-6 show a dramatic change at the composition range corresponding to the transition from crystalline to amorphous structure of Ni-P. The impedance characteristics of amorphous Ni-P are scarcely a&cted by P content. These findings point to the primary role of the alloy structure in governing the .x...-...,:..UIJD”L”U”ll Xnn,.l..&... ..C NT:LD n1L.m Dnnh ..A,.+ A. ,Xn. UlIWIb “1 1.r1 au”,U. LI_LI p”nL u1 1 a&u 4-6 represents averaged value calculated for at least three samples. Values of impedance parameters for amorphous alloys showed standard deviations less than 25%. Somewhat bigger deviations were found for crystalline samples (especially for capacitance values). However, definitely the highest scatter was
“---=ECORR -_+-O.ISV I.0
3 c
0.1
0.01
5
IO
20
15
25
50
%P Fig. 6. Time constant for charge transfer process of NCP in 0.1 N NaCl (at various potentials) as a function of the alloy composition.
The results of this work show clearly that, under the open circuit conditions, the corrosion resistance of Ni-P alloys in neutral solutions is comparable, regardless of their structure and composition. Anodic polarization, however, gives rise to evident differentiation in the dissolution mechanism. When exposed to anodic polarization, amorphous alloys exhibit suppression of dissolution, like passivation, but features of this process are similar to those observed at the corrosion potential. Under the same conditions, anodic process of crystalline alloys is modified, and active, intense dissolution occurs. Thus, anodic behavior of Ni-P is very strongly dependent on the structural state of this alloy. Variation in P content has a slight effect, although the contribution of alloying P to the dissolution process is self-evident.
REFERENCES 1. C. H. de Minjer and A. Brenner, Plating 44, 1297
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