The corrosion behavior of amorphous NiCrP alloys in concentrated hydrofluoric acid

Corrosion Science, Vol. 33, No. 10, pp. 1519-1528, 1992 Printed in Great Britain.

0010-938X/92 $5.00 + 0.00 © 1992 Pergamon Press Ltd

THE CORROSION BEHAVIOR ALLOYS IN CONCENTRATED

OF AMORPHOUS HYDROFLUORIC

Ni-Cr-P ACID

B o - P I N G ZHANG, HIROKI HABAZAKI, A S A H I KAWASHIMA, KATSUHIKO ASAMI

and

K o J I HASHIMOTO Institute for Materials Research, Tohoku University, Sendai 980, Japan

Abstract--In order to develop alloys resistant to aggressive hydrofluoric acids, the corrosion behavior of the amorphous Ni-Cr-19P alloys in 47% HF solution was examined by corrosion tests, electrochemical measurements and XPS analyses. The corrosion rate for the Ni-19P alloy is 0.7 mm y ~ at 30°C, which is about 20% of that for Ni. This high corrosion resistance is based on the formation of an elemental phosphorus layer on the alloy surface which acts as a diffusion barrier against alloy dissolution, as has been found in HCI solutions. The addition of Cr enhances the corrosion resistance due to spontaneous passivation. The corrosion rate of amorphous Ni-Cr-19P alloys containing -> 15 at% Cr is 2-7 × 10 .3 mm y - 1. The passive film on the chromium-containing alloys consists mainly of hydrated chromium and nickel oxyhydroxide.

INTRODUCTION

RECENTLY,the global pollution caused by the use of chlorofluorocarbons (CFC) on a large scale has become a serious problem. There is a need to find efficient methods to collect and/or decompose CFC. The decomposition products of CFC are fluorine, chlorine, hydrofluoric acid and hydrochloric acid. All of these are highly corrosive. A corrosion resistant material is therefore required which is resistant to the aggressive environments mentioned above in order to develop a CFC decomposition system. It is well known that some amorphous alloys possess excellent corrosion resistance. For example, some Fe-Cr-Mo-metalloid alloys become spontaneously passive even in hot concentrated hydrochloric acids.l'2 A number of highly corrosionresistant amorphous Ni-base alloys in aggressive solutions have been found. 3-s Among them, Ni-(20--30) at% Ta alloys showed the highest corrosion resistance and maintained a metallic luster even after immersion for several weeks in boiling concentrated HCI, 6 HNO35'6 and H3PO4 .7 Attempts were made recently to prepare amorphous Ni-P alloys with a high corrosion resistance to HCI by the addition of chromium. 9-12 Amorphous Ni-19P alloy shows a low corrosion rate, regardless of the concentration of HC1.12-13 Extremely corrosion-resistant amorphous Ni-Cr-P alloys in 6 M HC1 solution were obtained, although their corrosion behavior was quite sensitive to the presence of microcrystallites. 10-11 One might therefore expect that amorphous nickel-base alloys might be corrosion-resistant in hydrofluoric acid. Although hydrofluoric acid is a weak acid unlike the other hydrohalogenic acids, it attacks severely almost all metals except for gold and platinum even at low temperatures, Nickel and Monel are used industrially for the transport and storage of hydrofluoric acid only at ambient temperature. Corrosion data of metals and alloys Manuscript received 20 January 1992. 1519

1520

B.-P. ZHANGet al.

in hydrofluoric acid are restricted, b u t few data for a m o r p h o u s alloys are available.14 E l e c t r o c h e m i c a l m e a s u r e m e n t in hydrofluoric acid has n o t b e e n carried out. T h e p r e s e n t w o r k aims to investigate the c o r r o s i o n b e h a v i o r of a m o r p h o u s N i C r - 1 9 P alloys in a c o n c e n t r a t e d h y d r o f l u o r i c acid. F o r a b e t t e r u n d e r s t a n d i n g of the c o r r o s i o n m e c h a n i s m , the p o l a r i z a t i o n m e a s u r e m e n t was a t t e m p t e d a n d the resulta n t surface films were a n a l y s e d by XPS. E X P E R I M E N T A L METHOD Melt-spun Ni-xCr-19 at% P (x = 0, 5, 10, 15, 20, 25, 30 at%) alloy ribbons of about 0.5-1.0 mm width and 20-30 #m thickness were prepared by a method described elsewhere.9'12 The structure of the specimens was confirmed to be amorphous by X-ray diffraction. Commercial crystalline chromium stick (99.9%) and nickel sheet (99.7%) were also used for comparison. Before each experiment the specimens were polished mechanically with a silicon carbide paper up to No. 1000 in cyclohexane, degreased in acetone and dried in air. The electrolyte used was a reagent grade 47 weight% HF solution saturated with air. Tetrafluoroethylene vessels were used for polarization curve measurements and immersion tests. A platinum plate electrode and a saturated calomel electrode were used as counter and reference electrodes respectively. The potentials cited are with reference to the saturated calomel electrode (SCE). Polarization was carried out by sweeping the potential anodically and cathodically at 1.8 mV s-1 , starting from 50 mV below and above the corrosion potential, respectively. The current density change with time was measured potentiostatically. Corrosion rates were estimated from the weight loss after immersion for 1 week in 47% HF solution at temperatures of 30, 60, 90 and 110°C. X-Ray photo-electron spectra of amorphous Ni-Cr-19P alloys immersed in HF solution at 30°Cfor 3 h were measured by means of an ISS SSX-100electron spectrometer with A1 Ka radiation (hv = 1486.6eV). Binding energies of X-ray photo-electron spectra were calibrated by the method described elsewhereJ 5A6 The binding energies of the Cu 2p3/2and Cu 3s electrons of copper metal and the Au 4)~/2electrons of gold metal are taken as 932.53,122.4 and 84.07 eV, respectivelyJ s The compositions of the surface film and the underlying alloy were quantitatively determined from the XPS spectra intensities by the previously proposed methodJ 7'ls The binding energy of the F ls electrons for the fluoride ion was determined by using reagent grade CrF3 •xH20 powder by ESCA-850 electron spectrometer with Mg K~ radiation (hv = 1253.6 eV). The peak shift caused by charge-up effects was corrected against the binding energies of Au 4f7/2and 4fs/2electrons of gold metal evaporated on the CrF 3•xH20. E X P E R I M E N T A L RESULTS F i g u r e 1 shows the c o r r o s i o n rates of a m o r p h o u s N i - C r - 1 9 P alloys in 47% H F solutions at v a r i o u s t e m p e r a t u r e s of 30, 60, 90 a n d 110°C as a f u n c t i o n of alloy c h r o m i u m c o n t e n t . T h e c o r r o s i o n rates of c h r o m i u m a n d nickel m e t a l s in the s o l u t i o n at 30 a n d 90°C are also p r e s e n t e d for c o m p a r i s o n . T h e c o r r o s i o n rate of the a m o r p h o u s N i - 1 9 P alloy in h y d r o f l u o r i c acid at 30°C is a b o u t 0.7 m m y - l , which is a b o u t one-fifth a n d o n e - t h o u s a n d t h of those for nickel a n d c h r o m i u m metals, respectively. A n increase in the t e m p e r a t u r e of H F s o l u t i o n t e n d s to increase the c o r r o s i o n rate of the N i - 1 9 P alloy. E v e n t h o u g h the c o r r o s i o n rate of the N i - 1 9 P alloy at 90°C is a l m o s t the s a m e as that of the nickel m e t a l , it is still a b o u t t h r e e orders of m a g n i t u d e lower t h a n that of the c h r o m i u m metal. T h e a d d i t i o n of c h r o m i u m to the N i - 1 9 P alloy decreases the c o r r o s i o n rate, a l t h o u g h c h r o m i u m suffers severe c o r r o s i o n in the H F solution. T h e a m o r p h o u s N i - C r - 1 9 P alloys with 15 a t % or m o r e c h r o m i u m show excellent c o r r o s i o n resistance of 2 - 7 x 10 -3 m m y - 1 in H F s o l u t i o n at 30°C. Similarly to the N i - 1 9 P alloy, the N i - C r - 1 9 P alloys b e c o m e less c o r r o s i o n - r e s i s t a n t as the s o l u t i o n t e m p e r a t u r e is i n c r e a s e d . I n c r e a s i n g the c h r o m i u m c o n t e n t t e n d s to decrease the c o r r o s i o n rate w i t h i n a field t e m p e r a t u r e . F i g u r e 2 shows the cathodic a n d a n o d i c p o l a r i z a t i o n curves of the a m o r p h o u s

The corrosion of N i - C r - P alloys in HF 104

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FIG. 1. Corrosion rates of amorphous Ni-Cr-19P alloys measured in 47% HF at 30, 60, 90 and 110°C. Included are the results of nickel and chromium metals measured at 30 and 90°C.

Ni-Cr-19P alloys measured in 47% HF solution at room temperature. The most striking characteristic is that these alloys do not suffer pitting corrosion when subjected to anodic polarization in the concentrated HF solution. This fact is substantially different from the result in 12 M HC1 solution in which pitting corrosion occurred. 12 The Ni-19P alloy shows no active state nor apparent passive state in the anodic polarization curve. The anodic current density rises with the applied potential, as has been observed for the amorphous Ni-P alloys in other acidic solutions. 13"19-23 The corrosion potential of the Ni-19P alloy is 40 mV, and shifts toward a more noble value with chromium content although the shift is small. The cathodic and anodic current densities of the Ni-Cr-19P alloys decrease with increasing chromium content. This tendency is especially clear in the potential region of about 400 mV(SCE) or above. Chromium-bearing alloys show a passive region, and the current densities in the passive region decrease with increasing chromium

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1522

B.-P. ZIJAN6 et al. I0

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content. For a better understanding of the corrosion behavior under open circuit conditions, the change in current density with time during anodic polarization at a fixed potential very close to the open circuit potential was examined and the results are shown in Fig. 3. The current density for the amorphous Ni-19P alloy decreases with time until 4 min, whereas those for alloys containing chromium drop in the initial 1 min or shorter period of time. The steady current densities decrease with increasing chromium content. The corrosion behavior under the open circuit condition was further examined by XPS analysis of the surface film for amorphous Ni-Cr-19P alloys immersed in 47% HF solution for 3 h. All the spectra from the alloy constituents consisted of two peaks corresponding to the oxidized state in the surface film and the metallic state in the underlying substrate. The Ni °×, Cr °x and pox, where superscript ox indicates oxidized state, in the surface film were in the divalent, 7'24 trivalent2s'26 and pentavalent 7'27 states, respectively. All the peak binding energies of Ni °x 2,o3/2 and po× photoelectrons were not dependent on the alloy composition. They coincided with those measured from the specimens polished and immersed in HCI solutions. 12 The full width at half-maximum (FWHM) of Cr °x 2,o3/2 spectrum obtained from the specimens immersed in HF solution became, however, slightly larger when the alloy chromium content increased. This suggests the presence of two or more different Cr 3+ species. The O ls spectrum showed a peak at about 531.4 eV and had a large FWHM because of overlap of O ls spectra from O 2-, O H - , P O ] - and U20 , although they were not separated into four spectra in this work. Figure 4 shows the F ls electron spectra obtained from the surface films on the amorphous Ni-Cr-19P alloys immersed in HF solution for 3 h. The chromium-free alloy shows a very weak F ls signal at about 686.7 eV. There are two peaks at 684.6 and 686.7 eV for chromium-containing alloys. It can be seen that the increase in the alloy-chromium content leads to an increase in the intensity of the low binding

1523

The corrosion of Ni-Cr-P alloys in HF I

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energy peak and to a decrease in that of high binding energy peak. In contrast to the F ls spectrum, the shape of the X-ray induced F KL23L23 Auger spectra for the specimens containing chromium showed no compositional dependence. The relationship between the peak kinetic energy of the F KL23L23 Auger spectrum and the peak binding energies of the F ls spectrum was plotted as was shown by Wagner et al.28 The relationship is given in Fig. 5. The literature data of the NiF2 compound 28 and the result obtained from the powder CrF3-xH20 are also presented as references. Two groups of data which correspond to low and high binding energy peaks of the F ls spectra are located far from the region of nickel fluoride, indicating the absence of nickel fluoride species in the surface films on the specimens immersed in HF solution for 3 h. Even after immersion for 10 h, the peak characterized by the nickel fluoride was not found. This fact suggests that nickel fluoride film is not formed by immersion in 47% HF solution on the amorphous Ni-P alloy with or without chromium. The data for the low binding energy peaks are located on the line of the powder CrF 3 • xH20. Accordingly, some part of the fluoride ions seems to be bonded to the Cr 3+ ions. The peak binding energy of Cr 2/)3/2 electron for CrF3" xH20 was 580.6 eV, which is in good agreement with the value (580.1 eV) for CrF3 reported in the literature. 28 It is much higher than the binding energy of the chromium species in oxides,or hydroxides, which is known to be 576-577 eV. t6 The binding energy of the Cr 3+ 2/)3/2 electrons obtained from the specimens immersed in HF was about 577.4 eV, and FWHM increased slightly with increasing alloy chromium content. The Cr 2/73/2 spectrum might be composed of overlapped spectra corresponding to fluoride

1524

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F]6.5. Relation between the peak kinetic energy of the F KL23L23 Auger spectrum and the peak binding energy of F Is spectrum obtained for amorphous Ni-Cr-19P alloys immersed in 47% HF for 3 and 10 h. Included are the literature data 2s of NiF2 (/~) and the result obtained from the powder CrF 3 • xH20 ([~).

and oxide or oxyhydroxide, although chromium fluoride is the minor species. Even if the alloy chromium content was increased to 30 at%, the amount of the chromic fluoride was very low since the increase in FWHM was still small. The signal of the Cr 3+ for chromic fluoride did not give a separate peak in the Cr 2p3/2 spectrum. The only change appeared in the FWHM of the Cr 3+ 2p3/2electron spectra. The peak of F ls electron spectrum at about 686.7 eV could not be identified. Since HF molecules are linked together by hydrogen bonds, the F ls spectrum for HF should give the peak at a higher binding energy than that for F-. It can, therefore, be assumed that the peak at about 686.7 eV is the peak due to the F ls electron of adsorbed HF. Figure 6 shows the cationic fraction in the surface film formed on the Ni-Cr-19P

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alloys immersed in 47% H F solution for 3 h. No apparent change in film composition with time was found for all alloys after prolonged immersion up to 10 h. C h r o m i u m is significantly concentrated in the film formed on the chromium-containing alloys. On the other hand, phosphorus is significantly enriched on the Ni-19P alloy, and the enrichment of phosphorus tends to decrease with increasing alloy chromium content. Figure 7 gives the n u m b e r of anions and bound water per cation in the surface film formed in 47% H F solution as a function of alloying chromium content. The sum of numbers of O 2 , O H - and bound water is exhibited since they cannot correctly be separated from the O ls spectrum measured. The F - content of the surface film is very low for the Ni-19P alloy, but increases by the addition of chromium to the alloy. This is in agreement with the fact that fluoride ions are in the form of chromium fluoride. It can be said from Fig. 7 that all the films on the N i - C r - 1 9 P alloys are composed mainly of hydrated chromium and nickel oxyhydroxide, since [F ] is about a quarter of [O] in the surface film. DISCUSSION As has been reported previously,13 the change in the anodic current density with time for the a m o r p h o u s Ni-19P alloy at a potential near the open circuit potential in 1 M HCI follows Fick's second law, since the anodic polarization leads to the preferential dissolution of nickel with a consequent formation of an elemental phosphorus layer on the alloy which acts as a diffusion barrier for alloy dissolution. Because of the formation of the elemental phosphorus barrier layer, the corrosion rate of the a m o r p h o u s Ni-19P alloy was almost the same value of about 0.05 m m y 1 in l, 6 and 12 M HC1 solutions, that is, independent of the concentration of hydrochloric acid. 1~ According to the previous study, 13 the elemental phosphorus layer covering the Ni-19P specimen surface was readily oxidized when the specimen was exposed to air, and a high density of pS+ ions was found, when X-ray photo-electron spectra were measured for the specimen transferred through air after immersion or anodic polarization in 1 M HC1. In the present study, a large amount o f P 5+ ions was detected on the Ni-19P alloy

1526

B.-P. ZriAN6 et al.

after immersion in HF solution. This could be formed by air oxidation of the elemental phosphorus layer. On the other hand, when chromium was added to Ni-19P alloy, the increasing alloy chromium content led to a decrease in the extent of phorphorus enrichment and to an increase in the chromium concentration in the surface films. The surface films on alloys with higher chromium contents were composed mainly of hydrated chromium oxyhydroxide. It can therefore be assumed that the high corrosion resistance of the amorphous Ni-19P alloy is based on the formation of the elemental phosphorus barrier layer while that of chromiumcontaining alloys arises from passivation due to the formation of hydrated chromium oxyhydroxide film. If this is true for the Ni-19P alloy, the change in the anodic current density with time should follow Fick's second law. The relationship between the reciprocal of the square of the current density and the time was calculated from the results of the current decay curves shown in Fig. 3. The data are shown in Fig. 8. A linear relationship is obtained for the amorphous Ni-19P alloy, indicating that change in the dissolution current in HF solution obeys Fick's second law. Consequently, it can be said that the formation of an elemental phosphorus layer on the amorphous Ni-19P alloy surface is responsible for the high corrosion resistance in 47% HF solution as well as in concentrated HCI. In the same way, the curve for the amorphous Ni-5Cr-19P alloy and the initial period of the curve for the Ni-10Cr-19P alloy show the linear relationship. With increasing alloy chromium content, however, the relation in Fig. 8 becomes non-linear. This can be interpreted in terms of passivation by the formation of a chromium-enriched passive film. As reported previously, 12 the phosphorus concentration in the film after immersion for 3 h was higher than that for 10 h although the steady surface films formed on the Ni-20Cr-19P and Ni-30Cr-19P alloys in 6 M HCI solution consisted mainly of passive hydrated chromium oxyhydroxide. This suggests that the elemental phosphorus layer is formed in the initial period of immersion for high chromium alloys. The open circuit potentials of these two high chromium alloys in 6 M HCI solution were initially low at about - 100 mV(SCE), and increased about

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The corrosion of Ni-Cr-P alloys in HF

1527

300 m V during the initial period of time. A c c o r d i n g l y , the blocking action of elemental p h o s p h o r u s layer was initially quite effective in preventing active dissolution of alloy and in ennobling the o p e n circuit potential. Increasing o p e n circuit potential p r o m o t e d passivation by the f o r m a t i o n of c h r o m i u m - e n r i c h e d passive film, and aging of the passive film further e n n o b l e d the o p e n circuit potential. T h e same m e c h a n i s m can be applied to explain the anodic b e h a v i o r of the a m o r p h o u s N i - C r - P alloys in H F . CONCLUSIONS A m o r p h o u s N i - 1 9 P alloy has a high corrosion resistance in 47% H F at 30°C. T h e corrosion rate of the alloy is about 0.7 m m y-1 which is about one-fifth of that for nickel metal. T h e addition of c h r o m i u m to a m o r p h o u s N i - 1 9 P alloy further increases the corrosion resistance. T h e corrosion rate of a m o r p h o u s N i - C r - 1 9 P alloys with 15 at% or m o r e c h r o m i u m is 2-7 × 10 -3 m m y-1. T h e f o r m a t i o n of an elemental p h o s p h o r u s layer on the a m o r p h o u s N i - 1 9 P alloy surface is responsible for the high corrosion resistance, acting as a diffusion barrier for alloy dissolution. T h e elemental p h o s p h o r u s layer is f o r m e d as a result of selective dissolution of nickel as has b e e n f o u n d in hydrochloric acid solution. T h e high corrosion resistance of a m o r p h o u s N i - C r - P alloys with 15 a t % or m o r e c h r o m i u m is caused by the f o r m a t i o n of c h r o m i u m enriched passive o x y h y d r o x i d e film. REFERENCES 1. K. HASHIMOTO,K. KOBAYASHI,K. ASAMIand T. MASUMOTO,Proc. 8th Int. Congr. Metallic Corrosion, Vol. 1, p. 70. DECHEMA, Frankfurt (1981). 2. K. KOBAYASHI,K. HASHIMOTOand T. MASUMOTO,Sci. Rep. Inst. Tohoku Univ. A29, 284 (1981). 3. K. HASHIMOTO,M. KASAYA,K. ASAMIand T. MASUMOTO,Bosyuko Gijutsu (Corros. Engng) 26, 445

(1977). 4. M. NAKA,K. ASAMI,K. HASHIMOTOand T. MASUMOTO,Proc. 4th Int. Conf. Titanium (eds H. KIMURA and O. IZUMI), Vol. 4, p. 2677. The Metallurgical Society of AIME (1981). 5. A. KAWASHIMA,K. SHIMAMURA,S. CHIBA, T. MATSUNAGA,K. ASAMIand K. HASHIMOTO,Proc. 4th

Asian-Pacific Corrosion Control Conf., Tokyo, Vol. 2, p. 1042 (1985). 6. K. SHIMAMURA,A. KAWASHIMA,K. ASAMIand K. HASHIMOTO,Sci. Rep. Inst. Tohoku Univ. A33, 196

(1986). 7. A. M~TSUrIASm,K. ASAMI,A. KAWASHIMAand K. HASmMOTO,Corros. Sci. 27, 957 (1987). 8. K. SHIMAMURA,K. MIURA, A. KAWASHIMA,K. ASAMIand K. HASIflMOTO,Sci. Rep. Inst. Tohoku Univ. A34, 107 (1988). 9. B.-P. ZHANG,H. HAaAZAKI,A. KAWASHIMA,K. ASAMIand K. HASHIMOTO,Bosyuko Gijutsu (Corros. Engng) 38, 384 (1989). 10. A. KAWASmMA,B.-P. ZHAN6, H. HABAZAKI,K. ASAMIand K. HASmMOTO,Corros. Sci. 31, 355 (1990). 11. B.-P. ZHANG,H. HABAZAKI,A. KAWASI-nMA,K. ASAMIand K. HASmMOm, Corros. Sci. 32, 433 (1991). 12. B.-P. ZHAr~, H. HABAZAKI,A. KAWASHIMA,K. ASAMIand K. HASHIMOTO, Corros. Sci. 33, 667 (1992). 13. H. HABAZAKI,S.-Q. DING, A. KAWASHIMA,K. ASAM1,K. HASHIMOTO,A. INOUEand T. MASUMOTO,

Corros. Sci. 29, 1319 (1989). 14. M. A. TENrmVER,D. B. LUKCO,G. A. SHREVEand R. S. HENDERSON,J. Non-crystall. Solids 116,233 (1990). 15. K. ASAMI,J. Electron Spectrosc. 9, 469 (1976). 16. K. ASAMIand K. HASHIMOTO,Corros. Sci. 17, 559 (1977). 17. K. ASAMI,K. HASm~IOTOand S. SHIMODAmA,Corros. Sci. 17, 731 (1977). 18. K. HASmMOTOand K. ASAMI,Boshoku Gijutsu (Corros. Engng) 26, 375 (1977).

1528 19. 20. 21. 22. 23.

24. 25. 26. 27. 28.

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