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Hot corrosion of nickel in anhydrous sodium hydroxide
Materials Chemistry and Physics 45 (1996)
171-175
Materials Science Communication
Hot corrosion of nickel in anhydrous sodium hydroxide Manoj Komath Received 12 August 1994: accepted 9 October 1995
Abstract The action of hot NaOH melt on metallic nickel surfaces in the temperature range 700-850 “C is investigated. It is observed that nickel dissolves in NaOH to form sodium nickelate ( NaNiO,) and the melt-metal interface becomes coated with a film of nickel oxide (NiO) The
corrosion rates of Ni in NaOH under the experimental conditions are notably high, 2-3 orders of magnitude higher than those reported at lower temperatures. Keywords: Corrosion;
Nickel; Sodium hydroxide
1. Introduction
Nickel and nickel-based alloys are well known for their ability to resist corrosive environments, even under severe operating conditions. They offer good resistance to caustic alkalis and show remarkable immunity to caustic stress corrosion cracking [ 11. Specifically, in the case of resisting highly concentrated NaOH, nickel ranks next to silver. For this reason nickel (and also its alloys) is widely used in NaOH service [ 1,2]. Detailed investigations have been conducted by many workers [ 2261 on the corrosion of nickel caused by NaOH. All of these reports record very low corrosion rates, typically fractions of a millimeter per year. The present report contains studies on an experimental system which involves the reactions between metallic nickel and hot NaOH melt at high temperatures (above 700 “C) It is observed that the metal surface corrodes heavily and the nickel dissolved in NaOH forms some reaction products, in addition to the formation of an oxide coating at the metalalkali interface.
2. Experimental Details of the experimental system have already been given in a previous report [ 71. In this experiment, NaOH is melted in nickel crucibles inside an electric furnace. Cup-shaped crucibles ( 15 ml vohrme) made of pure nickel, and Analar grade anhydrous NaOH, are used as the reaction materials. 0254-0584/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSD10254~0584(95)01708-1
The furnace used is the vertical muffle type, with temperature control facilities. The furnace is set at the required temperature ( > 700 “C), and the crucible containing NaOH is kept inside the muffle for a few minutes. The action of NaOH on the nickel surface is visually observable. The crucible surface becomes tarnished owing to a coating of some kind of film, possibly an oxide. On cooling the melt, it can be observed that some reaction products are separating out. When the experiment is repeated with the same crucible, not much change is observed in the thickness of the film, but nickel continues to react with NaOH to form the reaction products. The kinetics of the dissolution in the present case has also been studied. The fresh crucible is allowed to react with NaOH at high temperatures for a sufficient period of time so that a uniform black coating is formed over the surface. It is then cleaned thoroughly, dried and weighed in a 5-digit metallar balance. One gram of NaOH is put in the crucible and inserted in the furnace, which is maintained at the constant experimental temperature. The reaction time is kept as 30 min, to obtain an appreciable dissolution. After this period the crucible is taken out and cooled, and the NaOH mass is dissolved away in a suitable solvent. It is washed, dried and weighed again. The difference in the weights gives the net weight of nickel dissolved, assuming the amount of black coating is preserved after each experiment. Knowing the bottom surface area of the crucible, where it comes in contact with the NaOH melt, the dissolution rate per unit area at the experimental temperature can be calculated. The experiment is repeated at steps of 25 “C within the
172
M. Komarh /Materials Chemistry and Physics 45 (1996) 171-l 75
temperature range 700-850 “C. The lower limit, 700 “C, is selected to obtain detectable dissolution, while the upper limit is fixed at 850 “C to eliminate the error due to evaporation of the melt. The film and the reaction products are characterized using various techniques such as optical and electron microscopy, X-ray diffractometry, atomic emission spectroscopy and infrared spectroscopy.
3. Results 3.1. Reaction kinetics I
A plot of the average dissolution rates versus temperature gave an exponential curve (Fig. 1). The corrosion rates ranged from 22.82 mm y-i at 700 “C to 122.42 mm y-’ at 850 “C. The corresponding Arrhenius plot is given in Fig. 2. It indicates that the dissolution of nickel is of an activated nature, obeying the relation [ 81
700
750
800
TEMPERATURE Fig. 1. Dissolution/corrosion temperature.
850
I
‘C
rates of Ni in NaOH
as a function
of
-7.2.
V=A exp( - E,IkT) where E, is the activation energy of the dissolution reaction, T the temperature, V the dissolution rate, k the Boltzmann constant and A the pre-exponential factor. The activation energy for the dissolution reaction is calculated to be 29.29 kcal mol- ‘.
-7.4L yE V -7.6. g Y J? -7.8-
3.2. Characterization of the reaction products
t -8.0
The film formed over the crucible was analysed in an Xray diffractometer to find its identity. The spectrum (Fig. 3) showed peaks corresponding to NiO along with the peaks of the underlying metal surface [9]. The scanning electron microscopy (SEM) picture of the film surface is shown in Fig. 4. The presence of the reaction products formed in the melt can be visually identified as the emergence of some shining platelets in the NaOH melt during the cooling. These products were separated by dissolving the NaOH mass in absolute alcohol. (Absolute alcohol is preferred, because with water and aqueous-based solvents, there is a risk of decomposition due to hydrolysis, if the products contain sodium). The precipitate was again washed and centrifuged in absolute alcohol and dried in vacuum. On observing under an optical microscope, the product was seen to contain lustrous platelets along with a yellowishbrown powder. The platelets were mostly hexagonal in shape, in the size range 0.1-0.5 mm. Distorted and coalesced platelets were also seen, along with needle-shaped and dendritic crystals. For the chemical identification of the reaction product, first of all, elemental analysis was done employing inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Only sodium and nickel were determined among the elements
0.90
0.!35 lg/~
1.00 K-’
Fig. 2. Arrhenius plot corresponding
to Fig. 1.
that can be identified with the technique. The overall composition was found to be 27.36% Na and 35.75% Ni. To find the specific compounds involved, the product was analysed in an X-ray diffractometer. The XRD spectrum, in the range 10” to 90” of the 28 value, contained as many as 35 peaks, which suggested the presence of a mixture of compounds. From the set of peaks, the corresponding individual compounds were traced out by Hanawalt’s method [lo]. The peak positions and the intensities were noted and compared with those reported [ 91. Among the possible compounds, the X-ray data reported for sodium nickelate ( NaNi02 - monoclinic as well as hexagonal forms), hydrated sodium peroxide ( NaZOz * 8H,O) and sodium carbonate (Na,CO,) matched the present data most satisfactorily (Table 1) . Infrared spectrometry of the product was done as a supporting test. The transmission IR spectrum (KBr pellet technique) scanned in the range 200-4000 cm- ’ is given in Fig. 5. The characteristic peak corresponding to the peroxide linkage (at 875 cm-‘) and that corresponding to the OH group
M. Komath / Materials Chemistry and Physics 45 (1996) 171-l 75
173
3 ANGLE Fig, 3. X-ray diffraction
Fig. 4. Scanning electron micrograph metal surface.
28
spectrum of the coating formed over the nickel metal surface.
of the coating formed over the nickel
of the bound HZ0 (centered around 1500 cm- ‘) appeared in the spectrum. This confirms the presence of Na,O, 8H20. The broad band centered at 3520 cm-i corresponds to the stretching of the OH group. The peaks seen at 570 and 3 15 cm-’ may be those caused by the NiO linkage in NaNiO,. The C03* peaks were totally absent in the spectrum [ 111.
The XRD analysis revealed the crystalline components of the products to be NaNiO, (hexagonal and monoclinic forms), Na,O, .8H,Oand Na2C03. However, the IR results make the occurrence of Na&O, doubtful. Reports show that the compound NaNiOz is of a decomposing nature, which absorbs moisture and CO, from the air to form Na&O, on the surface [ 121. The sodium carbonate that contributes peaks to the XRD spectrum may be that formed by the decomposition of NaNiO, during the analysis procedure. At the same time, the sample for IR analysis was prepared rapidly to avoid or minimise any decomposition of the products. The possibility that Na,C03 is present originally in the reaction product can therefore be ruled out. For confirmation, a hydrolysis test was done. The product obtained was treated with distilled water. The result of the hydrolysis on the product can be anticipated - Na202 will be converted to NaOH (which is soluble in water) and NaNiO, will be transformed to hydroxides and oxyhydroxides of nickel [ 121. Na2C0,, if present, will remain unaffected. The hydrolysed precipitate was separated through centrifuging, washed, dried and subjected to various characterization tests, the results of which were presented in an earlier
Fig. 5. Infrared spectrum of the reaction product.
114
M. Komath / Materials Chemistry and Physics 45 (1996) 171-l 75
Table 1 X-ray diffraction
data of the product obtained and a comparison
d-value (A)
IILn, (%)
d-values of NaNiO, (monoclinic) Card no. 6-63 1 d(A)
1 2 3 4 5
5.4922 5.2023 4.6965 3.4133 3.2219
62.56 100.00 1.15 0.34 0.44
6 I 8 9
2.9593 2.7544 2.6991 2.6101 2.5420
2.48 8.06 1.02 18.14 1.62
10
III, (%)
90
5.250 _
70
2.610 2.590
60 70
_
16 17 18 19 20
2.1893 2.0969 2.0335 2.0040 1.9749
0.81 2.19 0.22 0.33 0.41
_
21 22 23 24 25
1.9476
0.67 0.71 0.20 0.33 0.28
1.737
1.4568 1.4220
0.35 1.42 0.46 0.21 0.31
1.655 1.550 1.502 1.458 1.423
60 90 80 30 60
1.3795 1.3201 1.3069 1.2483 1.1983
0.90 0.55 2.35 0.25 0.18
1.371 1.320 1.305 1.249 1.207
50 70 50 70 70
31 32 33 34 35
(%)
d-values of NaPOz. 8H,O Card no. 15-32
d-values of Na,CO, Card no. 18-1208
d(A)
d(A)
I/I, (%) 90
5.50
2.420
_
2.99
60
2.71
100
60 50
2.57
95
70
2.42
3.43 3.28
16 18
2.91 2.75 2.62 2.61 2.55
100 30 40 45 65
2.31 2.26
100 50 -
40 -
_ 90
I/I, (%)
90
_
2.470 2.380 _
1.6592 1.5547 1.5025
_
5.240
_
0.62 0.97 2.94 1.03 2.35
26 27 28 29 30
_
_ _ _
2.4487 2.3970 2.3657 2.2552 2.2335
1.965
III,
_
_
2.100
d (A)
_
2.620
2.230
d-values of NaNiOz (hexagonal) Card no. 6-632
_
11 12 13 14 15
1.8828 1.7402 1.7092 1.6782
compounds
Reported diffraction data for the anticipated compounds (from JCPDS data cards)
X-ray diffraction data of the reaction product Peak no.
with the reported data for anticipated
-
_
-
2.130
100
1.978
tr. _
2.20 100 _ 20
35 -
2.04 -
12
1.96
35
1.89
30
1.71 1.68
14 14
60
1.93 40 1.684
60
1.557 1.475
70 70
_
1.423
50
1.342 1.313 1.289 1.250 1.216
60 30 60 20 60
report [ 131. The precipitate contained only y-NiOOH and /3Ni(OH)*.
4. Discussion The results of the kinetics study are very interesting. The corrosion rates recorded under the experimental conditions are notably higher (at least 3 orders of magnitude) than those reported at lower temperatures. The reported range of Ni dissolution in 75-100% NaOH solution at 250-475 “C is 0.05-0.06 mm y-i [2-51. Barkel [6] reported the highest rate to be 0.8 mm y -‘, in boiling 25% NaOH solution.
-
-
_ _ _
_ _
_
The heavy corrosion observed in this case is related to the factor that accelerates the Ni-NaOH reaction. In an earlier study, Williams et al. [ 141 observed that the high-temperature reaction of hot NaOH with Ni metal results in the oxidation of the metal surface to NiO along with the evolution of hydrogen: Ni + 2NaOH -
NiO + Na,O + H2
Formation of black NiO coatings over the nickel surface during the Ni-NaOH reaction has also been reported by others [2], and the mechanism of formation of this layer has been discussed [ 15,161. The NiO layer is proved to be a
M. Komuth /Materials
Chemistry and Physics 45 (1996) 171-l 75
passive one, and believed to impart good protection against further corrosion [ I]. Dyer et al. [ 121, on reacting NaOH with nickel in the presence of oxygen, observed the formation of NaNiO,. No detailed study on the reaction mechanism has been conducted. In the present experiment, NiO formation can be observed on the surface of the crucible. On continuing the reaction, nickel comes into the NaOH melt to form NaNiO,. Following Molenda and Stoklosa [ 171, a possible mechanism may be proposed. These workers made NiO react with Na,O, at 700 “C in the presence of oxygen to synthesize NaNiO,. Since the present experimental conditions are similar, it is possible that the NiO and Na20, obtained during the Ni-NaOH reaction react further to form NaNiO,. This is just a tentative mechanism; other possibilities cannot be ruled out. An extensive study is needed to confirm the mechanism that leads to the heavy corrosion of nickel.
Acknowledgements
The author is thankful to Dr K.A. Cherian and Dr Arabinda Ray for their interest in the work and encouragements. This work is carried out under the financial support of the CSIR, India.
17.5
References [ I] B.C. Craig (ed.), Handbook of Corrosion Data, ASM International, Metals Park, OH, 1989. [Z] D. Behrens (ed.), DECHEMA Corrosion Handbook, Vol. 1, DECHEMA, VCH, Weinheim, Germany, 1988. [3] G.P. Smith, M.E. Steidlitz and E.E. Hoffman, Corrosion, 13 (1957) 56lt. 627t; 17 (1958) 47t. [4] M. Yasuda, F. Takeya and F. Hine, Corrosion, 39 ( 1983) 369. [5] M. Yasuda. F. Fukumoto, H. Koizumi, Y. Ogata and F. Hine, Corrosron, 43 ( 1987) 492. [6] B.M. Barkel, Proc. Corrosion Conj:, Atlanta, GA, 1979, National Association of Corrosion Engineers, Houston, TX, 1979, p. 9. [7] K.A. Cherian, Suq! Gout. TechnoL, 47 (1991) 127. [8] G.K. Baranova and E.M. Nadagonyi, Sov. Phys-Crystallogr., 13 ( 1969) 722. [9] X-Ray Diffraction Data Cards, JCPDS, Swarthmore, PA, 1974. [ IO] Hanuwalt Search Manual, International Centre for Diffraction Data, Swarthmore, PA, 1983. [ 111 K. Nakamoto, Infrared Spectra of‘ Inorganic and Co-ordination Cornpun&, Wiley, New York, 1963. [ 121 L.D. Dyer, B.S. Borie and G.P. Smith, J. Am. Chem. Sot., 76 (1954) 1499. [ 131 M. Komath, S. Thomas, K.A. Cherian and A. Ray, Mu&r. Chem. Phys., 36 (1993) 190. [ 141 D.D. Wdliams, J.A. Grand and R.R. Miller, J. Am. Chem. Sot., 78 (1956) 5150. [ 151 P.J. Harrop, J. Mater. Sci., 3 (1968) 206. [ 161 Corrosion Basics, National Association of Corrosion Engineers, Houston, TX, 1984, p. 280. [ 171 J. Molenda and S.Stoklosa, Solid State Ionics, 38 ( 1990) 1.
171-175
Materials Science Communication
Hot corrosion of nickel in anhydrous sodium hydroxide Manoj Komath Received 12 August 1994: accepted 9 October 1995
Abstract The action of hot NaOH melt on metallic nickel surfaces in the temperature range 700-850 “C is investigated. It is observed that nickel dissolves in NaOH to form sodium nickelate ( NaNiO,) and the melt-metal interface becomes coated with a film of nickel oxide (NiO) The
corrosion rates of Ni in NaOH under the experimental conditions are notably high, 2-3 orders of magnitude higher than those reported at lower temperatures. Keywords: Corrosion;
Nickel; Sodium hydroxide
1. Introduction
Nickel and nickel-based alloys are well known for their ability to resist corrosive environments, even under severe operating conditions. They offer good resistance to caustic alkalis and show remarkable immunity to caustic stress corrosion cracking [ 11. Specifically, in the case of resisting highly concentrated NaOH, nickel ranks next to silver. For this reason nickel (and also its alloys) is widely used in NaOH service [ 1,2]. Detailed investigations have been conducted by many workers [ 2261 on the corrosion of nickel caused by NaOH. All of these reports record very low corrosion rates, typically fractions of a millimeter per year. The present report contains studies on an experimental system which involves the reactions between metallic nickel and hot NaOH melt at high temperatures (above 700 “C) It is observed that the metal surface corrodes heavily and the nickel dissolved in NaOH forms some reaction products, in addition to the formation of an oxide coating at the metalalkali interface.
2. Experimental Details of the experimental system have already been given in a previous report [ 71. In this experiment, NaOH is melted in nickel crucibles inside an electric furnace. Cup-shaped crucibles ( 15 ml vohrme) made of pure nickel, and Analar grade anhydrous NaOH, are used as the reaction materials. 0254-0584/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSD10254~0584(95)01708-1
The furnace used is the vertical muffle type, with temperature control facilities. The furnace is set at the required temperature ( > 700 “C), and the crucible containing NaOH is kept inside the muffle for a few minutes. The action of NaOH on the nickel surface is visually observable. The crucible surface becomes tarnished owing to a coating of some kind of film, possibly an oxide. On cooling the melt, it can be observed that some reaction products are separating out. When the experiment is repeated with the same crucible, not much change is observed in the thickness of the film, but nickel continues to react with NaOH to form the reaction products. The kinetics of the dissolution in the present case has also been studied. The fresh crucible is allowed to react with NaOH at high temperatures for a sufficient period of time so that a uniform black coating is formed over the surface. It is then cleaned thoroughly, dried and weighed in a 5-digit metallar balance. One gram of NaOH is put in the crucible and inserted in the furnace, which is maintained at the constant experimental temperature. The reaction time is kept as 30 min, to obtain an appreciable dissolution. After this period the crucible is taken out and cooled, and the NaOH mass is dissolved away in a suitable solvent. It is washed, dried and weighed again. The difference in the weights gives the net weight of nickel dissolved, assuming the amount of black coating is preserved after each experiment. Knowing the bottom surface area of the crucible, where it comes in contact with the NaOH melt, the dissolution rate per unit area at the experimental temperature can be calculated. The experiment is repeated at steps of 25 “C within the
172
M. Komarh /Materials Chemistry and Physics 45 (1996) 171-l 75
temperature range 700-850 “C. The lower limit, 700 “C, is selected to obtain detectable dissolution, while the upper limit is fixed at 850 “C to eliminate the error due to evaporation of the melt. The film and the reaction products are characterized using various techniques such as optical and electron microscopy, X-ray diffractometry, atomic emission spectroscopy and infrared spectroscopy.
3. Results 3.1. Reaction kinetics I
A plot of the average dissolution rates versus temperature gave an exponential curve (Fig. 1). The corrosion rates ranged from 22.82 mm y-i at 700 “C to 122.42 mm y-’ at 850 “C. The corresponding Arrhenius plot is given in Fig. 2. It indicates that the dissolution of nickel is of an activated nature, obeying the relation [ 81
700
750
800
TEMPERATURE Fig. 1. Dissolution/corrosion temperature.
850
I
‘C
rates of Ni in NaOH
as a function
of
-7.2.
V=A exp( - E,IkT) where E, is the activation energy of the dissolution reaction, T the temperature, V the dissolution rate, k the Boltzmann constant and A the pre-exponential factor. The activation energy for the dissolution reaction is calculated to be 29.29 kcal mol- ‘.
-7.4L yE V -7.6. g Y J? -7.8-
3.2. Characterization of the reaction products
t -8.0
The film formed over the crucible was analysed in an Xray diffractometer to find its identity. The spectrum (Fig. 3) showed peaks corresponding to NiO along with the peaks of the underlying metal surface [9]. The scanning electron microscopy (SEM) picture of the film surface is shown in Fig. 4. The presence of the reaction products formed in the melt can be visually identified as the emergence of some shining platelets in the NaOH melt during the cooling. These products were separated by dissolving the NaOH mass in absolute alcohol. (Absolute alcohol is preferred, because with water and aqueous-based solvents, there is a risk of decomposition due to hydrolysis, if the products contain sodium). The precipitate was again washed and centrifuged in absolute alcohol and dried in vacuum. On observing under an optical microscope, the product was seen to contain lustrous platelets along with a yellowishbrown powder. The platelets were mostly hexagonal in shape, in the size range 0.1-0.5 mm. Distorted and coalesced platelets were also seen, along with needle-shaped and dendritic crystals. For the chemical identification of the reaction product, first of all, elemental analysis was done employing inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Only sodium and nickel were determined among the elements
0.90
0.!35 lg/~
1.00 K-’
Fig. 2. Arrhenius plot corresponding
to Fig. 1.
that can be identified with the technique. The overall composition was found to be 27.36% Na and 35.75% Ni. To find the specific compounds involved, the product was analysed in an X-ray diffractometer. The XRD spectrum, in the range 10” to 90” of the 28 value, contained as many as 35 peaks, which suggested the presence of a mixture of compounds. From the set of peaks, the corresponding individual compounds were traced out by Hanawalt’s method [lo]. The peak positions and the intensities were noted and compared with those reported [ 91. Among the possible compounds, the X-ray data reported for sodium nickelate ( NaNi02 - monoclinic as well as hexagonal forms), hydrated sodium peroxide ( NaZOz * 8H,O) and sodium carbonate (Na,CO,) matched the present data most satisfactorily (Table 1) . Infrared spectrometry of the product was done as a supporting test. The transmission IR spectrum (KBr pellet technique) scanned in the range 200-4000 cm- ’ is given in Fig. 5. The characteristic peak corresponding to the peroxide linkage (at 875 cm-‘) and that corresponding to the OH group
M. Komath / Materials Chemistry and Physics 45 (1996) 171-l 75
173
3 ANGLE Fig, 3. X-ray diffraction
Fig. 4. Scanning electron micrograph metal surface.
28
spectrum of the coating formed over the nickel metal surface.
of the coating formed over the nickel
of the bound HZ0 (centered around 1500 cm- ‘) appeared in the spectrum. This confirms the presence of Na,O, 8H20. The broad band centered at 3520 cm-i corresponds to the stretching of the OH group. The peaks seen at 570 and 3 15 cm-’ may be those caused by the NiO linkage in NaNiO,. The C03* peaks were totally absent in the spectrum [ 111.
The XRD analysis revealed the crystalline components of the products to be NaNiO, (hexagonal and monoclinic forms), Na,O, .8H,Oand Na2C03. However, the IR results make the occurrence of Na&O, doubtful. Reports show that the compound NaNiOz is of a decomposing nature, which absorbs moisture and CO, from the air to form Na&O, on the surface [ 121. The sodium carbonate that contributes peaks to the XRD spectrum may be that formed by the decomposition of NaNiO, during the analysis procedure. At the same time, the sample for IR analysis was prepared rapidly to avoid or minimise any decomposition of the products. The possibility that Na,C03 is present originally in the reaction product can therefore be ruled out. For confirmation, a hydrolysis test was done. The product obtained was treated with distilled water. The result of the hydrolysis on the product can be anticipated - Na202 will be converted to NaOH (which is soluble in water) and NaNiO, will be transformed to hydroxides and oxyhydroxides of nickel [ 121. Na2C0,, if present, will remain unaffected. The hydrolysed precipitate was separated through centrifuging, washed, dried and subjected to various characterization tests, the results of which were presented in an earlier
Fig. 5. Infrared spectrum of the reaction product.
114
M. Komath / Materials Chemistry and Physics 45 (1996) 171-l 75
Table 1 X-ray diffraction
data of the product obtained and a comparison
d-value (A)
IILn, (%)
d-values of NaNiO, (monoclinic) Card no. 6-63 1 d(A)
1 2 3 4 5
5.4922 5.2023 4.6965 3.4133 3.2219
62.56 100.00 1.15 0.34 0.44
6 I 8 9
2.9593 2.7544 2.6991 2.6101 2.5420
2.48 8.06 1.02 18.14 1.62
10
III, (%)
90
5.250 _
70
2.610 2.590
60 70
_
16 17 18 19 20
2.1893 2.0969 2.0335 2.0040 1.9749
0.81 2.19 0.22 0.33 0.41
_
21 22 23 24 25
1.9476
0.67 0.71 0.20 0.33 0.28
1.737
1.4568 1.4220
0.35 1.42 0.46 0.21 0.31
1.655 1.550 1.502 1.458 1.423
60 90 80 30 60
1.3795 1.3201 1.3069 1.2483 1.1983
0.90 0.55 2.35 0.25 0.18
1.371 1.320 1.305 1.249 1.207
50 70 50 70 70
31 32 33 34 35
(%)
d-values of NaPOz. 8H,O Card no. 15-32
d-values of Na,CO, Card no. 18-1208
d(A)
d(A)
I/I, (%) 90
5.50
2.420
_
2.99
60
2.71
100
60 50
2.57
95
70
2.42
3.43 3.28
16 18
2.91 2.75 2.62 2.61 2.55
100 30 40 45 65
2.31 2.26
100 50 -
40 -
_ 90
I/I, (%)
90
_
2.470 2.380 _
1.6592 1.5547 1.5025
_
5.240
_
0.62 0.97 2.94 1.03 2.35
26 27 28 29 30
_
_ _ _
2.4487 2.3970 2.3657 2.2552 2.2335
1.965
III,
_
_
2.100
d (A)
_
2.620
2.230
d-values of NaNiOz (hexagonal) Card no. 6-632
_
11 12 13 14 15
1.8828 1.7402 1.7092 1.6782
compounds
Reported diffraction data for the anticipated compounds (from JCPDS data cards)
X-ray diffraction data of the reaction product Peak no.
with the reported data for anticipated
-
_
-
2.130
100
1.978
tr. _
2.20 100 _ 20
35 -
2.04 -
12
1.96
35
1.89
30
1.71 1.68
14 14
60
1.93 40 1.684
60
1.557 1.475
70 70
_
1.423
50
1.342 1.313 1.289 1.250 1.216
60 30 60 20 60
report [ 131. The precipitate contained only y-NiOOH and /3Ni(OH)*.
4. Discussion The results of the kinetics study are very interesting. The corrosion rates recorded under the experimental conditions are notably higher (at least 3 orders of magnitude) than those reported at lower temperatures. The reported range of Ni dissolution in 75-100% NaOH solution at 250-475 “C is 0.05-0.06 mm y-i [2-51. Barkel [6] reported the highest rate to be 0.8 mm y -‘, in boiling 25% NaOH solution.
-
-
_ _ _
_ _
_
The heavy corrosion observed in this case is related to the factor that accelerates the Ni-NaOH reaction. In an earlier study, Williams et al. [ 141 observed that the high-temperature reaction of hot NaOH with Ni metal results in the oxidation of the metal surface to NiO along with the evolution of hydrogen: Ni + 2NaOH -
NiO + Na,O + H2
Formation of black NiO coatings over the nickel surface during the Ni-NaOH reaction has also been reported by others [2], and the mechanism of formation of this layer has been discussed [ 15,161. The NiO layer is proved to be a
M. Komuth /Materials
Chemistry and Physics 45 (1996) 171-l 75
passive one, and believed to impart good protection against further corrosion [ I]. Dyer et al. [ 121, on reacting NaOH with nickel in the presence of oxygen, observed the formation of NaNiO,. No detailed study on the reaction mechanism has been conducted. In the present experiment, NiO formation can be observed on the surface of the crucible. On continuing the reaction, nickel comes into the NaOH melt to form NaNiO,. Following Molenda and Stoklosa [ 171, a possible mechanism may be proposed. These workers made NiO react with Na,O, at 700 “C in the presence of oxygen to synthesize NaNiO,. Since the present experimental conditions are similar, it is possible that the NiO and Na20, obtained during the Ni-NaOH reaction react further to form NaNiO,. This is just a tentative mechanism; other possibilities cannot be ruled out. An extensive study is needed to confirm the mechanism that leads to the heavy corrosion of nickel.
Acknowledgements
The author is thankful to Dr K.A. Cherian and Dr Arabinda Ray for their interest in the work and encouragements. This work is carried out under the financial support of the CSIR, India.
17.5
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