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Corrosion behavior of rf-sputtered Ni45Cr55 film
Volume
4, number
MATERIALS
5,6,7
CORROSION BE~VIOR R.S. BHATTACHARYA, Unioersui Energy System, Received
OF ~-SPU~ERED C.N. RAFFOUL
Inc., 4401 Dayton-Xenia
July 1986
LETTERS
Ni,Cr,,
FILM
and A.K. RAI
Road, Dayton, ON 45432, USA
7 April 1986
Homogeneous Nid5CrSS films of thickness = 6800 A have been fabricated by rf sputtering and their corrosion behavior was investigated in acidic and basic solutions. The sputtered films showed very low corrosion rates in oxidizing acid solutions. However, the corrosion resistance in Cl-containing solution has been found to be rather poor due to pitting attack.
A considerable number of commercial nickelchromium (Ni-Cr) alloys are available for various ap plications [ 11. The most widely used Ni-Cr alloy in chemical and other process industries is Inconel alloy 600 which contains about 76 wt% Ni, 15.5 wt% Cr and about 8 wt% Fe as major constituents. Iron is being used primarily to reduce cost. This alloy shows good corrosion resistance in highly oxidizing acid solutions as well as in chloride containing stress corrosion cracking environment. Increasing Cr content apparently provides higher resistance to oxidizing, carbur~zing and sulfidation at elevated temperatures. In many of the industrial applications requiring stronger materials with better corrosion resistance, one can use cheaper substrate with low Cr content but desirable mechanical properties if it can be coated with highCr-based alloy suitable for providing desirable corrosion resistant behavior. Sputtering techniques [2] such as rf and magnetron have met with increasing acceptance in industrial deposition applications owing to their high deposition rates, minimum substrate heating and extreme versatility. Since no melting is involved in the sputtering process, it is possible to deposit multicomponent films of desired composition by fabricating the sputtering target from alloys of powders. Very few studies have been performed on the corrosion behavior of sputter-deposited coatings. We have undertaken a systematic study of the corrosion behavior of some Ni-based alloy coatings fabricated by rf sputtering, the final objective being industrial applications as protective 0 167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (Noah-Homed Physics Pub~sh~g Division)
layers. In this paper, results concerning the fabrication of rf-sputter-deposited NiCr films and their corrosion behavior in acid and salt solutions are reported. Substrate Ni samples were cut into 1 cm X 2 cm pieces from 0.5 mm rolled Ni sheet stock of 99.997% purity. These samples were polished to mirror finish and cleaned in trichloroethylene, acetone and deionized water prior to film deposition. rf-sputter deposition was performed by using the deposition system at the University of Illinois, Urbana-Champaign. Both target and substrate were sputter cleaned for 45 min at bias voltages of - 1 kV and -120 V respectively prior to film deposition using argon discharge. The deposition wasmade at a base pressure of 5 X lo-? Torr. A homogeneous Ni45 Cr55 film of thickness ~6800 A was deposited from a composite Ni-fr target on polished Ni as well as on polished device-quality Si substrates. The overall composition of the fiIm and homogeneity were determined from Rutherford backscattering measurements. The deposition on Si was made for the convenience of sample preparation for tran~ission electron microscopic (TEM) analysis. Cross-section specimens were prepared by ion milling using a 5 keV Ar’ beam at a shallow angle. Electrochemical experiments were performed with an EG&G PARC model 332 Soft Corr corrosion software in conjunction with a PARC model 173 potentiostat/galvanostat, in solutions of 1 N HNO, .0.1 N NaCl and 1 N H,SO, at 25°C. Figs. 1a and 1b show the transmission electron diffraction (TED) pattern and the cross-section bright 271
July 1986
MATERIALS LETTERS
Volume 4, number 5,6,7
SURFKE
Fig. 1. (a) TED pattern and (b) cross-section bright field micrograph of the sputter deposited Nick film on a Si substrate.
field micrograph of the sputter deposited NiCr film on a Si substrate. The presence of broken diffraction rings in the TED pattern indicates the highly textured nature of the film. The Bragg diffraction spots present in the TED pattern are due to underlying Si substrate and they serve as a standard pattern for analysing the diffraction rings. The analysis of the TED pattern shows the presence of a tetragonal Ni-Cr phase hav-
ing lattice parameters a = 6.42 A and c = 4.61 A. The cross-section micrograph shows the columnar nature of the grains with the long axis of the grains lying perpendicular to the surface. The potentiodynamic polarization curves of the sputtered film along with those of pure Ni and Cr in 1 N HNO, solution are shown in fig. 2. The polarization curves of Ni and Ni-Cr film are similar, showing
.
N,4,Cr05
SPUTTERED
0 NI l cr 1000 -
.IN 700
-
400
-
NaCl
s E
Y v) ? w
100 -
-200
-5001 -2 IO
1
lo-’
I
I
IO0 IO’ CURRENT
I
to2 DENSITY
I
IO’
I
IO4
I
IO’
J IO6
(p A/cm’)
Fig. 2. Potentiodynamic polarization curves of the sputtered NiCr film, pure Ni and Cr metals in 1 N HNOa solution.
212
-
16’
IO0
CURRENT
IO’
IO2
DENSITY
IO’
IO4
(pA/cm2)
Fig. 3. Potentiodynamic polarization curves of the sputtered NiCr film, pure Ni and Cr metals in 0.1 N NaCl solution.
Volume 4, number 5,6,7
MATERIALS LETTERS
active dissolution followed by passivation. However, the polarization curve of Cr does not show an active passive transition. Passivity in Cr is readily attained at a much lower current density than in Ni. Sputtered NiCr showed a significantly lower corrosion rate than Ni. The critical current density for passivation for NiCr has been reduced to about three orders of magnitude compared to that of Ni. The passivation occurred at about 300 mV for both Ni and NiCr, but at a lower current density for NiCr. The passivation current density for NiCr is about the same as that for Cr. The behavior of NiCr film can be tentatively ascribed to dissolution of Ni from the surface and the gradual formation of passive Cr,03 layer. The potentiodynamic polarization curves of Ni, Cr and NiCr film in 0.1 N NaCl solution are shown in fig. 3. The Ni and the sputtered NiCr film show an active dissolution, although Cr showed a small range of passivation in the voltage range below x650 mV. It is obvious that the NiCr film undergoes pitting and crevice corrosion similar to that of Ni, but only at a lower rate. This was confirmed by viewing the corroded surface under the optical microscope. Fig. 4 shows the potentiodynamic polarization curve of Ni, Cr and NiCr film in 1 N H,SO4 solution. Both Ni and Cr showed a similar behavior compared to that in 1 N HNO, solution, although passivation for Ni is attained at a lower current density. The NiCr film showed a significantly lower corrosion rate compared to both Ni and Cr. The current density below about 600 mV was so low that it was not measurable with our instrument. Myers et al. [3] have reported the anodic polarization behavior of bulk NiCr alloys of various compositions in HZ-saturated H,SO4 solutions. Our result of NiCr film (Ni45Cr55) is in general agreement with their result on Ni49Cr51 except that they observed active dissolution followed by passivation, although the critical current density was rather low (~10 pA/cm2). The only difference in the experimental condition was that their solution was saturated with H,. In conclusion, sputtered Ni45Cr55 alloy film exhibits significantly low corrosion rate in oxidizing acid
July 1986
1100
5
700
E Y ul ;
300
w
-100
-500
1 16”
I
,
14’
IO0
\ IO’
CURRENT
I 102
IO’
DENSITY
I
I
IO4
IO5
IO6
(pA/cm2)
Fig. 4. Potentiodynamic polarization curves of the sputtered NiCr film, pure Ni and Cr metals in 1 N Hz SO4 solution.
solutions. However, the corrosion resistance in Clcontaining solution is rather poor. Thus, the sputtered NiCr coating can be used in chemical or other industry on components subjected to highly oxidizing environment. This research was supported by the US Department of Energy under SBIR contract #DE-AC01 -83ER-80020.
References [II W.Z. Friend, Corrosion of nickel and nickel-base alloys (Wiley, New York, 1980) p. 136. PI R.F. Bunshah, ed., Deposition technologies for films and [31
coatings (Noyes Publications, NJ, 1982). 3.R. Myers, F.H. Beck and M.G. Fontana, (1965) 277.
Corrosion
2
273
4, number
MATERIALS
5,6,7
CORROSION BE~VIOR R.S. BHATTACHARYA, Unioersui Energy System, Received
OF ~-SPU~ERED C.N. RAFFOUL
Inc., 4401 Dayton-Xenia
July 1986
LETTERS
Ni,Cr,,
FILM
and A.K. RAI
Road, Dayton, ON 45432, USA
7 April 1986
Homogeneous Nid5CrSS films of thickness = 6800 A have been fabricated by rf sputtering and their corrosion behavior was investigated in acidic and basic solutions. The sputtered films showed very low corrosion rates in oxidizing acid solutions. However, the corrosion resistance in Cl-containing solution has been found to be rather poor due to pitting attack.
A considerable number of commercial nickelchromium (Ni-Cr) alloys are available for various ap plications [ 11. The most widely used Ni-Cr alloy in chemical and other process industries is Inconel alloy 600 which contains about 76 wt% Ni, 15.5 wt% Cr and about 8 wt% Fe as major constituents. Iron is being used primarily to reduce cost. This alloy shows good corrosion resistance in highly oxidizing acid solutions as well as in chloride containing stress corrosion cracking environment. Increasing Cr content apparently provides higher resistance to oxidizing, carbur~zing and sulfidation at elevated temperatures. In many of the industrial applications requiring stronger materials with better corrosion resistance, one can use cheaper substrate with low Cr content but desirable mechanical properties if it can be coated with highCr-based alloy suitable for providing desirable corrosion resistant behavior. Sputtering techniques [2] such as rf and magnetron have met with increasing acceptance in industrial deposition applications owing to their high deposition rates, minimum substrate heating and extreme versatility. Since no melting is involved in the sputtering process, it is possible to deposit multicomponent films of desired composition by fabricating the sputtering target from alloys of powders. Very few studies have been performed on the corrosion behavior of sputter-deposited coatings. We have undertaken a systematic study of the corrosion behavior of some Ni-based alloy coatings fabricated by rf sputtering, the final objective being industrial applications as protective 0 167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (Noah-Homed Physics Pub~sh~g Division)
layers. In this paper, results concerning the fabrication of rf-sputter-deposited NiCr films and their corrosion behavior in acid and salt solutions are reported. Substrate Ni samples were cut into 1 cm X 2 cm pieces from 0.5 mm rolled Ni sheet stock of 99.997% purity. These samples were polished to mirror finish and cleaned in trichloroethylene, acetone and deionized water prior to film deposition. rf-sputter deposition was performed by using the deposition system at the University of Illinois, Urbana-Champaign. Both target and substrate were sputter cleaned for 45 min at bias voltages of - 1 kV and -120 V respectively prior to film deposition using argon discharge. The deposition wasmade at a base pressure of 5 X lo-? Torr. A homogeneous Ni45 Cr55 film of thickness ~6800 A was deposited from a composite Ni-fr target on polished Ni as well as on polished device-quality Si substrates. The overall composition of the fiIm and homogeneity were determined from Rutherford backscattering measurements. The deposition on Si was made for the convenience of sample preparation for tran~ission electron microscopic (TEM) analysis. Cross-section specimens were prepared by ion milling using a 5 keV Ar’ beam at a shallow angle. Electrochemical experiments were performed with an EG&G PARC model 332 Soft Corr corrosion software in conjunction with a PARC model 173 potentiostat/galvanostat, in solutions of 1 N HNO, .0.1 N NaCl and 1 N H,SO, at 25°C. Figs. 1a and 1b show the transmission electron diffraction (TED) pattern and the cross-section bright 271
July 1986
MATERIALS LETTERS
Volume 4, number 5,6,7
SURFKE
Fig. 1. (a) TED pattern and (b) cross-section bright field micrograph of the sputter deposited Nick film on a Si substrate.
field micrograph of the sputter deposited NiCr film on a Si substrate. The presence of broken diffraction rings in the TED pattern indicates the highly textured nature of the film. The Bragg diffraction spots present in the TED pattern are due to underlying Si substrate and they serve as a standard pattern for analysing the diffraction rings. The analysis of the TED pattern shows the presence of a tetragonal Ni-Cr phase hav-
ing lattice parameters a = 6.42 A and c = 4.61 A. The cross-section micrograph shows the columnar nature of the grains with the long axis of the grains lying perpendicular to the surface. The potentiodynamic polarization curves of the sputtered film along with those of pure Ni and Cr in 1 N HNO, solution are shown in fig. 2. The polarization curves of Ni and Ni-Cr film are similar, showing
.
N,4,Cr05
SPUTTERED
0 NI l cr 1000 -
.IN 700
-
400
-
NaCl
s E
Y v) ? w
100 -
-200
-5001 -2 IO
1
lo-’
I
I
IO0 IO’ CURRENT
I
to2 DENSITY
I
IO’
I
IO4
I
IO’
J IO6
(p A/cm’)
Fig. 2. Potentiodynamic polarization curves of the sputtered NiCr film, pure Ni and Cr metals in 1 N HNOa solution.
212
-
16’
IO0
CURRENT
IO’
IO2
DENSITY
IO’
IO4
(pA/cm2)
Fig. 3. Potentiodynamic polarization curves of the sputtered NiCr film, pure Ni and Cr metals in 0.1 N NaCl solution.
Volume 4, number 5,6,7
MATERIALS LETTERS
active dissolution followed by passivation. However, the polarization curve of Cr does not show an active passive transition. Passivity in Cr is readily attained at a much lower current density than in Ni. Sputtered NiCr showed a significantly lower corrosion rate than Ni. The critical current density for passivation for NiCr has been reduced to about three orders of magnitude compared to that of Ni. The passivation occurred at about 300 mV for both Ni and NiCr, but at a lower current density for NiCr. The passivation current density for NiCr is about the same as that for Cr. The behavior of NiCr film can be tentatively ascribed to dissolution of Ni from the surface and the gradual formation of passive Cr,03 layer. The potentiodynamic polarization curves of Ni, Cr and NiCr film in 0.1 N NaCl solution are shown in fig. 3. The Ni and the sputtered NiCr film show an active dissolution, although Cr showed a small range of passivation in the voltage range below x650 mV. It is obvious that the NiCr film undergoes pitting and crevice corrosion similar to that of Ni, but only at a lower rate. This was confirmed by viewing the corroded surface under the optical microscope. Fig. 4 shows the potentiodynamic polarization curve of Ni, Cr and NiCr film in 1 N H,SO4 solution. Both Ni and Cr showed a similar behavior compared to that in 1 N HNO, solution, although passivation for Ni is attained at a lower current density. The NiCr film showed a significantly lower corrosion rate compared to both Ni and Cr. The current density below about 600 mV was so low that it was not measurable with our instrument. Myers et al. [3] have reported the anodic polarization behavior of bulk NiCr alloys of various compositions in HZ-saturated H,SO4 solutions. Our result of NiCr film (Ni45Cr55) is in general agreement with their result on Ni49Cr51 except that they observed active dissolution followed by passivation, although the critical current density was rather low (~10 pA/cm2). The only difference in the experimental condition was that their solution was saturated with H,. In conclusion, sputtered Ni45Cr55 alloy film exhibits significantly low corrosion rate in oxidizing acid
July 1986
1100
5
700
E Y ul ;
300
w
-100
-500
1 16”
I
,
14’
IO0
\ IO’
CURRENT
I 102
IO’
DENSITY
I
I
IO4
IO5
IO6
(pA/cm2)
Fig. 4. Potentiodynamic polarization curves of the sputtered NiCr film, pure Ni and Cr metals in 1 N Hz SO4 solution.
solutions. However, the corrosion resistance in Clcontaining solution is rather poor. Thus, the sputtered NiCr coating can be used in chemical or other industry on components subjected to highly oxidizing environment. This research was supported by the US Department of Energy under SBIR contract #DE-AC01 -83ER-80020.
References [II W.Z. Friend, Corrosion of nickel and nickel-base alloys (Wiley, New York, 1980) p. 136. PI R.F. Bunshah, ed., Deposition technologies for films and [31
coatings (Noyes Publications, NJ, 1982). 3.R. Myers, F.H. Beck and M.G. Fontana, (1965) 277.
Corrosion
2
273