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The pitting corrosion behavior of aluminum ion implanted with tantalum
Nuclear Instruments North-Holland
and Methods
in Physics Research
The pitting corrosion with tantalum P.M. Natishan, E. McCafferty VaualResearch Laboramy, Washington.
B59/60
behavior
841
(1991) 841-844
of aluminum
ion implanted
and G.K. Hubler DC 20375, USA
The corrosion behavior of Ta-Al surface alloys produced by ion implantation at room and liquid nitrogen temperatures was examined in de-aerated, O.lM NaCI. Surface analysis using Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy showed that the surface concentration of Ta was greater in samples that were ion implanted at liquid nitrogen temperatures. Anodic polarization curves showed that Ta ion implanted into Al at room temperature and at liquid nitrogen temperature increased the pitting potential of Al by 50 mV and 175 mV, respectively. From the standpoint of the pH of zero charge model, this behavior is explained by the presence of the implanted cations in the aluminum-oxide lattice.
I. Introduction The mechanism of anion adsorption, the first step in passive film breakdown leading to localized corrosion, has been described by a model based on surface charge considerations and the pH of zero charge of an oxide, pH,,, [l-2]. Briefly, at the pH,, the surface of an oxide has no net charge. At pH values lower than the pH,,,, the surface has a net positive charge and anions such as chloride are electrostatically attracted to the surface and can become incorporated into the oxide film. The incorporated anions cause film disruption and loss of passivIty. At pH values higher than the pH,,,. the surface has 3 net negative charge and anion adsorption is inhibited. The PH,,, model predicts that the surface charge of ur oxide such as aluminum oxide can be changed by Introducing other oxide-forming elements into the oxide film and tion
and
thereby the
increasing
susceptibility
or decreasing to localized
anion
adsorp-
corrosion.
Ex-
for binary aluminum surface alloys produced by ion implantation provide qualitative support for this model [1,2]. One factor that prevented a more quantitative correlation between pitting potentials and the pH,,, was that the surface concentration of the implanted species was lower than desired and varied with the implanted species. Also, in some cases, the surface concentration of the implanted species was independent of the implantation dose. For example, when MO was ion implanted under conditions that were to yield nominal surface concentrations of 4, 12, or 20 at.%, the actual surface concentration for all alloys was approximately 1 at.% [1.2]. These effects were caused by radiation enhanced diffusion or radiation induced segregation. Similar effects were observed on ion beam mixed samples perimental
[3]. Thus, the ability to control the surface concentration of alloying elements during ion implantation and ion beam mixing was complicated by ion induced migration phenomenon. It is known that vacancy defects are mobile at room temperature in Al, so the observations of ion induced migration are not surprising. Because Al appeared in large concentrations at the surface [3], it was possible to deduce that the mobile species was Al. In order to further improve the pitting corrosion behavior of aluminum and to further study the pH,,, model, it is important to produce surface alloys with higher, more consistent concentrations of the alloying element. It should be possible to control the surface concentration of the alloying element by performing cold implantations. This communication reports on the behavior of Ta-Al surface alloys produced by ion implantation at room and at liquid nitrogen temperatures.
results
0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
2. Experimental Samples were cut from a $ in. diameter aluminum rod (99.999% pure) and were polished automatically. The final polish was performed on a polishing wheel using 3 pm diamond spray. After polishing, the aluminum samples were implanted with Ta ions at a dose of 4 x lOI6 ions/cm* at 100 keV. The samples were implanted at liquid nitrogen and room temperatures. Samples were analyzed before and after pitting by Rutherford backscattering spectroscopy (RBS) and Xray photoelectron spectroscopy (XPS). RBS profiles were obtained with a 2 MeV alpha-particle beam produced by the NRL 3 MV tandem Van de Graaff accelerator. A glancing angle of detection of 8’ from the
B.V. (North-Holland)
VII. METALS
/ TRIBOLOGY
P.M. Natishan et al. / Corrosion behavior of Ta-implanted Al
842 ENERGY 0.6 I
0.8
1.0 I
- - -
Ta(AT)AI
surface plane was used to enhance the depth resolution: the scattering angle was 135 O. X-ray photoelectron spectroscopy (XPS) measurements were made using a monochromatic Al Ka X-ray source. The spot size was 600 ym. the pass energy was 100 eV. and the base pressure was 8 X 10m9 Torr or better. The XPS spectra were corrected for charge shifts by normalizing binding energies to that of the adventitious carbon Is peak at 284.6 eV [4]. Samples were attached to electrode holders, and the sample sides and the edge of the face to be tested were masked with several coats of an alkyd varnish. The electrochemical measurements were made in a deaerated O.lM NaCl solution using a conventional corrosion cell. The solution pH was 5.8. The samples were immersed in the solution for 24 h prior to polarization to establish a steady state open circuit potential. The pitting potentials were then determined potentiostatitally by stepping the potential in 25 to 50 mV incre-
(MeV)
1.2 I
1.4 I
1.6 I
1.6 I
1
\ 200
300
400
CHANNEL
Fig. 1. Rutherford backscattering spectroscopy spectra for aluminum ion implanted with tantalum at room temperature (dashed line) and at liquid nitrogen temperature (solid line).
0
0
9 SPUTTER
Fig. 2. X-ray photoelectron
spectroscopy
0
Fig. 3. X-ray photoelectron
spectroscopy
TIME (SECONDS)
depth concentration profile for an aluminum nitrogen temperature.
sample
ion implanted
with tantalum
at liquid
900 SPUTTER
depth concentration
TIME (SECONDS)
profile
for an aluminum
temperature.
sample
ion implanted
with tantalum
at room
843
P.M. Natishan et al. / Corrosion behavior of Ta-implanted AI ments from the corrosion potential in the anodic direction allowing the current to reach steady state values. Usually 16 to 20 min were required at each potential.
103
62
0.1MNaCl Ta(RT)AI
P
3. Results and discussion 3.1, Surface analysis Fig. 1 shows RBS profiles for aluminum that was ion implanted with Ta at room and liquid nitrogen temperatures. The peak concentration of Ta for the Al sample ion implanted with Ta at liquid nitrogen temperatures, Ta(LN,)Al, is lower than that of the Al sample ion implanted with Ta at room temperature Ta(RT)Al. The peak concentrations for the Ta(LN,)Al and Ta(RT)Al samples occur at approximately the same depth. Also, the RBS profile for the Ta(LN,)Al sample is broader, indicating that more Ta may be present in the near surface region. XPS, which is a more surface sensitive analysis technique, was used to further examine the distribution of Ta in the ion implanted samples. Figs. 2 and 3 show XPS depth concentration profiles for the Ta(LN,)Al and Ta(RT)AI samples. These spectra are similar to the RBS spectra in that they show that the peak concentrations of Ta for the Ta(LN,)Al and Ta(RT)Al samples occur at approximately the same depth. The XPS curves also show a greater concentration of Ta in the nearsurface region of the Ta(LN,)Al sample. In addition, the Ta concentration of the Ta(LN,)Al sample rises more quickly than that of the Ta(RT)Al sample. Table 1 lists concentration values at selected sputter intervals. The sputter rate was approximately 15 A/min. The values listed in table 1. again, show that ion implantation at liquid nitrogen temperature produced higher concentrations of the implanted species in the nearsurface region.
Table 1 Concentration of Ta in samples implanted at room ture and at liquid nitrogen temperature as determined photoelectron spectroscopy Sputter
time *
Ta concentration
temperaby X-ray
[at.&]
[sl
Room temp.
Liquid Nz temp.
0 30 60 90 120 150 180
0.4 0.6 1.0 1.8 2.5 4.0 6.3
0.75 1.8 3.5 5.5 7.5 10.6 13.3
a Sputter
rate is approximately
15 A/min.
1
Potential (Vsce) Fig. 4. Anodic polarization curves for aluminum, aluminum ion implanted with tantalum at room temperature. and aluminum ion implanted with tantalum at liquid nitrogen temperature.
3.2. Anodic polarization Fig. 4 shows anodic polarization curves for Al. a Ta(RT)Al sample, and a Ta(LN,)Al sample. The important characteristic of each curve is the pitting potential. i.e. the electrode potential at which there is a sudden increase in current density due to the initiation of corrosion pits. At potentials below (less positive) than the pitting potential, pits do not initiate or grow. A higher (more positive) pitting potential represents an increased resistance to pitting corrosion. The effect of implanting Ta into Al at room and liquid nitrogen temperatures was to increase the pitting potential of Al by 50 mV and 150 mV, respectively. Table 2 lists average pitting potential values for the various samples. The average pitting potential values for the samples implanted at room temperature and at liquid nitrogen temperature were 50 mV and 175 mV more positive than the pitting potential value for pure aluminum. As noted above. the pH,, affects the surface charge and therefore, the adsorption characteristics of an oxide. In solutions of pH less than 9, the surface of aluminum oxide consists of acidic sites [5.6] which are receptors for Lewis bases such as Cl-. In solutions with a pH
Table 2 Pitting potentials aluminum Implanted ion
for
aluminum
Conditions
and
Ta
ion
Dose [XlO’b ions/cm*]
_
_
Ta Ta
room temp. liquid N, temp.
4 4
a Compared
with the pitting
potential
implanted
A.$,,
a
[VI -0.700 - 0.650 - 0.525
+ 0.050 +0.175
of pure aluminum.
VII. METALS
,’ TRIBOLOC
Y
844
P.M. Nalishan et al. / Corrosion behaoior of Ta-implanred
greater than 2.8. tantalum oxide is composed of basic sites [7] to which Cl- would not be attracted. Thus, the addition of tantalum oxide, which acts as a Lewis base, into the aluminum oxide should increase the pitting potential, as was observed. As shown in figs. 2 and 3. and previously [1.2], the implanted cations are contained in the surface oxide film so that the effect of ion implantation is to replace a portion of the aluminum-oxygen bonds in the passive film with bonds formed between oxygen and the implanted ions. Therefore, ion beam modification offers the possibility of inhibiting Cl- ion adsorption by changing the pH,,, of the surface thereby extending the passive range.
Al
the pH of zero charge model this behavior is explained by the presence of the implanted cations in the aluminum oxide lattice.
Acknowledgements
The authors gratefully acknowledge the financial support and technical interaction of A. John Sedriks. Office of Naval Research, Arlington, Virginia. The authors also acknowledge the contributions of Randy Walker and Gabrielle T. Peace in sample preparation and electrochemical testing.
4. Summary
References
Surface analysis using Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy showed that ion implantation at liquid nitrogen temperature produced a higher concentration of Ta in the near-surface region compared with ion implantation at room temperature. In both cases, the concentrations at the outer portion of the oxide film were still relatively low. Anodic polarization showed that the average pitting potential value for the Ta(LN,)Al samples was 175 mV higher than the average value obtained for the Ta(RT)Al samples, indicating that even a small increase in the surface concentration of Ta can improve the pitting behavior of aluminum. From the standpoint of
PI P.M. Natishan, E. McCafferty and G.K. Hubler, J. Electrothem. Sot. 135 (1988) 321.
PI P.M. Natishan. E. McCafferty and G.K. Hubler. Mater. Sci. Eng. All6 (1989) 41. [31 P.M. Natishan, E. McCafferty and G.K. Hubler, Con. Sci.. in press. [41 C.D. Wagner, in: Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, eds. D. Briggs and M.P. Seah (Wiley. New York. 1983) p. 478. 2nd. ed. [51 W. Stumm and J.J. Morgan, Aquatic Chemistry, (Wiley. New York, 1981) pp. 5999640. [61 G.A. Parks, Chem. Rev. 65 (1965) 177. V.A. Kolesnikov. A.F. Gubin and A.A. [71 G.A. Kokarev. Korobanov, Sov. Electrochem. 18 (1982) 407.
and Methods
in Physics Research
The pitting corrosion with tantalum P.M. Natishan, E. McCafferty VaualResearch Laboramy, Washington.
B59/60
behavior
841
(1991) 841-844
of aluminum
ion implanted
and G.K. Hubler DC 20375, USA
The corrosion behavior of Ta-Al surface alloys produced by ion implantation at room and liquid nitrogen temperatures was examined in de-aerated, O.lM NaCI. Surface analysis using Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy showed that the surface concentration of Ta was greater in samples that were ion implanted at liquid nitrogen temperatures. Anodic polarization curves showed that Ta ion implanted into Al at room temperature and at liquid nitrogen temperature increased the pitting potential of Al by 50 mV and 175 mV, respectively. From the standpoint of the pH of zero charge model, this behavior is explained by the presence of the implanted cations in the aluminum-oxide lattice.
I. Introduction The mechanism of anion adsorption, the first step in passive film breakdown leading to localized corrosion, has been described by a model based on surface charge considerations and the pH of zero charge of an oxide, pH,,, [l-2]. Briefly, at the pH,, the surface of an oxide has no net charge. At pH values lower than the pH,,,, the surface has a net positive charge and anions such as chloride are electrostatically attracted to the surface and can become incorporated into the oxide film. The incorporated anions cause film disruption and loss of passivIty. At pH values higher than the pH,,,. the surface has 3 net negative charge and anion adsorption is inhibited. The PH,,, model predicts that the surface charge of ur oxide such as aluminum oxide can be changed by Introducing other oxide-forming elements into the oxide film and tion
and
thereby the
increasing
susceptibility
or decreasing to localized
anion
adsorp-
corrosion.
Ex-
for binary aluminum surface alloys produced by ion implantation provide qualitative support for this model [1,2]. One factor that prevented a more quantitative correlation between pitting potentials and the pH,,, was that the surface concentration of the implanted species was lower than desired and varied with the implanted species. Also, in some cases, the surface concentration of the implanted species was independent of the implantation dose. For example, when MO was ion implanted under conditions that were to yield nominal surface concentrations of 4, 12, or 20 at.%, the actual surface concentration for all alloys was approximately 1 at.% [1.2]. These effects were caused by radiation enhanced diffusion or radiation induced segregation. Similar effects were observed on ion beam mixed samples perimental
[3]. Thus, the ability to control the surface concentration of alloying elements during ion implantation and ion beam mixing was complicated by ion induced migration phenomenon. It is known that vacancy defects are mobile at room temperature in Al, so the observations of ion induced migration are not surprising. Because Al appeared in large concentrations at the surface [3], it was possible to deduce that the mobile species was Al. In order to further improve the pitting corrosion behavior of aluminum and to further study the pH,,, model, it is important to produce surface alloys with higher, more consistent concentrations of the alloying element. It should be possible to control the surface concentration of the alloying element by performing cold implantations. This communication reports on the behavior of Ta-Al surface alloys produced by ion implantation at room and at liquid nitrogen temperatures.
results
0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
2. Experimental Samples were cut from a $ in. diameter aluminum rod (99.999% pure) and were polished automatically. The final polish was performed on a polishing wheel using 3 pm diamond spray. After polishing, the aluminum samples were implanted with Ta ions at a dose of 4 x lOI6 ions/cm* at 100 keV. The samples were implanted at liquid nitrogen and room temperatures. Samples were analyzed before and after pitting by Rutherford backscattering spectroscopy (RBS) and Xray photoelectron spectroscopy (XPS). RBS profiles were obtained with a 2 MeV alpha-particle beam produced by the NRL 3 MV tandem Van de Graaff accelerator. A glancing angle of detection of 8’ from the
B.V. (North-Holland)
VII. METALS
/ TRIBOLOGY
P.M. Natishan et al. / Corrosion behavior of Ta-implanted Al
842 ENERGY 0.6 I
0.8
1.0 I
- - -
Ta(AT)AI
surface plane was used to enhance the depth resolution: the scattering angle was 135 O. X-ray photoelectron spectroscopy (XPS) measurements were made using a monochromatic Al Ka X-ray source. The spot size was 600 ym. the pass energy was 100 eV. and the base pressure was 8 X 10m9 Torr or better. The XPS spectra were corrected for charge shifts by normalizing binding energies to that of the adventitious carbon Is peak at 284.6 eV [4]. Samples were attached to electrode holders, and the sample sides and the edge of the face to be tested were masked with several coats of an alkyd varnish. The electrochemical measurements were made in a deaerated O.lM NaCl solution using a conventional corrosion cell. The solution pH was 5.8. The samples were immersed in the solution for 24 h prior to polarization to establish a steady state open circuit potential. The pitting potentials were then determined potentiostatitally by stepping the potential in 25 to 50 mV incre-
(MeV)
1.2 I
1.4 I
1.6 I
1.6 I
1
\ 200
300
400
CHANNEL
Fig. 1. Rutherford backscattering spectroscopy spectra for aluminum ion implanted with tantalum at room temperature (dashed line) and at liquid nitrogen temperature (solid line).
0
0
9 SPUTTER
Fig. 2. X-ray photoelectron
spectroscopy
0
Fig. 3. X-ray photoelectron
spectroscopy
TIME (SECONDS)
depth concentration profile for an aluminum nitrogen temperature.
sample
ion implanted
with tantalum
at liquid
900 SPUTTER
depth concentration
TIME (SECONDS)
profile
for an aluminum
temperature.
sample
ion implanted
with tantalum
at room
843
P.M. Natishan et al. / Corrosion behavior of Ta-implanted AI ments from the corrosion potential in the anodic direction allowing the current to reach steady state values. Usually 16 to 20 min were required at each potential.
103
62
0.1MNaCl Ta(RT)AI
P
3. Results and discussion 3.1, Surface analysis Fig. 1 shows RBS profiles for aluminum that was ion implanted with Ta at room and liquid nitrogen temperatures. The peak concentration of Ta for the Al sample ion implanted with Ta at liquid nitrogen temperatures, Ta(LN,)Al, is lower than that of the Al sample ion implanted with Ta at room temperature Ta(RT)Al. The peak concentrations for the Ta(LN,)Al and Ta(RT)Al samples occur at approximately the same depth. Also, the RBS profile for the Ta(LN,)Al sample is broader, indicating that more Ta may be present in the near surface region. XPS, which is a more surface sensitive analysis technique, was used to further examine the distribution of Ta in the ion implanted samples. Figs. 2 and 3 show XPS depth concentration profiles for the Ta(LN,)Al and Ta(RT)AI samples. These spectra are similar to the RBS spectra in that they show that the peak concentrations of Ta for the Ta(LN,)Al and Ta(RT)Al samples occur at approximately the same depth. The XPS curves also show a greater concentration of Ta in the nearsurface region of the Ta(LN,)Al sample. In addition, the Ta concentration of the Ta(LN,)Al sample rises more quickly than that of the Ta(RT)Al sample. Table 1 lists concentration values at selected sputter intervals. The sputter rate was approximately 15 A/min. The values listed in table 1. again, show that ion implantation at liquid nitrogen temperature produced higher concentrations of the implanted species in the nearsurface region.
Table 1 Concentration of Ta in samples implanted at room ture and at liquid nitrogen temperature as determined photoelectron spectroscopy Sputter
time *
Ta concentration
temperaby X-ray
[at.&]
[sl
Room temp.
Liquid Nz temp.
0 30 60 90 120 150 180
0.4 0.6 1.0 1.8 2.5 4.0 6.3
0.75 1.8 3.5 5.5 7.5 10.6 13.3
a Sputter
rate is approximately
15 A/min.
1
Potential (Vsce) Fig. 4. Anodic polarization curves for aluminum, aluminum ion implanted with tantalum at room temperature. and aluminum ion implanted with tantalum at liquid nitrogen temperature.
3.2. Anodic polarization Fig. 4 shows anodic polarization curves for Al. a Ta(RT)Al sample, and a Ta(LN,)Al sample. The important characteristic of each curve is the pitting potential. i.e. the electrode potential at which there is a sudden increase in current density due to the initiation of corrosion pits. At potentials below (less positive) than the pitting potential, pits do not initiate or grow. A higher (more positive) pitting potential represents an increased resistance to pitting corrosion. The effect of implanting Ta into Al at room and liquid nitrogen temperatures was to increase the pitting potential of Al by 50 mV and 150 mV, respectively. Table 2 lists average pitting potential values for the various samples. The average pitting potential values for the samples implanted at room temperature and at liquid nitrogen temperature were 50 mV and 175 mV more positive than the pitting potential value for pure aluminum. As noted above. the pH,, affects the surface charge and therefore, the adsorption characteristics of an oxide. In solutions of pH less than 9, the surface of aluminum oxide consists of acidic sites [5.6] which are receptors for Lewis bases such as Cl-. In solutions with a pH
Table 2 Pitting potentials aluminum Implanted ion
for
aluminum
Conditions
and
Ta
ion
Dose [XlO’b ions/cm*]
_
_
Ta Ta
room temp. liquid N, temp.
4 4
a Compared
with the pitting
potential
implanted
A.$,,
a
[VI -0.700 - 0.650 - 0.525
+ 0.050 +0.175
of pure aluminum.
VII. METALS
,’ TRIBOLOC
Y
844
P.M. Nalishan et al. / Corrosion behaoior of Ta-implanred
greater than 2.8. tantalum oxide is composed of basic sites [7] to which Cl- would not be attracted. Thus, the addition of tantalum oxide, which acts as a Lewis base, into the aluminum oxide should increase the pitting potential, as was observed. As shown in figs. 2 and 3. and previously [1.2], the implanted cations are contained in the surface oxide film so that the effect of ion implantation is to replace a portion of the aluminum-oxygen bonds in the passive film with bonds formed between oxygen and the implanted ions. Therefore, ion beam modification offers the possibility of inhibiting Cl- ion adsorption by changing the pH,,, of the surface thereby extending the passive range.
Al
the pH of zero charge model this behavior is explained by the presence of the implanted cations in the aluminum oxide lattice.
Acknowledgements
The authors gratefully acknowledge the financial support and technical interaction of A. John Sedriks. Office of Naval Research, Arlington, Virginia. The authors also acknowledge the contributions of Randy Walker and Gabrielle T. Peace in sample preparation and electrochemical testing.
4. Summary
References
Surface analysis using Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy showed that ion implantation at liquid nitrogen temperature produced a higher concentration of Ta in the near-surface region compared with ion implantation at room temperature. In both cases, the concentrations at the outer portion of the oxide film were still relatively low. Anodic polarization showed that the average pitting potential value for the Ta(LN,)Al samples was 175 mV higher than the average value obtained for the Ta(RT)Al samples, indicating that even a small increase in the surface concentration of Ta can improve the pitting behavior of aluminum. From the standpoint of
PI P.M. Natishan, E. McCafferty and G.K. Hubler, J. Electrothem. Sot. 135 (1988) 321.
PI P.M. Natishan. E. McCafferty and G.K. Hubler. Mater. Sci. Eng. All6 (1989) 41. [31 P.M. Natishan, E. McCafferty and G.K. Hubler, Con. Sci.. in press. [41 C.D. Wagner, in: Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, eds. D. Briggs and M.P. Seah (Wiley. New York. 1983) p. 478. 2nd. ed. [51 W. Stumm and J.J. Morgan, Aquatic Chemistry, (Wiley. New York, 1981) pp. 5999640. [61 G.A. Parks, Chem. Rev. 65 (1965) 177. V.A. Kolesnikov. A.F. Gubin and A.A. [71 G.A. Kokarev. Korobanov, Sov. Electrochem. 18 (1982) 407.