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Ion implantation through aluminum thin film deposited on iron
Nuclear Instruments and Methods 209/210 (1983) 941-945 North-Holland Publishing Company
ION IMPLANTATION THROUGH
941
ALUMINUM THIN FILM DEPOSITED ON IRON *
M. I W A K I , Y. O K A B E **, K. T A K A H A S H I a n d K. Y O S H I D A The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako, Saitama, 351 Japan
The effect of ion implantation into aluminum-deposited-iron plates has been investigated with reference to the concentration profiles of aluminum and iron, and the corrosion inhibition. Aluminum thin films ten nanometers thick, were prepared on pure iron substrates by ion beam sputter coating. Argon and oxygen molecule implantations were performed with a dose of 1016/cm2 at an energy of 150 keV. Concentration profiles of aluminum and iron were measured by means of secondary ion mass spectrometry. The corrosion behaviour of these specimens was evaluated by means of the multi-sweep cyclic voltammetry in 0.5 mol/dm 3 acetate buffer solutions of pH 5.0 and 3.8. The results show that ion implantation through the deposited thin film results in making the interface of the film-substrate dispersed and improves the corrosion resistance.
1. Introduction F o r ten years, ion i m p l a n t a t i o n has been used
for the m o d i f i c a t i o n of m a t e r i a l surface-layers, a n d c o n s i d e r a b l e a t t e n t i o n has been directed tow a r d s altering the non-electric properties, such as the chemical a n d m e c h a n i c a l properties, of metals [1]. A q u e o u s c o r r o s i o n p h e n o m e n a , which are strongly affected b y the c o m p o s i t i o n a n d structure of the surface-layer of metals, are c o n s i d e r e d to be of first o r d e r i m p o r t a n c e in the m a t e r i a l surface sciences, M a n y e x p e r i m e n t a l results have shown that ion i m p l a n t a t i o n is a useful technique for the i m p r o v e m e n t of the corrosion resistance of steel, e.g. the electrochemical p r o p e r t i e s of c h r o m i u m i m p l a n t e d steel plates with the dose of 1 x 1017 C r + / c m 2 were similar to those of a stainless steel ( F e - 1 8 % C r alloy) [2]. Recently, the d o p i n g of the given species in m a t e r i a l s has been carried out not only b y direct ion i m p l a n t a t i o n b u t also b y ion b e a m mixing. T h e d o p i n g technique b y mixing is as follows: a thin film of the given species is d e p o s i t e d on the s u b s t r a t e a n d then the film a n d s u b s t r a t e are i n t e r m i x e d b y ion i m p l a n t a t i o n t h r o u g h the f i l m s u b s t r a t e interface. M a n y studies have revealed that ion b e a m mixing is a p o t e n t i a l technique for p r o d u c i n g m a t e r i a l s with c o n t r o l l e d s u r f a c e - p r o p erties [3]. In this report, the effects of i n t e r m i x i n g the * Work supported financially in part by the Kurata Foundation. ** Permanent address: Faculty of Engineering, Saitama Institute of Technology, Okabe-machi, Saitama, 369-02 Japan. 0167-5087/83/0000-0000/$03.00
a l u m i n u m thin film a n d iron substrate are investigated with reference to the c o n c e n t r a t i o n p r o files a n d aqueous c o r r o s i o n behaviour. M i x i n g was carried out b y O f - or A r + - i m p l a n t a t i o n , a n d conc e n t r a t i o n profiles of a l u m i n u m a n d iron were m e a s u r e d b y m e a n s of s e c o n d a r y ion mass spectrometry. T h e c o r r o s i o n resistance e x a m i n e d b y multi-sweep cyclic v o l t a m m e t r y in solution is discussed in c o n j u n c t i o n with the d e p t h profiles and S E M observations.
2. Experimental 2.1. Sample preparation
The substrates used were p u r e iron plates ( F u r uchi C h e m i c a l Lab. L t d C o d e No. F E M 3 0 0 1 1 A , 99.9% Iron) m e c h a n i c a l l y m i r r o r - p o l i s h e d b y a buffing wheel. A f t e r being u l t r a s o n i c a l l y cleaned in trichloroethylene, the specimens were s p u t t e r e d b y 5 keV A r + b e a m s at an angle of 75 ° from the normal. A l u m i n u m thin films ten n a n o m e t e r s thick, were p r e p a r e d on the iron substrates b y sputter c o a t i n g at r o o m t e m p e r a t u r e at a b o u t 10 -4 Pa b y using the ion b e a m milling e q u i p m e n t , I M M I - V ( C o m m o n w e a l t h Scientific Co. Ltd). I n this process, several iron substrates were p l a c e d in such a w a y as to m a k e an angle of 30 ° with an a l u m i n u m plate, which was s p u t t e r e d at an angle of 60 ° from the incidence of 5 keV A r + beams. T h e iron substrates were r o t a t e d at r a n d o m so that s p u t t e r e d a l u m i n u m a t o m s were u n i f o r m l y d e p o s i t e d on the iron surface with the d e p o s i t i o n rate of 5 n m / h .
© 1983 N o r t h - H o l l a n d
vii. METALS
942
M. lwaki et a L / Ion implantation through AI thin film
2.2. Ion beam mixing Ions of oxygen molecules or argon were implanted with a dose of 1 × 1016/cm 2 at an energy of 150 keV by using the R I K E N 200 kV Low Current Implanter [4]. The conditions of the dose and the energy were selected in such a way so as to reduce the amount of aluminum film removed by sputtering during ion implantation and for the most implanted ions to penetrate into the deeper region than the film-substrate interface. The beam current density was about 1 # A / c m 2, as a result of which the target temperature during ion implantation rose to a temperature slightly higher than room temperature.
2.3. Measurements of concentration profiles The concentration profiles of aluminum and iron were measured by means of secondary ion mass spectrometry, which was carried out by an Ion-Beam Surface Mass Analyzer (Commonwealth ScientifiC Co. Ltd). Primary ions for profiling were produced from oxygen gas and bombarded on the specimen at an angle of 45° from the normal. The ion energy and current density was 7 - 8 keV and about 60 /~A/cm 2, respectively. The sputtering removal rates used for measuring the concentration profiles were about 1 n m / m i n for aluminum and about 2 n m / m i n for iron.
2.4. Electrochemical measurements The aqueous corrosion behaviour of the specimens was investigated by means of a conventional voltammetric system with a three electrode type cell. The potential sweep-rate and sweep region in the cyclic voltammetry were 50 m V / s and - 0 . 7 to + 0.7 V versus SCE, respectively. The electrolytic solutions used were 0.5 m o l / d m 3 acetate buffer solutions of pH 5.0 and 3.8. All measurements were made at (25.0 _+ 0.1)°C. The cell construction and the instrument are given in detail in ref. 5.
All
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o~ ,~ O
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20
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SPUTTERING TIME /rain Fig. I. Concentration profiles of aluminum (open) and iron (closed) in unimplanted (O, O), Of-implanted (D, I ) and Ar% implanted (z~, A) specimens, which are pure iron plates deposited with aluminum.
planted and argon-implanted specimens are shown in fig. 1. In the case of the as-deposited specimen, it was found that the surface of the iron substrate was covered by deposited aluminum atoms, but it seems that aluminum atoms have invaded the iron substrate. It seems that no appearance of the sharp interface may be casued by building up unexpected roughness of deposited thin films or intermixing the film and substrate during ion bombardment for depth profiling. When comparing concentration profiles of unimplanted and implanted specimens, it is obvious that ion implantation only reduced the total numI
I
I
!Oth positive-going sweep 3
-25th 13
~20th bJ ev" ee"
~lOth
3
3. Results I
3.1. Concentration profiles The concentration profiles of aluminum and iron in unimplanted (as-deposited), oxygen-ira-
-0.5
I
I
0 0.5 ELECTRODE POTENTIAL vs. SCE//V
Fig. 2. Typical multi-sweep cyclic vohammograms for aluminum-deposited iron with argon implantation in 0.5 tool/din 3 acetate buffer solution of pH 5.0.
M. lwaki et al. / l o n implantation through AI thin film
ber of aluminum atoms to a small extent, and caused the iron atoms to move into the aluminum thin film and aluminum atoms to penetrate into the iron substrate. Especially, in the case of argon implantation, the iron atoms came out on the surface through the thin film. These results show that ion implantation through the thin film causes the thin film to intermix with the substrate.
3.2. Aqueous corrosion Fig. 2 shows typical multi-sweep voltammograms obtained for the ion beam mixing specimen produced by argon implantation in aluminum-deposited-iron plates. The anodic current peaks were observed in the potential region A (-0.65 to - 0 . 2 V versus SCE), which corresponds to the anodic dissolution of iron [5]. The potential region B (0 to +0.7 V versus SCE), where the current was depressed, corresponds to the passivation of the metal electrode. In the case of the argon-implanted specimen indicated in the figure, the anodic peak current increases with an increase in the number of potential sweep cycles. Such multisweep voltammograms suggest that the surface layer is gradually dissolved with repetition of the potential sweep cycles. In order to compare the dissolution-passivation characteristics for these specimens, the peak current density of the anodic dissolution, ip, was plotted against the number of the potential sweep 30
943
cycles, n c, as shown in fig. 3. In the i p - n c c u r v e of the pure iron substrate, a relatively high anodic peak current appears even in the 2nd positive-going potential sweep and becomes stationary after several cycles of the potential sweep. The anodic current densities for the aluminumdeposited-iron substrate became lower than those for virgin iron. In the 30th sweep, the anodic peak current density of the aluminum-deposited-iron is half as large as that of the virgin iron. These results show that aluminum deposition improves the corrosion inhibition of iron substrates. In the case of specimens intermixed by ion implantation through the aluminum iron interface, the anodic peak currents became very low, as compared with that of the as-deposited specimen. Consequently, ion implantation through the thin film is considered to be an important technique for improvement of the corrosion inhibition, 4. Discussion
4.1. Effect of voltammetric measurements on the depth profiles In order to see the effect of electrochemical reactions on the composition of a surface layer, the concentration profiles of aluminum and iron were measured before and after the cyclic voltammetry. Fig. 4 shows the concentration profiles of aluminum and iron in the as-deposited specimens before and after the electrochemical measurements. Most of atoms on the surface are aluminum
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Fig. 3. Relationship between the anodic peak current density, ip, and the number of potential sweep cycles, n~, for pure iron (o), aluminum deposited iron without implantation (O), and with Ar +- (A) or O ~ - (•) implantation in the solution of pH 5.0.
1
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1
e
J
10 20 30 40 SPUTTERING TIME /rain
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Fig. 4. Concentration profiles of aluminum ( O , O ) and iron (A, A) in aluminum deposited iron before (open) and after (closed) the electrochemical measurements. VII. METALS
M. lwaki et al. / lon implantation through AI thin film
944
30 00
00
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Fig. 5. Relationship between the anodic peak current density, i v, and the number of potential sweep cycles, n c, for pure iron
(e), aluminum deposited iron without implantation (O), and with Ar +- (zx) or O2- (A) implantation in the solution of pH 3.8. even after the electrochemical measurement. Some of the iron atoms, which were hidden by deposited aluminum atoms, appeared on the surface after electrochemical measurements. Even in the ion beam mixing specimen, iron atoms could also be observed on the surface, but the number of iron atoms on the surface is smaller than that of the as-deposited specimen. The existence of iron atoms on the surface observed in the depth profiles corresponds to the observation of the anodic dissolution of iron as shown in fig. 3. 4.2. A q u e o u s corrosion p h e n o m e n a
The information obtained from the metal electrode/0.5 m o l / d m 3 acetate buffer solution of pH 5.0 systems indicates that aluminum deposition
and subsequent ion implantation are useful techniques for the improvement of the corrosion inhibition of iron. However, in this experimental condition, aluminum deposition showed the major effect on the aqueous corrosion inhibition rather than subsequent ion implantation. In order to clarify the effect of the ion beam mixing on corrosion inhibition, the solution was made to acidify. The relationship between i v and n c obtained from the multi-sweep cyclic voltammograms in the solution of pH 3.8 is shown in fig. 5. The peak current of pure iron becomes stationary in the early stage of potential sweep. The anodic peak current for the aluminum depositing specimen gradually increases with repetition of potential sweep cycles and is nearly equal to that of pure iron in the 30th sweep. On the contrary, ion implantation through the thin film results in a lowering of the anodic peak current, which is almost the same as that observed for the solution of pH 5.0. These results suppose that the ion beam mixing is a potential technique for improving the corrosion inhibition of aluminum-deposited-iron. The anodic peak current for the intermixed specimens, however, slightly increases with repetition of the potential sweeps. This result and the shape of the v o l t a m m o g r a m s at each of the potential sweeps suggest that the dissolution of iron from the specimen surface increases with increasing the potential sweep cycles. The concentration profiles measured after the cyclic voltammetry also suggest that the iron atoms hidden by de"positing aluminum appeared on the surface by the voltammetric treatment. This voltammetric effect on the surface structure was examined by SEM observations.
Fig. 6. SEM micrographs of unimplanted (a), Ar+-implanted (b) and O~+-implanted (c) specimens electrochemically lreated in the solution of pH 3.8.
M. lwaki et al. / Ion implantation through AI thin film
The SEM micrographs shown in fig. 6 were the surfaces of the specimens after the cyclic voltammetry with 30 potential sweep cycles, whose ip-n~ curves were shown in fig. 5. These micrographs show the nonuniformly corroded surfaces, where holes from 5 to 10 ktm in diameter on the surface can be seen. Iron atoms detected in concentration profiles and voltammograms could be obtained from the holes. The unimplanted specimen has m a n y holes in a cluster and the argon- or oxygen-implanted specimen has a little holes on the aluminum surface. These results suggest that ion implantation through the thin film improves the uniformity of the thin film layer in the horizontal direction of the surface. These specimens before electrochemical treatments were not different from each other in the SEM observations. Therefore, it is proposed that the electrochemical observation such as multisweep cyclic voltammetry is a very useful way to evaluate the characteristic of the thin film-substrate system. 5. Summary Ion implantation through the aluminum thin film deposited on iron was investigated with reference to the effect on the concentration profiles and the aqueous corrosion. The concentration profiles were measured by means of secondary ion mass
945
spectrometry. The aqueous corrosion of the specimens was evaluated by multi-sweep cyclic voltammetry in 0.5 m o l / d m 3 acetate buffer solutions of p H 3.8 and 5.0. Results were as follows: (1) Ion implantation through the aluminum thin film results in intermixing the aluminum film and iron substrate. (2) Anodic dissolution currents become low by depositing aluminum on iron and even lower by subsequent ion implantation. The phenomena become clear in the p H 3.8 solution. (3) The small anodic current may be caused by the nonuniform corrosion with small holes. From these results, it is concluded that ion implantation through the thin film results in making the interface of the film-substrate dispersed and improves the aqueous corrosion inhibition.
References [1] J.K. Hirvonen, Ion Implantation, Treatise on Materials Science and Technology, vol. 18 (Academic Press, New York, 1980). [2] Y. Okabe, M. Iwaki, K. Takahashi, S. Namba and K. Yoshida, Surf. Sci. 86 (1979) 257. [3] J.W. Mayer, B.Y. Tsaur, S.S. Lau and L-S. Hung, Nucl. Instr. and Meth. 182/183 (1981) 1. [4] M. lwaki, Y. Okabe, S. Namba and K. Yoshida, Nucl. Instr. and Meth. 189 (1981) 155. [5] K. Takahashi, Y. Okabe and M. lwaki, Nucl. Instr. and Meth. 182/183 (1981) 1009.
VII. METALS
ION IMPLANTATION THROUGH
941
ALUMINUM THIN FILM DEPOSITED ON IRON *
M. I W A K I , Y. O K A B E **, K. T A K A H A S H I a n d K. Y O S H I D A The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako, Saitama, 351 Japan
The effect of ion implantation into aluminum-deposited-iron plates has been investigated with reference to the concentration profiles of aluminum and iron, and the corrosion inhibition. Aluminum thin films ten nanometers thick, were prepared on pure iron substrates by ion beam sputter coating. Argon and oxygen molecule implantations were performed with a dose of 1016/cm2 at an energy of 150 keV. Concentration profiles of aluminum and iron were measured by means of secondary ion mass spectrometry. The corrosion behaviour of these specimens was evaluated by means of the multi-sweep cyclic voltammetry in 0.5 mol/dm 3 acetate buffer solutions of pH 5.0 and 3.8. The results show that ion implantation through the deposited thin film results in making the interface of the film-substrate dispersed and improves the corrosion resistance.
1. Introduction F o r ten years, ion i m p l a n t a t i o n has been used
for the m o d i f i c a t i o n of m a t e r i a l surface-layers, a n d c o n s i d e r a b l e a t t e n t i o n has been directed tow a r d s altering the non-electric properties, such as the chemical a n d m e c h a n i c a l properties, of metals [1]. A q u e o u s c o r r o s i o n p h e n o m e n a , which are strongly affected b y the c o m p o s i t i o n a n d structure of the surface-layer of metals, are c o n s i d e r e d to be of first o r d e r i m p o r t a n c e in the m a t e r i a l surface sciences, M a n y e x p e r i m e n t a l results have shown that ion i m p l a n t a t i o n is a useful technique for the i m p r o v e m e n t of the corrosion resistance of steel, e.g. the electrochemical p r o p e r t i e s of c h r o m i u m i m p l a n t e d steel plates with the dose of 1 x 1017 C r + / c m 2 were similar to those of a stainless steel ( F e - 1 8 % C r alloy) [2]. Recently, the d o p i n g of the given species in m a t e r i a l s has been carried out not only b y direct ion i m p l a n t a t i o n b u t also b y ion b e a m mixing. T h e d o p i n g technique b y mixing is as follows: a thin film of the given species is d e p o s i t e d on the s u b s t r a t e a n d then the film a n d s u b s t r a t e are i n t e r m i x e d b y ion i m p l a n t a t i o n t h r o u g h the f i l m s u b s t r a t e interface. M a n y studies have revealed that ion b e a m mixing is a p o t e n t i a l technique for p r o d u c i n g m a t e r i a l s with c o n t r o l l e d s u r f a c e - p r o p erties [3]. In this report, the effects of i n t e r m i x i n g the * Work supported financially in part by the Kurata Foundation. ** Permanent address: Faculty of Engineering, Saitama Institute of Technology, Okabe-machi, Saitama, 369-02 Japan. 0167-5087/83/0000-0000/$03.00
a l u m i n u m thin film a n d iron substrate are investigated with reference to the c o n c e n t r a t i o n p r o files a n d aqueous c o r r o s i o n behaviour. M i x i n g was carried out b y O f - or A r + - i m p l a n t a t i o n , a n d conc e n t r a t i o n profiles of a l u m i n u m a n d iron were m e a s u r e d b y m e a n s of s e c o n d a r y ion mass spectrometry. T h e c o r r o s i o n resistance e x a m i n e d b y multi-sweep cyclic v o l t a m m e t r y in solution is discussed in c o n j u n c t i o n with the d e p t h profiles and S E M observations.
2. Experimental 2.1. Sample preparation
The substrates used were p u r e iron plates ( F u r uchi C h e m i c a l Lab. L t d C o d e No. F E M 3 0 0 1 1 A , 99.9% Iron) m e c h a n i c a l l y m i r r o r - p o l i s h e d b y a buffing wheel. A f t e r being u l t r a s o n i c a l l y cleaned in trichloroethylene, the specimens were s p u t t e r e d b y 5 keV A r + b e a m s at an angle of 75 ° from the normal. A l u m i n u m thin films ten n a n o m e t e r s thick, were p r e p a r e d on the iron substrates b y sputter c o a t i n g at r o o m t e m p e r a t u r e at a b o u t 10 -4 Pa b y using the ion b e a m milling e q u i p m e n t , I M M I - V ( C o m m o n w e a l t h Scientific Co. Ltd). I n this process, several iron substrates were p l a c e d in such a w a y as to m a k e an angle of 30 ° with an a l u m i n u m plate, which was s p u t t e r e d at an angle of 60 ° from the incidence of 5 keV A r + beams. T h e iron substrates were r o t a t e d at r a n d o m so that s p u t t e r e d a l u m i n u m a t o m s were u n i f o r m l y d e p o s i t e d on the iron surface with the d e p o s i t i o n rate of 5 n m / h .
© 1983 N o r t h - H o l l a n d
vii. METALS
942
M. lwaki et a L / Ion implantation through AI thin film
2.2. Ion beam mixing Ions of oxygen molecules or argon were implanted with a dose of 1 × 1016/cm 2 at an energy of 150 keV by using the R I K E N 200 kV Low Current Implanter [4]. The conditions of the dose and the energy were selected in such a way so as to reduce the amount of aluminum film removed by sputtering during ion implantation and for the most implanted ions to penetrate into the deeper region than the film-substrate interface. The beam current density was about 1 # A / c m 2, as a result of which the target temperature during ion implantation rose to a temperature slightly higher than room temperature.
2.3. Measurements of concentration profiles The concentration profiles of aluminum and iron were measured by means of secondary ion mass spectrometry, which was carried out by an Ion-Beam Surface Mass Analyzer (Commonwealth ScientifiC Co. Ltd). Primary ions for profiling were produced from oxygen gas and bombarded on the specimen at an angle of 45° from the normal. The ion energy and current density was 7 - 8 keV and about 60 /~A/cm 2, respectively. The sputtering removal rates used for measuring the concentration profiles were about 1 n m / m i n for aluminum and about 2 n m / m i n for iron.
2.4. Electrochemical measurements The aqueous corrosion behaviour of the specimens was investigated by means of a conventional voltammetric system with a three electrode type cell. The potential sweep-rate and sweep region in the cyclic voltammetry were 50 m V / s and - 0 . 7 to + 0.7 V versus SCE, respectively. The electrolytic solutions used were 0.5 m o l / d m 3 acetate buffer solutions of pH 5.0 and 3.8. All measurements were made at (25.0 _+ 0.1)°C. The cell construction and the instrument are given in detail in ref. 5.
All
z 2
~10 z
D./ U
z
oD
;I
o~
O (.9
O&
o~ ,~ O
I21 /~x O 131
10
1'0
zx
3b
20
50
SPUTTERING TIME /rain Fig. I. Concentration profiles of aluminum (open) and iron (closed) in unimplanted (O, O), Of-implanted (D, I ) and Ar% implanted (z~, A) specimens, which are pure iron plates deposited with aluminum.
planted and argon-implanted specimens are shown in fig. 1. In the case of the as-deposited specimen, it was found that the surface of the iron substrate was covered by deposited aluminum atoms, but it seems that aluminum atoms have invaded the iron substrate. It seems that no appearance of the sharp interface may be casued by building up unexpected roughness of deposited thin films or intermixing the film and substrate during ion bombardment for depth profiling. When comparing concentration profiles of unimplanted and implanted specimens, it is obvious that ion implantation only reduced the total numI
I
I
!Oth positive-going sweep 3
-25th 13
~20th bJ ev" ee"
~lOth
3
3. Results I
3.1. Concentration profiles The concentration profiles of aluminum and iron in unimplanted (as-deposited), oxygen-ira-
-0.5
I
I
0 0.5 ELECTRODE POTENTIAL vs. SCE//V
Fig. 2. Typical multi-sweep cyclic vohammograms for aluminum-deposited iron with argon implantation in 0.5 tool/din 3 acetate buffer solution of pH 5.0.
M. lwaki et al. / l o n implantation through AI thin film
ber of aluminum atoms to a small extent, and caused the iron atoms to move into the aluminum thin film and aluminum atoms to penetrate into the iron substrate. Especially, in the case of argon implantation, the iron atoms came out on the surface through the thin film. These results show that ion implantation through the thin film causes the thin film to intermix with the substrate.
3.2. Aqueous corrosion Fig. 2 shows typical multi-sweep voltammograms obtained for the ion beam mixing specimen produced by argon implantation in aluminum-deposited-iron plates. The anodic current peaks were observed in the potential region A (-0.65 to - 0 . 2 V versus SCE), which corresponds to the anodic dissolution of iron [5]. The potential region B (0 to +0.7 V versus SCE), where the current was depressed, corresponds to the passivation of the metal electrode. In the case of the argon-implanted specimen indicated in the figure, the anodic peak current increases with an increase in the number of potential sweep cycles. Such multisweep voltammograms suggest that the surface layer is gradually dissolved with repetition of the potential sweep cycles. In order to compare the dissolution-passivation characteristics for these specimens, the peak current density of the anodic dissolution, ip, was plotted against the number of the potential sweep 30
943
cycles, n c, as shown in fig. 3. In the i p - n c c u r v e of the pure iron substrate, a relatively high anodic peak current appears even in the 2nd positive-going potential sweep and becomes stationary after several cycles of the potential sweep. The anodic current densities for the aluminumdeposited-iron substrate became lower than those for virgin iron. In the 30th sweep, the anodic peak current density of the aluminum-deposited-iron is half as large as that of the virgin iron. These results show that aluminum deposition improves the corrosion inhibition of iron substrates. In the case of specimens intermixed by ion implantation through the aluminum iron interface, the anodic peak currents became very low, as compared with that of the as-deposited specimen. Consequently, ion implantation through the thin film is considered to be an important technique for improvement of the corrosion inhibition, 4. Discussion
4.1. Effect of voltammetric measurements on the depth profiles In order to see the effect of electrochemical reactions on the composition of a surface layer, the concentration profiles of aluminum and iron were measured before and after the cyclic voltammetry. Fig. 4 shows the concentration profiles of aluminum and iron in the as-deposited specimens before and after the electrochemical measurements. Most of atoms on the surface are aluminum
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20
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30
%
91,c
Fig. 3. Relationship between the anodic peak current density, ip, and the number of potential sweep cycles, n~, for pure iron (o), aluminum deposited iron without implantation (O), and with Ar +- (A) or O ~ - (•) implantation in the solution of pH 5.0.
1
I
0
1
e
J
10 20 30 40 SPUTTERING TIME /rain
L 5O
Fig. 4. Concentration profiles of aluminum ( O , O ) and iron (A, A) in aluminum deposited iron before (open) and after (closed) the electrochemical measurements. VII. METALS
M. lwaki et al. / lon implantation through AI thin film
944
30 00
00
• •
•
0
0
0
0
O 2C
o
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.,,,~"10
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3o
Fig. 5. Relationship between the anodic peak current density, i v, and the number of potential sweep cycles, n c, for pure iron
(e), aluminum deposited iron without implantation (O), and with Ar +- (zx) or O2- (A) implantation in the solution of pH 3.8. even after the electrochemical measurement. Some of the iron atoms, which were hidden by deposited aluminum atoms, appeared on the surface after electrochemical measurements. Even in the ion beam mixing specimen, iron atoms could also be observed on the surface, but the number of iron atoms on the surface is smaller than that of the as-deposited specimen. The existence of iron atoms on the surface observed in the depth profiles corresponds to the observation of the anodic dissolution of iron as shown in fig. 3. 4.2. A q u e o u s corrosion p h e n o m e n a
The information obtained from the metal electrode/0.5 m o l / d m 3 acetate buffer solution of pH 5.0 systems indicates that aluminum deposition
and subsequent ion implantation are useful techniques for the improvement of the corrosion inhibition of iron. However, in this experimental condition, aluminum deposition showed the major effect on the aqueous corrosion inhibition rather than subsequent ion implantation. In order to clarify the effect of the ion beam mixing on corrosion inhibition, the solution was made to acidify. The relationship between i v and n c obtained from the multi-sweep cyclic voltammograms in the solution of pH 3.8 is shown in fig. 5. The peak current of pure iron becomes stationary in the early stage of potential sweep. The anodic peak current for the aluminum depositing specimen gradually increases with repetition of potential sweep cycles and is nearly equal to that of pure iron in the 30th sweep. On the contrary, ion implantation through the thin film results in a lowering of the anodic peak current, which is almost the same as that observed for the solution of pH 5.0. These results suppose that the ion beam mixing is a potential technique for improving the corrosion inhibition of aluminum-deposited-iron. The anodic peak current for the intermixed specimens, however, slightly increases with repetition of the potential sweeps. This result and the shape of the v o l t a m m o g r a m s at each of the potential sweeps suggest that the dissolution of iron from the specimen surface increases with increasing the potential sweep cycles. The concentration profiles measured after the cyclic voltammetry also suggest that the iron atoms hidden by de"positing aluminum appeared on the surface by the voltammetric treatment. This voltammetric effect on the surface structure was examined by SEM observations.
Fig. 6. SEM micrographs of unimplanted (a), Ar+-implanted (b) and O~+-implanted (c) specimens electrochemically lreated in the solution of pH 3.8.
M. lwaki et al. / Ion implantation through AI thin film
The SEM micrographs shown in fig. 6 were the surfaces of the specimens after the cyclic voltammetry with 30 potential sweep cycles, whose ip-n~ curves were shown in fig. 5. These micrographs show the nonuniformly corroded surfaces, where holes from 5 to 10 ktm in diameter on the surface can be seen. Iron atoms detected in concentration profiles and voltammograms could be obtained from the holes. The unimplanted specimen has m a n y holes in a cluster and the argon- or oxygen-implanted specimen has a little holes on the aluminum surface. These results suggest that ion implantation through the thin film improves the uniformity of the thin film layer in the horizontal direction of the surface. These specimens before electrochemical treatments were not different from each other in the SEM observations. Therefore, it is proposed that the electrochemical observation such as multisweep cyclic voltammetry is a very useful way to evaluate the characteristic of the thin film-substrate system. 5. Summary Ion implantation through the aluminum thin film deposited on iron was investigated with reference to the effect on the concentration profiles and the aqueous corrosion. The concentration profiles were measured by means of secondary ion mass
945
spectrometry. The aqueous corrosion of the specimens was evaluated by multi-sweep cyclic voltammetry in 0.5 m o l / d m 3 acetate buffer solutions of p H 3.8 and 5.0. Results were as follows: (1) Ion implantation through the aluminum thin film results in intermixing the aluminum film and iron substrate. (2) Anodic dissolution currents become low by depositing aluminum on iron and even lower by subsequent ion implantation. The phenomena become clear in the p H 3.8 solution. (3) The small anodic current may be caused by the nonuniform corrosion with small holes. From these results, it is concluded that ion implantation through the thin film results in making the interface of the film-substrate dispersed and improves the aqueous corrosion inhibition.
References [1] J.K. Hirvonen, Ion Implantation, Treatise on Materials Science and Technology, vol. 18 (Academic Press, New York, 1980). [2] Y. Okabe, M. Iwaki, K. Takahashi, S. Namba and K. Yoshida, Surf. Sci. 86 (1979) 257. [3] J.W. Mayer, B.Y. Tsaur, S.S. Lau and L-S. Hung, Nucl. Instr. and Meth. 182/183 (1981) 1. [4] M. lwaki, Y. Okabe, S. Namba and K. Yoshida, Nucl. Instr. and Meth. 189 (1981) 155. [5] K. Takahashi, Y. Okabe and M. lwaki, Nucl. Instr. and Meth. 182/183 (1981) 1009.
VII. METALS