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The pitting corrosion behavior of sputter-deposited amorphous AlTi alloys in a neutral chloride-containing solution
Journal of Non-Crystalline Solids 125 (1990) 25-31 North-Holland
25
The pitting corrosion behavior of sputter-deposited amorphous A1-Ti alloys in a neutral chloride-containing solution Q. Yan 1, H. Yoshioka 2, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendal 980, Japan Received 23 March 1990 Revised manuscript received 30 May 1990
Pitting corrosion behavior of sputter-deposited amorphous A1-Ti alloys in a neutral borate-boric acid solution with C1 ions has been compared with that of argon arc-melted crystalline counterparts. The sputter-deposited amorphous A1-Ti alloys possess a better passivating ability and pitting corrosion resistance. They exhibit lower passive current densities and pitting potentials which are about 500 mV higher than their crystalline counterparts. Crystallization of the amorphous alloys by heat treatment leads to a decrease in the pitting potential. Pits formed on the amorphous A1-Ti alloys are featureless smooth holes, and those on the argon arc-melted alloys show facetting dissolution. The pit bottom of the argon arc-melted AI-36Ti alloy is enriched with titanium; thus pitting corrosion involves preferential dissolution of aluminum. Sputter-deposited alloys containing ~< 30 at.% titanium consist of a single fcc a-Al phase supersaturated with titanium, and their pitting potential is raised with increasing titanium content.
I. Introduction Amorphous alloys containing passivating elements, such as chromium, tantalum, niobium etc., are known to have a high pitting corrosion resistance. This is associated with their high passivating ability along with chemically homogeneous single phase nature [1]. However, even if a single amorphous phase structure is not formed, rapid solidification increases the corrosion resistance. For example, the good pitting corrosion resistance of rapidly solidified crystalline Fe-Cr alloys has been attributed to an increase in alloy homogeneity, since the formation of large precipitates is prevented [2]. The pitting corrosion resistance of rapidly solidified aluminum alloys is also higher than that of aluminum metal [3]. This has been interpreted in terms of an enhancement of corrosion resistance of the a-Al phase by supersaturai Visiting Researcher from Beijing Aeronautical Manufacturing Technology Research Institute, Beijing, P,R. China. 2 Visiting Researcher from YKK, Kurobe, Toyama, Japan.
tion with alloying elements, since the most susceptible phase to corrosion in aluminum alloys is generally the a-A1 phase. Amorphous alloys containing tantalum a n d / o r niobium possess a particularly high corrosion resistance. Amorphous Ni-Ta alloys are spontaneously passive in boiling 6 N HC1 and do not suffer pitting corrosion [4]. Sputter-deposited amorphous Cu-Ta and Cu-Nb alloys do not suffer pitting corrosion by anodic polarization in 12 N HC1 [5]. Amorphous A1-Ta and A1-Nb alloys have the highest corrosion resistance among amorphous aluminum-base alloys but suffer pitting corrosion in 1 N HC1 by anodic polarization [6]. This has been attributed to the fact that no enrichment of valve metal cations occurs in the passive film formed on the aluminum-valve metal alloys in 1 N HC1, while nickel- and copper-valve metal alloys form quite protective passive films consisting exclusively of valve metal ions as cations even in concentrated hydrochloric acids. The pitting potential of sputter-deposited aluminum alloys in
0022-3093/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
26
Q. Yan et al. / Corrosion behavior of sputter-deposited amorphous A I- Ti alloys
1 N HC1 is ennobled by an increase in content of refractory metals, such as titanium, zirconium, niobium, tantalum, molybdenum and tungsten [68]. The ennoblement of the pitting potentials of sputter-deposited aluminum alloys in neutral [912] and slightly alkaline [13] chloride solutions has also been confirmed, although the formation of the amorphous structure for those alloys has not been examined. The sputter-deposited A1-Ti alloys containing 30-60 at.% titanium consist of a single amorphous phase [8]. The pitting potential of sputter-deposited A1-Ti alloys in a chloride-containing borate-boric acid solution increases with titanium content [8,14]. The relationship between the structure and corrosion behavior of sputter-deposited A1-Ti alloys is not yet clarified. The amorphous AI-Ti alloys should be more corrosion-resistant than conventionally processed crystalline counterparts containing a titanium-poor phase which would be preferentially corroded. Accordingly, the present work has been performed to compare the corrosion behavior of sputter-deposited A1-Ti alloys with crystalline counterparts. Particular attention has been paid to the effect of changes in alloy structure with titanium content.
2. Experimental procedures A1-Ti alloys were prepared by dc magnetron sputtering. Targets were 99.9% pure aluminum discs of 100 mm diameter and 6 mm thickness, on the erosion region of which 99.9% pure titanium discs of 20 mm diameter were placed. Titanium discs placed on the aluminum disc were utilized to change the composition of the sputter-deposited alloys. Sputtering apparatus and conditions used were the same as those described elsewhere [5,6]. Alloys containing 15-75 at.% titanium of 4 - 6 ~m thickness were prepared on glass substrates. Alloy compositions hereafter are all denoted in at.%. The composition of the sputter-deposits was determined by inductively coupled plasma emission spectrometry (ICP) after being dissolved into 1 N HC1 solution.
To compare the corrosion behavior of amorphous alloys with that of crystalline counterparts, the amorphous alloys were crystallized by annealing. After a glass tube containing the amorphous alloy specimen was evacuated to 1 0 - 4 Torr, the glass tube was sealed. Then the specimen was annealed for 30 min at intervals of 50°C from 200 to 550°C, followed by furnace-cooling. Some A1Ti alloy ingots were also prepared by argon arcmelting of aluminum and titanium metals. The structure of the AI-Ti alloys was identified by the diffractometer X-ray method with Cu K a radiation. Electrochemical experiments were carried out in a deaerated chloride-containing borate-boric acid solution consisting of 0.075M Na2B407, 0.3M H3BO 3 and 0.5M NaC1 at pH 8.4 and 25°C. The solution was deaerated by bubbling purified nitrogen gas for about 12 h before each experiment. The pitting corrosion behavior was examined by potentiodynamic polarization measurements with a potential sweep rate of 0.5 mV/s. The specimens used for observation of the pitting corrosion morphology were potentiostatically passivated for 1 h at 100 mV below the pitting potential determined by the potentiodynamic method, followed by potentiostatic polarization at 100 mV above the pitting potential. The morphology of pitting corrosion was observed by scanning electron microscopy (SEM), and the alloy composition at the pitting sites was qualitatively determined by energy dispersive Xray spectroscopy (EDX).
3. Experimental results Table 1 shows phases identified by X-ray diffraction for sputter-deposited and argon arcmelted A1-Ti alloys used. The sputter-deposited A1-Ti alloys up to 23 at.% titanium showed intense crystalline diffraction patterns and no trace of the amorphous phase. The crystalline diffraction patterns were ascribed to the fcc c~-A1 phase. Lattice spacings of four most intense diffraction lines of the c~-A1 phase are almost the same as those of the tetragonal A1Ti phase. However, the fifth most intense (110) reflection of the tetragonal
Q. Yan et a L / Corrosion behavior of sputter-deposited amorphous A I - Ti alloys 0.235
Table 1 Phase identified by X-ray diffraction
A1- x Ti alloy (at.%)
Sputtered A I - Ti Alloys
Arc-melted
x<30 30Ti 36Ti
170
Sputtered \ 0.234 r~
a-Al amorphous amorphous
AI 3Ti + AITi A1Ti + unknown phase AITi >> A1Ti 2 A1Ti << A1Ti2
49Ti 60Ti 6475
27
50
5
/,,/
0.233
amorphous amorphous amorphous + a-Ti a-Ti
--0
0.232 60
I
I 70
V-~
30
•
[
] 80
I
I 90
1
tO tOO
Ti Content o f Alloy / o t ~
A1Ti phase was not generally detected. Accordingly, the crystalline diffraction patterns are attributed to a-Al phase. As shown in fig. 1, the diffraction lines broadened with increasing titanium content. The grain size estimated from the full width at half maximum of (111) reflection of the a-Al phase decreases almost linearly with titanium content. The intersection of the straight line with the abscissa almost coincides with the lower limit of the titanium content of the alloy consisting of a single amorphous phase. Consequently, alloying of aluminum with titanium by sputter deposition continuously decreases the crystal grain size with increasing supersaturation of titanium and ultimately results in the formation of the amorphous phase. The alloys with 30-60 at.% titanium were amorphous as defined by X-ray diffraction. 0,7~
Sputtered AI-Ti Alloys
70
~ O.6 \ g o.5
5o
0.4
40
0.3
Jo
~o
~
\
eo
0 0
tO
20
30
7"1Content o f A l l o y / o f ~
Fig. 1. Full width at half m a x i m u m of (111) reflection of the a-AI phase, which is the only phase present in sputter-deposited AI and A I - T i alloys, and the crystal grain size estimated from the full width at half maximum.
Fig. 2. The (002) lattice spacing of the hcp a-Ti phase in the sputter-deposited Ti and Ti-A1 alloys, and the crystal grain
size estimated from the full width at half maximum of the (002) reflectionof the a-Ti phase.
On the other hand, when aluminum is added to titanium, only the hcp a-Ti phase is formed in the wide composition region where the a-Ti phase is the equilibrium state. Alloys containing 64-69 at.% titanium, which is close to the border of the stable a-Ti phase, consist of a mixture of amorphous and a-Ti phases. As shown in fig. 2, the lattice constant of the a-Ti phase decreases continuously with aluminum content, while the grain size is unaffected by alloying. Because the stability region of a-Ti phase is wide, sputtering does not result in a supersaturation with aluminum, and hence does not result in grain refining. The a-Ti phase showed a strong 001 preferred orientation perpendicular to the substrate plane. The argon arc-melted A1-Ti alloys, corresponding to the composition range where the sputter-deposited alloys became amorphous, were composed of two phases, such as A13Ti and A1Ti, or A1Ti and A1Ti 2For investigating the effect of crystallization on the pitting corrosion behavior, the amorphous A136Ti and A1-60Ti alloys were annealed at different temperatures. The amorphous structure of the sputter-deposited A1-36Ti alloy was maintained after annealing at 300°C, but crystallization occurred at 350°C with a consequent formation of AITi phase. The amorphous A1-60Ti alloy was stable at 500°C, but was crystallized by annealing at 550°C, forming tetragonal A1Ti and hexagonal
Q. Yan et at / Corrosion behaoior of sputter-deposited amorphous A l - Ti alloys
28
AITi 2 phases. The crystallization temperature of the amorphous A1-60Ti alloy is about 200°C higher than that of the amorphous A1-36Ti alloy. Accordingly, increasing titanium content of the amorphous A1-Ti alloys raises the crystallization temperature. Figure 3 shows the effect of structure change of the Al-36Ti alloy on the anodic polarization behavior. Comparison of the polarization curve of the amorphous alloy with that of the argon arcmelted counterpart clearly reveals that the amorphous alloy possesses a higher passivating ability and a higher pitting corrosion resistance. Because of oxidation of the amorphous and argon arcmelted alloy specimens during annealing, annealing leads to ennoblement of the open circuit potential and to decrease in the anodic current density. However, pitting occurs at weak points of a passive film, and hence the pitting potentials of the argon arc-melted specimens before and after annealing are almost the same as each other and are about 550 mV lower than the pitting potential of the amorphous alloy. Similarly, crystallization of the amorphous alloy by isothermal annealing leads to a decrease of about 450 mV in the pitting potential, because crystallization introduces heterogeneity in the homogeneous amorphous alloy with a consequent formation of the passive film containing weak points where passivity breakdown occurs.
Deaerated
~
A I -,36 77
0,075M Naz,B,~Or-O.3M H.,BOn-O.5MNoCI 25*C
10~
\
Ar c-mel t e~dj
i ld ~ L) 16
/
~
e
red
~Sputtered and Annealed ot 350°C
k
-0.8
-0.4
i
0.4
J
0.8
Potentiol / V vs SCE Fig. 3. Anodic polarization curves of sputter-deposited amorphous and argon arc-melted crystalline A1-36Ti alloys before and after annealing.
I
[
ao ].6
~
1.2
0.4
I
iI [
NoCti 25°C ~ t Sputtered Crysto~one Amorphous Crystol lized
• e~reu
~" --0,4 /" ~0.8
I
Deoerated O,O 75M Nagd~Oz-O, JM HjBOj
0.8 ~-
I
A I- Ti Alloys
/
~ 0
• Crystoltme I 20
I 40
I 60
I 80
d I(30
Ti Content of Alloy / at~ Fig. 4. Change in pitting potential of sputter-deposited and arc melted AI-Ti alloys estimated from potentiodynamic polarization curves measured in a deaerated borate-boric acid solution containing chloride ion, as a function of alloy titanium content. Pitting potentials measured after crystallization of sputter-deposited amorphous AI-36Ti and A1-60Ti alloys are also shown for comparison.
The pitting potential of each alloy in the chloride-containing borate-boric acid solution was estimated from a sharp current increase in the potentiodynamic polarization curve. The pitting potential of sputter-deposited and argon arcmelted alloys as a function of alloy titanium content is shown in figure 4. The potentials of the alloys crystallized by annealing are also given for comparison. The pitting potentials of sputter-deposited alloys are ennobled with titanium content. There appears to be a plateau in the composition range of the single amorphous structure. After crystallization by annealing, the pitting potentials of amorphous A1-36Ti and A1-60Ti alloys shift toward more negative potentials which are close to those of the corresponding argon arc-melted alloys. This suggests that the formation of a single amorphous phase is beneficial in improving the pitting corrosion resistance. When the major phase became the a-Ti phase, that is, when the titanium content was 75 at.% or more, pitting was not observed up to about 2 V (SCE) which was the limit of the potential obtainable with the potentioslat used.
Q. Yah et al. / Corrosion behavior of sputter-deposited amorphous AI-Ti alloys
Amorphous
'
29
Crystalline
3 #m
_4 #m
AI-36Ti Alloy
,:3 ,urn
,:3 ,urn
AI-6OTi Alloy Fig. 5. Scanning electron micrographs of pitting for sputter-deposited amorphous and argon arc-melted crystalline AI-36Ti and A1-60Ti alloys.
A scanning electron microscope has been used for observation of the pitting corrosion morphology. Figure 5 shows typical surfaces after initiation of pitting corrosion for sputter-deposited amorphous and argon arc-melted crystalline AI-36Ti and A1-60Ti alloys. The pit morphology of amorphous alloys differs from that of argon arc-melted counterparts. There are no
specific features around and inside the pit of the amorphous alloys. By contrast, the shape of the pit of the crystalline alloys is irregular, showing the difference in the dissolution rate of crystallographic planes. The EDX analysis was performed for the free surface and the bottom of a pit on the argon arc-melted AI-36Ti alloy. Figure 6 shows the EDX spectra. A remarkable deficiency of
30
Q. Yen et a L / Corrosion behavior of sputter-deposited amorphous AI-Ti alloys
Deoereted 0.0 75M No2B407 -0.3M H3BO3 -0.5M NaCI 25"C
Al
Free Surface
~q ,¢
\
4 r c - m e l ted Al-36Ti
A~
Bottom o f pit
J
5 Energy /
IO /(V
Fig. 6. EDX spectra of" free surface and pit bottom for the argon arc-melted crystalline A1-36Ti alloy.
aluminum can be seen at the bottom of the pit. Consequently, pitting corrosion occurs by preferential dissolution of aluminum.
4. Discussion
In sputter-deposited A1-Ti alloys, et-A1, amorphous and a-Ti phases were detected. Sputtering generally leads to the formation of a single phase alloy exceeding the solubility limit of the stable phase, although a two-phase field, including the amorphous phase, was found close to the border of the a-Ti phase. By contrast, equilibrium compound phases were found for the argon arc-melted A1-Ti alloys of the composition range where sputter-deposited alloys became amorphous. These compound phases were not detected in the sputter-deposited alloys. This suggests that the alloy compositions which form these compounds in the stable state are suitable for the formation of the amorphous structure by sputter-deposition. EDX analysis showed that pitting corrosion occurs by preferential dissolution of aluminum. This is consistent with the fact that, when the ct-Al phase supersaturated with titanium is formed, the
pitting potential is raised with increasing alloy titanium content. As shown in fig. 4 the pitting potentials of sputter-deposited amorphous alloys are generally higher than those of argon arc-melted and annealed alloys. The presence of the titanium-poor phase in the argon arc-melted and crystallized alloys seems responsible for their lower pitting potentials in comparison with homogeneous amorphous alloys consisting of a single phase. As can be seen in the aluminum rich region, since sputtering tends to form a single phase solid solution supersaturated with titanium, the pitting potential increases continuously with titanium content. According to more detailed investigation [15], argon arc-melted alloys containing 5-25 at.% titanium consist of a two phase mixture of a-Al and A13Ti, and their pitting potentials are almost independent of the average titanium contents of the alloys and 250-600 mV lower than sputter-deposited counterparts. In this manner, single phase solid solution formed by sputtering is generally more corrosion-resistant than stable crystalline counterparts composed of heterogeneous phases. Accordingly, the transition from the single c~-Al phase to the amorphous phase with increasing alloy titanium content leads to no clear discrepancy in the continuous increase of the pitting potential, despite the fact that the pitting potentials of the amorphous alloys are significantly higher than the heterogeneous stable crystalline counterparts.
5. Conclusions
The pitting corrosion behavior of sputter-deposited A1-Ti alloys in a neutral borate-boric acid solution containing chloride ions was investigated in relation to the change in the structure. Sputter-deposited alloys containing up to 23 at.% titanium consist of a single fcc c~-AI phase supersaturated with titanium, and their pitting potential is raised with titanium content. Sputterdeposited alloys containing 30-60 at.% titanium consist of a single amorphous phase, and their pitting potential is significantly higher than those of crystalline counterparts. EDX analysis reveals
Q, Yan et al. / Corrosion behavior of sputter-deposited amorphous AI-Ti alloys that pitting occurs by preferential dissolution of aluminum. Sputter-deposited alloys containing 6 4 - 6 9 at.% t i t a n i u m a r e c o m p o s e d o f a m o r p h o u s and hcp a-Ti phases, and suffer no pitting in the potential region examined.
[7]
[8]
References [1] K. Hashimoto, in: Passivity of Metals and Semiconductors, ed. M. Froment (Elsevier, Amsterdam, 1983) p. 235. [2] A. Kawashima and K. Hashimoto, Corros. Sci. 26 (1986) 311. [3] H. Yoshioka, S. Yoshida, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci. 26 (1986) 795. [4] K. Shimamura, K. Miura, A. Kawashima, K. Asami and K. Hashimoto, Sci. Rep. Res. Inst. Tohoku University A-34 (1988) 107. [5] K. Shimamura, K. Miura, A. Kawashima, K. Asami and K. Hashimoto, in: Corrosion, Electrochemistry and Catalysis of Metallic Glasses, eds. R.B. Diegle and K. Hashimoto (The Electrochemical Society, 1988) p. 232. [6] H. Yoshioka, A. Kawashima, K. Asami and K. Hashimoto,
[9] [10] [11] [12] [13] [14] [15]
31
in: Corrosion, Electrochemistry and Catalysis of Metallic Glasses, eds. R.B. Diegle and K. Hashimoto (The Electrochemical Society, 1988) p. 242, H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, in: Proc. MRS Int. Meeting on Advanced Materials, Vol. 3 (Materials Research Society, Pittsburgh, PA, 1989) p. 429. H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci. 31 (1990) 349. W.C. Mioshier, G.D. Davis, J.S. Ahearn and H.F. Hough, J. Electrochem. Soc. 133 (1986) 1063. W.C. Mioshier, G.D. Davis, J.S. Ahearn and H.F. Hough, J. Electrochem. Soc. 134 (1986) 2677. W.C. Mioshier, G.D. Davis, and G.O. Cote, J. Electrochem. Soc. 136 (1986) 356. W.C. Mioshier, G.D. Davis, T.L. Fritz and G.O. Cote, J. Electrochem. Soc. 137 (1990) 422. G.F. Frankel, M.A. Russak, C.V. Jahnes, M. Mirzamaani and V.A. Brusic, J. Electrochem. Soc. 136 (1989) 1243. Q. Yam H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci, 31 (1990) 401. H. Yoshioka, Q. Tan, K. Asami and K. Hashimoto, in: Proc. 7th Int. Conf. on Rapidly Quenched Metals, Stockholm, August 1990.
25
The pitting corrosion behavior of sputter-deposited amorphous A1-Ti alloys in a neutral chloride-containing solution Q. Yan 1, H. Yoshioka 2, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendal 980, Japan Received 23 March 1990 Revised manuscript received 30 May 1990
Pitting corrosion behavior of sputter-deposited amorphous A1-Ti alloys in a neutral borate-boric acid solution with C1 ions has been compared with that of argon arc-melted crystalline counterparts. The sputter-deposited amorphous A1-Ti alloys possess a better passivating ability and pitting corrosion resistance. They exhibit lower passive current densities and pitting potentials which are about 500 mV higher than their crystalline counterparts. Crystallization of the amorphous alloys by heat treatment leads to a decrease in the pitting potential. Pits formed on the amorphous A1-Ti alloys are featureless smooth holes, and those on the argon arc-melted alloys show facetting dissolution. The pit bottom of the argon arc-melted AI-36Ti alloy is enriched with titanium; thus pitting corrosion involves preferential dissolution of aluminum. Sputter-deposited alloys containing ~< 30 at.% titanium consist of a single fcc a-Al phase supersaturated with titanium, and their pitting potential is raised with increasing titanium content.
I. Introduction Amorphous alloys containing passivating elements, such as chromium, tantalum, niobium etc., are known to have a high pitting corrosion resistance. This is associated with their high passivating ability along with chemically homogeneous single phase nature [1]. However, even if a single amorphous phase structure is not formed, rapid solidification increases the corrosion resistance. For example, the good pitting corrosion resistance of rapidly solidified crystalline Fe-Cr alloys has been attributed to an increase in alloy homogeneity, since the formation of large precipitates is prevented [2]. The pitting corrosion resistance of rapidly solidified aluminum alloys is also higher than that of aluminum metal [3]. This has been interpreted in terms of an enhancement of corrosion resistance of the a-Al phase by supersaturai Visiting Researcher from Beijing Aeronautical Manufacturing Technology Research Institute, Beijing, P,R. China. 2 Visiting Researcher from YKK, Kurobe, Toyama, Japan.
tion with alloying elements, since the most susceptible phase to corrosion in aluminum alloys is generally the a-A1 phase. Amorphous alloys containing tantalum a n d / o r niobium possess a particularly high corrosion resistance. Amorphous Ni-Ta alloys are spontaneously passive in boiling 6 N HC1 and do not suffer pitting corrosion [4]. Sputter-deposited amorphous Cu-Ta and Cu-Nb alloys do not suffer pitting corrosion by anodic polarization in 12 N HC1 [5]. Amorphous A1-Ta and A1-Nb alloys have the highest corrosion resistance among amorphous aluminum-base alloys but suffer pitting corrosion in 1 N HC1 by anodic polarization [6]. This has been attributed to the fact that no enrichment of valve metal cations occurs in the passive film formed on the aluminum-valve metal alloys in 1 N HC1, while nickel- and copper-valve metal alloys form quite protective passive films consisting exclusively of valve metal ions as cations even in concentrated hydrochloric acids. The pitting potential of sputter-deposited aluminum alloys in
0022-3093/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
26
Q. Yan et al. / Corrosion behavior of sputter-deposited amorphous A I- Ti alloys
1 N HC1 is ennobled by an increase in content of refractory metals, such as titanium, zirconium, niobium, tantalum, molybdenum and tungsten [68]. The ennoblement of the pitting potentials of sputter-deposited aluminum alloys in neutral [912] and slightly alkaline [13] chloride solutions has also been confirmed, although the formation of the amorphous structure for those alloys has not been examined. The sputter-deposited A1-Ti alloys containing 30-60 at.% titanium consist of a single amorphous phase [8]. The pitting potential of sputter-deposited A1-Ti alloys in a chloride-containing borate-boric acid solution increases with titanium content [8,14]. The relationship between the structure and corrosion behavior of sputter-deposited A1-Ti alloys is not yet clarified. The amorphous AI-Ti alloys should be more corrosion-resistant than conventionally processed crystalline counterparts containing a titanium-poor phase which would be preferentially corroded. Accordingly, the present work has been performed to compare the corrosion behavior of sputter-deposited A1-Ti alloys with crystalline counterparts. Particular attention has been paid to the effect of changes in alloy structure with titanium content.
2. Experimental procedures A1-Ti alloys were prepared by dc magnetron sputtering. Targets were 99.9% pure aluminum discs of 100 mm diameter and 6 mm thickness, on the erosion region of which 99.9% pure titanium discs of 20 mm diameter were placed. Titanium discs placed on the aluminum disc were utilized to change the composition of the sputter-deposited alloys. Sputtering apparatus and conditions used were the same as those described elsewhere [5,6]. Alloys containing 15-75 at.% titanium of 4 - 6 ~m thickness were prepared on glass substrates. Alloy compositions hereafter are all denoted in at.%. The composition of the sputter-deposits was determined by inductively coupled plasma emission spectrometry (ICP) after being dissolved into 1 N HC1 solution.
To compare the corrosion behavior of amorphous alloys with that of crystalline counterparts, the amorphous alloys were crystallized by annealing. After a glass tube containing the amorphous alloy specimen was evacuated to 1 0 - 4 Torr, the glass tube was sealed. Then the specimen was annealed for 30 min at intervals of 50°C from 200 to 550°C, followed by furnace-cooling. Some A1Ti alloy ingots were also prepared by argon arcmelting of aluminum and titanium metals. The structure of the AI-Ti alloys was identified by the diffractometer X-ray method with Cu K a radiation. Electrochemical experiments were carried out in a deaerated chloride-containing borate-boric acid solution consisting of 0.075M Na2B407, 0.3M H3BO 3 and 0.5M NaC1 at pH 8.4 and 25°C. The solution was deaerated by bubbling purified nitrogen gas for about 12 h before each experiment. The pitting corrosion behavior was examined by potentiodynamic polarization measurements with a potential sweep rate of 0.5 mV/s. The specimens used for observation of the pitting corrosion morphology were potentiostatically passivated for 1 h at 100 mV below the pitting potential determined by the potentiodynamic method, followed by potentiostatic polarization at 100 mV above the pitting potential. The morphology of pitting corrosion was observed by scanning electron microscopy (SEM), and the alloy composition at the pitting sites was qualitatively determined by energy dispersive Xray spectroscopy (EDX).
3. Experimental results Table 1 shows phases identified by X-ray diffraction for sputter-deposited and argon arcmelted A1-Ti alloys used. The sputter-deposited A1-Ti alloys up to 23 at.% titanium showed intense crystalline diffraction patterns and no trace of the amorphous phase. The crystalline diffraction patterns were ascribed to the fcc c~-A1 phase. Lattice spacings of four most intense diffraction lines of the c~-A1 phase are almost the same as those of the tetragonal A1Ti phase. However, the fifth most intense (110) reflection of the tetragonal
Q. Yan et a L / Corrosion behavior of sputter-deposited amorphous A I - Ti alloys 0.235
Table 1 Phase identified by X-ray diffraction
A1- x Ti alloy (at.%)
Sputtered A I - Ti Alloys
Arc-melted
x<30 30Ti 36Ti
170
Sputtered \ 0.234 r~
a-Al amorphous amorphous
AI 3Ti + AITi A1Ti + unknown phase AITi >> A1Ti 2 A1Ti << A1Ti2
49Ti 60Ti 64
27
50
5
/,,/
0.233
amorphous amorphous amorphous + a-Ti a-Ti
--0
0.232 60
I
I 70
V-~
30
•
[
] 80
I
I 90
1
tO tOO
Ti Content o f Alloy / o t ~
A1Ti phase was not generally detected. Accordingly, the crystalline diffraction patterns are attributed to a-Al phase. As shown in fig. 1, the diffraction lines broadened with increasing titanium content. The grain size estimated from the full width at half maximum of (111) reflection of the a-Al phase decreases almost linearly with titanium content. The intersection of the straight line with the abscissa almost coincides with the lower limit of the titanium content of the alloy consisting of a single amorphous phase. Consequently, alloying of aluminum with titanium by sputter deposition continuously decreases the crystal grain size with increasing supersaturation of titanium and ultimately results in the formation of the amorphous phase. The alloys with 30-60 at.% titanium were amorphous as defined by X-ray diffraction. 0,7~
Sputtered AI-Ti Alloys
70
~ O.6 \ g o.5
5o
0.4
40
0.3
Jo
~o
~
\
eo
0 0
tO
20
30
7"1Content o f A l l o y / o f ~
Fig. 1. Full width at half m a x i m u m of (111) reflection of the a-AI phase, which is the only phase present in sputter-deposited AI and A I - T i alloys, and the crystal grain size estimated from the full width at half maximum.
Fig. 2. The (002) lattice spacing of the hcp a-Ti phase in the sputter-deposited Ti and Ti-A1 alloys, and the crystal grain
size estimated from the full width at half maximum of the (002) reflectionof the a-Ti phase.
On the other hand, when aluminum is added to titanium, only the hcp a-Ti phase is formed in the wide composition region where the a-Ti phase is the equilibrium state. Alloys containing 64-69 at.% titanium, which is close to the border of the stable a-Ti phase, consist of a mixture of amorphous and a-Ti phases. As shown in fig. 2, the lattice constant of the a-Ti phase decreases continuously with aluminum content, while the grain size is unaffected by alloying. Because the stability region of a-Ti phase is wide, sputtering does not result in a supersaturation with aluminum, and hence does not result in grain refining. The a-Ti phase showed a strong 001 preferred orientation perpendicular to the substrate plane. The argon arc-melted A1-Ti alloys, corresponding to the composition range where the sputter-deposited alloys became amorphous, were composed of two phases, such as A13Ti and A1Ti, or A1Ti and A1Ti 2For investigating the effect of crystallization on the pitting corrosion behavior, the amorphous A136Ti and A1-60Ti alloys were annealed at different temperatures. The amorphous structure of the sputter-deposited A1-36Ti alloy was maintained after annealing at 300°C, but crystallization occurred at 350°C with a consequent formation of AITi phase. The amorphous A1-60Ti alloy was stable at 500°C, but was crystallized by annealing at 550°C, forming tetragonal A1Ti and hexagonal
Q. Yan et at / Corrosion behaoior of sputter-deposited amorphous A l - Ti alloys
28
AITi 2 phases. The crystallization temperature of the amorphous A1-60Ti alloy is about 200°C higher than that of the amorphous A1-36Ti alloy. Accordingly, increasing titanium content of the amorphous A1-Ti alloys raises the crystallization temperature. Figure 3 shows the effect of structure change of the Al-36Ti alloy on the anodic polarization behavior. Comparison of the polarization curve of the amorphous alloy with that of the argon arcmelted counterpart clearly reveals that the amorphous alloy possesses a higher passivating ability and a higher pitting corrosion resistance. Because of oxidation of the amorphous and argon arcmelted alloy specimens during annealing, annealing leads to ennoblement of the open circuit potential and to decrease in the anodic current density. However, pitting occurs at weak points of a passive film, and hence the pitting potentials of the argon arc-melted specimens before and after annealing are almost the same as each other and are about 550 mV lower than the pitting potential of the amorphous alloy. Similarly, crystallization of the amorphous alloy by isothermal annealing leads to a decrease of about 450 mV in the pitting potential, because crystallization introduces heterogeneity in the homogeneous amorphous alloy with a consequent formation of the passive film containing weak points where passivity breakdown occurs.
Deaerated
~
A I -,36 77
0,075M Naz,B,~Or-O.3M H.,BOn-O.5MNoCI 25*C
10~
\
Ar c-mel t e~dj
i ld ~ L) 16
/
~
e
red
~Sputtered and Annealed ot 350°C
k
-0.8
-0.4
i
0.4
J
0.8
Potentiol / V vs SCE Fig. 3. Anodic polarization curves of sputter-deposited amorphous and argon arc-melted crystalline A1-36Ti alloys before and after annealing.
I
[
ao ].6
~
1.2
0.4
I
iI [
NoCti 25°C ~ t Sputtered Crysto~one Amorphous Crystol lized
• e~reu
~" --0,4 /" ~0.8
I
Deoerated O,O 75M Nagd~Oz-O, JM HjBOj
0.8 ~-
I
A I- Ti Alloys
/
~ 0
• Crystoltme I 20
I 40
I 60
I 80
d I(30
Ti Content of Alloy / at~ Fig. 4. Change in pitting potential of sputter-deposited and arc melted AI-Ti alloys estimated from potentiodynamic polarization curves measured in a deaerated borate-boric acid solution containing chloride ion, as a function of alloy titanium content. Pitting potentials measured after crystallization of sputter-deposited amorphous AI-36Ti and A1-60Ti alloys are also shown for comparison.
The pitting potential of each alloy in the chloride-containing borate-boric acid solution was estimated from a sharp current increase in the potentiodynamic polarization curve. The pitting potential of sputter-deposited and argon arcmelted alloys as a function of alloy titanium content is shown in figure 4. The potentials of the alloys crystallized by annealing are also given for comparison. The pitting potentials of sputter-deposited alloys are ennobled with titanium content. There appears to be a plateau in the composition range of the single amorphous structure. After crystallization by annealing, the pitting potentials of amorphous A1-36Ti and A1-60Ti alloys shift toward more negative potentials which are close to those of the corresponding argon arc-melted alloys. This suggests that the formation of a single amorphous phase is beneficial in improving the pitting corrosion resistance. When the major phase became the a-Ti phase, that is, when the titanium content was 75 at.% or more, pitting was not observed up to about 2 V (SCE) which was the limit of the potential obtainable with the potentioslat used.
Q. Yah et al. / Corrosion behavior of sputter-deposited amorphous AI-Ti alloys
Amorphous
'
29
Crystalline
3 #m
_4 #m
AI-36Ti Alloy
,:3 ,urn
,:3 ,urn
AI-6OTi Alloy Fig. 5. Scanning electron micrographs of pitting for sputter-deposited amorphous and argon arc-melted crystalline AI-36Ti and A1-60Ti alloys.
A scanning electron microscope has been used for observation of the pitting corrosion morphology. Figure 5 shows typical surfaces after initiation of pitting corrosion for sputter-deposited amorphous and argon arc-melted crystalline AI-36Ti and A1-60Ti alloys. The pit morphology of amorphous alloys differs from that of argon arc-melted counterparts. There are no
specific features around and inside the pit of the amorphous alloys. By contrast, the shape of the pit of the crystalline alloys is irregular, showing the difference in the dissolution rate of crystallographic planes. The EDX analysis was performed for the free surface and the bottom of a pit on the argon arc-melted AI-36Ti alloy. Figure 6 shows the EDX spectra. A remarkable deficiency of
30
Q. Yen et a L / Corrosion behavior of sputter-deposited amorphous AI-Ti alloys
Deoereted 0.0 75M No2B407 -0.3M H3BO3 -0.5M NaCI 25"C
Al
Free Surface
~q ,¢
\
4 r c - m e l ted Al-36Ti
A~
Bottom o f pit
J
5 Energy /
IO /(V
Fig. 6. EDX spectra of" free surface and pit bottom for the argon arc-melted crystalline A1-36Ti alloy.
aluminum can be seen at the bottom of the pit. Consequently, pitting corrosion occurs by preferential dissolution of aluminum.
4. Discussion
In sputter-deposited A1-Ti alloys, et-A1, amorphous and a-Ti phases were detected. Sputtering generally leads to the formation of a single phase alloy exceeding the solubility limit of the stable phase, although a two-phase field, including the amorphous phase, was found close to the border of the a-Ti phase. By contrast, equilibrium compound phases were found for the argon arc-melted A1-Ti alloys of the composition range where sputter-deposited alloys became amorphous. These compound phases were not detected in the sputter-deposited alloys. This suggests that the alloy compositions which form these compounds in the stable state are suitable for the formation of the amorphous structure by sputter-deposition. EDX analysis showed that pitting corrosion occurs by preferential dissolution of aluminum. This is consistent with the fact that, when the ct-Al phase supersaturated with titanium is formed, the
pitting potential is raised with increasing alloy titanium content. As shown in fig. 4 the pitting potentials of sputter-deposited amorphous alloys are generally higher than those of argon arc-melted and annealed alloys. The presence of the titanium-poor phase in the argon arc-melted and crystallized alloys seems responsible for their lower pitting potentials in comparison with homogeneous amorphous alloys consisting of a single phase. As can be seen in the aluminum rich region, since sputtering tends to form a single phase solid solution supersaturated with titanium, the pitting potential increases continuously with titanium content. According to more detailed investigation [15], argon arc-melted alloys containing 5-25 at.% titanium consist of a two phase mixture of a-Al and A13Ti, and their pitting potentials are almost independent of the average titanium contents of the alloys and 250-600 mV lower than sputter-deposited counterparts. In this manner, single phase solid solution formed by sputtering is generally more corrosion-resistant than stable crystalline counterparts composed of heterogeneous phases. Accordingly, the transition from the single c~-Al phase to the amorphous phase with increasing alloy titanium content leads to no clear discrepancy in the continuous increase of the pitting potential, despite the fact that the pitting potentials of the amorphous alloys are significantly higher than the heterogeneous stable crystalline counterparts.
5. Conclusions
The pitting corrosion behavior of sputter-deposited A1-Ti alloys in a neutral borate-boric acid solution containing chloride ions was investigated in relation to the change in the structure. Sputter-deposited alloys containing up to 23 at.% titanium consist of a single fcc c~-AI phase supersaturated with titanium, and their pitting potential is raised with titanium content. Sputterdeposited alloys containing 30-60 at.% titanium consist of a single amorphous phase, and their pitting potential is significantly higher than those of crystalline counterparts. EDX analysis reveals
Q, Yan et al. / Corrosion behavior of sputter-deposited amorphous AI-Ti alloys that pitting occurs by preferential dissolution of aluminum. Sputter-deposited alloys containing 6 4 - 6 9 at.% t i t a n i u m a r e c o m p o s e d o f a m o r p h o u s and hcp a-Ti phases, and suffer no pitting in the potential region examined.
[7]
[8]
References [1] K. Hashimoto, in: Passivity of Metals and Semiconductors, ed. M. Froment (Elsevier, Amsterdam, 1983) p. 235. [2] A. Kawashima and K. Hashimoto, Corros. Sci. 26 (1986) 311. [3] H. Yoshioka, S. Yoshida, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci. 26 (1986) 795. [4] K. Shimamura, K. Miura, A. Kawashima, K. Asami and K. Hashimoto, Sci. Rep. Res. Inst. Tohoku University A-34 (1988) 107. [5] K. Shimamura, K. Miura, A. Kawashima, K. Asami and K. Hashimoto, in: Corrosion, Electrochemistry and Catalysis of Metallic Glasses, eds. R.B. Diegle and K. Hashimoto (The Electrochemical Society, 1988) p. 232. [6] H. Yoshioka, A. Kawashima, K. Asami and K. Hashimoto,
[9] [10] [11] [12] [13] [14] [15]
31
in: Corrosion, Electrochemistry and Catalysis of Metallic Glasses, eds. R.B. Diegle and K. Hashimoto (The Electrochemical Society, 1988) p. 242, H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, in: Proc. MRS Int. Meeting on Advanced Materials, Vol. 3 (Materials Research Society, Pittsburgh, PA, 1989) p. 429. H. Yoshioka, Q. Yan, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci. 31 (1990) 349. W.C. Mioshier, G.D. Davis, J.S. Ahearn and H.F. Hough, J. Electrochem. Soc. 133 (1986) 1063. W.C. Mioshier, G.D. Davis, J.S. Ahearn and H.F. Hough, J. Electrochem. Soc. 134 (1986) 2677. W.C. Mioshier, G.D. Davis, and G.O. Cote, J. Electrochem. Soc. 136 (1986) 356. W.C. Mioshier, G.D. Davis, T.L. Fritz and G.O. Cote, J. Electrochem. Soc. 137 (1990) 422. G.F. Frankel, M.A. Russak, C.V. Jahnes, M. Mirzamaani and V.A. Brusic, J. Electrochem. Soc. 136 (1989) 1243. Q. Yam H. Yoshioka, H. Habazaki, A. Kawashima, K. Asami and K. Hashimoto, Corros. Sci, 31 (1990) 401. H. Yoshioka, Q. Tan, K. Asami and K. Hashimoto, in: Proc. 7th Int. Conf. on Rapidly Quenched Metals, Stockholm, August 1990.