The corrosion behavior of sputter-deposited AlZr alloys in 1 M HCl solution

Corrosion Science, Vol. 33, No. 3, pp. 425-436, 1992 Printed in Great Britain.

THE

0010-938X/92$5.00+ 0.00 © 1992PergamonPress plc

CORROSION BEHAVIOR OF SPUTTER-DEPOSITED A1-Zr ALLOYS IN 1 M HC1 SOLUTION

H. YOSHIOKA,* H. HABAZAKI, A. KAWASHIMA, K. ASAMI and K. HASHIMOTO Institute for Materials Research, Tohoku University, Sendai 980, Japan Abstract--Using electrochemical measurements and XPS analysis, the corrosion behavior of sputterdeposited AI-Zr alloys in 1 M HC1 solution at 30°C was investigated. Their open circuit corrosion resistance, pitting corrosion resistance and passivating ability were enhanced with increasing zirconium. Alloying of aluminum with zirconium extends the time to reach the steady passive state by potentiostatic polarization and decreases the steady state current density. XPS analysis revealed that when potentiostatic polarization was continued in the passive region, Zr4+ cations in the surface film was slightly concentrated simultaneously with the concentration of zirconium in the underlying alloy surface, due to preferential dissolution of aluminum. The cationic fraction of the surface film finally consisted of AI 3 . and Zr 4+ according to the bulk alloy composition, although the Zr4+ content was apt to be higher than the alloy zirconium content. Therefore, the higher the alloy zirconium content, the higher the corrosion resistance of the sputter-deposited AI-Zr alloys. INTRODUCTION ALUMINUM and its alloys are widely used in various fields, because of their light weight, high strength and g o o d corrosion resistance in neutral environments. T h e y suffer pitting corrosion in the presence of chloride anions and easily dissolve in acidic and basic solutions. Alloying elements which i m p r o v e the disadvantages of alumin u m generally have a limited solubility in a l u m i n u m in the equilibrium state. R e c e n t l y various techniques have offered to extend the solid solubility of a l u m i n u m , for example, rapid solidification, 1'2 ion implantation 3'4 and sputter deposition, s-8 In these investigations attention has b e e n paid to the pitting corrosion b e h a v i o r in neutral solutions containing chloride anions such as 0.1 M NAG1, 3'4'7 0.5 M NaC12 and 0.1 M KC1 at p H 8.0. 5,6,8 Little is k n o w n , h o w e v e r , a b o u t a l u m i n u m alloys corrosion-resistance in acidic solutions, where commercial a l u m i n u m alloys are severely c o r r o d e d . T h e corrosion b e h a v i o r of a m o r p h o u s Al-early transition metal (Ti, Zr, Nb, Ta, M o , W ) alloys p r e p a r e d by m a g n e t r o n sputtering in wide composition ranges 9-11 have b e e n studied. T h e s e alloys, except for A1-Ti alloys, possess significantly lower corrosion rates than a l u m i n u m metal, and their pitting potentials in 1 M HCI, except for A I - M o and A 1 - W alloys, are e n n o b l e d with increasing alloying elements. A c c o r d i n g to XPS analysis, the cationic compositions of the surface films f o r m e d on A 1 - T a and A I - N b alloys polarized in 1 M H C I are not so different f r o m those of underlying a m o r p h o u s alloys. 9 T h e pitting corrosion of A I - T a and A I - N b alloys is, therefore, interpreted in terms of the fact that alloying elements responsible for high corrosion resistance c a n n o t be c o n c e n t r a t e d in the surface films, although pitting potentials are r e m a r k a b l y e n n o b l e d in 1 M HCI. * Present address: YKK Corporation, Kurobe, Toyama 938, Japan. Manuscript received 2 January 1990. 425

426

H. YOSHIOKAet al.

Z i r c o n i u m is o n e of the valve m e t a l e l e m e n t s t o g e t h e r with t a n t a l u m a n d n i o b i u m , a n d the c o r r o s i o n b e h a v i o r of s p u t t e r - d e p o s i t e d A 1 - Z r alloys m a y therefore be similar to those of A I - T a a n d A 1 - N b alloys. A 1 - Z r alloys have lower c o r r o s i o n rates a n d higher pitting p o t e n t i a l s t h a n a l u m i n u m m e t a l n o t only in a n e u t r a l s o l u t i o n c o n t a i n i n g chloride a n i o n s 12 b u t also in 1 M HC1.11 D e t a i l e d i n v e s t i g a t i o n is necessary for a b e t t e r u n d e r s t a n d i n g of the c o r r o s i o n b e h a v i o r of these alloys. A c c o r d i n g l y , the p u r p o s e of this p a p e r is to study the p a s s i v a t i o n b e h a v i o r of s p u t t e r - d e p o s i t e d a m o r p h o u s A 1 - Z r alloys in 1 M HC1 at 30°C by a n o d i c p o l a r i z a t i o n m e a s u r e m e n t a n d X P S analysis. P a r t i c u l a r a t t e n t i o n has b e e n paid to the change in the passive film c o m p o s i t i o n with time of p o t e n t i o s t a t i c p o l a r i z a t i o n , since the f o r m a t i o n of the steady state passive film o n the alloys c o n t a i n i n g valve m e t a l s g e n e r a l l y r e q u i r e s p o l a r i z a t i o n for m a n y hours. E X P E R I M E N T A L METHOD A d.c. magnetron sputtering method was used for the preparation of A1-Zr alloys. The targets were composed of a 99.99% pure aluminum disc of 100 mm diameter and 6 mm thickness, and several 99.9% pure zirconium discs of 20 mm diameter placed symmetrically on the sputter-erosion region of the aluminum disc. The number of zirconium discs placed on the aluminum disc was chosen to change the composition of the sputter-deposited alloys. The composition of these alloys was determined by electron probe micro-analysis. Pure aluminum and zirconium metals were also sputter-deposited. Sputtering apparatus and conditions used were the same as those described elsewhere. 13 X-Ray diffraction patterns of the alloys were measured with the X-ray diffractometer using Cu Ka radiation. These alloys, except for AI-11 at% Zr alloy which showed an fcc a-A1 pattern, showed only halo patterns characteristic of amorphous structure. 12 The corrosion rates of these metals and alloys in 1 M HCI at 30°Cwere estimated from ICP analysis of dissolved elements after immersion for 72-120 h (4 h for sputtered aluminum). Electrochemical measurements were carried out in de-aerated 1 M HCI solution which was prepared from a reagent grade chemical and deionized water. The specimens were polished with silicon carbide paper up to No. 1500 in cyclohexane, and were set in the electrochemical cell as soon as possible. Potentiodynamic polarization curves were measured with a potential sweep rate of 30 mV min-1 from the open circuit potential. Potentiostatic polarization curves of sputter-deposited AI-24Zr and AI-57Zr alloys and sputter-deposited zirconium metal were measured after polarization for 1 h at various potentials. In order to obtain the current-time curves, some sputter-deposited metals and alloys were polarized at passive potentials for different periods of time. X-ray photo-electron spectra for the surface analysis of sputter-deposited A1-24Zr and A1-57Zr alloys were measured by SHIMADZU ESCA 850 with Mg Ka excitation, after immersion or potentiostatic polarization. Binding energies of electrons were calibrated by a method described elsewhere.14'Is The thickness and composition of the surface film and the composition of the underlying alloy immediately under the surface film were quantitatively determined by a previously proposed method. 16 The photoionization cross sections of AI 2p and Zr 3d electrons relative to the O ls electrons used were 1.86817 and 2.561, respectively. The latter was estimated from the photo-ionization cross sections calculated theoretically by Scofield.18 E X P E R I M E N T A L RESULTS T a b l e I shows the c o r r o s i o n rates in 1 M H C l at 30°C e s t i m a t e d from I C P analysis. I n this table, z i r c o n i u m m e t a l used is n o t s p u t t e r - d e p o s i t e d b u t c o m m e r c i a l sheet of the s a m e purity. T h e c o r r o s i o n rates of A I - Z r alloys decrease almost logarithmically with i n c r e a s i n g z i r c o n i u m c o n t e n t . F i g u r e 1 shows the p o t e n t i o d y n a m i c p o l a r i z a t i o n curves of s p u t t e r - d e p o s i t e d A l Z r alloys in d e - a e r a t e d 1 M H C I at 30°C. T h e p o t e n t i o d y n a m i c p o l a r i z a t i o n curves of s p u t t e r - d e p o s i t e d a l u m i n u m a n d z i r c o n i u m are i n c l u d e d in this figure for c o m p a r i -

Corrosion of sputter-deposited A l - Z r alloys TABLE 1.

427

CORROSION RATES IN 1M HCI AT

30°C Alloy (at%)

Corrosion rate (mm y 1)

Sputtered A1

5.29 x 10 1

Al-17Zr AI-24Zr AI-34Zr AI-45Zr AI-57Zr AI-71Zr

1.48 9.93 1.13 4.39 1.02 4.14

Zr metal

2.28 x 10 6

× x x x x x

10 t0 -2 10-2 10 3 10 3 10 4

son. The open circuit potentials of the alloys are higher than aluminum metal and are ennobled by increasing zirconium content. This tendency is in agreement with the open circuit corrosion rates in Table 1. The A1-57Zr alloy has higher open circuit potential than zirconium metal in spite of the higher corrosion rate of A1-57Zr alloy than that of zirconium metal. The pitting potential is also increased by the addition of zirconium. The main disadvantage of zirconium as an alloying element is that zirconium suffers pitting by anodic polarization in this solution. Nevertheless, the addition of zirconium is effective in ennobling the pitting potential. Furthermore, the addition of zirconium decreases the current density in the passive region. In particular, the passive current density of AI-57Zr alloy is almost the same as that of sputter-deposited zirconium. Figure 2 shows the potentiostatic polarization curves of sputter-deposited AI24Zr and A1-57Zr alloys and sputter-deposited zirconium, after immersion or polarization at various potentials for 1 h. Passive current densities and pitting potentials measured by potentiostatic polarization are lower than those measured potentiodynamically. The decrease in the passive current density of AI-24Zr alloy by potentiostatic polarization is less remarkable than A1-57Zr alloy and zirconium 1 0 3'

~

,

i

j

AI-xZr 1021-

~.

Deaerated IM HCl, 30*C

~ I

Sputtered /

At :

10'

I /

llZr

I

~Tz r lI

•

I /I4 5 Z r / I

I

10r

57Zr

]d'

;'

24Zr ..

10

FIG. 1.

-1.0

-0.8

-0.6 - 04 Potential

-0.2 /

S.p.utter..ed_Zr_.~

0 0.2 V vs SCE

0.4

Potentiodynamic polarization curves of sputter-deposited A I - Z r alloys in de-

aerated 1 M HCI at 30°C. Included in this figure for comparison are potentiodynamic polarization curves of sputter-deposited aluminum and zirconium metals.

428

H. YOSHIOKAet al. 102

i

I

Al-xZr E ;~ 10' - -

Deaerated 1M HCl, 30"C

~, 10 °

57~ ~ i0 "l

24Zr

"1.0

-O.d

~ O

-0.6

-0.4

Potential F1G. 2.

/

-0.2

0

V vs SEE

Potentiostatic polarization curves of sputter-deposited AI-24Zr and AI-57Zr alloys and sputter-deposited Zr metal in de-aerated 1 M HCI at 30°C.

metal. The passive current density of the AI-57Zr alloy is slightly lower than that of sputter-deposited zirconium, despite the fact that the AI-57Zr alloy has higher corrosion rate and lower pitting potential than those of zirconium metal. In order to resolve the conflict mentioned above, current decay curves of these metals and alloys during potentiostatic polarization were examined in the passive region, as shown in Fig. 3. The anodic current is the sum of film formation and dissolution currents. The current density of sputter-deposited aluminum gradually decreases up to about 20 s and becomes constant. By contrast, the current density of sputter-deposited zirconium decreases logarithmically with logarithm of time up to 15 h. Accordingly, aluminum easily reaches the steady state dissolution, while the ~0 ~

r

....

i

....

i

, ,,,

i

i i,i

i

, ,,,

A 1- x Z r

1°° " ~

I0"I

~

2';ir E='O.6V

E= -Ct35V "%~_¢5puttered Zr 10u

Oeoeroted

Ca

~

E=-0.35V

, M HCI, 30"C ,

10

,,~

i

101

,

,,,

i

~ ,

,,,

102

Time

i

103

/

,

,,,

1 1

104

. . . .

10 5

sec

FIG. 3, Current decay curves of sputter-deposited AI-Zr alloy and aluminum and zirconium metals during potentiostatic polarization in de-aerated 1 M HCI at 30°C.

Corrosion of sputter-deposited AI-Zr alloys

429

film growth of zirconium continues even after 15 h. Alloying of aluminum with zirconium decreases the passive current density and extends the time to reach the steady state dissolution, despite that the alloys with higher zirconium contents are polarized at higher potentials. In particular, the AI-57Zr alloy requires about 5 h to reach the steady state at -0.35 V(SCE). Its current density finally becomes higher than that of sputter-deposited zirconium, although it is lower until 4 h. In this manner, the steady state current density of the A1-57Zr alloy is higher than that of sputter-deposited zirconium, and hence the surface film formed on the A1-57Zr alloy is less protective than that on the sputter-deposited zirconium. For a better understanding of the corrosion behavior of these alloys, an XPS analysis for sputter-deposited A1-24Zr and A1-57Zr alloys was performed after immersion or potentiostatic polarization for 1 h. Figure 4 shows the binding energies of the A1 2p and Zr 3d5/2 electrons in both metallic [M] and oxidized [OX] states and those of O ls electrons as a function of polarization potential. The oxidized states of aluminum and zirconium were assigned to AI 3+ and Zr 4+ , respectively. The O ls spectrum was composed of two overlapping peaks which were called [OM] and [OH] oxygen. 14,16 The [OM] oxygen is bonded to

AI-Zr

•

" o

I!

,

•

A 1-24Zr AI-57Zr

D

I

i

i

72 • ~o

73

~ e

A/[M]

e~e

Al2p

M OX

74 o - - o

-

•

•

--

•

Al (OX3

75

G _ _ m _ _ m _ _ m _ _ O _ _ u . _ ,

~

n

n

n

rfM]

M

:::[

~8o~.

Z r 3d 5/2 OX

r[OX]

184~. 530 53

=

0 IS

. Zr 02

[OM3

•

A ~ A

A

A--A

A/203

[OHJ 53,

533 e

As ~

FiG. 4.

~

t

-Od

t

-0-6 Potential

I

/

-0.4 V vs

t

-0.2 5CE

Binding energies of photo-electrons from AI-24Zr and AI-57Zr alloys as a function of polarization potential.

430

H. YOSHIOKAet al.

5.(3 AI-5?Zr

Ec

E 4.0 Ec

oI A 1-36 Ti

P Ec

AI-24Zr

,,--0-%

3.0 4¢

.~ 2.0

E Deaerated

LL I

0

As P.

~

i

I

-0.8 -0.6 Potential /

! M HCI, 30"C I

-04. V

I

vs

-0.2 $CE

0

FI6.5. Thickness of the surface film formed on AI-24Zr, At-57Zr and A1-36Ti alloys as a function of polarization potential. "As P." and "Ec" in this figure denote the as-polished specimen and the specimens immersed under the open circuit conditions, respectively.

only metal ions and hence corresponds to 0 2- ions. The [OH] oxygen means oxygen linked to a proton in the film, and is composed of O H - ions and bound water in the film. The photo-electron spectrum of CI arising from the solution species was less than the detectable level. It can be seen that the peak binding energies for all species are independent of polarization potential. However, there is a difference in the binding energies except for Zr [M] between two kinds of alloys, that is, the A1-24Zr alloy gives the higher binding energy peaks than the AI-57Zr alloy. The difference in the binding energy of A1 [M] 2,o is attributable to the alloying effect. The change in the binding energies of oxygen and metallic ions with alloy composition is based on the difference in the film composition as shown later. This suggests that the surface films are not composed of a simple mixture of aluminum and zirconium oxides but 4+ 2consist of a solid solution, 12 such as (Ala3+ Zrl_a)Ob (OH)4-a-2b. After integrated intensities of photo-electron spectra were obtained for individual species, the thickness and composition of the surface film and the composition of the underlying alloy were quantitatively determined. Figure 5 shows the film thickness of the surface film formed on the sputter-deposited A1-Zr alloys as a function of polarization potential. The film thickness of the sputter-deposited AI-36Ti alloy polarized (for 15 min, because of their relatively high current densities of 1 A m -2) in the same solution is included in this figure for comparison. The thicknesses on these alloys tend to increase with increasing polarization potential. When pitting occurs, however, the film thickness decreases, possibly because the current concentration inside pits prevents polarization of the specimen. The fractior] of Zr 4+ in the surface film formed on the sputter-deposited Al-Zr alloys is shown in Fig. 6. Immersion or anodic polarization of these alloys in the passive region leads to a slight increase in the concentration of Zr 4+ cations compared with bulk alloy composition. Its fraction is almost independent of polarization potential. This reveals that the surface film consists of both Zr 4+ and A13+ cations, even though aluminum metal easily dissolves in this solution. When pitting occurs at higher potentials, AI 3+ cations in the surface film increase. As shown in Fig. 7, the atomic fractions of the alloy surfaces just below the surface films are almost the same as the bulk alloy compositions but aluminum in the underlying

Corrosion of sputter-deposited AI-Zr alloys

431

alloy decreases by pitting. It can, therefore, be said that, when pitting prevents polarization of specimen, aluminum is preferentially oxidized. Figures 8 and 9 show the amounts of oxygen species relative to cations in the surface films formed on A1-24Zr and AI-57Zr alloys, respectively. The concentration of O 2- ions in the surface film formed on the A1-57Zr alloy is higher than that on the A1-24Zr alloy. The 0 2- species in the surface film formed on both alloys increase slightly with polarization potential, although the cationic fractions of the surface film are independent of polarization potential in the passive region. This indicates that the surface film is dehydrated with increasing polarization potential. The average compositions of the passive films formed on the A1-24Zr and A1-57Zr alloys after 1 h polarization are approximately as follows; A1-24Zr

( A l o3+ . 7 2 Z r o 4+ . 2 8 ) O 1 .23 2 O H 0 . 6-4

A1-57Zr

3+ 4+ 2(Alo.37Zr0.63)O1.60OI-~).43.0.21H2 O at - 0 . 3 5 V(SCE).

• 0.23H20

at - 0 . 6 V(SCE)

Accordingly, the passive films formed on the A I - Z r alloys are hydrated oxyhydroxide of aluminum and zirconium in which the concentration of zirconium ions are slightly higher than the alloy zirconium content. The XPS analysis was carried out after potentiostatic polarization for 1 h. The decrease in the anodic current of AI-57Zr alloy continued after 1 h as shown in Fig. 5. It is expected that the thickness and composition of the surface film changed with polarization time. Figure 10 shows the thickness of the surface film formed on the A157Zr alloy at - 0 . 3 5 V(SCE) as a function of polarization time. The surface film thickness increases asymptotically to about 4.1 nm with polarization time up to 4 h: this is almost the same as the time when the anodic current becomes constant in Fig. 5. In this manner, the formation of the surface film on the AI-57Zr alloy continues accompanied by an anodic current decrease. Figure 11 shows the cationic and atomic fractions of zirconium on the AI-57Zr alloy as a function of polarization time. The fraction of Zr 4+ in the surface film after polarization for 1 h is slightly higher than that in the air-formed film on the aspolished alloy, while the atomic fraction in the underlying alloy is not appreciably changed. Prolonged polarization leads to an enrichment of zirconium in both the i.o

II

i

i

Cationic Fraction in F i l m O4 AI-57Zr o

o~ "-. \ 04

A1-24 Z r • --0

.,. 0 ,

0~

~ o.2 Deoerated

0

L

As P.

I

L

-0.8

IM

t

-0. 6

Potential

HCI, 30"C ~

-0.4

/

-0,2

V vs $CE

FIG. 6. The ratio of Z r 4+ cations to the sum of A I 3+ a n d Z r 4+ cations in the surface films formed on AI-24Zr and AI-57Zr alloys as a function of polarization potential.

432

H. YOSHIOKAet al. 1.0 Atomic Fraction in Underlying Alloy

O4

AI-5?Zr

0~ "- 0.4 \

o _ o .°

o

o

A1-24Zr •

0--0

0 ~ 0 o 0

02 Deaerated I

|

I

As P.

-0 ~

I

-0. 6

-0.4

Potential FIG. 7.

IM

I

/

HCI

30"C I

-02

0

V vs 5CE

T h e atomic fraction of zirconium in the underlying alloy just below the surface film as a function of polarization potential.

surface film and the underlying alloy as a result of preferential dissolution of aluminum. Figure 12 shows the amounts of oxygen species relative to the cations in the surface film as a function of polarization time. The ratio of 0 2- ions to cations is 1.60 after 1 h polarization and decreases evidently with prolonged polarization. Sputter-deposited zirconium showed the same tendency; the ratios of 0 2- ions to Zr 4+ cations are 1.56, 1.49 and 1.29 for as-polished specimen and specimens polarized for 1 h and 15 h at -0.35 V(SCE), respectively. The compositions of the passive film on the A1-57Zr alloy polarized for 1 h and 2 h at -0.35 V(SCE) are respectively as follows; 3+

4+

2-

--

(Alo 37Zro.63)O1.60OH~.43.0.21H2O after 1 h 3+ 4+ 2(Alo.31Zro.69)O1.32OHl.05-0.02H20

after 2 h.

As shown in Figs 8 and 9, the rise of potential leads to an increase in the ratio of 02ions in the surface film, while prolonged polarization at the same potential does not promote dehydration but brings about an increase in O H - ions in the surface film. Consequently, the stable passive film formed on the alloy at relatively low potentials

2.0,

i

E

U

p

i

AI-24Zr

I

F

Deaereted IM HCI, 30"C

1.5 o

0

~

o 2-

,o i

(D

0.~ Cb

©~e--'----°'----------e"~'©

OH -

A

H~O

o z~

0

i

As P, FIG. 8.

~

u

I

~ I

-0.8 -0.6 Potential

I

I

-0.4 -0.2 / V vs SCE

A m o u n t s of oxygen species relative to cations in the surface film formed on the A l - 2 4 Z r alloy as a function of polarization potential.

Corrosion of sputter-deposited AI-Zr alloys

433

E 2.0 t~

0 20

0

0

~ 1.5 AI-57Zr

Deaerated 1M HCI, 30°C

~1.0 OH-

~:" 0.5

0

^

H20 A

II

L

I

-05

As P.

I

-0.4

Potential

FIG. 9.

I

-0.3

I

-02

/

-0.1

V vs 5CE

Amounts of oxygen species relative to cations in the surface film formed on the A1-57Zr alloy as a function of polarization potential.

for valve metals such as aluminum and zirconium may not be oxide but oxyhydroxide. DISCUSSION

In a previous study, 12 it was found that sputter deposition extended the solid solubility of zirconium in aluminum up to 11 at%, forming a single a-Al phase supersaturated with zirconium, and that further increase in zirconium gave rise to the formation of a single amorphous phase. Accordingly, these alloys have no chemical heterogeneity based on phase separation. Furthermore, the surface film formed on these alloys consists of a single solid solution oxide at all potentials examined, and the cationic fraction in the surface film is close to the bulk alloy composition. Consequently, the formation of single phase alloys by sputter deposition leads to the formation of homogeneous solid solution oxide whose cationic composition is similar to the alloy composition, and hence their corrosion rates, passive current densities and pitting potentials are dependent upon the alloy zirconium content. Prolonged polarization shows that the high zirconium alloy needs longer time to reach the steady state current density than the low zirconium alloy. When valve

i

II

5.01

i

i

AI-5?Zr

v} ~ 4.0

.

.Ic

~

i

E---O.35V ~

o

•

•

s"

3.0

*"

Oeaerated IM

E

H C I , 300C

~ 2,0 0[

FIG. 10.

.~

As t~

i

t

t

i

i

i

2 3 4 5 Time / hr

I

tO

20

Thickness change of the surface film on the AI-57Zr alloy with polarization time during polarization at - 0 . 3 5 V(SCE).

434

H. YOSHIOKAet al. 1.0

,41

I

,

'

'

A 1-57Zr

i

E=-0.35 v

L.

N

O.d

C a t i o n i c Fraction e" .

< o.c

o

Atomic Fraction ,.~

04

Oeaera ted

ot

IM HCI , 30"C I

II

I

AsP.

I

I

I

2

3

Time

I

/

i

4 5 hr

T

I

I

10

20

FIG. 11. Change in fraction of Zr4+ in the surface film and atomic fraction of zirconium in the underlying alloy with polarization time during polarization of the A1-57Zr alloy at -0.35 V(SCE).

metals, such as aluminum and zirconium, are anodized in a suitable electrolyte, the current becomes very small and often time-dependent, a9 In 1 M HCI solution, aluminum is easily dissolved, but the anodic current density of zirconium metal depends upon the polarization time for a long time. This suggests that zirconium requires a long time to reach the steady state dissolution. The single phase A1-Zr alloys are endowed with such a characteristic of zirconium. As shown in Fig. 4, the current density of A1-57Zr alloy is slightly lower than that of sputter-deposited zirconium during polarization at -0.35 V(SCE) from about 10 s to 5 h. Similar results are obtained at various potentials after 1 h polarization as shown in Fig. 2. It was also observed that sputter-deposited A1--69Ti and AI-75Ti alloys showed relatively lower current densities compared to titanium metal in the passive region, e° Hashimoto e t al. 21 made a proposal to explain these phenomena in terms of the higher activity of the amorphous state. According to them, the initial current density of amorphous Fe-10Cr-13P-7C alloy within 2 s is higher than that of

l$

.S .o

i

i

i

i

Al-57Zr

E-'-0.35 V

2 02 -

L)

£i

©f®

OH -

C)

H~O

4" 0 0 I

g

I

I

I

AsP.

10

Time FIG.

12.

I

20

hr

Amounts of oxygen species relative to the cations in the surface film formed on the AI-57Zr alloy as a function of time of polarization at -0.35 V(SCE).

Corrosion of sputter-deposited Al-Zr alloys

435

crystalline 18Cr-8Ni stainless steel due to higher activity of the amorphous state, and rapid film formation on the amorphous alloy leads to lower current density after 2 s. The AI-57Zr alloy is in the amorphous state and its initial current density less than 10 s is slightly higher than that of sputter-deposited crystalline zirconium metal. Therefore, the proposed mechanism described above may be applied to explain the difference in current density between A1-57Zr alloy and zirconium metal. XPS analysis after prolonged polarization reveals that the surface film grows, being accompanied by the current decrease. In this period, the fraction of Zr 4+ in the surface film gradually increases. The atomic fraction of zirconium in the underlying alloy also increases. This implies that preferential dissolution of aluminum occurs in this period. Further prolonged polarization no longer thickens the surface film and does not change the cationic and atomic fractions. It is difficult to determine dissolved elements in the steady state. According to ICP analysis of the open circuit corrosion loss, aluminum dissolves preferentially during the early stage of immersion and finally the composition of dissolved elements becomes the same as that of the bulk alloy composition. 22 A similar change seems to occur during potentiostatic polarization in the passive region, because both aluminum and zirconium are passive under the open circuit conditions. CONCLUSIONS

The following conclusions can be drawn from the immersion test, anodic polarization and XPS analysis of sputter-deposited A1-Zr alloys in 1 M HCl solution at 30°C. The corrosion rates of sputter-deposited A1-Zr alloys are 1-3 orders of magnitude lower than that of aluminum metal and decrease with increasing zirconium content. Anodie polarization shows that the open circuit and pitting potentials are continuously ennobled with increasing zirconium content. The passive current densities also decrease by the addition of zirconium. This is attributable to the fact that the alloy zirconium content continuously increases due to formation of single phase, without phase separation, such as a-A1 phase supersaturated with zirconium or amorphous phase. Prolonged polarization in the passive state exhibits that logarithm of the current densities of AI-Zr alloys and zirconium metal decreases linearly with logarithm of polarization time, and the constant currents appears earlier for A1-Zr alloys than zirconium metal. The high zirconium alloy needs a longer time to reach the steady state current density. According to XPS analysis, while the current continues to decay, preferential dissolution of aluminum occurs as well as film growth. The surface film finally consists of A13+ and Zr 4+ cations although the concentration of Zr ~+ is slightly higher than that of bulk alloy composition. Because of presence of Al 3+ cations in the surface film, the corrosion resistance of sputter-deposited Al-Zr alloys cannot exceed that of zirconium metal. REFERENCES l. A. 8AITO and R. M. LATANIS1ON,Proc. 9th Int. Cong. Metallic Corrosion, Vol. 3, p. 122. National Research Council of Canada, Ottawa (1984). 2. H. YOSHIOKA,S. YOSHIDA,A. KAWASH1MA,K. ASAMIand K. HASHIMOTO,Corros. Sci. 26,795 (1986). 3. P. M. NATISHAN,E. MCCAFFERTYand G. K. HUBLER,J. electrochem. Soc. 133, 1061 (1986). 4. P. M. NATISHAN,E. MCCAFFERTYand G. K. HUBLER,J. electrochem. Soc. 135,321 (1988). 5~ W. C. MOSHIER,G. D. DAVIS,J. S. AHEARNand H. F. HouGH, J. electrochem. Soc. 133, 1063 (1986).

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