High corrosion resistance of amorphous CoCrMoZr alloys

Scripta M E T A L L U R G I C A

Vol. 17, pp. 1293-1297, Printed in the U.S.A.

HIGH CORROSION

RESISTANCE

OF AMORPHOUS

1983

Pergamon Press Ltd. All rights reserved

Co-Cr-Mo-Zr

ALLOYS

M. Naka*, M. Miyake**, M. Maeda***, I. Okamoto* and Y. Arata* * Welding Research Institute of Osaka University, Ibaraki, Osaka 567, .Japan ** University of Osaka Prefecture, Osaka, .Japan *** Osaka Industrial University, Osaka, Japan (Received June 6, 1983) (Revised July 20, 1983) Introduction One of outstanding characteristics of amorphous alloys is a high corrosion resistance. Naka et al. (1,2) have reported that amorphous FeCrPI3C ? and FeCrNiPI3C v alloys containing 8 at.% or more chromium show an extremely high corrosion resistance in various solutions, especially to pitting in acid and neutral solutions containing chloride. This high corrosion resistance is attributable to rapid formation of homogeneous protective passive film. Large amounts of phosphorus which accelerate the active dissolution of alloys cause the rapid formation of passive film and enrichment of chromium in the film. Amorphous alloys containing phosphorus often cause embrittlement prior to crystallization. It is preferable that amorphous alloys composed of only metallic elements, therefore, reveal high corrosion resistance. This paper deals with the corrosion behavior of amorphous CoCr-Mo-Zr alloys in IN HCI. Experimental Amorphous ribbons 2mm wide and 30 pm thick were prepared using a meltspinning technique. A series of compositions of Co75 xCrl5MOxZrl0 with X= 3 ~ i 0 and Co65 xCr25MoxZrlo with 3 ~ 7 were investigated. The formation of an amorphous structure was conflrmed by X-ray examlnation. Corro.~on tests and electrical measurements were carried out in IN HCI, which was exposed to air. The number attached to the respective elements denotes the nominal content in atomic percent. Corrosion rates were estimated from weight losses after immersion in acid at 303 K. Polarization measurements were made by a p o t e n t i o d y n a m i c method with a potential sweep rate of 2.5X10 -3 Vs -I starting from the corrosion potential at room temperature. X-ray photoelectron and Auger spectra from the alloy surface after'60 min immersion in 1N HCI were measured. Results

and Discussions

Figure 1 shows the change in corrosion rate of amorphous Co75 - -_Crl5MOxZrlo A alloys in IN HCI as a function of chromium content. Included for comparlson in the figure are corrosion rates of crystalline Coss_xCrl5Mo x and Co75_xCr15MoxZrlo alloys. The corrosion rates of the amorphous cobalt alloys decrease effectively with an increase in m o l y b d e n u m content. Immunity to corrosion in IN HCI is attained at 5 at.% molybdenum. On the other hand, an increase in m o l y b d e n u m decreases gradually the corrosion rates of crystalline Co85_xCr15Mo x alloys. Though alloying with m o l y b d e n u m up to 5 at.% is effective in increasing the corrosion resistance of crystalline C o 7 5 _ x C r 1 5 M o x Z r l o alloys, the additional increase in molybdenum is not effective . The addition of m o l y b d e n u m to a amorphous Co-Zr alloy contain--

1293 0036-9748/83 $3.00 + .00 Copyright (c) 1983 Pergamon Press

Ltd.

1294

CORROSION RESISTANCE OF Co-Cr-Mo-Zr

Vol. 17, No. ii

ing 25 at.% chromium provides a high corrosion resistance as shown in Fig. 2. I ~ i t y to corrosion is attained at only 3 at.% molybdenum. Crystalline Co75_xCr25Mo x and Co65_xCr25MoxZrlo alloys give higher corrosion rates than that of amorphous Co65_xCr25MoxZrlo alloys. Potentiodynamic polarization measurements show the effect of molybdenum on corrosion resistance of amorphous Co-Cr-Zr alloys, crystalline Co-Cr-Mo and Co-Cr-Mo-Zr alloys. Figs. 3 and 4 show anodic polarization curves of amorphous CoTs_xCrI5MOxZrlo and Co65_xCr25MoxZrlo alloys in IN HCI, respectively. Anodic current densities of fio75_xCr15MoxZrlo and Co65_xCr25MoxZr10 alloys are much lower with an increase in molybdenum content. An outstand z ing characteristic is that pitting corrosion is suppressed by 8dding molybdenum to the amorphous alloys in IN HCI. Co65CrlsMOloZrlo and C°6_5-xCr~M°~Zrln_ . . . .alloys containing 3 at.% or more molybdenum content exhibit the spontaneous passlva~ion, and no pitting potential up to the transpassivation of chromium, Figs. 5 and 6 show anodic polarization curves of crystalline Co85_xCr15Mo x and Co75 xCrl5MoxZrlo alloys in 1N HCI. Though the pitting corrosion is suppressed-to some extent by adding molybdenum, the anodic current densities of the crystalline alloys are higher than that of amorphous alloys containing molybdenum. Figs. 7(a) to (d) show the microphotographs of amorphous Co75Cr15Zr10(a), C o ~ C r ~ Z r l n ( b ) , Co72Cr15Mo3Zrlo(c) and Co62Cr25Mo3Zrlo(d) alloys cor~6&e~-at-~0.2 V(SCE) for 2min. The pitting pores are decreased by the addition of molybdenum to amorphous alloys in IN HCI. After 3.6 ks immersion in IN HCI, an Auger analyser with I0 keV for amorhous Co58Cr25Mo7Zr$0 alloy was performed as shown in Fig. 8. Composition profiles were obtalned by sputtering in a 2.67 mPa argon atmosphere with 2 keV argon ion at a current of 3 pA, Auger peaks of Co, Cr~ Mo, Zr and O are 787, 538, 196, 155 and 520 (eV), respectively, Molybdenum and zirconium are not present in the top surface of the film formed during immersion, and their content gradually increases in the film. Chromium is enriched on the top surface in the film. The peak height and relative sensitivity factors of cobalt and chromium give 86.8 at.% of chromium fraction on the top surface of film. XPS spectrum measurements with M g - K ~ (10keV,20mA) reveal the state of chromium in the film formed during 3.6 ks immersion in IN HCI. Fig. 9 shows Cr2Pl/2, Cr2P3/2 electron spectra which result from the oxide state of Cr. Fig. i0 shows Ols electron spectra from specimen. The peak due to oxygen in Cr-O is indicated by an arrow. The sub-peak in the higher energy arises from the metal-OH and bound water. The film formed during immersion is composed mainly of hydrated chromium oxy-hydroxide CrOx(OH)y . Hashimoto et al. (3) have discussed the role of the alloying element of molybdenumin in corrosion. The amorphous structure of alloys imparts the formtion of a homogeneous alloy possessingasmaller number and size of micropores than that of a chemical heterogeneous crystalline alloy. The micropores f o r m e d on amorphous a l l o y s a r e e a s i l y c o v e r e d w i t h c o r r o s i o n p r o d u c t s . The c o r r o s i o n p r o d u c t s o f molybdenum and z i r c o n i u m a r e u n s t a b l e in t h e a c t i v e s t a t e , and d i s s o l v e i n t h e s o l u t i o n . C o n s e q u e n t l y , molybdenum and z i r c o n i u m a r e n o t f o u n d i n p a s s i v e f i l m f o r m e d on a C068Cr25MoTZr10 a l l o y a f t e r immers i o n i n 1N HC1, b u t t h e y d e f i n i t e l y assist the passivation of alloys. References i.

M. Naka, K. Hashimoto and T. Masumoto, J. Japan Inst. Metals,

38, 385

(1974) 2. 3.

M. Naka, K. Hashimoto and T. Masumoto, Corrosion, 32, 146 (1976) K. Hashimoto, M. Naka, K. Asami and T. Masumoto, Corrosion Science,

165(1979)

19,

Vol.

17, No. ii

CORROSION

1000

RESISTANCE

OF Co-Cr-Mo-Zr

1295

100(

IN HCI

100

1N HCI

10(

~.co~_,c~z~

•

10,

10

Cryst Co85.xC%MOx

E

[] o-yst. C ~ . x C ~ s ~

o

I "

i 1~

1'

A~Co~_~Cr2~Z ~

~' C r ~ C o ~ C ~

°c.~C~.~z~

.6 o.1 0 U

001 0.005 ,

,

J

i

--

i

i

I

i

0

f5

.

McWybdenum , X , at%

Molybdenum

FIG. 1 Corrosion rates of amorphous Co75_xCr15MoxZrlo, crystalline Co85_xCr15Mo x and Co75_xCr15MoxZrlo alloys in IN HCI at 303 K.

,d < I~

"

1¢

-f, t

X;0

d

.

,5

.

FIG. 2 Corrosion rates of amorphous Co65_xCr25MoxZrlo, crystalline Co75-xCr25Mo x and Co65-xCr25MoxZrlo alloys in IN HCI at 303 K.

AmC%&r~MOxZr~o

1NHCi

.

, X, at%

1N HCI

Am. CO~xCr~l~

ld <

lO~ X=(

a

j

]

X=3

U

ld <~

o Potential

U

ld

7

"10

os

~o

15

V(SCE)

FIG. 3 Anodic polarization curves of amorphous Co75_xCrlsMOxZrlo alloys in IN HCI.

Ij

41)

-o'.5

0

Potential

&

110

15

V(SCE)

FIG. 4 Anodic polarization curves of amorphous Co65_xCr25MoxZrlo alloys in 1N HCI.

1296

CORROSION

ld IN HCI

RESISTANCE

OF Co-Cr-Mo-Zr

10" ]N HCI

Cryst C%.xCr~Mox

~E

Vol.

17, No.

Cryst C%s_:,.Cr15 MOxZ%

:f

~E

¢-

I_1

U

ld

Id I

-o.s

6 Potent,al

ds

1.'o

10=-10 -05

15

V(SCE)

FIG. 5 Anodic polarization curves line C085_xCr~5Mo x alloys IN HCI.

of crystalin ~,

0

Potenhal

015

I

10

1.5

V(SCE)

FIG. 6 Anodic polarization curves of crystalline Co75Cr15MoxZrlo alloys in IN HCI.

.....

:~

FIG. 7 Microphotographs of amorphous C075Cr15Zrlo(a), Co65Cr25Zrlo(b), Co72Cr15Mo3Zrlo(c ) and Co62Cr25Mo3Zrlo(d) alloys corroded at +0.2 V(SCE) for 2 min in IN HCI.

ii

Vol.

17, No.

Ii

CORROSION RESISTANCE

OF Co-Cr-Mo-Zr

ICr

12!I

L2~2 2P~j2

~I

Cr

C

C" . . . . .

0

---"

I

I

I

I

10 20 30 Sputtenng Time min

FIG. 8 Depth profile of the film formed on amorphous Co58Cr25MovZrlo alloy after immersion in IN HC] as a function of sputtering time.

01s Cr-O I

(b

cr

r-

3

j Binding Energy

eV

FIG. I0 Ols spectra of amorphous C058Cr2sMo?Zrlo alloy after immersion in IN HCI.

i

i

570

1

575

i

i

5E0 585 Binding Energy

i

1

590 eV

595

FIG. 9 Cr2Pl/2 and Cr2P3/2 electron spectra of ambrphous Co58Er25MOTZrlo a l l o y a f t e r immersion in 1N HC1.