INTERMETALLIC INFLUENCE

ON

COMPOUNDS THE

A.I, Evstyukhin, and V.V. Osipov

OF

CORROSION I.I.

ZIRCONIUM PROPERTIES

AND OF

THEIR ZIRCONIUM

ALLOYS

Korobkov, UDC 620.193.23:669.296

The m o s t i m p o r t a n t z i r c o n i u m a l l o y s f o r n u c l e a r e n e r g e t i c s usually contain alloying a d d i t i v e s of tin, iron, nickel, copper, molybdenum, and Other e l e m e n t s [1]. These a d d i t i v e s have low s o l u b i l i t y in a - z i r conium and f o r m compounds of the type of ZrFe2, Zr2Ni, Zr4Sn, etc. with it. The p r o p e r t i e s of z i r c o n i u m a l l o y s depend g r e a t l y on the p r o p e r t i e s of the i n t e r m e t a l l i c compounds f o r m e d , t h e i r amount, and the n a t u r e of t h e i r d i s t r i b u t i o n [2]. In view of this we undertook to study the p r o p e r t i e s of i n c l u s i o n s of i n t e r m e t a [ l i c compounds in the pure f o r m and to evaluate t h e i r influence on the c o r r o s i o n p r o p e r t i e s of a l l o y s of z i r c o n i u m in w a t e r and high-parameter steam. In addition, we made a d e t a i l e d i n v e s t i g a t i o n of the m i c r o h a r d n e s s at v a r i o u s t e m p e r a t u r e s and the s t r u c t u r e of the i n t e r m e t a l i i c compounds, a s well as the c o m p o s i t i o n of the oxide f i l m s f o r m e d . MATERIALS

AND

METHODS

Z i r c o n i u m i n t e r m e t a l l i c compounds w e r e produced f r o m z i r c o n i u m iodide and the c o r r e s p o n d i n g pure m e t a l s by m e l t i n g them t o g e t h e r in an M I F I - 9 - 3 a r c f u r n a c e with a tungsten e l e c t r o d e . The a l l o y s w e r e p r e p a r e d in the f o r m of r o d s up to 80 m m long and weighing 80-100 g each. To obtain an e q u i i i b r i u m s t r u c t u r e the a l l o y s w e r e annealed for 50 h under a vacuum of ~ 5.10 -~ m m Hg at the t e m p e r a t u r e 780~ (ZrFe2, Zr2Ni and Zr~Cu) o r 950~ (ZrMo 2 and Zr4Sn ). Such a l l o y s had a fine c r y s t a l l i n e homogeneous s t r u c t u r e , while the r e s u l t s of c h e m i c a l and x - r a y a n a l y s i s c o n f i r m s the c l o s e n e s s of t h e i r c o m p o s i t i o n to the s t o i e h i o m e t r i c . F o r c o r r o s i o n t e s t s , p l a t e s 12 x 10 • 8 m m in d i m e n s i o n s w e r e cut out of the p r e p a r e d a l l o y s , ground on e m e r y p a p e r of v a r i o u s n u m b e r s and p o l i s h e d on cloth. A f t e r this the s u r f a c e of the s a m p l e s was etched in a m i x t u r e c o n s i s t i n g of 20% HF, 20% HNO3, and 60% g l y c e r i n , and w a s h e d in w a t e r and alcohol. The c o r r o s i o n s t a b i l i t y in s t e a m - w a t e r medium w a s studied in m i c r o a u t o c l a v e s with a volume of 6 cm 3 at 400~ and a p r e s s u r e of 280 atm. The d e g r e e of c o r r o s i o n was e s t i m a t e d a c c o r d i n g to the method of weight gains, which w e r e d e t e r m i n e d on an M-20 m i c r o a n a l y t i c a l b a l a n c e with a s e n s i t i v i t y of 1.10 -~ g. The k i n e t i c s of the oxidation of s a m p l e s of the a l l o y s in oxygen and a i r was i n v e s t i g a t e d by the method of continuous weighing on a vacuum a u t o m a t i c t h e r m o b a l a n c e with a s e n s i t i v i t y of 2.10 -~ g at t e m p e r a t u r e s of 400 and 500~ which w e r e m a i n t a i n e d with an a c c u r a c y of 9 5 ~ A study of the s t r u c t u r e and c o m p o s i t i o n of c o m p a r a t i v e l y thin oxide f i l m s (down to 1000.~) was conducted on an ]~G-100A e l e c t r o n d i f f r a c t o m e t e r . A thin fitm of aluminum was used as the s t a n d a r d . T h i c k e r l a y e r s of s c a l e (0.1-0.5 mm) w e r e i n v e s t i g a t e d by an x - r a y method. The x - r a y d i f f r a c t i o n p i c t u r e s w e r e taken in this c a s e on a s h a r p - f o c u s tube with c o p p e r r a d i a t i o n and a nickel f i l t e r in an RKU-86 c a m e r a . The combination of the i n d i c a t e d two m e t h o d s made it p o s s i b l e to obtain d a t a on the s t r u c t u r e and c o m p o s i tion of the o x i d e s on the s u r f a c e of the i n t e r m e t a l l i c compounds and in d e e p e r l a y e r s . The t o p o g r a p h y of the s u r f a c e of the oxide film f r o m the s a m p l e s was studied on an EM-5 e l e c t r o n m i c r o s c o p e with c a r b o n r a p l i e a s , shadowed with platinum. T r a n s l a t e d f r o m A t o m n a y a t~nergiya, Vol. 28, No,3, pp. 201-206, M a r c h , 1970. s u b m i t t e d A p r i l 30, 1969.

Original article

Consultants l~ureau, a di~,ision of Plem~m F ~k,lishin~ Corporation. 227 We.st i Tth. Street, New York, i N. )'. i001]. -ill riKhts reserved, T/tie ttrticle r'anlot be reproduced for any ~urpoze whatsoever withou.t permission o/ the publisher. I cot) )" of this article is available from the publisher for $]5.00. ] 01970

J 262

T A B L E 1. V a l u e s of the W e i g h t G a i n s f o r S a m p l e s of I n t e r m e t a l l i e C o m p o u n d s of Z i r c o n i u m a f t e r E x p o s u r e to S t e a m (280 a i m ) and to A i r a t 400~ !ntermetallic Time ofex-i compound iposure, min ZrFez ZrzNi Zr~Sn ZrzCu ZrMo2

{ , I

t20 50 70 t0 t20

Weight gain, rag/ore z

steam

air

0,3 Decomp.

0,3 Decomp.

-

0',4

ZrFe 2

ZrzNi Zr~Sn ZrsCu ZrMoz

Fe203

~owder after oxide layer complete oxi~ation of sample Fe203 @ ZrO2

* M - monoclinie lattice; T - tetragonal or cubic lattice. T During corrosion in stean% a mixture of the oxides MozOs+ ZrOz (M) is detected.

T A B L E 3. V a l u e s of the i n t e r p l a n e D i s t a n e e d and R e l a t i v e I n t e n s i t i e s J of the L i n e s oll the E l e c t r o n D i f f r a c t i o n P a t t e r n and X - r a y D i f f r a c t i o n P a t t e r n of the O x i d e F o r m e d on the I n t e r m e t a l l i e C o m p o u n d Z r M o 2 a t 500~ Electron diffrac- rX-ray diffractioni X-ray diffraction tion pattern of i pattern of oxide l oxide film llayer I pattern ~ p~

2,96 Strong 2,54 Meditim . 2,34 --

--

1,9t Weak 1,84 Very strong i,69 Medium 1,63 Strong

-- 'i

--

2,96 S~rong 2,57 Mediu ke 2,33 m ~ a; w

J

~s

--

j/

ho

+

[

i

I

50

700

eYO

s

ZOO

--

Time, h

bj

20 d 75--

Fe203 @ ZrO 2

(~§ * (~,+ ~) NiO NiO~-Zr02 (M) NiO+Zr02 (M) ZrO2 (M) ZrO2 (M)+SnO2 Zr02 (-u)+SnO2 CuO CuOTZrO2(u) ZrO2(M)+CuO ZrMozO~eC ZrMoz05 ZrMozOs

3,9 ,Medium

--

20,07

Strucltre and composition of oxides

metallic [thin films .~ompoundland surface ioxide layer

3

2

T A B L E 2. P h a s e C o m p o s i t i o n and S t r u c t u r e of O x i d e s F o r m e d on I n t e r m e t a l t i e C o m p o u n d s of Z i r c o n i u m d u r i n g O x i d a t i o n in the I n t e r v a l 400- 800~ Inter-

/1

3,91 Very strong 2,97 Strong 2,56 Medium 2,35 Weak

20

~#

80

30

700

72Y

/~tOTime.

h

F i g . 1. D e p e n d e n c e of the w e i g h t g a i n s on the t i m e f o r t n t e r m e t a l l i c c o m p o u n d s of z i r e o n i u m d u r i n g o x i d a t i o n in a i r and in o x y g e n a t 400 (a) and 500~ (b): 1) Zr2Ni; 2) Zr4Sn; 3) Zr2Cu; 4) Z r F e 2 ; 5) ZrMo2; 6 ) z i r conium iodide. The m e t h o d of m i e r o h a r d n e s s , w h i c h d o e s not c a u s e a b r e a k d o w n of the s u r f a c e of b r i t t l e a l t o y s , w a s u s e d to e v a l u a t e the s t r e n g t h and p l a s t i e i t y of s a m p I e s of the i n t e r m e t a l l i c c o m p o u n d s w i t h i n the t e m p e r a t u r e i n t e r v a ! 20-700~ The mierohardness was measured u n d e r a l o a d of 100 g on a s p e c i a l l y d e s i g n e d and p r e p a r e d v a c u u m m i e r o h a r d n e s s gauge, using a s t a n d a r d h e a d to f a s t e n the i n d e n t e r f r o m the P M T - 3 i n s t r u m e n t . T h e t i m e of a c t i o n of the s a p p h i r e t i p with s t a n d a r d a n g l e s of p o i n t i n g on the s a m p l e w a s 60 s e c . B e f o r e m e a s u r e m e n t of the m i e r o h a r d n e s s , the s a m p l e w a s k e p t a t the t e m p e r a t u r e of t e s t i n g f o r 20 min, and then s i x i m p r i n t s were made for eaeh temperature. DISCUSSION

OF

RESULTS

1,99

1,93 !Medium t,84 Strong ~ I _ 1~9 IStrong

1~93 Strong t,84 Very strong f,70 t,63

o b t a i n e d in the o x i d a t i o n of t h e s e f r o m T a b l e 1, a f t e r e x p o s u r e of

C o r r o s i o n S t a b i l i t y of P u r e I n t e r m e t a l l i c C o r n p o u n d s . The r e s u l t s of the m e a s u r e m e n t s of the r a t e of Medium Weak i n t e r a c t i o n of i n t e r m e t a l l i c c o m p o u n d s with s t e a m at 400~ a r e p r e s e n t e d in T a b l e 1. D a t a on the w e i g h t g a i n s s a m p l e s in a i r a r e a l s o p r e s e n t e d t h e r e f o r c o m p a r i s o n . A s can be s e e n the s a m p l e s to s t e a m , the w e i g h t g a i n s e o u l d be e s t i m a t e d f o r o n l y two

263

Fig. 2. Electron micrograph of the surface of samples of intermeta[lic compounds of zirconium after oxidation at 500~ a) ZrFe2, time of oxidation I h; b) Zr2Ni , i h; c) Zr4Sn , lh; d) Zr2Cu, 38h; e) ZrMo 2, 15 h.

F i g . 3. E l e c t r o n m i c r o g r a p h s of the s t r u c t u r e of z i r c o n i u m a l l o y s : a) Z r +1% Cn; b) Z r +0.8% F e +0.2% NL

i n t e r m e t a l l [ c compounds: Z r M o 2 and Z r F e 2 . The r e m a i n i n g s a m p l e s b r o k e down d u r i n g t e s t i n g and w e r e c o n v e r t e d to the oxide in the f o r m of a powder. This was e v i d e n t l y due to the b r i t t l e n e s s of the [ n t e r m e t a ' lie compounds and to the action of two f a c t o r s in the c o r r o s i o n p r o c e s s : h y d r o s t a t i c p r e s s u r e and oxidation~ F r o m Table 1 it is a l s o evident that when the shape of the s a m p l e s is p r e s e r v e d , t h e r e is a definite c o r r e s pondence between the p r o c e s s e s of c o r r o s i o n in s t e a m and a i r . F i g . 1 p r e s e n t s the k i n e t i c s of the oxidation of i n t e r m e t a t l i c compounds of z i r c o n i u m in a i r and in oxygen at 400 and 500~ It is evident that the oxidation of the i n t e r m e t a l l i c compounds at 400~ d u r i n g t h e

264

first 80 h proceeds according to a parabolic law. Such a shape of the oxidation curves is due to the formation of a protective oxide film on the surface of the samples. In the further testing, the rate of oxidation begins to increase, which is evidently due to cracking of the continuous oxide film. However, in the sites of cracks on the film, a new protective layer of oxide is formed, which to some degree stabilizes the corrosion process, and therefore the rate of oxidation in an established system obeys a linear law. It is interesting to note that an analogous shape of the kinetic curves is also characteristic of pure zirconium iodide [3]. An exception is the intermetallic compound Zr2Ni , the rate of oxidation of which has high values at the very beginning, although a continuous dark gray film is formed on the surface of this intermetallic compound according to visual observations. The greatest stability to oxidation at 400~ is possessed by the intermetallic compound ZrMo2, the rate of oxidation of which is close to the rate of oxidation of pure zirconium. When the temperature of testing is increased to 500~ the nature of the kinetic curves of the oxidation of intermetallic compounds changes. There either are no points of inflection on the kinetic curves of oxidation in this case, or they are very weakly manifested. The ratios of the rates of oxidation of intermetallic compounds at 500 and 400~ are somewhat different. The greatest stability to oxidation at 500~ is exhibited by the intermetallic compounds ZrMo 2 and ZrFe2; the intermetallic compound Zr2Cu is close to them, and Zr4Sn is inferior to it. Structure of the Oxide Films on Intermetallic Compounds. The method of electron diffraction and xray diffraction studies of oxide films was used to study the structure and composition of the oxides at various depths of the oxide layer, formed during heating in steam and oxygen. Table 2 presents the results of a phase analysis of such oxide layers, arising on the surface of the samples during oxidation in oxygen at 500~ When the temperature of oxidation was increased to 800~ no appreciable changes were detected in the phase composition and structure of the oxides, with the exception of the formation of a tetragonal or monoclinic modification of ZrO 2 on the intermetallic compound ZrFe 2.

formed

During the corrosion of intermetallic compounds which are close in structure and composition

of zirconium in steam at 400~ oxide layers are to those that arise in oxygen at 800~

Fig. 2 presents the electron micrographs of the surface of samples of intermetaIlic compounds of zirconium after their oxidation at 500~ Analyzing these photographs and considering the data of a study of the structure of the samples (see Table 2), we can conclude that a two-phase oxide film consisting of crystals of the oxides Fe203 and ZrO 2 is formed on the surface of the intermetallic compound ZrFe 2 (see Fig. 2a); moreover, the crystals of Fe203 emerge above the surface of the intermetallic compound, while portions of a solid oxide film of ZrO 2 are visible between them. The microstructure of the oxide film on the intermetallic compound Zr2Ni is shown in Fig. 2b. Here also a two-phase character of the oxide layer is detected. The large crystals correspond to the oxide NiO, while the ZrO 2 phase has a fine crystalline structure. Numerous microcracks are observed in the solid film around the deposits of NiO crystals. The oxide film on the surface of the intermetallic compound Zr4Sn , just as on pure zirconium, consists of fine crystals of ZrO 2 (see Fig. 2c). On the intermetallic compound Zr2Cu , the oxide film has a twophase structure (see Fig. 2d); moreover, most of the surface is occupied by Cu20 crystals. The oxide film on the intermetaIlic compound ZrMo 2 is homogeneous and possesses a clearly pronounced structure (see F i g . 2e). The v a l u e s of the i n t e r p l a n e d i s t a n c e s of the oxide arising" on the s u r f a c e of tile i n t e r m e t a l l i c c o m pound Z r M o 2 a r e given in T a b l e 3. F r o m this table it can be concluded that the oxide f o r m e d d i f f e r s in s t r u c t u r e f r o m the o x i d e s of z i r c o n i u m and molybdenum. The s t r u c t u r e of the oxide obtained as a r e s u l t of c o m p l e t e oxidation of the i n t e r m e t a l l i e compound Z r M o 2 at 500 and 800~ c o i n c i d e s with the s t r u c t u r e of the thin oxide f i l m . The d a t a on the amount of oxygen n e c e s s a r y f o r the c o m p l e t e oxidation of one mole of the i n t e r m e t a l l i e compound ZrMo2, found by the w e i g h t - g a i n method, p e r m i t us to a s s u m e that a compound c o r r e s p o n d i n g to the c o m p o s i t i o n ZrMo205 is f o r m e d d u r i n g oxidation. Influence of I n t e r m e t a l l i e I n c l u s i o n s on the C o r r o s i o n R e s i s t a n c e of A l l o y s . F i g . 3 p r e s e n t s e l e c tron m i c r o g r a p h s of the s t r u c t u r e of known z i r c o n i u m a l l o y s with a d d i t i o n s of copper, iron, and nickel. T h e s e photographs give an i d e a of the n a t u r e of the d i s t r i b u t i o n of v a r i o u s i n t e r m e t a l l i c compounds in the a l l o y s , t h e i r d i s p e r s i o n , a n d f r o m a f t e r the usual heat t r e a t m e n t f o r these a l l o y s . The c o r r o s i o n s t a b i l i t y of t h e s e a l l o y s should e v i d e n t l y depend on the c o r r o s i o n p r o p e r t i e s of the i n t e r m e t a l l t c i n c l u s i o n s . Thus, f o r example, under the action of w a t e r and h i g h - p a r a m e t e r s t e a m on an a l l o y containing the i n t e r m e t a l l i c

265

70oo

compounds Zr2Ni , the inclusions of Zr2Ni a r e broken down f i r s t . At the site of breakdown of these i n t e r m e t a l l i c compounds, m i e r o s e o p m channels a r e f o r m e d in the oxide film, through which hydrogen p e n e t r a t e s into the alloy in substantial amounts, which leads to its hydrogenation. The g r e a t tendeney of z i r c o n i u m a i l o y s containing nickel for hydrogen s a t u r a t i o n d u r i n g c o r rosion in w a t e r and h i g h - p a r a m e t e r s t e a m [4, 51 is e v i d e n t l y due to the i n d i c a t e d p r o c e s s e s , which o c c u r in the oxide film.

~00 .a g00 m~ m

9

?00

0

100

200

300

~,00

YO0

500

700

Temperature, ~ F i g . 4. Dependence of the m i e r o h a r d n e s s on the t e m p e r a t u r e f o r i n t e r m e t a l l i c compounds of z i r c o n i u m : 1) ZrMo2; 2) ZrFe2; 3) Zr4Sn; 4) Zr2Ni; 5) Zr2Cu; 6) z i r c o n i u m iodide.

The most s t a b l e i n t e r m e t a l l i c compounds in w a t e r and s t e a m a r e ZrMo 2 and Z r F e 2. I n c l u s i o n s o[ these i n t e r m e t a l l i e compounds, e x p e e i a i l y in the finely divided state, a c c o r d i n g to the d a t a of the a u t h o r s of this work, m a k e s z i r c o n i u m a l l o y s m o r e e o r r o s i o n ~ r e s i s t a n t in c o m p a r i s o n with a l l o y s containing Zr2Nio

!nfluenee of the MechanieaI P r o p e r t i e s of I n t e r m e t a l l i c Inclusions on the Mechanical P r o p e r t i e s of the A l l o y s . The c o m p a r a t i v e m e c h a n i c a l s t r e n g t h of pure i n t e r m e t a l l i e compounds can be judged a c c o r d ing to the changes in the h a r d n e s s and m i e r o h a r d n e s s during heating. F i g . 4 p r e s e n t s the c u r v e s of the dependence of the m i e r o h a r d n e s s on the t e m p e r a t u r e for the i n t e r m e t a l l i c compounds studied in the work within the i n t e r v a l 20-700~ It can be seen that the i n t e r m e t a l l i c compounds ZrMo 2 and Z r t r e 2 do not change the high initial hardnes~ ~within the indicated t e m p e r a t u r e i n t e r v a l , w h e r e a s the h a r d n e s s of o t h e r i n t e r m e t a l l i e compounds (Zr~C~, Zr2Ni, and Zr~Sn) is s u b s t a n t i a l l y r e d u c e d upon heating above 300-400~

In view of this, the g r e a t e s t strengthening effect m z i r e o m u m should be expected f r o m the i n t e r m e t a l lie compounds ZrMo 2 and ZrFe2, since t h e i r e l a s t i c influence on the z i r c o n i u m m a t r i x is a l s o m a i n t a i n e d during heating. The other i n t e r m e t a l l i e compounds studied in the work, as a r e s u l t of weakening, can thems e l v e s b e c o m e d e f o r m e d d u r i n g heating, and thus wilt have no s t r e n g t h e n i n g effect on the a l l o y s . CONCLUSIONS 1. It was e s t a b l i s h e d that the c o r r o s i o n of pure i n t e r m e t a l l i c eompounds (ZrFe2, ZrMo2, Zr2Ni , Zr4Sn , Zr2Cu ) in the t e m p e r a t u r e i n t e r v a l 400-800~ o c c u r s at a h i g h e r r a t e than that of pure z i r c o n i u m , both in oxygen and in w a t e r v a p o r . 2. The l e a s t r a t e of oxidation is p o s s e s s e d by the i n t e r m e t a l l i e compound ZrMo2, the g r e a t e s t by the i n t e r m e t a l l i e compound Zr2Ni. The i n t e r m e t a l l i c compound Zr4Sn has a low r a t e of oxidation at 300-400~ but with i n c r e a s i n g t e m p e r a t u r e the r a t e of its oxidation r i s e s s h a r p l y . 3. A r e l a t i o n s h i p was d e t e c t e d between the k i n e t i c s of the oxidation, s t r u c t u r e , and c o m p o s i t i o n of oxide f i l m s a r i s i n g on the s u r f a c e of i n t e r m e t a l l i c compounds. The high c o r r o s i o n r e s i s t a n c e of the i n t e r m e t a l l i c compound ZrMo 2 is due to the f o r m a t i o n of a homogeneous oxide film_ on its s u r f a c e , c o n s i s t i n g of the compound ZrMo20 5. The c a t a s t r o p h i c oxidation of the i n t e r m e t a l l i e compound Zr2Ni is explained by the t w o - p h a s e s t r u c t u r e of the film and its c r a c k i n g during growth. On the i n t e r m e t a l l i e compounds Z r F e 2 , Zr2Ni, and Zr2Cu oxide f i l m s a r e f o r m e d , the s u r f a e e l a y e r s of which c o n s i s t e n t i r e l y of the oxides Fe203, NiO, andCu20, r e s p e c t i v e l y , while the d e e p e r l a y e r s c o n s i s t of a m i x t u r e of the i n d i c a t e d oxides and Z re 2. 4. It was shown by the method of measuring the microhardness during heating that the intermetallie compounds Zr2Ni , Zr4Sn, and Zr2Cu become weakened in the interval 350-450~ whereas the intermetallie compounds ZrMo 2 and ZrFe 2 retain their original hardness up to 7000C. LITERATURE Io 2.

266

CITED

V.V. Gagarin, Atomnaya Tekhika za Rubezhom, No. 3, 23 (1968). V . S . E m e l ' y a n o v et a l . , R e p o r t No. 341, P r e s e n t e d by the USSR at the Third I n t e r n a t i o n a l Conference on the P e a c e f u l U s e s of A t o m i c E n e r g y (Geneva, 1964) [in Russian].

3.

4~ 5.

I. L Korobkov eta[., in: Transactions of the Second International Conference on the Peaceful Uses of Atomic Energy (Geneva, 1958). Reports of the Soviet Scientists [in Russian], Vol. 3, Atomizdat, Moscow (1959), p. 474. Electrochem. Teehnol., 4, No. 3-4; 5-6; 7-8 (1966). B.G. Parfenov, V.V. Gerasimov and G.I. Venediktova, The Corrosion of Zirconium and Its Alloys [in Russian], Atomizdat, Moscow (1967) p. 186.

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