Corrosion Resistance of Metals and Alloys to Sodium and Lithium

Corrosion Resistance of Metals and Alloys to Sodium and Lithium HANDLING AND USES OF THE ALKALI METALS Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 06/02/15. For personal use only.

Ε. E. HOFFMAN and W. D. MANLY Metallurgy Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee

Sodium and lithium offer several advantages as heat-transfer media. A comparison is made of the corrosion resistance of various metals and alloys in these liquid metals at elevated tempera tures. The principal variables which affect the extent and form of the corrosion noted are tem perature, time, temperature differentials in the system, and purity of the liquid metal.

A l t h o u g h the liquid alkali metals — particularly sodium and lithium — have a great number of industrial and chemical uses, the discussion in this paper is confined to their application as heat-transfer media in various high-temperature nuclear-reactor applications. Because of their very attractive heat-transfer prop erties, use of these liquid metals might also be extended to more conventionally fueled power plants. S o d i u m , in particular, has found application for many years in solving heat-transfer problems. O n e of the most extensive uses of liquid metals, and one with which almost everyone comes in contact at least remotely, is the cooling of automobile, truck, and aircraft engine valve seats b y sodium ( 4 ) . In any conventional electricity-generating plant in which water is used as the heat-transfer medium, a major difficulty is the high pressures encountered at high operating temperatures. T h e higher temperatures lead to increased efficiency of the cycle, but a temperature limit is reached (approximately 1200°F.) beyond which the structural materials used today do not have sufficient strength to withstand the high pressures. L i q u i d metals, with their high boiling tempera tures and excellent heat-transfer properties, afford a solution to this pressure problem. T h e problem confronting the metallurgists and corrosion engineers is to find materials which have both good strength and good resistance to corrosion in the liquid-metal environments. T h e attempt to find such materials has neces sitated vigorous research to supplement the present liquid-metal technology. T h e best sources of information covering the various aspects of liquid-metal (sodium, lithium, and many others) technology are the latest editions of the " L i q u i d Metals H a n d b o o k "

(3,4).

Some of the most important properties of sodium and lithium for high-tem perature nuclear-reactor applications are listed in T a b l e I. S e v e r a l other popu lar and potential heat-transfer fluids are shown for comparison purposes. T h e ad vantages and disadvantages of various coolants are considered in relation to their application at temperatures in excess of 1200 °F. T h e undesirable properties of a particular coolant are underlined. Water is not particularly suitable because of its very low boiling point and its poor thermal conductivity. S o d i u m and the sodium-potassium alloy have properties to which there are no major objections. ( A n y statement made in this paper concerning the corrosiveness of sodium may be considered as applicable to the sodium-potassium alloys, as differences found 82

HOFFMAN AND

M A N L Y — C O R R O S I O N RESISTANCE TO SODIUM A N D

LITHIUM

83

Table I. Important Properties of Heat-Transfer Fluids for Reactor Applications

21

H 0 Na Na(56%) Κ (44%) Pb Li Hg

HANDLING AND USES OF THE ALKALI METALS Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 06/02/15. For personal use only.

2

a

Melting Point, °F.

Boiling Point, °F.

32 208

212 1616

66 622 367 -37

Density at M . P . , G./Cc.

Absorption Thermal Heat Cross Section, Conductivity Capacity Barns at M . P . , at M . P . , C a l . / G . - ° C . C a l . / S e c . - C m . - C. (10- C m . ) 0

24

1.0 0.92

1.0 0.33

0.001 0.21

0.6 0.45

1518

0.87

0.26

0.06

1.1

3170 2403 675

10.4 0.50 13.6

0.04 1.0 0.03

0.04 0.09 0.02

0.2 65 430

2

U n d e s i r a b l e properties are u n d e r l i n e d .

to date h a v e been v e r y slight.) T h e l o w m e l t i n g point of the s o d i u m - p o t a s s i u m alloy s h o u l d be considered a definite advantage, as it w o u l d not be necessary to a p p l y heat to m a i n t a i n the alloy i n the l i q u i d state w h i l e a p o w e r plant (nuclear or conventional) is c o m i n g u p to p o w e r or d u r i n g shutdowns. L i t h i u m has m a n y attractive properties; however, one n u c l e a r p r o p e r t y limits its application i n n u c l e a r power plants — the h i g h absorption cross section of the l i t h i u m - 6 isotope for t h e r m a l neutrons. S u c h a h i g h - a b s o r p t i o n cross section w o u l d l e a d to inefficient utilization of neutrons p r o d u c e d b y the fission process. T h e l i t h i u m - 7 isotope, w h i c h comprises 92.5% of n a t u r a l l y o c c u r r i n g l i t h i u m , fortunately has a v e r y l o w absorption cross section (0.033 b a r n ) . A n isotopic separation of the l i t h i u m - 7 f r o m the l i t h i u m - 6 isotope w o u l d be necessary before l i t h i u m c o u l d be considered as a p r i m a r y coolant for a n u c l e a r reactor. A n o t h e r disadvantage of l i t h i u m is that it is more corrosive t h a n the other a l k a l i metals. In s u m m a r y , the outstanding properties of the l i q u i d metals, especially s o d i u m a n d l i t h i u m , are their h i g h t h e r m a l conductivities, their h i g h b o i l i n g points, a n d their c h e m i c a l a n d t h e r m a l stabilities. M a n y types of l i q u i d - m e t a l corrosion tests have been and are b e i n g p e r f o r m e d to determine the most satisfactory container m a t e r i a l for a p a r t i c u l a r set of e n v i r o n m e n t a l conditions. A s i n a l l tests of this nature, the object is to stimulate, as n e a r l y as possible, the actual operating conditions w h i c h w i l l p r e v a i l i n the ultimate use of the m a t e r i a l . E a c h test involves a compromise o n one or m o r e of the operating conditions. T h e l i q u i d - m e t a l corrosion tests w h i c h are discussed are classified as either static or d y n a m i c , depending o n whether or not the m o l t e n m e t a l moves w i t h respect to the container m a t e r i a l . Static tests conducted u n d e r i s o t h e r m a l conditions are easy to p e r f o r m a n d u s u a l l y simple to interpret. T h e attack suffered b y a container m a t e r i a l is i n almost a l l cases greater i n a d y n a m i c system t h a n i n a static system; therefore, the purpose of the static test is to screen out materials that show no promise. T h e extent of corrosion is evaluated b y w e i g h t - c h a n g e data, c h e m i c a l analysis of the l i q u i d m e t a l , a n d x - r a y a n d spectrographic e x a m i n a t i o n of the specimen surface, but the greatest importance is g i v e n to metallographic e x a m i n a t i o n of the test specimens a n d the container walls. Because of the c h e m i c a l a c t i v i t y of s o d i u m a n d l i t h i u m , extreme care must be t a k e n i n l o a d i n g the test containers i n order to protect the l i q u i d metals f r o m atmospheric contamination (2). T h e r e f o r e a n inert atmosphere must be m a i n tained over the l i q u i d m e t a l d u r i n g the l o a d i n g operation a n d the test p e r i o d . T h e v a r i o u s test systems m a y be a r b i t r a r i l y classified as follows: Static tests. N o m o v e m e n t of l i q u i d m e t a l ; no temperature gradients D y n a m i c tests, l o w velocity (2 to 10 feet per minute) Seesaw or t i l t i n g tests. A sealed tube p a r t i a l l y filled w i t h a l i q u i d m e t a l is tilted up a n d d o w n T h e r m a l - c o n v e c t i o n loops. A sealed loop filled w i t h a l i q u i d m e t a l is heated i n certain sections a n d cooled i n others to cause c i r c u l a t i o n due to changes i n density of the l i q u i d m e t a l D y n a m i c tests, h i g h velocity (2 to 50 feet per second), flow i n d u c e d b y e l e c tromagnetic or m e c h a n i c a l p u m p s

84

A D V A N C E S IN CHEMISTRY SERIES


T h e p r i n c i p a l types of l i q u i d - m e t a l corrosion are (5) : S i m p l e solution A l l o y i n g between l i q u i d m e t a l a n d solid m e t a l Integranular penetration d u e to selective r e m o v a l I m p u r i t y reactions T e m p e r a t u r e - g r a d i e n t mass transfer C o n c e n t r a t i o n - g r a d i e n t transfer o r d i s s i m i l a r m e t a l transfer S o l u t i o n of the s o l i d - c o n t a i n e r m a t e r i a l i n the l i q u i d m e t a l , a n d a l l o y i n g between the l i q u i d a n d s o l i d m e t a l , c o u l d r e a d i l y b e p r e d i c t e d i f adequate p h a s e d i a g r a m i n f o r m a t i o n was a l w a y s available. T h e s e a r e the simplest forms of l i q u i d - m e t a l corrosion, a n d static-capsule tests a r e u s u a l l y sufficient to determine the extent of these reactions (solution or a l l o y i n g ) . Inter g r a n u l a r penetration of a container m a t e r i a l occurs as a result of p r e f e r e n t i a l attack o n a constituent of the m e t a l w h i c h segregates i n the g r a i n boundaries. T h e t w o most troublesome forms of l i q u i d - m e t a l corrosion are t e m p e r a t u r e - g r a d i e n t mass transfer a n d d i s s i m i l a r - m e t a l mass transfer. T h e s e corrosion p h e n o m e n a are v e r y often difficult to observe i n a s i m p l e capsule test, a n d i n some cases they m a y b e detected o n l y after a large d y n a m i c p u m p system has b e e n operated f o r a n extended time p e r i o d . T h e m e c h a n i s m of t e m p e r a t u r e - g r a d i e n t mass transfer is i l l u s t r a t e d i n F i g u r e 1. T h i s type of corrosion m a y b e studied i n a t h e r m a l - c o n v e c t i o n loop test ( F i g u r e 2 ) . Because the s o l u b i l i t y of most container materials i n a p a r t i c u l a r l i q u i d m e t a l is t e m p e r a t u r e - d e p e n d e n t , solution i n the h o t section a n d subsequent deposition i n a cooler section m a y occur. T h e results of this t y p e of corrosion m a y be seen i n F i g u r e s 3 a n d 4. D i s s i m i l a r - m e t a l transfer o r c o n c e n t r a t i o n - g r a d i e n t transfer ( F i g u r e 5) m a y occur i n solid m e t a l - l i q u i d m e t a l systems, e v e n w h e n n o temperature differentials are present. W h e r e t w o o r m o r e s o l i d metals a r e i n contact w i t h the same l i q u i d metal, the l i q u i d m e t a l m a y act as a c a r r i e r i n t r a n s f e r r i n g atoms of one of the solid metals to the surface of the other solid m e t a l . T h e effect of s u c h a l l o y i n g m a y be seen i n F i g u r e 6. S u c h a l l o y i n g w o u l d , i n most cases, h a v e a n adverse effect o n the m e c h a n i c a l properties of the d i s s i m i l a r metals, a n d m i g h t e v e n cause p l u g g i n g o f s m a l l tubes i n some cases. T h e p r i n c i p a l variables affecting l i q u i d - m e t a l corrosion a r e : Temperature T e m p e r a t u r e gradient C y c l i c temperature fluctuation R a d i o of surface area to v o l u m e P u r i t y of l i q u i d m e t a l F l o w velocity or Reynolds number S u r f a c e c o n d i t i o n of container m a t e r i a l T w o o r m o r e materials i n contact w i t h l i q u i d m e t a l C o n d i t i o n of container m a t e r i a l ( g r a i n - b o u n d a r y precipitation, second phase, stressed o r annealed)

CAftftlCO TO COLO PORTION OF SYSTEM

Figure 1. Temperature gradient mass transfer

T h e relative importance o f the n u m e r o u s variables m a y change, d e p e n d i n g o n the l i q u i d m e t a l a n d the container system. A l t h o u g h i t is difficult to generalize, it m a y b e said that, f o r most l i q u i d m e t a l - s o l i d m e t a l systems, these v a r i a b l e s a r e listed i n order of decreasing importance. T h e corrosion resistance of some metals a n d alloys i n h i g h - t e m p e r a t u r e l i q u i d s o d i u m is s h o w n i n F i g u r e 7. F o r most p r a c t i c a l e n g i n e e r i n g - t y p e a p p l i c a -


H O F F M A N A N D M A N L Y — C O R R O S I O N RESISTANCE TO SODIUM A N D LITHIUM

85

Figure 2. Thermal convection test loop in operation B a t h metal flowing i n clockwise d i r e c t i o n due to temperature differences i n various sections of loop

tions i n v o l v i n g operating temperatures i n the range f r o m 1000° to 1500°F., some grade of austenitic stainless steel — such as T y p e 347 ( 1 8 % C r - 8 % N i - b a l a n c e F e ; N b stabilized) — i s the most suitable container m a t e r i a l f o r s o d i u m . N i c k e l base alloys s u c h as Inconel ( n o m i n a l composition of 7 7 % N i - 1 5 % C r - 7 % F e ) suffer t e m p e r a t u r e - g r a d i e n t mass transfer i n d y n a m i c s o d i u m systems above 1400°F. T h e m e t a l crystals deposited i n the c o l d sections of s u c h systems analyze a p p r o x i m a t e l y 9 0 % n i c k e l , 8% c h r o m i u m , a n d less t h a n 0.5% i r o n , w h i c h indicates the p r e f e r e n t i a l transfer of n i c k e l b y s o d i u m . T h e precious metals seem to b e consistent, i n that a l l h a v e v e r y poor resistance to l i q u i d a l k a l i metals.

Figure 3. Hot leg and cold leg sections from Inconelsodium thermal convection loop Operated at i n d i c a t e d temperature levels 1000 h o u r s ; a i r blast directed o n cold l e g to increase temperature gradient



Figure 4. Cold section of Inconel-sodium high-temperature dynamic system Note h o w mass-transfer crystals tend to grow i n opposite d i r e c t i o n of s o d i u m flow

1

METAL A

^ METAL

MOVEMENT BY DIFFUSION OR LIQUID MOVEMENT

Figure

5.

Dissimilar

DIFFUSION INTO METAL Β AND ALLOY FORMATION

metal transfer

or concentration

gradient transfer

Figure 6.

Alloying of nickel with molybdenum during a

static test (1830°F. 100 hours) M o l y b d e n u m specimen i n contact w i t h s o d i u m i n a n i c k e l container. G r e a t hardness of transfer l a y e r i n d i cated b y V i c k e r s hardness n u m b e r s . E t c h e d w i t h oxalic acid #


87

TEMPERATURE (°F.) Ο

ο IRON LOW-ALLOY STEELS FERRITIC (Fe-Cr) STAINLESS STEELS AUSTENITIC (Fe-Ni-Cr) STAINLESS STEELS


NICKEL NICKEL-BASE ALLOYS (INCONEL) COBALT COBALT-BASE ALLOYS (STELLITE 25) REFRACTORY METALS Mo, Nb To.Ti.W.Vo.Zr COPPER. Cu-BASE ALLOYS PRECIOUS METALS (Ag, Au.Pt)

Figure 7. Corrosion resistance of various metals and alloys in sodium

Figure 8.

Inconel—boiling sodium loop

S o d i u m oxide is the most objectionable i m p u r i t y i n s o d i u m , i n so f a r as h i g h temperature heat-transfer systems are concerned. E v e n t h o u g h great care m a y be t a k e n i n k e e p i n g s o d i u m free of s o d i u m oxide d u r i n g its p r e p a r a t i o n , s o d i u m m a y easily b e contaminated w i t h the oxide, e v e n i n a gas-tight system. O x i d e s of n i c k e l , i r o n , a n d c h r o m i u m o n the walls of a stainless steel system, for e x a m p l e ,

A D V A N C E S IN


88

CHEMISTRY SERIES

m a y be r e d u c e d b y the s o d i u m to f o r m s o d i u m oxide. T h i s r e d u c t i o n occurs e v e n m o r e r e a d i l y i n the case of l i t h i u m , the oxide of w h i c h is e x t r e m e l y stable. O n e of the effects of o x y g e n i n s o d i u m is that i n h i g h concentrations it increases the a m o u n t of mass transfer observed i n the c o l d sections of n i c k e l - b a s e alloy a n d austenitic stainless steel systems (5). D y n a m i c tests h a v e been conducted i n w h i c h o n l y distilled s o d i u m has come i n contact w i t h the test section ( F i g u r e 8). T h e d i s t i l l e d s o d i u m was r e t a i n e d for a short p e r i o d i n two traps i n the condenser leg of this loop. T h e traps w e r e h e l d at different temperatures d u r i n g the e x p e r i ment, a n d the cooler t r a p was e x a m i n e d for mass-transfer crystals f o l l o w i n g the test. T h e results of this e x p e r i m e n t are s h o w n i n F i g u r e 9. A l t h o u g h the o x y g e n concentration of the s o d i u m i n the test section a n a l y z e d less t h a n 25 p.p.m., some mass transfer still o c c u r r e d . L o w e r i n g the o x y g e n concentration seems to decrease the rate of mass transfer but does not eliminate it i n a n i c k e l - b a s e a l l o y s u c h as Inconel. C a r b u r i z a t i o n of container metals is another p r o b l e m i n v o l v e d i n the h a n d l i n g of l i q u i d metals, especially s o d i u m a n d l i t h i u m . C a r b o n is r e a d i l y t r a n s f e r r e d between two metals of different c a r b o n contents i n contact w i t h s o d i u m , or a m o n o m e t a l l i c system m a y be c a r b u r i z e d o w i n g to a h i g h c a r b o n concentration i n the s o d i u m . A n e x a m p l e of the extent to w h i c h this c a r b u r i z a t i o n m a y proceed is s h o w n i n F i g u r e 10, w h i c h is a p h o t o m i c r o g r a p h of the w a l l of a stainless steel

Figure 9.

Sections of condenser leg Note mass-transfer

of

boiling loop (see

Figure

crystals i n cold trap

Figure 10. Effect of carbon in Type 304 stainless steelsodium system held at 1500°F. for 100 hours Note h e a v y c a r b u r i z a t i o n of exposed (right) surface of tube; specimen n i c k e l plated after test to preserve edges d u r i n g metallographic p o l i s h i n g

8)


89


capsule f o l l o w i n g a 100-hour 1500°F. test w i t h the capsule i n contact w i t h s o d i u m i n t e n t i o n a l l y contaminated w i t h c a r b o n . T h e walls of the capsule w e r e h e a v i l y c a r b u r i z e d to a d e p t h of 15 m i l s . C o n t a m i n a t i o n of s o d i u m b y v a r i o u s h y d r o carbons m a y also result i n subsequent c a r b u r i z a t i o n of m e t a l containers. T h e corrosion resistance of various metals a n d alloys i n h i g h - t e m p e r a t u r e l i q u i d l i t h i u m is s h o w n i n F i g u r e 11. U n f o r t u n a t e l y , l i t h i u m is m u c h m o r e c o r rosive t h a n s o d i u m . Consequently, it w i l l be impossible to take f u l l advantage of its m a n y attractive h e a t - t r a n s f e r properties u n t i l a satisfactory container m a t e r i a l is f o u n d . T h e most corrosion-resistant p u r e metals i n a static i s o t h e r m a l system are m o l y b d e n u m , n i o b i u m , t a n t a l u m , tungsten, a n d i r o n . O f the c o m m e r c i a l l y available s t r u c t u r a l materials, no alloys tested to date h a v e h a d satisfactory corrosion resistance at a temperature above 1400 ° F . f o r extended time periods i n systems w h e r e temperature differentials exist. E v e n t h o u g h i r o n has good r e s i s t ance i n static isothermal l i t h i u m , i r o n a n d i r o n - b a s e alloys suffer f r o m mass t r a n s TEMPERATURE (°F.) ->

ο

©

Ο

Ο

IRON LOW-ALLOY STEELS FERRITIC (Fe-Cr) STAINLESS STEELS

STATIC SYSTEMS DYNAMIC SYSTEMS

AUSTENITIC (Fe-Ni-Cr STAINLESS STEELS

BARS INDICATE APPROXIMATE TEMPERATURES BELOW WHICH A SYSTEM MIGHT BE OPERATED FOR 1000 hr WITH LESS THAN 0.005 in. OF ATTACK OR CONTAINER SURFACE REMOVAL.

NICKEL-BASE ALLOYS (INCONEL)

REFRACTORY METALS (Mo,Nb,To,2r,Ti.W. Vo) • • • • P R E D I C T E D

PRECIOUS METALS (Ag.Au.Pt)

Figure 11. Corrosion resistance of various metals and alloys in lithium

H o t zone, 538°C.

(1000°F.)

C o l d zone, 325°C.

(618°F.)

Figure 12. Sections from Type 347 stainless steel loop in which lithium was circulated for 3000 hours


90


fer to a serious degree i n d y n a m i c systems where t h e r m a l gradients exist. T h e best c o m m e r c i a l l y available s t r u c t u r a l alloys to use w h i c h contain l i t h i u m i n d y n a m i c systems operating at temperatures below 1100°F. are the austenitic grades ( i r o n - n i c k e l - c h r o m i u m ) a n d f e r r i t i c grades ( i r o n - c h r o m i u m ) of stainless steels. F i g u r e 12 shows the hot a n d c o l d zones of a T y p e 347 stainless steel loop f o l l o w i n g 3000 hours of operation at the i n d i c a t e d temperatures. D i s s i m i l a r - m e t a l mass transfer c a n be v e r y serious i n l i t h i u m systems, especially i f one of the metals is n i c k e l o r a n a l l o y c o n t a i n i n g n i c k e l . F i g u r e 13 illustrates h o w this c o r rosion process m a y increase attack i n such systems. A n austenitic stainless steel specimen containing 8% n i c k e l was attacked to a d e p t h of less t h a n 2 m i l s w h e n tested i n l i t h i u m i n a stainless steel container. A s i m i l a r stainless steel specimen tested i n a n i r o n container was attacked to a d e p t h of 20 m i l s . T h i s great increase i n attack was due to transfer of n i c k e l f r o m the s p e c i m e n to the w a l l of the i r o n container. T h e corrosion resistance of the r e f r a c t o r y - t y p e metals i n l i t h i u m is characterized b y m o l y b d e n u m ( F i g u r e 14), w h i c h has s h o w n excellent resistance

Figure 13. Phase change (austenite to ferrite) on exposed surface of Type 304 stainless steel caused by preferential leaching of nickel Note v e r y h e a v y attack o n the right encountered w h e n d i s s i m i l a r metal container was used. E t c h e d w i t h g l y c e r i a regia

Figure 14. Edge of molybdenum specimen following 100hour exposure to static lithium at 1500°F. Note absence of attack o n surface of specimen. E t c h e d w i t h 50% h y d r o g e n peroxide a n d 50% a m m o n i u m hydroxide


HOFFMAN AND

M A N L Y — C O R R O S I O N RESISTANCE TO SODIUM A N D

LITHIUM

91

Figure 15. Type 316 stainless steel tube after a 100-hour exposure to lithium P l u s 0.1%

l i t h i u m nitride at

1600°F.

i n static, i s o t h e r m a l test capsules. Tests must be conducted i n d y n a m i c systems w i t h temperature differentials present i n order to complete the evaluation of the corrosion resistance of the refractory metals. M a n y other problems, such as the poor oxidation resistance of m o l y b d e n u m a n d n i o b i u m , must be solved or c i r c u m vented before utilization of these metals is p r a c t i c a l . T h e precious metals have v e r y poor corrosion resistance, even at moderately low temperatures. T h e most h a r m f u l contaminant f o u n d i n l i t h i u m is l i t h i u m n i t r i d e . L i t h i u m n i t r i d e is f o r m e d on the surface of l i t h i u m exposed to the atmosphere, a n d t h e r e fore such exposure must always be avoided. T h e h a r m f u l effects of m i n o r a d d i tions of this m a t e r i a l to l i t h i u m m a y be seen i n F i g u r e 15. T h e w a l l of a T y p e 316 stainless steel container was completely penetrated (32 m i l s ) , b y w a y of the g r a i n boundaries, w h e n l i t h i u m n i t r i d e was added to the l i t h i u m test b a t h . I n a standard test w i t h no addition the attack u n d e r s i m i l a r conditions was 2 to 4 mils.

Summary F o r containing s o d i u m i n systems that are to operate at temperatures below 1500°F., the austenitic (300 series) stainless steels are the most satisfactory s t r u c t u r a l materials. O x y g e n is the most troublesome i m p u r i t y a n d should be avoided if mass transfer is to be m i n i m i z e d . T o date, no s t r u c t u r a l alloy has b e e n d i s c o v ered w h i c h is satisfactory as a container for l i t h i u m i n d y n a m i c systems above 1200°F. A t temperatures b e l o w 1000°F., the stainless steels have good corrosion resistance, but contamination of the l i t h i u m w i t h l i t h i u m n i t r i d e should be avoided.

Acknowledgment T h e authors w o u l d l i k e to acknowledge the assistance r e n d e r e d b y L . R. T r o t t e r a n d other m e m b e r s of the M e t a l l u r g y D i v i s i o n of the O a k R i d g e N a t i o n a l L a b o r a t o r y i n obtaining the i n f o r m a t i o n i n c l u d e d i n this paper. T h e m e t a l lographic w o r k was p e r f o r m e d b y a group u n d e r the d i r e c t i o n of R. J . G r a y .

Literature Cited (1) (2) (3) (4) (5)

B r u s h , E. G., K o e n i g , R. F . , " N u c l e a r M e t a l l u r g y , " Institute of Metals D i v i s i o n Special R e p o r t Series N o . 2, A I M M E (Feb. 20, 1956). Hoffman, Ε. E., M a n l y , W . D . , V r e e l a n d , D . C., Nucleonics 11, N o . 11, 36-9 (1953). J a c k s o n , C . B . , "Liquid Metals H a n d b o o k , S o d i u m - N a K S u p p l e m e n t , " A t o m i c E n e r g y C o m m i s s i o n , D e p a r t m e n t of N a v y , Washington, D . C., 1955. L y o n , R. N . , "Liquid Metals H a n d b o o k , " N A V E X O S - P - 7 3 3 (rev.), p. 5, U . S. G o v e r n m e n t P r i n t i n g Office, Washington, D . C., 1954. M a n l y , W . D . , Corrosion 12, 336-42 ( J u l y , 1956).

Corrosion Resistance of Metals and Alloys to Sodium and Lithium

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