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Oxidation of beryllium in carbon dioxide and oxygen
JOURNAL
OF NUCLEAR
MATERIALS
OXIDATION
5, No. I (1962) 67-80, NORTH-HOLLAND
OF BERYLLIUM
IN CARBON DIOXIDE
Received
by a variety
of
principalement
different
routes, in dry carbon dioxide
at 500-f000°
C,
et qui pouvait
at
600-1000” C and vapour
in
carbon
for
eertains
dioxide
at, 700” C for times
ment
up to
Protective
films were formed
up to
1000” C breakaway
de
850” C. At
obtained
at
of 3 vol. y0 water
I atm
The subsequent dependent
the
oxide
samples
Most
were
from in dry
carbon
with
Le
d’oxyde
in
celui
the
the
view
No
of fabrication.
courbes
de gain
de poids
der
Zeit
fur
whilst) in
Kohlendioxyd
stoff
600-1000” C
bei
Wasserdampf
bei
a 600”-1000
11 y a form&on des Rohantillons
d’eau
du temps
Bei
see a 500”-
schon
hiihnisse
bei
der Methode,
sea
wnrde
des Materials
La presence de 3 oh en volume de vapeur d’eau dens le gaz carbonique sous 1 atm provoquait le “break-
mit
away”
die
2x700” C. Succedant
au ‘*breakaway”
8,
dans 8. 700’
als Funktion
aufgestellt, wurden,
und
in
die
auf
einmal
in
Kohlendioxyd Zeiten
mit
bis zu
10 000
von
l’attaque 67
auf
den
Bei
1000” C
trat
Wasserdampf
in
3 Die
Oxyden in
me&en
ein.
Vol. ye
folgenden
mit, einem
konnten
linearen
sind merklich
FBllen. Die meisten
abhangig
des Metalls Faktor
von
Differenzen
Vervon
benutzt 1000 in
im Betragen
mit dem Verunroinigungsgrad
werden. Sie standen im Zusammenhanq
dem Schutz auf
wurden gebildet.
die zur Herst,ellung
und differ&t
gewissen
1500 heures.
von
700” C ein.
nicht korreliert
apres
Btait observable
“breakaway”
fiir den Angriff
rupture
(breakaway)
que
unt,er 1 atm Druck tritt das Zerbrechen
sous une pression dune atmosphere et aux t,emp&atures inferieures b 850”. A 1000” C, on observe une du film
similaire
bei einer atm Druok und einer
Schutzfilme
Gegenwart
a 700” C. carbonique
see Dans
bei 500-1000” C, in Sauer-
850” C
1500 Stunden
C et de gaz carbonique
de gaz
bis
Matlerialien
de films prot.ert~eurs sur la plupart en presence
t&s
see tandis
700” C iiber
Kohlendioxyd
Kohlendioxyd
de la vapeur
I’oxygene
Stunden.
moyens,
1000” C, d’oxygene
&ait
hergestellt
trockenem
nach
de gaz carbonique
dans
de fabrication.
Berylliumproben Wege
(jusqu’a 10 000 heures) ont 6th det~erminees pour des Bchantillons de tciles de beryllium &bore par difftrents en presence
“breakaway”
Es werden Kurven der Gewichtszunahme
fabricated
at 700” and 850” C.
en fonction
du
en
pas une
--
In some cases it was
was obt,ained
metal
carbonique
network
also varied
dioxide,
vis-St-vis
cas, le “breakaway”
verschiedene
in dry oxygen
Blabores
set n’apportait
la methode
le gaz
d’oxyde probable
et I, 850” C .
dioxide.
of metal
Bchantillons
cas ce comportement
dans
Unter
contenant
a la
de gaz humide.
significant
on the
les
du
aussi av8c
d’autres
that
sur
comportement
Temperatur Les
de comporte-
pas etre reliee
La croissance
carbonique
significative
certains
of 1000 in
de metal.
en presence
variait
th8
in wet. gas was provided grown
to that in dry carbon
ot,hers breakaway
fabricate
differences
of metal.
breakaway
with the method
observe
were markedly to
with an intergranular
batches
previously
The behaviour similar
the
protection
could not be relat,ed to impurity
is associated
films
of
consistent
in certain
protection
used
by as much as a factor
of materials but
of oxide
method
in carbon 700’ C.
lots
de gaz
at
vapour
in breakaway
linear rates of attack
upon
instances.
protection
very
resulted
and differed
content
films
presence
The presence
by
was
1500 h.
behaviour
des differences
ne pouvait
du metal
de I B 1000 dans
tion Btait associee a un reseau intergramrlaire
on most mat,erials in
dans certains
certain
d’elaboration
dans le rapport
cas. La plupart
at 1 atm pressure and temperatures
metal
varier
des materiaux
dry carbon dioxide
dioxide
dont la valeur dependait
de la methode
teneur en impureties mais plutBt au fait que la protec-
10 000 h.
after
AND OXYGEN
une vitesse Iineaire
sheet fabricated
water
obtained
adopt&it
gain/time of beryllium
oxygen
been
CO., AMSTERDAM
1961
Weight
in
have
27 April
samples
containing
curves
PUBLISHING
bei
durch
ein intergranulares
gewissen
trockenem
Proben.
Kohlendioxyd
Die
Netzwerk Oxydfilms,
hergestellten
68
.J.
l’robm
geaachsen
Sohutz
fiir
sintl,
Hetragen
des Metalls
ebenfalls
in Abhkngigkeit,
1.
birtcw
das “breakaway”
K.
keincn
HIGGINS
AND
betlerltenden
in fiinc~htcm (;a~.
in trocakencrrl Gauerstoff van dcr Mct,hotlr
Ijas
use of beryllium
material in the Advanced in the
UK
halten
In oinigen in
tlrr Her-
erhalten
wurde.
of
its
oxidation behaviour in gaseous environments. Preliminary work on the reaction with carbon dioxide was done by Munro and Williams and by Gregg et al. Munro and Williams 1) found that beryllium formed protective + films at temperatures up to 600” C in dry carbon dioxide at atmospheric pressure whilst Gregg et al. 2) oxidizing for shorter time intervals, (300 as compared with 3000 h), found that the oxide was protective at temperatures up to 700” C. Breakaway i + occurred at 650” C and 750” C according to Munro and Williams and Gregg et al. respectively. Water vapour has been shown to have a pronounced effect upon the reaction, causing very rapid attack and lowering the temperature at which breakaway occurs. For example Munro and Williams observed rapid non-protective oxidation at 600” C in gas containing 4-7 vol. %
Fdlen
trockenem
Fdlen
as a canning
a knowledge
stellung.
ANTILL
anderen
Gas Cooled Reactor
necessitates
E.
variiert
Introduction The possible
J.
ein
war es iihnlich tlem VW-
Kohlentlioxyd, “breakaway”
wiihrend hei
in
700-850” (I
In oxygen the rates of attack are normally similar to those in carbon dioxide. Aylmore, Gregg and Jepson 4, found that the oxide was protective up to 650” C in dry oxygen whilst breakaway
occurred
at 700” C. These workers
were unable to verify the findings of Cubicciotti 5) and Gulbransen and Andrew 6) that the oxidation followed a parabolic law from 350” to 970” C over the first 34 h. The present work was undertaken to widen the range of conditions under which the reactions in carbon dioxide and oxygen have been carried out, and to study the behaviour of samples of metal fabricated by a wide variety of techniques. Beryllium from different sources, namely Pechiney electrolytic flake and Brush thermally reduced powder, has been oxidized at atmospheric pressure in dry carbon dioxide at 500-1000” C. in oxygen at 600-1000” C and in carbon dioxide containing 3 vol. “/o water vapour at 700” C for times up to 10 000 h. 2.
Experimental
Hz0 1). The chemical nature of the reaction products
The oxidation was followed by measuring the gain in weight of specimens and constructing
formed in dry gas has been examined by Gregg, Hussey and Jepson 3) as well as the particular reactions favoured from among the many
weight gain/time curves. The more reactive samples were weighed continuously on a silica balance with a reproducibility of spring
thermodynamically possible. They have deduced that beryllium oxide and to a lesser extent beryllium carbide are formed at temperatures from 550” to 750’ C by the following reactions:
5 1 mg/cmz whilst the more inert materials were cooled periodically and weighed with a reproducibility of 50.01 mg/cma on a semmicrobalance. All the samples were suspended in silica reaction vessels by platinum hooks which did not visibly react with the beryllium. Samples of electrolytic flake supplied by t,he L’echiney Co., France, and thermally reduced powder from the Brush Beryllium (‘0.. IJSA. were fabricated as shown below:
Bet-CO,
+
BeOiCO
2Ke+-CO7. + 2BeO-I~ (1 -F Be&
2Ke I (‘ i
A film is defined
attack
cont,innously
as protective
derreascs
when t,he rate of
during
the oxidation.
tt Hreakaway is defined as t,he point at which the oxide film loses it,s protective propnt~ies and the rate of attack increases.
then remains
constant
or continuously
Flake No. 1 ~ Rolled French Flake. French flake was ground to ,200 mesh, leached in 10 s’; oxalic acid solution and rolled in a mild
OXIDATION
OF TABLE
Typical Sample
Fe
1
Si
j
(ppm)
1
(mm)
. . . . i 0.3-0.8 1.0 . . . . 1
:
200-300
I
0.05-0.9
(
twt %)
Flake
No.
3 .
Flake
No.
4
.
.
. . . .. Flake No. 5 . . . . Flake No. 7 . . . . FlakeNo.8. . . . Powder No. 2 . . .
.
. . . . .
.
. . . . .
. . I~ 0.1-0.2 0.3-1.0 . 0.01-0.03 . . 0.4 .
/
1000
600
) /
Rolled Brush Powder.
NN50
40
80 27
400-1000
Brush
powder was ground to ~200 mesh, outgassed at 600-700’ C and rolled as for Flake No. 1.
~
m 200
600 m 50
175
were
200 350.-1000
30
~ 400-1000 % 30
!a 30
700 W 200
% 30
<50
Halogens
(ppm)
(PPm)
50
~ 1
MMg
Al
1
100-300
500 400
steel can at 1000” C. The basal planes mainly parallel to the surface. Powder No. 1 -
1
analyses for the different batches of beryllium Be0
Flake Nos. 1 & 2 Powder No. 1 . .
69
BERYLLIUM
<30
(ppm)
~
I ( , :
200 20-50 %50 50 100 O-30
55
300
i 1000-3000
Flake No. 8 ~ Single Crystal Beryllium. French flake was vacuum cast, extruded at 1050” C and zone melted to give large grains which were subsequently
isolated.
Powder
2
No.
-
Forged and Rolled Brush < 200 mesh was treated
French Flake. French
Powder. Brush Powder
flake < 200 mesh was leached in 10 ‘$(, oxalic acid solution and extruded at 1050” C as tube.
as for Flake No. 6. The rolled samples
The basal planes were mainly perpendicular the surface.
of approximately 3 x 1 x 0.1 cm using a Norton alumina grinding wheel No. 38860 K5VBE. The extruded specimens were sections cut from a tube of diameter 2.7 cm and the specimen of high purity beryllium and single crystal were irregular shaped pieces of approximate area 1.5 cm2. The density of all t’he samples was > 1.8 g/cn13. The final surface preparation consisted of a chemical polish in a phosphoric/
Flake No. 2 -
Extruded
to
Flake No. 3 - Cast French Flake. Flake No. 2 was consumable arc melted into a cylindrical ingot from which specimens were cut. Flake No. 4 - Vacuum Cast and Rolled French Flake. French flake was vacuum cast, ground to powder and rolled as for Flake No. 1. Flake No. 5 - Tungsten Arc Melted and Rolled French Flake. Leached French flake was sintered at 1200” C to a density of 1.8 g/cm3 and then melted in a tungsten arc in argon. The resulting buttons were ground to ~200 mesh and rolled as for Flake No. 1. Flake No. 6 - Forged and Rolled French Flake. French flake ~200 mesh was hydrostatically pressed, sintered at 1225” C, forged in a mild steel can at 1050” C and then finally cross rolled at 900” C. Flake No. 7 - High Purity Beryllium. Beryllium was evaporated from the molten state at 1400' C to produce a mass of coarse crystals which were then cast into an ingot.
were ground
to a size
chromic acid bath for 30 set at 100” C followed by a rinse in distilled water and acetone. Analyses could not be obtained for each batch of material, but typical impurity contents for metal fabricated by some of the routes are shown in table 1. Carbon dioxide was obtained from a solid carbon dioxide (“Cardice”) convertor with oxygen and water vapour as its main impurities. Dry gas containing < 20 vpm + water vapour and < 300 vpm of gas not condensable in liquid air was obtained by passing the carbon dioxide through columns of manganous oxide at 150" C and anhydrous magnesium perchlorate to remove oxygen and water vapour respectively. t
vpm represents parts per million by volume.
70
J.
K.
HIGGINS
AND
J.
J3.
ANTILL
It was found at temperatures of 850’ C and above that t’he water vapour concentration of the dried
gas rose to
passage through culty
> 100 vpm
the reaction
was overcome
during
its
vessel. The diffi-
by first passing
20.0
the gas
through a steel tube furnace at 800” C and then removing
any water vapour formed with anhy-
drous magnesium the concentration the reaction
perchlorate.
By this means
5.0
of water vapour in gas leaving
vessel was kept to
-: 20 vpm.
2.0
Carbon dioxide containing 3 vol. 94 water vapour was prepared by bubbling the converter supply through distilled water at room temperature. The oxygen used was industrial cylinder oxygen, which was passed through a “Deoxo” unit followed by a steel tube furnace at 800” (I
a-
1.0
:
s z
-z 0.50
I
0.20
0.10
and a bed of anhydrous magnesium perchlorate to lower the hydrogen and water vapour contents to <20 vpm.
0.05
0.02
Results
3.
The weight gain versus time graphs obtained for the various experimental conditions are presented in Figs. 1-6. 3.1.
DRY
CARBON
;
0.01
I
L 100
;
3
500 TIME
Fig.
1.
Weight
Powder
Ko.
gain/time 1 in dry
,000
2000
5’
(h)
curves for Flake No. 1 and COz at 500-1000” C.
DIOXIDE
The results for Flake No.
1 (rolled French
flake) and Powder No. 1 (rolled Brush powder) at temperatures from 500” to 1000“ C are shown in fig. 1. The reaction rate increased with temperature, and protective films were formed from 500” to 850” C for times up to 10 000 h. At 1000” C the oxide was protective initially but broke down after 1500 h to give a rate of attack which increased with time. At all temperatures except 1000” C Flake No. 1 was attacked somewhat more rapidly than Powder No. 1. At 1000” C the samples were covered with a hard white scale whilst at the lower temperatures the films were grey or merely produced interference colours. At 1000” C some of the samples were bent (Flake No. 1) and the oxide blistered (Powder No. 1). The attack of specimens fabricated by the more complicated techniques (fig. 2) did not
differ greatly from that of Flake No. 1 and Powder No. 1 at 700” C. Whilst the film on the high purity sample (Flake No. 7) was not protective and the cast French flake (Flake No. 3) showed signs of an increasing
reaction
rate towards the end of the oxidation, the films on the rest of the materials, including the single crystal (Flake No. S), were protective for times up to 10 000 h. The graph for Flake No. 4 (vacuum cast and rolled French flake) is interesting as the attack was less than for the other materials during the first 2500 h, but then increased markedly during the next 1000 h finally to slow down to a rate similar to the other materials. Thermal cycling did not increase the reaction rate of Flake No. 1 at 700” C in tests in which the sample was cooled once a day to room temperature in 15 min ; the weight gains with and without thermal cycling were 0.25 and 0.27 mg/cm respectively after 3000 h.
OXIDATION
OF
However,
10.0
71
BERYLLIUM
at 850” C thermal cycling resulted in
breakaway after 700 h; the weight gain at that time was only 0.13 mg/cmz but during the next
5.0
1500 h it increased
by 1.4 mg/cmz,
compared
with an increase of 0.25 mg/cm2 for the sample
2.0
--.-
FLAKE
No.7
+
FLAKE
NOB
not thermally
cycled.
I.0
3.2. 0.50 ri-
CARBON
DIOXIDE+~
VAPOUR
AT 700°C
VOL. %
WATER
E
The addition
2 5 z
of 3 vol. y0 water vapour
to
the carbon dioxide resulted in breakaway and thereby increased the rate of attack of all the samples, which may be classified into two groups according to the size of this increase.
0.20
2 ; 0.10
0.05
The weight gain/time graphs for the less reactive group are plotted in fig. 3. Comparison with the data for dry carbon dioxide shows that the addition of 3 vol. oh water vapour increased
0.02
0.01 I
100
200
500
1000
2000
5000
IC
)O
TIME(tt)
Fig. 2. Weight gain/time curves for Flake Nos. 2, 3, 4, 5, 7 and 8 in dry CO2 at 700” C.
t
the weight gain after 3000 h oxidation for Flake No. 1, Flake No. 2 (extruded French flake) and Powder No. 1 by factors of 3.3, 9.6 and 150 respectively. The weight gains for Flake Nos. 1 and 2, but not Powder No. 1, tend towards a constant value after roughly 4000 h. The second group of samples having little resistance to the wet gas comprises all the other materials fabricated by the more complicated routes involving vacuum casting, arc melting, forging, etc. as well as the high purity specimen (Flake No. 7) and the single crystal (Flake No. 8). These all oxidized
100
200
500
1000
2(
TIME(h)
Fig. 3. Weight gain/time curves for Flake Nos. 1, 2 and Powder No. 1 in GOa + 3 vol. y0 Ha0 at 700’ C.
at a fast linear rate
either immediately or after a short induction period. The weight gain/time graphs are given in fig. 4 and the linear rate constants in table 2. Some idea of the variation in reaction rates may be gained by comparing the linear rate constant of the most reactive specimen, Flake No. 4 (vacuum cast and rolled French flake) with that of Flake No. 1, the lease reactive of all the sample calculated for the first 3000 h of oxidation. The values are 1 x 1O-7 and 8 xlO-ll g/cm2 set respectively, a change of more than a thousand-fold. The order of decreasing resistance to oxidation in wet gas derived from the linear rate constants in table 2 is:
J. K. HIGGINS AND J. E. ANTILL
72
+
FLAKE
-+-
FLAKE
No. 4
-e
FLAKE
No. 5
-+-
FLAKE
No. 6
+
FLAKE
No. 7
&
POWDER
+
FLAKE
Ne. 3
No 2 No. 8.
TIME(h)
Fig. 4.
Weight
gain/time
curves for Flake
Nos.
3-8 and Powder
Flake No. 1 > Flake No. 2 > Powder No. 1 > Flake No. 8>Flake No. 5>Flake No. 6> Powder No. 2>Flake No. 3>Flake No. 7> Flake No. 4. The appearance of the samples which little resistance to the wet gas indicated
had two
distinct types of attack: for the single crystal and polycrystalline material with large grains (Flake No. 3) the product was mainly a light grey or white nonadherent powder formed at the geometric surface, whilst appreciable internal oxidation had occurred for the polycrystalline materials with small grains, resulting in the formation of dark grey blisters or thick adherent scale. Fig. 7 is a photomicrograph of a section of the polycrystalline material with large grains from which it is apparent that although most of the attack was at the geometric surface intergranular attack had occurred. As expected the effect was more pronounced with the materials containing a large number of small grains, as for instance with Flake No. 4. Fig. 8 shows photomicrographs of two faces at right angles of a section cut from Flake No. 4 after the sample had been oxidized in the wet gas at 700” C for 70 h. The different orientation of the metal grains at the two faces can be seen; oxidation left isolated grains or particles of metal
No. 2 in COZ + 3 vol.
y. Hz0
at 700” C.
embedded in the oxide either perpendicular or parallel to the direction of attack, depending upon the orientation of the grains. In spite of the difference in the appearance of the attack at the two faces, the extent of the reaction is similar. It appears that oxidation first occurred between the grains and was then followed by attack of the adjacent grains, some of which appear more reactive than others. So far only polycrystalline material has been considered but in Fig. 9 photomicrographs are shown of the oxidized (0001) and (1010) faces of the single crystal (Flake No. 8). Although the two faces oxidized at comparable speeds, preferred paths of attack appear to exist at the lOi face, indicating preferred attack of the basal planes which are perpendicular to this face. 3.3.
DRY OXYGEN
In fig. 5 are given the weight gain/time graphs for Flake No. 1 and Powder No. 1 in dry oxygen at temperatures from 600” to 1000” C. Comparison with fig. 1 for the same samples in dry carbon dioxide, shows that the weight gains were similar in the two gases for Powder NO. 1 at all temperatures and for Flake No. 1 at 600” and 1000” C. However, Flake No. 1 exhibited a rapid breakaway reaction at 700” C and 850” C in oxygen whilst in carbon dioxide
OXIDATION
OF BERYLLIUM
‘OO.O I 50.0
20.0
-@-
FLAKE No.4
-.-
FLAKE No.7
+
FLAKE No.8
10.0
5.0
N;
2.0
z E” - 1.00 z G r 0.50 3
0.20
0.10
0.05
0.01
0.02
100 0.01
I
I
TIME(h)
0
Fig. 5. Weight gain/time curves for Flake No. 1 and Powder No. 1 in dry 02 at 600-1000” C.
Fig. 6.
200
500
1000 TIME(h)
2000
5000
10000
Weight gain/time curves for Flake Nos. 4, 7 and 8 in dry 02 at 600-700” C.
at these temperatures the films were protective. Flake No. 4 (vacuum cast and rolled French flake) behaved similarly to Flake No. 1 at 700” C but the attack on the single crystal (Flake No. 8) and the high purity sample (Flake No. 7) was less in oxygen than in carbon dioxide (cf figs. 2 and 6).
3.4.
X-RAY
AND ELECTRON DIFFRACTION
MEASUREMENTS ON THE PRODUCTS
An X-ray diffraction investigation showed that the products obtained from samples oxidized in dry and moist carbon dioxide and in dry oxygen at temperatures up to 850” C consisted of beryllia which in many cases was mixed with unattacked metal. An electron diffraction examination showed that as the temperature was raised from 500” to 1000” C
Fig. 7. Photomicrographsof & section of Flake No. 3 after oxidation in CO2 + 3 vol. ‘$& Hz0 at 700” C for 170 h. x 55
the crystallite size of the oxide grown in dry carbon dioxide increased from < 100 A to > 10 000 A.
74
J.
K.
HIGGINS
AND
J.
E.
ANTILL
(.A) Fig. S.
~‘tl~,tolnicrograplls
(13)
of two
faces of a specimen
at 700° C for
70 Il.
(A)
of Flake
No. 4 after
oxidation
in CO2 + :I vol.
Side
(B) Fig.
9.
Photomicrographs
of
the
single
at 700” C for
4.
crystal
700 h.
(A)
(Flake
0001 face.
Discussion
The oxidation may equation of the form
be represented
by
No.
an
wn=kt where w = weight gain t = time k = rate constant
8) after (B)
oxidat,ion
10 10 face.
x
in COZ +
3 vol.
“/o H&I
120
may be classified according to the value of n, If n = 1 the rate of oxidation will be constant, whilst if n < 1 the rate of oxidation will continuously increase with time. In both cases the film formed is classified as non-protective and breakaway is said to occur. However, if n> 1 then the rate of oxidation will continuously decrease and the film is classified as protective. On this basis the oxidation in dry carbon dioxide at 1 atmosphere pressure is protective for nearly all the samples. This is clear from table 2 where the values of n for Flake No. 1 and Powder No. 1 from 500" to 1000"C are given together with those at 700'C forsamples fabricated by the more complicated routes. For Flake No. 1
OXIDATION
OF
the reaction is said to obey a parabolic law. It should be borne in mind, however, that after 1500 h oxidation at 1000” C, l/n increases
and Powder No. 1 n lies between 3.1 and 00 at temperatures from 500” to 700’ C whilst at 850” and 1000” C the value approaches 2 and TABLE
TTalues for the constants Sample
j /
in an equation
2
representing Values
Temp.
(“C)
75
BERYLLIUM
~~
the weight of n and
(:urv-e
k in equation
E
rl
Dry
gain/time
wn =
I
kt
Time
(h)
COZ
No.
1 .........
500
3.8
350-
7 000
3,
No.
1 .........
600
9.5
500-
8 000
7,
No.
1 .........
700
4.3
500-10
1,
No.
1 .........
850
2.7
500-
1 000
Flake
000 8 000
27&
1 500
3.1
300-
7 000
600
00
1 300-
6 000
700
9.0
300-
7 000
1 .........
850
2.1
No.
1 .........
1 000
Flak;
No.
2 .........
700
6.5
3s
No.
3
0
No.4
,,
No.
1 ........
Po;vder
No.
1 ..........
500
1, ,,
No.
1 .........
No.
1 .........
:,
No.
~1.3
2.2 t
X lo-l1
1.3 x lo-r4 ml.8
1.7 t
x lo-11
g2/cm4
set
g2/cm4
set
250-
5 000
gz/cm4
set
170-
1500
1700-10000
.........
700
2.0
3.3 x lo-14
g2jcm4
set
450-
4 000
.........
700
2.1
3.5 X lo-l5
g2/cm4
see
500-
2 500
No.
5 .........
700
2.5
350-
5 500
1,
No.
7 .........
700
1.0
200-
4 500
I,
No.
8 .........
700
2.3
300-
4 000
3 000
~
COz + 3 vol ye Hz0 Flake
No.
1
Powder
No.
1
Flake
No.
2 .........
......... .........
700
1.0
8 x 10-n
g/cm2
set
500-
700
1.1
1 X 1O-g
g/cm2
see
600-
7 000
700
0.9
4.9
g/cm2
set
50s
3 000
4.4 X lo-*
loo-
170
,t
No.
3 .........
700
1.0
??
No.
4 .........
700
1.0
,t
No.
5 .........
700
1.0
X 10-r”
g,cms
set
g/cm2
set
o-
70
2.3 x 10-s
g/cm2
set
0-
180
1 X 1O-7
,,
No.
6 .........
700
1.0
4.2 x 10-s
g/cm2
see
0-
280
,f
No.
7 .........
700
1.0
5.7 X 10-s
g/cm2
set
240-
410
No.
8 .......... ........
700
1.0
1.2 X lo-*
g/cm2
set
300-
600
Pdkder
No.
2
700
1.0
4.3 x 10-s
g/cm2
see
20-
280
Dry Oz Flake
No.
1 .........
600
6.8
,v
No.
1 .........
700
0.4
,,
No.
1 .........
850
0.4
No.
1
........
1 000
1.0
No.
1 ..........
600
,,
No.
1 .........
,,
No.
1 .........
No.
1
Pikder
Flak:
......... No. 4 .........
2,
No.4
,,
No.
.........
t
Breakaway
8
I
.........
500-
2.8 X 10-s
after
set
4 500
1 500-
4 000
250-
700
6.8
500-
5 000
700
3.1
300-
5 000
850
3.7
150-
8 000
1 000
1.3
250-
1000
600
3.8 0.3
55O-
6 000
55@-
2 000
700 700
4.0
~ 1 OOO- 5 000
L
occurred
g/cm2
5 000
1 50s
an initial
period
of parabolic
oxidation.
76
J.
K.
HIGGINS
AND
J.
E.
rapidly for both Flake No. 1 and Powder No. 1 and the parabolic law breaks down (fig. 1).
specimens
ANTILL
of Flake
700” C in dry carbon
No.
5 were
dioxide
oxidized
at
for 500 h. They
This may be due to the thermal cycling occurring
were then bound
when removing since breakaway
the specimens for weighing, was obtained after 700 h upon
and heated for a further 2000 h at 850” C. On
thermal
samples from
that the specimens were firmly knit together, and microscopic examination revealed the presence of a continuous layer of oxide between
cycling
temperature
350” C to room
once a day. Although
the number
of cycles at 1000” C was considerably
less than
in the test at 850” C, the higher temperature and thicker oxide film may compensate for this and assist the rupture of the oxide film. The value of n in the region of 2 obtained during the formation of thick films at high temperatures is consistent with Wagner’s parabolic rate law, and the reaction should therefore be controlled by the diffusion of ions through the film via lattice defects. The high values of n (e.g. n m 10) cannot be explained quantitatively by a well established theory, but may be explained qualitatively by a change in the composition of the film during film growth due to the diffusion of impurities, or to a change in the factors determining t’he rate of film growth. The film thickness corresponding to the high values of n is approximately 1000-10 000 A which may be considered the limit of the thin film region. The growth of thin films may be determined by several factors, including diffusion of ions in an electrostatic field, whilst thicker films can only grow by the normal diffusion mechanism proposed by Wagner. It follows that the high values of n could correspond to the point at which the factors determining thin film growth are exerting little influence and the temperature is insufficient for appreciable diffusion. According to Kubaschewski and Hopkins 7) beryllium oxide is a cation excess semiconductor. whilst Albrecht and Mandeville *) deduced the presence of anion vacancies in the oxide from a study of the photostimulated luminescence of X-ray irradiated beryllia. It follows that appreciable diffusion of the cations and anions may be expected in the film. In order to obtain more of the diffusion process precise knowledge governing the growth of the films, the following experiment was carried out. Two separate
together
with platinum
wire
cooling and removing the platinum it was found
the pieces of metal, showing that beryllium ion diffusion had occurred through the films. That the effect was not due to sintering was shown by repeating the above experiment using two pieces of metal which had been initially oxidized at 850” C for 500 h: the oxidation during the subsequent heat treatment at 850” C for 2000 h was only half that in the previous experiment and the specimens were not knit together, but fell apart when the platinum wire was removed. These results do not rule out the possibility of anion as well as cation diffusion. Indeed, it is probable that anion diffusion occurs in the presence of moisture because when the single crystal was oxidized in COs+ 3 vol %, Hz0 at 700” C, particles of unattacked metal were present in the oxide (fig. 9). Although the product is porous and a linear rate law is obeyed in wet gas, the kinetics of the reaction are probably still controlled by a diffusion mechanism. According to the commonly accepted theory of linear oxidation, a continuous film of oxide first forms on the metal surface and then cracks due to the stresses set up in it. Gas penetrates down the cracks and the cycle is repeated, the rate of oxidation being controlled by diffusion through the thin barrier film adhering to the metal. It is probable that the high rates of oxidation occurring in wet carbon dioxide are due to the presence of the element hydrogen. Analysis showed that the oxide formed on Flake No. 3 3 vol o/o Ha0 at 700” C in gas containing contained 300 ppm hydrogen, whilst the original metal contained only 45 ppm. The method of analysis was to heat the sample in a graphite boat at 1400” C and collect the hydrogen evolved. The hydrogen was found to be firmly bound in the oxide as its concentration was still
OXIDATION
150 ppm
after
3 h heating
OF
in dry argon
at
1000” C. The hydrogen may act in three possible ways: it may lead to the formation of hydroxyl ions in the film and hence alter the defect
77
BERYLLIUM
tinuous
inclusion
network,
which
corresponds
with the grain boundaries (cf fig. lOB), whilst in the more reactive sample Flake No. 4 (fig. 1 IB) which was vacuum cast before rolling,
structure and diffusion rates of ions through the oxide ; or it may hinder the flow of dislocations,
the network is replaced by isolated inclusion particles scattered at random throughout the
making
material. It is thought that the inclusions consist
the
oxide
more
brittle
and thereby
reducing
the thickness of the untracked
through
which the ions have to diffuse;
may rupture the film by evolving hy~ogen
oxide or it gas
at the oxide/metal interface 9). It is unlikely that extensive interstitial hydroxyl ion diffusion occurs because the ionic radius is greater than for the oxygen ion, the respective values being 1.7 and l.4A (ref. 9)). However protons may attach themselves temporarily to oxygen ions in the lattice and may move through the oxide by jumping from one lattice point to the next 9). Any one of the three explanations appear adequate to explain the behaviour of the single crystal but the marked differences in behaviour obtained for the polycrystalline materials indicate that additional factors have to be considered for these materials. It is clear that the presence of metallic impurities is not the cause of the rapid attack since the high purity sample (Flake No. 7) was one of the most reactive polycrystalline materials in wet gas. Any theory which relies solely on the presence of metallic impurities to explain the behaviour in wet gas will therefore be inadequate. This reasoning does not however apply to carbon as its concentration (M 200 ppm) was unaltered by the purification process. ~nter~anular penetration seems to be the most important feature of the oxidation of the more reactive polycrystalline materials in the wet gas (fig. 8 for Flake No. 4) whilst the less reactive materials show hardly any intergranular penetration (fig. 10A for Flake No. 1). It is considered by the authors that the difference is connected with the presence of oxide between the grains of the less reactive materials. An examination of sections of the unoxidized materials under dark ground illumination reveals for the relatively unreactive Flake No. 1. (fig. 11A) the presence of a more or less con-
of oxide
and that samples fabricated
more complicated resistance
by the
routes show poor oxidation
in the wet gas because
the inter-
granular oxide network is destroyed by melting and other fabrication techniques. In this connection it is worthy of note that Smith 10) oxidizing extruded beryllium bar at 700” C in carbon dioxide saturated with water vapour at 30-40” C found that the edge which contained less in~rgranular
of the bar oxide than
the central core oxidized at a faster rate. When there is no continuous oxide network present before oxidation, the grain boundaries appear to be especially suitable sites for the possible action of hydrogen, and attack proceeds down the grain boundaries to produce a cracked porous oxide through which water vapour may migrate to continue the attack. Since there are a large number of grains, the oxidation rate of polycrystalline material is greater than that for the single crystal (table 2). Once intergranular attack has started the grains may be forced apart due to the high volume ratio of the oxide to metal (1.68) and thereby lead to more rapid ingress of gas and distortion of the specimen. When a more or less continuous intergranular oxide network exists throughout the sample, the hydrogen must diffuse through the network to affect the reaction by one of the three mechanisms mentioned earlier. It is probable that since the diffusion path is considerably longer than that through a surface film, the reaction is limited to the surface layers of the specimen by this diffusion process and the rate of attack is consequently reduced (fig. lOA). If the above theory is correct, then destroying the continuity of the network by altering the position of the grain boundaries should increase the reactivity. This was confirmed by a test in which a sample of the unreactive Flake No. 1
78
Fig.
J.
10.
Photomicrographs (A) After
K.
HIGGINS
AND
J.
E.
ANTILL
of Flake NO. 1 before and after oxidation in COz + 3 vol. “/bHa0 at 700” Ce. 10 000 h oxidation. x 270. (R) Before oxidation. x 180
Fig. 11. Photomicrographs of oxide within the fabricated
showing
the distribution
metal before commencing
the oxidation tests. (A) Flake No. 1. (B) Flake No. 4. (C) Electrolytic flake oxidized for 1 11 before rolling. ‘ii I x0 Dark ground illumination.
OXIDATION
lost its protection crystallized
against
OF
Experiments
wet gas when reat 1225’ C for
by annealing
79
BERYLLIUM
were also carried out to deter-
mine the protection
1 h.
against wet gas alforded
in
by surface films grown in the absence of water
CO2 + 3 vol y0 Ha0 at 700’ C were 2 x 1O-8 and 8 x lo-11 g/cm2 set for the annealed and un-
vapour. Samples of the single crystal and the reactive polycrystalline materials (Flake Nos 3,
treated
4 and 5) were heated in dry carbon dioxide at 700” C for 6146 and 500 h respectively, before
The linear rate constants
samples
for the reaction
respectively.
Further
if
an
oxide network can be formed round the grains of the more reactive materials, then their oxida-
being exposed to gas containing
tion resistance should be improved.
vapour
point, electrolytic
To check this
at the
same
effect of the preformed
flake, which had been vacuum
behaviour of beryllium in dry gas after it has been subject to breakaway in the wet gas. Samples of the reactive Flake Nos. 4 and 5 which had been oxidized in CO2 + 3 vol o/o Hz0 at 700” C for 160 and 230 h respectively, were placed in the dry gas at the same temperature. Linear oxidation commenced after 450 h, but was then followed by parabolic oxidation after 1000 h. If beryllia is accepted as a cation excess semiconductor, increasing the partial pressure of oxygen by conducting the experiments in oxygen should slightly reduce the concentration of interstitial cations and hence the reaction rate. Whilst most of the results are consistent with this reasoning, there is no satisfactory
TABLE
3
Linear rate constants for samples of pre-treated beryllium tested in CO2 + I
’
I
.I
:>
No.
4
.
>9
No.
4
.
,,
No.
5
,,
No.
5
.
8
. . . . . . . . . . . . . . . . . .
1,
No.8.
9,
No.
.
.
.
.
.
.
.
.
.
.
. . . .
. . .
. .
.
. i .
.
~
.I
(g/cm2
set x lo-s)
None 500
h dry
CO2
at
700” C
COz
Itt 700” (’
None 500
I h dry
CO2
at
700” C
None 6146
~
h dry
CO2
at
700” C
at 700
Tim,
t0
4.4
x0 70 0 70
>4
C
linear rat)e (II)
4.6 10
h dry
None 500
3 vol y0 Hz0
Linear rate constant
Pre-treatment
)
only
Another point of considerable technological importance as well as scientific interest is the
high purity sample was not low in carbon and the pre-oxidation treatment could have preferentially oxidized the carbide at the surface of the powder particles.
Flake No. 3 . . . . . . . . No. 3 . . . . . . . :1
The
films was to delay the
onset of breakaway on Flake Nos. 4 and 5 by 70-140 h (table 3). The linear rates after breakaway were similar to those for the untreated samples.
cast and ground to powder, was heated for 1 h in air at 750” C before rolling. The oxygen and were 0.4 o/o and 0.25 o/0 nitrogen pickups respectively and the continuous network present in the fabricated metal is shown in fig. 11C. Specimens gained only 0.5 mg/cm2 after 3000 h oxidation in the wet gas at 700” C compared with a weight gain of 40 mg/cmz after 70 h for a standard sample made from untreated powder. Although there is no positive evidence to support the view that carbon present as an impurity in the metal is responsible for intergranular attack, it should be pointed out that most of the data are consistent with it. Microscopic examination did not indicate carbon or beryllium carbide in the grain boundaries but if a thin layer of carbide was present it might well be attacked preferentially. In addition the
Sample
3 vol y. water
temperature.
2.3
0
1.5
140
1.2
1%;
1.3
150
80
J.
explanation
at
present
for
K.
the
HIGGINS
AND
J.
breakaway
of all the published
data
reveals that there is considerable disagreement concerning the time and temperature at which breakaway
occurs in various environments,
the
importance
demonstrated
of
the
fabrication
3. The differences in the linear rates of attack for wet gas cannot be related to impurity content but are consistent with the view
but
that protection
route
rate constants obtained for carbon dioxide (table 2) ; for example the values at 1000” C for the present and previous work, extra-
is associated
with an inter-
granular network of oxide in certain batches of metal.
by the present work. The para-
bolic rate constants obtained by Gulbransen and Andrew and Cubicciotti for oxygen are greater than those for Flake No. 4 and Powder No. 1 in carbon dioxide at 700” and X50” C respectively, by factors of 10-100, but are in good agreement with the other three parabolic
the metal and may differ
by as much as a factor of 1000 between different batches of metal.
this now appears reasonable upon consideration of
ANTILL
used to fabricate
obtained in oxygen with Flake No. 1 at 700” and 850” C and Flake No. 4 at 700” C. An examination
E.
4 ’
The behaviour of the metal in dry oxygen also varies with the method of fabrication. In some cases it is very similar to that in dry carbon dioxide whilst severe breakaway can be obtained with some materials at 700” and 850” C.
Acknowledgements
polated 0; an Arrheniis plot to 1000” C, are 1.3-1.8 x lo-11 and 0.5-1.8 x 10-11 g2/cm4 set
The authors wish to thank Mr. N. Hill for the fabrication and supply of most of the
respectively.
beryllium
Conclusions The data established
which
have
been
obtained
several facts although
have
some of the
explanations of the observed effects can only be considered tentative at the present time. Protective films are normally formed on consolidated metal in dry carbon dioxide at 1 atm and temperatures up to 850’ C for 10 000 h ; at 1000” C breakaway
occurs after
References and W. Mum-o, AERE (Harwell), Report M/M 108 2J S. J. Gregg et al.,J. Nucl. Mat. 3 (1961) 175 3) S. J. Gregg et al., J. Nucl. Mat. 2 (1960) 225
‘)
J. Williams
4) D. W. Aylmore et al., J. Nucl. Mat. 2 (1960) 169 5J D. Cubicciotti, J. Amer. Chem. Sot. 72 (1950) 2084 0) E. A. Gulbransen and K. F. Andrew, J. Electrothem.
7,
1500 h. The presence of 3 vol y0 of water vapour in carbon dioxide at 1 atm results in breakaway at 700” C ; the subsequent linear rates of attack depend markedly upon the method
samples.
Sot.
of
Metals
Publications
8,
97 (1950)
0. Kubaschewski and
383
and B. E. Hopkins, Alloys
(Butterworths
Scientific,
(1953) 36
H. 0. Albrecht and C. E. Mandeville, 101 (1956)
0xidat)ion
Phys. RVY.
1250
9, D. W. Aylmore et al., .J. Nucl. Mat. 3 (1961) 190 10) R. Smith, International Atomic Energy Agem::Conference ( Vienna, 1960) FE/2/1\‘..?
OF NUCLEAR
MATERIALS
OXIDATION
5, No. I (1962) 67-80, NORTH-HOLLAND
OF BERYLLIUM
IN CARBON DIOXIDE
Received
by a variety
of
principalement
different
routes, in dry carbon dioxide
at 500-f000°
C,
et qui pouvait
at
600-1000” C and vapour
in
carbon
for
eertains
dioxide
at, 700” C for times
ment
up to
Protective
films were formed
up to
1000” C breakaway
de
850” C. At
obtained
at
of 3 vol. y0 water
I atm
The subsequent dependent
the
oxide
samples
Most
were
from in dry
carbon
with
Le
d’oxyde
in
celui
the
the
view
No
of fabrication.
courbes
de gain
de poids
der
Zeit
fur
whilst) in
Kohlendioxyd
stoff
600-1000” C
bei
Wasserdampf
bei
a 600”-1000
11 y a form&on des Rohantillons
d’eau
du temps
Bei
see a 500”-
schon
hiihnisse
bei
der Methode,
sea
wnrde
des Materials
La presence de 3 oh en volume de vapeur d’eau dens le gaz carbonique sous 1 atm provoquait le “break-
mit
away”
die
2x700” C. Succedant
au ‘*breakaway”
8,
dans 8. 700’
als Funktion
aufgestellt, wurden,
und
in
die
auf
einmal
in
Kohlendioxyd Zeiten
mit
bis zu
10 000
von
l’attaque 67
auf
den
Bei
1000” C
trat
Wasserdampf
in
3 Die
Oxyden in
me&en
ein.
Vol. ye
folgenden
mit, einem
konnten
linearen
sind merklich
FBllen. Die meisten
abhangig
des Metalls Faktor
von
Differenzen
Vervon
benutzt 1000 in
im Betragen
mit dem Verunroinigungsgrad
werden. Sie standen im Zusammenhanq
dem Schutz auf
wurden gebildet.
die zur Herst,ellung
und differ&t
gewissen
1500 heures.
von
700” C ein.
nicht korreliert
apres
Btait observable
“breakaway”
fiir den Angriff
rupture
(breakaway)
que
unt,er 1 atm Druck tritt das Zerbrechen
sous une pression dune atmosphere et aux t,emp&atures inferieures b 850”. A 1000” C, on observe une du film
similaire
bei einer atm Druok und einer
Schutzfilme
Gegenwart
a 700” C. carbonique
see Dans
bei 500-1000” C, in Sauer-
850” C
1500 Stunden
C et de gaz carbonique
de gaz
bis
Matlerialien
de films prot.ert~eurs sur la plupart en presence
t&s
see tandis
700” C iiber
Kohlendioxyd
Kohlendioxyd
de la vapeur
I’oxygene
Stunden.
moyens,
1000” C, d’oxygene
&ait
hergestellt
trockenem
nach
de gaz carbonique
dans
de fabrication.
Berylliumproben Wege
(jusqu’a 10 000 heures) ont 6th det~erminees pour des Bchantillons de tciles de beryllium &bore par difftrents en presence
“breakaway”
Es werden Kurven der Gewichtszunahme
fabricated
at 700” and 850” C.
en fonction
du
en
pas une
--
In some cases it was
was obt,ained
metal
carbonique
network
also varied
dioxide,
vis-St-vis
cas, le “breakaway”
verschiedene
in dry oxygen
Blabores
set n’apportait
la methode
le gaz
d’oxyde probable
et I, 850” C .
dioxide.
of metal
Bchantillons
cas ce comportement
dans
Unter
contenant
a la
de gaz humide.
significant
on the
les
du
aussi av8c
d’autres
that
sur
comportement
Temperatur Les
de comporte-
pas etre reliee
La croissance
carbonique
significative
certains
of 1000 in
de metal.
en presence
variait
th8
in wet. gas was provided grown
to that in dry carbon
ot,hers breakaway
fabricate
differences
of metal.
breakaway
with the method
observe
were markedly to
with an intergranular
batches
previously
The behaviour similar
the
protection
could not be relat,ed to impurity
is associated
films
of
consistent
in certain
protection
used
by as much as a factor
of materials but
of oxide
method
in carbon 700’ C.
lots
de gaz
at
vapour
in breakaway
linear rates of attack
upon
instances.
protection
very
resulted
and differed
content
films
presence
The presence
by
was
1500 h.
behaviour
des differences
ne pouvait
du metal
de I B 1000 dans
tion Btait associee a un reseau intergramrlaire
on most mat,erials in
dans certains
certain
d’elaboration
dans le rapport
cas. La plupart
at 1 atm pressure and temperatures
metal
varier
des materiaux
dry carbon dioxide
dioxide
dont la valeur dependait
de la methode
teneur en impureties mais plutBt au fait que la protec-
10 000 h.
after
AND OXYGEN
une vitesse Iineaire
sheet fabricated
water
obtained
adopt&it
gain/time of beryllium
oxygen
been
CO., AMSTERDAM
1961
Weight
in
have
27 April
samples
containing
curves
PUBLISHING
bei
durch
ein intergranulares
gewissen
trockenem
Proben.
Kohlendioxyd
Die
Netzwerk Oxydfilms,
hergestellten
68
.J.
l’robm
geaachsen
Sohutz
fiir
sintl,
Hetragen
des Metalls
ebenfalls
in Abhkngigkeit,
1.
birtcw
das “breakaway”
K.
keincn
HIGGINS
AND
betlerltenden
in fiinc~htcm (;a~.
in trocakencrrl Gauerstoff van dcr Mct,hotlr
Ijas
use of beryllium
material in the Advanced in the
UK
halten
In oinigen in
tlrr Her-
erhalten
wurde.
of
its
oxidation behaviour in gaseous environments. Preliminary work on the reaction with carbon dioxide was done by Munro and Williams and by Gregg et al. Munro and Williams 1) found that beryllium formed protective + films at temperatures up to 600” C in dry carbon dioxide at atmospheric pressure whilst Gregg et al. 2) oxidizing for shorter time intervals, (300 as compared with 3000 h), found that the oxide was protective at temperatures up to 700” C. Breakaway i + occurred at 650” C and 750” C according to Munro and Williams and Gregg et al. respectively. Water vapour has been shown to have a pronounced effect upon the reaction, causing very rapid attack and lowering the temperature at which breakaway occurs. For example Munro and Williams observed rapid non-protective oxidation at 600” C in gas containing 4-7 vol. %
Fdlen
trockenem
Fdlen
as a canning
a knowledge
stellung.
ANTILL
anderen
Gas Cooled Reactor
necessitates
E.
variiert
Introduction The possible
J.
ein
war es iihnlich tlem VW-
Kohlentlioxyd, “breakaway”
wiihrend hei
in
700-850” (I
In oxygen the rates of attack are normally similar to those in carbon dioxide. Aylmore, Gregg and Jepson 4, found that the oxide was protective up to 650” C in dry oxygen whilst breakaway
occurred
at 700” C. These workers
were unable to verify the findings of Cubicciotti 5) and Gulbransen and Andrew 6) that the oxidation followed a parabolic law from 350” to 970” C over the first 34 h. The present work was undertaken to widen the range of conditions under which the reactions in carbon dioxide and oxygen have been carried out, and to study the behaviour of samples of metal fabricated by a wide variety of techniques. Beryllium from different sources, namely Pechiney electrolytic flake and Brush thermally reduced powder, has been oxidized at atmospheric pressure in dry carbon dioxide at 500-1000” C. in oxygen at 600-1000” C and in carbon dioxide containing 3 vol. “/o water vapour at 700” C for times up to 10 000 h. 2.
Experimental
Hz0 1). The chemical nature of the reaction products
The oxidation was followed by measuring the gain in weight of specimens and constructing
formed in dry gas has been examined by Gregg, Hussey and Jepson 3) as well as the particular reactions favoured from among the many
weight gain/time curves. The more reactive samples were weighed continuously on a silica balance with a reproducibility of spring
thermodynamically possible. They have deduced that beryllium oxide and to a lesser extent beryllium carbide are formed at temperatures from 550” to 750’ C by the following reactions:
5 1 mg/cmz whilst the more inert materials were cooled periodically and weighed with a reproducibility of 50.01 mg/cma on a semmicrobalance. All the samples were suspended in silica reaction vessels by platinum hooks which did not visibly react with the beryllium. Samples of electrolytic flake supplied by t,he L’echiney Co., France, and thermally reduced powder from the Brush Beryllium (‘0.. IJSA. were fabricated as shown below:
Bet-CO,
+
BeOiCO
2Ke+-CO7. + 2BeO-I~ (1 -F Be&
2Ke I (‘ i
A film is defined
attack
cont,innously
as protective
derreascs
when t,he rate of
during
the oxidation.
tt Hreakaway is defined as t,he point at which the oxide film loses it,s protective propnt~ies and the rate of attack increases.
then remains
constant
or continuously
Flake No. 1 ~ Rolled French Flake. French flake was ground to ,200 mesh, leached in 10 s’; oxalic acid solution and rolled in a mild
OXIDATION
OF TABLE
Typical Sample
Fe
1
Si
j
(ppm)
1
(mm)
. . . . i 0.3-0.8 1.0 . . . . 1
:
200-300
I
0.05-0.9
(
twt %)
Flake
No.
3 .
Flake
No.
4
.
.
. . . .. Flake No. 5 . . . . Flake No. 7 . . . . FlakeNo.8. . . . Powder No. 2 . . .
.
. . . . .
.
. . . . .
. . I~ 0.1-0.2 0.3-1.0 . 0.01-0.03 . . 0.4 .
/
1000
600
) /
Rolled Brush Powder.
NN50
40
80 27
400-1000
Brush
powder was ground to ~200 mesh, outgassed at 600-700’ C and rolled as for Flake No. 1.
~
m 200
600 m 50
175
were
200 350.-1000
30
~ 400-1000 % 30
!a 30
700 W 200
% 30
<50
Halogens
(ppm)
(PPm)
50
~ 1
MMg
Al
1
100-300
500 400
steel can at 1000” C. The basal planes mainly parallel to the surface. Powder No. 1 -
1
analyses for the different batches of beryllium Be0
Flake Nos. 1 & 2 Powder No. 1 . .
69
BERYLLIUM
<30
(ppm)
~
I ( , :
200 20-50 %50 50 100 O-30
55
300
i 1000-3000
Flake No. 8 ~ Single Crystal Beryllium. French flake was vacuum cast, extruded at 1050” C and zone melted to give large grains which were subsequently
isolated.
Powder
2
No.
-
Forged and Rolled Brush < 200 mesh was treated
French Flake. French
Powder. Brush Powder
flake < 200 mesh was leached in 10 ‘$(, oxalic acid solution and extruded at 1050” C as tube.
as for Flake No. 6. The rolled samples
The basal planes were mainly perpendicular the surface.
of approximately 3 x 1 x 0.1 cm using a Norton alumina grinding wheel No. 38860 K5VBE. The extruded specimens were sections cut from a tube of diameter 2.7 cm and the specimen of high purity beryllium and single crystal were irregular shaped pieces of approximate area 1.5 cm2. The density of all t’he samples was > 1.8 g/cn13. The final surface preparation consisted of a chemical polish in a phosphoric/
Flake No. 2 -
Extruded
to
Flake No. 3 - Cast French Flake. Flake No. 2 was consumable arc melted into a cylindrical ingot from which specimens were cut. Flake No. 4 - Vacuum Cast and Rolled French Flake. French flake was vacuum cast, ground to powder and rolled as for Flake No. 1. Flake No. 5 - Tungsten Arc Melted and Rolled French Flake. Leached French flake was sintered at 1200” C to a density of 1.8 g/cm3 and then melted in a tungsten arc in argon. The resulting buttons were ground to ~200 mesh and rolled as for Flake No. 1. Flake No. 6 - Forged and Rolled French Flake. French flake ~200 mesh was hydrostatically pressed, sintered at 1225” C, forged in a mild steel can at 1050” C and then finally cross rolled at 900” C. Flake No. 7 - High Purity Beryllium. Beryllium was evaporated from the molten state at 1400' C to produce a mass of coarse crystals which were then cast into an ingot.
were ground
to a size
chromic acid bath for 30 set at 100” C followed by a rinse in distilled water and acetone. Analyses could not be obtained for each batch of material, but typical impurity contents for metal fabricated by some of the routes are shown in table 1. Carbon dioxide was obtained from a solid carbon dioxide (“Cardice”) convertor with oxygen and water vapour as its main impurities. Dry gas containing < 20 vpm + water vapour and < 300 vpm of gas not condensable in liquid air was obtained by passing the carbon dioxide through columns of manganous oxide at 150" C and anhydrous magnesium perchlorate to remove oxygen and water vapour respectively. t
vpm represents parts per million by volume.
70
J.
K.
HIGGINS
AND
J.
J3.
ANTILL
It was found at temperatures of 850’ C and above that t’he water vapour concentration of the dried
gas rose to
passage through culty
> 100 vpm
the reaction
was overcome
during
its
vessel. The diffi-
by first passing
20.0
the gas
through a steel tube furnace at 800” C and then removing
any water vapour formed with anhy-
drous magnesium the concentration the reaction
perchlorate.
By this means
5.0
of water vapour in gas leaving
vessel was kept to
-: 20 vpm.
2.0
Carbon dioxide containing 3 vol. 94 water vapour was prepared by bubbling the converter supply through distilled water at room temperature. The oxygen used was industrial cylinder oxygen, which was passed through a “Deoxo” unit followed by a steel tube furnace at 800” (I
a-
1.0
:
s z
-z 0.50
I
0.20
0.10
and a bed of anhydrous magnesium perchlorate to lower the hydrogen and water vapour contents to <20 vpm.
0.05
0.02
Results
3.
The weight gain versus time graphs obtained for the various experimental conditions are presented in Figs. 1-6. 3.1.
DRY
CARBON
;
0.01
I
L 100
;
3
500 TIME
Fig.
1.
Weight
Powder
Ko.
gain/time 1 in dry
,000
2000
5’
(h)
curves for Flake No. 1 and COz at 500-1000” C.
DIOXIDE
The results for Flake No.
1 (rolled French
flake) and Powder No. 1 (rolled Brush powder) at temperatures from 500” to 1000“ C are shown in fig. 1. The reaction rate increased with temperature, and protective films were formed from 500” to 850” C for times up to 10 000 h. At 1000” C the oxide was protective initially but broke down after 1500 h to give a rate of attack which increased with time. At all temperatures except 1000” C Flake No. 1 was attacked somewhat more rapidly than Powder No. 1. At 1000” C the samples were covered with a hard white scale whilst at the lower temperatures the films were grey or merely produced interference colours. At 1000” C some of the samples were bent (Flake No. 1) and the oxide blistered (Powder No. 1). The attack of specimens fabricated by the more complicated techniques (fig. 2) did not
differ greatly from that of Flake No. 1 and Powder No. 1 at 700” C. Whilst the film on the high purity sample (Flake No. 7) was not protective and the cast French flake (Flake No. 3) showed signs of an increasing
reaction
rate towards the end of the oxidation, the films on the rest of the materials, including the single crystal (Flake No. S), were protective for times up to 10 000 h. The graph for Flake No. 4 (vacuum cast and rolled French flake) is interesting as the attack was less than for the other materials during the first 2500 h, but then increased markedly during the next 1000 h finally to slow down to a rate similar to the other materials. Thermal cycling did not increase the reaction rate of Flake No. 1 at 700” C in tests in which the sample was cooled once a day to room temperature in 15 min ; the weight gains with and without thermal cycling were 0.25 and 0.27 mg/cm respectively after 3000 h.
OXIDATION
OF
However,
10.0
71
BERYLLIUM
at 850” C thermal cycling resulted in
breakaway after 700 h; the weight gain at that time was only 0.13 mg/cmz but during the next
5.0
1500 h it increased
by 1.4 mg/cmz,
compared
with an increase of 0.25 mg/cm2 for the sample
2.0
--.-
FLAKE
No.7
+
FLAKE
NOB
not thermally
cycled.
I.0
3.2. 0.50 ri-
CARBON
DIOXIDE+~
VAPOUR
AT 700°C
VOL. %
WATER
E
The addition
2 5 z
of 3 vol. y0 water vapour
to
the carbon dioxide resulted in breakaway and thereby increased the rate of attack of all the samples, which may be classified into two groups according to the size of this increase.
0.20
2 ; 0.10
0.05
The weight gain/time graphs for the less reactive group are plotted in fig. 3. Comparison with the data for dry carbon dioxide shows that the addition of 3 vol. oh water vapour increased
0.02
0.01 I
100
200
500
1000
2000
5000
IC
)O
TIME(tt)
Fig. 2. Weight gain/time curves for Flake Nos. 2, 3, 4, 5, 7 and 8 in dry CO2 at 700” C.
t
the weight gain after 3000 h oxidation for Flake No. 1, Flake No. 2 (extruded French flake) and Powder No. 1 by factors of 3.3, 9.6 and 150 respectively. The weight gains for Flake Nos. 1 and 2, but not Powder No. 1, tend towards a constant value after roughly 4000 h. The second group of samples having little resistance to the wet gas comprises all the other materials fabricated by the more complicated routes involving vacuum casting, arc melting, forging, etc. as well as the high purity specimen (Flake No. 7) and the single crystal (Flake No. 8). These all oxidized
100
200
500
1000
2(
TIME(h)
Fig. 3. Weight gain/time curves for Flake Nos. 1, 2 and Powder No. 1 in GOa + 3 vol. y0 Ha0 at 700’ C.
at a fast linear rate
either immediately or after a short induction period. The weight gain/time graphs are given in fig. 4 and the linear rate constants in table 2. Some idea of the variation in reaction rates may be gained by comparing the linear rate constant of the most reactive specimen, Flake No. 4 (vacuum cast and rolled French flake) with that of Flake No. 1, the lease reactive of all the sample calculated for the first 3000 h of oxidation. The values are 1 x 1O-7 and 8 xlO-ll g/cm2 set respectively, a change of more than a thousand-fold. The order of decreasing resistance to oxidation in wet gas derived from the linear rate constants in table 2 is:
J. K. HIGGINS AND J. E. ANTILL
72
+
FLAKE
-+-
FLAKE
No. 4
-e
FLAKE
No. 5
-+-
FLAKE
No. 6
+
FLAKE
No. 7
&
POWDER
+
FLAKE
Ne. 3
No 2 No. 8.
TIME(h)
Fig. 4.
Weight
gain/time
curves for Flake
Nos.
3-8 and Powder
Flake No. 1 > Flake No. 2 > Powder No. 1 > Flake No. 8>Flake No. 5>Flake No. 6> Powder No. 2>Flake No. 3>Flake No. 7> Flake No. 4. The appearance of the samples which little resistance to the wet gas indicated
had two
distinct types of attack: for the single crystal and polycrystalline material with large grains (Flake No. 3) the product was mainly a light grey or white nonadherent powder formed at the geometric surface, whilst appreciable internal oxidation had occurred for the polycrystalline materials with small grains, resulting in the formation of dark grey blisters or thick adherent scale. Fig. 7 is a photomicrograph of a section of the polycrystalline material with large grains from which it is apparent that although most of the attack was at the geometric surface intergranular attack had occurred. As expected the effect was more pronounced with the materials containing a large number of small grains, as for instance with Flake No. 4. Fig. 8 shows photomicrographs of two faces at right angles of a section cut from Flake No. 4 after the sample had been oxidized in the wet gas at 700” C for 70 h. The different orientation of the metal grains at the two faces can be seen; oxidation left isolated grains or particles of metal
No. 2 in COZ + 3 vol.
y. Hz0
at 700” C.
embedded in the oxide either perpendicular or parallel to the direction of attack, depending upon the orientation of the grains. In spite of the difference in the appearance of the attack at the two faces, the extent of the reaction is similar. It appears that oxidation first occurred between the grains and was then followed by attack of the adjacent grains, some of which appear more reactive than others. So far only polycrystalline material has been considered but in Fig. 9 photomicrographs are shown of the oxidized (0001) and (1010) faces of the single crystal (Flake No. 8). Although the two faces oxidized at comparable speeds, preferred paths of attack appear to exist at the lOi face, indicating preferred attack of the basal planes which are perpendicular to this face. 3.3.
DRY OXYGEN
In fig. 5 are given the weight gain/time graphs for Flake No. 1 and Powder No. 1 in dry oxygen at temperatures from 600” to 1000” C. Comparison with fig. 1 for the same samples in dry carbon dioxide, shows that the weight gains were similar in the two gases for Powder NO. 1 at all temperatures and for Flake No. 1 at 600” and 1000” C. However, Flake No. 1 exhibited a rapid breakaway reaction at 700” C and 850” C in oxygen whilst in carbon dioxide
OXIDATION
OF BERYLLIUM
‘OO.O I 50.0
20.0
-@-
FLAKE No.4
-.-
FLAKE No.7
+
FLAKE No.8
10.0
5.0
N;
2.0
z E” - 1.00 z G r 0.50 3
0.20
0.10
0.05
0.01
0.02
100 0.01
I
I
TIME(h)
0
Fig. 5. Weight gain/time curves for Flake No. 1 and Powder No. 1 in dry 02 at 600-1000” C.
Fig. 6.
200
500
1000 TIME(h)
2000
5000
10000
Weight gain/time curves for Flake Nos. 4, 7 and 8 in dry 02 at 600-700” C.
at these temperatures the films were protective. Flake No. 4 (vacuum cast and rolled French flake) behaved similarly to Flake No. 1 at 700” C but the attack on the single crystal (Flake No. 8) and the high purity sample (Flake No. 7) was less in oxygen than in carbon dioxide (cf figs. 2 and 6).
3.4.
X-RAY
AND ELECTRON DIFFRACTION
MEASUREMENTS ON THE PRODUCTS
An X-ray diffraction investigation showed that the products obtained from samples oxidized in dry and moist carbon dioxide and in dry oxygen at temperatures up to 850” C consisted of beryllia which in many cases was mixed with unattacked metal. An electron diffraction examination showed that as the temperature was raised from 500” to 1000” C
Fig. 7. Photomicrographsof & section of Flake No. 3 after oxidation in CO2 + 3 vol. ‘$& Hz0 at 700” C for 170 h. x 55
the crystallite size of the oxide grown in dry carbon dioxide increased from < 100 A to > 10 000 A.
74
J.
K.
HIGGINS
AND
J.
E.
ANTILL
(.A) Fig. S.
~‘tl~,tolnicrograplls
(13)
of two
faces of a specimen
at 700° C for
70 Il.
(A)
of Flake
No. 4 after
oxidation
in CO2 + :I vol.
Side
(B) Fig.
9.
Photomicrographs
of
the
single
at 700” C for
4.
crystal
700 h.
(A)
(Flake
0001 face.
Discussion
The oxidation may equation of the form
be represented
by
No.
an
wn=kt where w = weight gain t = time k = rate constant
8) after (B)
oxidat,ion
10 10 face.
x
in COZ +
3 vol.
“/o H&I
120
may be classified according to the value of n, If n = 1 the rate of oxidation will be constant, whilst if n < 1 the rate of oxidation will continuously increase with time. In both cases the film formed is classified as non-protective and breakaway is said to occur. However, if n> 1 then the rate of oxidation will continuously decrease and the film is classified as protective. On this basis the oxidation in dry carbon dioxide at 1 atmosphere pressure is protective for nearly all the samples. This is clear from table 2 where the values of n for Flake No. 1 and Powder No. 1 from 500" to 1000"C are given together with those at 700'C forsamples fabricated by the more complicated routes. For Flake No. 1
OXIDATION
OF
the reaction is said to obey a parabolic law. It should be borne in mind, however, that after 1500 h oxidation at 1000” C, l/n increases
and Powder No. 1 n lies between 3.1 and 00 at temperatures from 500” to 700’ C whilst at 850” and 1000” C the value approaches 2 and TABLE
TTalues for the constants Sample
j /
in an equation
2
representing Values
Temp.
(“C)
75
BERYLLIUM
~~
the weight of n and
(:urv-e
k in equation
E
rl
Dry
gain/time
wn =
I
kt
Time
(h)
COZ
No.
1 .........
500
3.8
350-
7 000
3,
No.
1 .........
600
9.5
500-
8 000
7,
No.
1 .........
700
4.3
500-10
1,
No.
1 .........
850
2.7
500-
1 000
Flake
000 8 000
27&
1 500
3.1
300-
7 000
600
00
1 300-
6 000
700
9.0
300-
7 000
1 .........
850
2.1
No.
1 .........
1 000
Flak;
No.
2 .........
700
6.5
3s
No.
3
0
No.4
,,
No.
1 ........
Po;vder
No.
1 ..........
500
1, ,,
No.
1 .........
No.
1 .........
:,
No.
~1.3
2.2 t
X lo-l1
1.3 x lo-r4 ml.8
1.7 t
x lo-11
g2/cm4
set
g2/cm4
set
250-
5 000
gz/cm4
set
170-
1500
1700-10000
.........
700
2.0
3.3 x lo-14
g2jcm4
set
450-
4 000
.........
700
2.1
3.5 X lo-l5
g2/cm4
see
500-
2 500
No.
5 .........
700
2.5
350-
5 500
1,
No.
7 .........
700
1.0
200-
4 500
I,
No.
8 .........
700
2.3
300-
4 000
3 000
~
COz + 3 vol ye Hz0 Flake
No.
1
Powder
No.
1
Flake
No.
2 .........
......... .........
700
1.0
8 x 10-n
g/cm2
set
500-
700
1.1
1 X 1O-g
g/cm2
see
600-
7 000
700
0.9
4.9
g/cm2
set
50s
3 000
4.4 X lo-*
loo-
170
,t
No.
3 .........
700
1.0
??
No.
4 .........
700
1.0
,t
No.
5 .........
700
1.0
X 10-r”
g,cms
set
g/cm2
set
o-
70
2.3 x 10-s
g/cm2
set
0-
180
1 X 1O-7
,,
No.
6 .........
700
1.0
4.2 x 10-s
g/cm2
see
0-
280
,f
No.
7 .........
700
1.0
5.7 X 10-s
g/cm2
set
240-
410
No.
8 .......... ........
700
1.0
1.2 X lo-*
g/cm2
set
300-
600
Pdkder
No.
2
700
1.0
4.3 x 10-s
g/cm2
see
20-
280
Dry Oz Flake
No.
1 .........
600
6.8
,v
No.
1 .........
700
0.4
,,
No.
1 .........
850
0.4
No.
1
........
1 000
1.0
No.
1 ..........
600
,,
No.
1 .........
,,
No.
1 .........
No.
1
Pikder
Flak:
......... No. 4 .........
2,
No.4
,,
No.
.........
t
Breakaway
8
I
.........
500-
2.8 X 10-s
after
set
4 500
1 500-
4 000
250-
700
6.8
500-
5 000
700
3.1
300-
5 000
850
3.7
150-
8 000
1 000
1.3
250-
1000
600
3.8 0.3
55O-
6 000
55@-
2 000
700 700
4.0
~ 1 OOO- 5 000
L
occurred
g/cm2
5 000
1 50s
an initial
period
of parabolic
oxidation.
76
J.
K.
HIGGINS
AND
J.
E.
rapidly for both Flake No. 1 and Powder No. 1 and the parabolic law breaks down (fig. 1).
specimens
ANTILL
of Flake
700” C in dry carbon
No.
5 were
dioxide
oxidized
at
for 500 h. They
This may be due to the thermal cycling occurring
were then bound
when removing since breakaway
the specimens for weighing, was obtained after 700 h upon
and heated for a further 2000 h at 850” C. On
thermal
samples from
that the specimens were firmly knit together, and microscopic examination revealed the presence of a continuous layer of oxide between
cycling
temperature
350” C to room
once a day. Although
the number
of cycles at 1000” C was considerably
less than
in the test at 850” C, the higher temperature and thicker oxide film may compensate for this and assist the rupture of the oxide film. The value of n in the region of 2 obtained during the formation of thick films at high temperatures is consistent with Wagner’s parabolic rate law, and the reaction should therefore be controlled by the diffusion of ions through the film via lattice defects. The high values of n (e.g. n m 10) cannot be explained quantitatively by a well established theory, but may be explained qualitatively by a change in the composition of the film during film growth due to the diffusion of impurities, or to a change in the factors determining t’he rate of film growth. The film thickness corresponding to the high values of n is approximately 1000-10 000 A which may be considered the limit of the thin film region. The growth of thin films may be determined by several factors, including diffusion of ions in an electrostatic field, whilst thicker films can only grow by the normal diffusion mechanism proposed by Wagner. It follows that the high values of n could correspond to the point at which the factors determining thin film growth are exerting little influence and the temperature is insufficient for appreciable diffusion. According to Kubaschewski and Hopkins 7) beryllium oxide is a cation excess semiconductor. whilst Albrecht and Mandeville *) deduced the presence of anion vacancies in the oxide from a study of the photostimulated luminescence of X-ray irradiated beryllia. It follows that appreciable diffusion of the cations and anions may be expected in the film. In order to obtain more of the diffusion process precise knowledge governing the growth of the films, the following experiment was carried out. Two separate
together
with platinum
wire
cooling and removing the platinum it was found
the pieces of metal, showing that beryllium ion diffusion had occurred through the films. That the effect was not due to sintering was shown by repeating the above experiment using two pieces of metal which had been initially oxidized at 850” C for 500 h: the oxidation during the subsequent heat treatment at 850” C for 2000 h was only half that in the previous experiment and the specimens were not knit together, but fell apart when the platinum wire was removed. These results do not rule out the possibility of anion as well as cation diffusion. Indeed, it is probable that anion diffusion occurs in the presence of moisture because when the single crystal was oxidized in COs+ 3 vol %, Hz0 at 700” C, particles of unattacked metal were present in the oxide (fig. 9). Although the product is porous and a linear rate law is obeyed in wet gas, the kinetics of the reaction are probably still controlled by a diffusion mechanism. According to the commonly accepted theory of linear oxidation, a continuous film of oxide first forms on the metal surface and then cracks due to the stresses set up in it. Gas penetrates down the cracks and the cycle is repeated, the rate of oxidation being controlled by diffusion through the thin barrier film adhering to the metal. It is probable that the high rates of oxidation occurring in wet carbon dioxide are due to the presence of the element hydrogen. Analysis showed that the oxide formed on Flake No. 3 3 vol o/o Ha0 at 700” C in gas containing contained 300 ppm hydrogen, whilst the original metal contained only 45 ppm. The method of analysis was to heat the sample in a graphite boat at 1400” C and collect the hydrogen evolved. The hydrogen was found to be firmly bound in the oxide as its concentration was still
OXIDATION
150 ppm
after
3 h heating
OF
in dry argon
at
1000” C. The hydrogen may act in three possible ways: it may lead to the formation of hydroxyl ions in the film and hence alter the defect
77
BERYLLIUM
tinuous
inclusion
network,
which
corresponds
with the grain boundaries (cf fig. lOB), whilst in the more reactive sample Flake No. 4 (fig. 1 IB) which was vacuum cast before rolling,
structure and diffusion rates of ions through the oxide ; or it may hinder the flow of dislocations,
the network is replaced by isolated inclusion particles scattered at random throughout the
making
material. It is thought that the inclusions consist
the
oxide
more
brittle
and thereby
reducing
the thickness of the untracked
through
which the ions have to diffuse;
may rupture the film by evolving hy~ogen
oxide or it gas
at the oxide/metal interface 9). It is unlikely that extensive interstitial hydroxyl ion diffusion occurs because the ionic radius is greater than for the oxygen ion, the respective values being 1.7 and l.4A (ref. 9)). However protons may attach themselves temporarily to oxygen ions in the lattice and may move through the oxide by jumping from one lattice point to the next 9). Any one of the three explanations appear adequate to explain the behaviour of the single crystal but the marked differences in behaviour obtained for the polycrystalline materials indicate that additional factors have to be considered for these materials. It is clear that the presence of metallic impurities is not the cause of the rapid attack since the high purity sample (Flake No. 7) was one of the most reactive polycrystalline materials in wet gas. Any theory which relies solely on the presence of metallic impurities to explain the behaviour in wet gas will therefore be inadequate. This reasoning does not however apply to carbon as its concentration (M 200 ppm) was unaltered by the purification process. ~nter~anular penetration seems to be the most important feature of the oxidation of the more reactive polycrystalline materials in the wet gas (fig. 8 for Flake No. 4) whilst the less reactive materials show hardly any intergranular penetration (fig. 10A for Flake No. 1). It is considered by the authors that the difference is connected with the presence of oxide between the grains of the less reactive materials. An examination of sections of the unoxidized materials under dark ground illumination reveals for the relatively unreactive Flake No. 1. (fig. 11A) the presence of a more or less con-
of oxide
and that samples fabricated
more complicated resistance
by the
routes show poor oxidation
in the wet gas because
the inter-
granular oxide network is destroyed by melting and other fabrication techniques. In this connection it is worthy of note that Smith 10) oxidizing extruded beryllium bar at 700” C in carbon dioxide saturated with water vapour at 30-40” C found that the edge which contained less in~rgranular
of the bar oxide than
the central core oxidized at a faster rate. When there is no continuous oxide network present before oxidation, the grain boundaries appear to be especially suitable sites for the possible action of hydrogen, and attack proceeds down the grain boundaries to produce a cracked porous oxide through which water vapour may migrate to continue the attack. Since there are a large number of grains, the oxidation rate of polycrystalline material is greater than that for the single crystal (table 2). Once intergranular attack has started the grains may be forced apart due to the high volume ratio of the oxide to metal (1.68) and thereby lead to more rapid ingress of gas and distortion of the specimen. When a more or less continuous intergranular oxide network exists throughout the sample, the hydrogen must diffuse through the network to affect the reaction by one of the three mechanisms mentioned earlier. It is probable that since the diffusion path is considerably longer than that through a surface film, the reaction is limited to the surface layers of the specimen by this diffusion process and the rate of attack is consequently reduced (fig. lOA). If the above theory is correct, then destroying the continuity of the network by altering the position of the grain boundaries should increase the reactivity. This was confirmed by a test in which a sample of the unreactive Flake No. 1
78
Fig.
J.
10.
Photomicrographs (A) After
K.
HIGGINS
AND
J.
E.
ANTILL
of Flake NO. 1 before and after oxidation in COz + 3 vol. “/bHa0 at 700” Ce. 10 000 h oxidation. x 270. (R) Before oxidation. x 180
Fig. 11. Photomicrographs of oxide within the fabricated
showing
the distribution
metal before commencing
the oxidation tests. (A) Flake No. 1. (B) Flake No. 4. (C) Electrolytic flake oxidized for 1 11 before rolling. ‘ii I x0 Dark ground illumination.
OXIDATION
lost its protection crystallized
against
OF
Experiments
wet gas when reat 1225’ C for
by annealing
79
BERYLLIUM
were also carried out to deter-
mine the protection
1 h.
against wet gas alforded
in
by surface films grown in the absence of water
CO2 + 3 vol y0 Ha0 at 700’ C were 2 x 1O-8 and 8 x lo-11 g/cm2 set for the annealed and un-
vapour. Samples of the single crystal and the reactive polycrystalline materials (Flake Nos 3,
treated
4 and 5) were heated in dry carbon dioxide at 700” C for 6146 and 500 h respectively, before
The linear rate constants
samples
for the reaction
respectively.
Further
if
an
oxide network can be formed round the grains of the more reactive materials, then their oxida-
being exposed to gas containing
tion resistance should be improved.
vapour
point, electrolytic
To check this
at the
same
effect of the preformed
flake, which had been vacuum
behaviour of beryllium in dry gas after it has been subject to breakaway in the wet gas. Samples of the reactive Flake Nos. 4 and 5 which had been oxidized in CO2 + 3 vol o/o Hz0 at 700” C for 160 and 230 h respectively, were placed in the dry gas at the same temperature. Linear oxidation commenced after 450 h, but was then followed by parabolic oxidation after 1000 h. If beryllia is accepted as a cation excess semiconductor, increasing the partial pressure of oxygen by conducting the experiments in oxygen should slightly reduce the concentration of interstitial cations and hence the reaction rate. Whilst most of the results are consistent with this reasoning, there is no satisfactory
TABLE
3
Linear rate constants for samples of pre-treated beryllium tested in CO2 + I
’
I
.I
:>
No.
4
.
>9
No.
4
.
,,
No.
5
,,
No.
5
.
8
. . . . . . . . . . . . . . . . . .
1,
No.8.
9,
No.
.
.
.
.
.
.
.
.
.
.
. . . .
. . .
. .
.
. i .
.
~
.I
(g/cm2
set x lo-s)
None 500
h dry
CO2
at
700” C
COz
Itt 700” (’
None 500
I h dry
CO2
at
700” C
None 6146
~
h dry
CO2
at
700” C
at 700
Tim,
t0
4.4
x0 70 0 70
>4
C
linear rat)e (II)
4.6 10
h dry
None 500
3 vol y0 Hz0
Linear rate constant
Pre-treatment
)
only
Another point of considerable technological importance as well as scientific interest is the
high purity sample was not low in carbon and the pre-oxidation treatment could have preferentially oxidized the carbide at the surface of the powder particles.
Flake No. 3 . . . . . . . . No. 3 . . . . . . . :1
The
films was to delay the
onset of breakaway on Flake Nos. 4 and 5 by 70-140 h (table 3). The linear rates after breakaway were similar to those for the untreated samples.
cast and ground to powder, was heated for 1 h in air at 750” C before rolling. The oxygen and were 0.4 o/o and 0.25 o/0 nitrogen pickups respectively and the continuous network present in the fabricated metal is shown in fig. 11C. Specimens gained only 0.5 mg/cm2 after 3000 h oxidation in the wet gas at 700” C compared with a weight gain of 40 mg/cmz after 70 h for a standard sample made from untreated powder. Although there is no positive evidence to support the view that carbon present as an impurity in the metal is responsible for intergranular attack, it should be pointed out that most of the data are consistent with it. Microscopic examination did not indicate carbon or beryllium carbide in the grain boundaries but if a thin layer of carbide was present it might well be attacked preferentially. In addition the
Sample
3 vol y. water
temperature.
2.3
0
1.5
140
1.2
1%;
1.3
150
80
J.
explanation
at
present
for
K.
the
HIGGINS
AND
J.
breakaway
of all the published
data
reveals that there is considerable disagreement concerning the time and temperature at which breakaway
occurs in various environments,
the
importance
demonstrated
of
the
fabrication
3. The differences in the linear rates of attack for wet gas cannot be related to impurity content but are consistent with the view
but
that protection
route
rate constants obtained for carbon dioxide (table 2) ; for example the values at 1000” C for the present and previous work, extra-
is associated
with an inter-
granular network of oxide in certain batches of metal.
by the present work. The para-
bolic rate constants obtained by Gulbransen and Andrew and Cubicciotti for oxygen are greater than those for Flake No. 4 and Powder No. 1 in carbon dioxide at 700” and X50” C respectively, by factors of 10-100, but are in good agreement with the other three parabolic
the metal and may differ
by as much as a factor of 1000 between different batches of metal.
this now appears reasonable upon consideration of
ANTILL
used to fabricate
obtained in oxygen with Flake No. 1 at 700” and 850” C and Flake No. 4 at 700” C. An examination
E.
4 ’
The behaviour of the metal in dry oxygen also varies with the method of fabrication. In some cases it is very similar to that in dry carbon dioxide whilst severe breakaway can be obtained with some materials at 700” and 850” C.
Acknowledgements
polated 0; an Arrheniis plot to 1000” C, are 1.3-1.8 x lo-11 and 0.5-1.8 x 10-11 g2/cm4 set
The authors wish to thank Mr. N. Hill for the fabrication and supply of most of the
respectively.
beryllium
Conclusions The data established
which
have
been
obtained
several facts although
have
some of the
explanations of the observed effects can only be considered tentative at the present time. Protective films are normally formed on consolidated metal in dry carbon dioxide at 1 atm and temperatures up to 850’ C for 10 000 h ; at 1000” C breakaway
occurs after
References and W. Mum-o, AERE (Harwell), Report M/M 108 2J S. J. Gregg et al.,J. Nucl. Mat. 3 (1961) 175 3) S. J. Gregg et al., J. Nucl. Mat. 2 (1960) 225
‘)
J. Williams
4) D. W. Aylmore et al., J. Nucl. Mat. 2 (1960) 169 5J D. Cubicciotti, J. Amer. Chem. Sot. 72 (1950) 2084 0) E. A. Gulbransen and K. F. Andrew, J. Electrothem.
7,
1500 h. The presence of 3 vol y0 of water vapour in carbon dioxide at 1 atm results in breakaway at 700” C ; the subsequent linear rates of attack depend markedly upon the method
samples.
Sot.
of
Metals
Publications
8,
97 (1950)
0. Kubaschewski and
383
and B. E. Hopkins, Alloys
(Butterworths
Scientific,
(1953) 36
H. 0. Albrecht and C. E. Mandeville, 101 (1956)
0xidat)ion
Phys. RVY.
1250
9, D. W. Aylmore et al., .J. Nucl. Mat. 3 (1961) 190 10) R. Smith, International Atomic Energy Agem::Conference ( Vienna, 1960) FE/2/1\‘..?