T h e Intermittent Oxidation of Some Nickel-Chromium Base Alloys
by 5. Lustrnan
has been known for a number of years that the
I Taddition
of certain alkaline-earth and rare-earth
4DDlTlVE
HOURS LIF5 IN
ELEMENT
4 S T M TEST
NONE
25
12246
Zr-Ca-Al
86
O 12046
Zr-CO-41-5
CONTINUOUS OXlO4TlON PT 2150DF
MEAN OF I L L ALLOYS
ALLOY
0 13246
metals to nickel-chromium base electric resistance
alloys causes marked increase in their oxidation resistance as measured in an intermittent oxidation
test similar to the ASTM-B 76-39 test. However,
the mechanism whereby such additions confer protection is still unknown. On the basis of the experimental observation that the increase in life in intermittent tests of nickel-base alloys is proportional to
the atomic radius of the added element, Hessenbruch' has proposed that the added metal ions occupy
holes in the oxide lattice which prevent diffusion of
nickel ions in the oxide. Horn2 has postulated that
the effect of such additions is to accelerate the rate
of diffusion of chromium atoms in the metal and
thus to favor the formation of CrnOsscales. Hickrnan
and Gulbransen%oncluded from electron diffraction
observation that the addition of the elements in
question results in the formation of a barrier to the
diffusion of nickel ions to the surface.
---
I
o=
NUMBER OF CYCLESTIME IN HOURS-50
100
I50
200
250
300
350
Fig. I-Results
of intermittent oxidation tests and
continuous tests on bulk specimens at 2150°F.
lographically and samples of the spalled oxide collected for X-ray diffraction identification of the
phases.
The specimens used were from the same lot of
material used by Hickrnan and Gulbransen%nd had
the compositions shown in table I. Included in table
I are results of ASTM life tests on the materials in
the form of wires. These materials, their composition and the ASTM life test data thereon were all
supplied by F. E. Bash of the Driver-Harris Co. It
may be noted that alloy 13246 of 80 pct nickel, 20
pct chromium nominal composition showed the
shortest life in the ASTM test; alloy 12246 with the
addition of small amounts of the elements zirconium,
calcium, and aluminum, intermediate life; and alloy
12046 with the further addition of silicon, the longest life.
In fig. 1 are shown the results of intermittent
oxidation tests made on bulk specimens at 2150°F
as well as continuous tests conducted at the same
temperature. Duplicate tests were made on each
specimen. The ordinate scale, inches of metal
oxidized, was derived from the experimentally observed weight gain of oxygen by assuming the scale
to be entirely Cr208.It may be noted that the metalloss curves during intermittent oxidation show a
constant or continuously increasing rate of weight
B. LUSTMAN, Member AIME, is associated with the
Atomic Power Division, Westinghouse Electric Co~p.,
Pittsburgh, Pa.
AIME Chicago'Meeting, October 1950.
TP 2895 E. Discussion ( 2 copies) may be sent to
Transactions AIME before Dec. 15, 1950. Manuscript
received Feb. 27, 1950.
In connection with some experiments conducted
to develop a test for the resistance to oxidation of
alloys in massive form under conditions of intermittent heating and cooling, some features of the
oxidation of nickel-chromium base alloys were revealed which may serve to elucidate the mechanism
of protection conferred by rare-earth and alkalineearth elements. Specimens of cylindrical shape 0.360
in. diam x 0.37 in. long were placed in alumina
thimbles and inserted into and withdrawn from a
Globar-heated furnace operating a t 2150°F at 7.5min intervals. The weight of oxygen absorbed as
well as the weight of oxide spalled were measured
at intervals. With the particular experimental conditions used, the specimens in a 7.5-min cycle were
at temperature for 2 min which corresponds to the
heating time of the ASTM test. The weight changes
of similar specimens under conditions of continuous
heating in air at 2150°F were also measured. The
specimens after oxidation were examined metal-
Table I. Analysis of Samples
Heat
No.
Mn
C
-
12046
12246
13246
0.08
0.08
0.12
0.01
0.01
1.70
!
-
)I
Cr
1.39
0.30
0.30
19.91
19.08
19 98
-
N1
Fe
Zr
Ca
A1
0.34
0.32
0.20
0.10
0.05
0.024
0.029
0.07
0.08
Mg
UsefuF
Life
at
21500F
0.006
157
86
25
Bal
Bal.
Bal.
-
The "useful life" is the tlme m hours required for a 10 pet Increase in resistance in the ASTM test B 76-39
- -
-
TRANSACTIONS AIME, VOL. 188, AUGUST 1950, JOURNAL OF METALS-995
Figs. 2-4--Oxidized 1529 cycles at 2150°F.
Fig. 2. Sample 13246. Fig. 3. Sample 12246. Fig. 4. Sample 12046. The gray portions are oxide, the white portions are metal.
Unetched. X730. Area reduced approximately three quarters in reproduction.
gain of oxygen (or thickness of metal oxidized) with
time or cycles of oxidation. Likewise, there is a
direct correlation between life measured in the
ASTM test and thickness of metal oxidized during
the intermittent test. On continuous oxidation, on the
other hand, the rate of metal loss by oxidation decreases continuously with time and furthermore, the
oxidation curves of each of the three alloys are practically identical within the limits of experimental
error. Such differences as could be noted during continuous oxidation indicated greater metal loss for
the alloy which showed least loss on intermittent
oxidation.
X-ray diffraction results on both the spalled and
adhering oxide present on each alloy after oxidation
showed, similar to the electron diffraction findings
of Hickman and Gulbransen on these same alloys,
that Cr,O, was the principal constituent of the scale
in the case of the alloy 12046 showing longest ASTM
life and least weight loss in the intermittent test,
and that admixtures of NiO, Cr,O,, and spinel were
present in the other alloys."
* X-ray measurements were made by S . Beatty of the Westinghouse Research Laboratories.
The microstructures of the alloys after oxidation
are quite instructive in indicating the possible cause
for the considerable difference in behavior between
intermittent and continuous oxidation as well as the
mechanism of protection by the addition of the
alkaline-earth metals and silicon to the base composition. In figs. 2, 3, and 4 are shown the surfaces
of the oxidized alloys 13246, 12246, and 12046 respectively after 1529 cycles of oxidation at 2150°F.
It may be noted that the oxide-metal interface is
quite smooth in case of alloy 13246 which showed
most oxidation in the intermittent test and shortest
life in the ASTM test; in case of alloy 12246, containing Zr, Ca, and Al, whose oxidation rate in the
intermittent test and life in the ASTM test were
intermediate, there was appreciable internal penetration of the oxide; while alloy 12046 containing
Si as well as Zr, Ca, and Al, showed considerable
internal penetration of oxide, least oxidation in the
intermittent test and longest ASTM life. A similar
trend of internal penetration of oxide was found in
case of continuous oxidation. It was likewise found
that the amount of oxide spalled during the intermittent test relative to the total weight of oxide
formed increased with increasing weight loss or decreasing depth of internal oxidation. Results similar
to those found here have also been noted in alloys
of base composition 61.5 pct Ni, 16.5 pct Cr, 22 pct
Fe.
At the oxide-metal interface, oxygen diffuses from
the oxide into the base metal and metal ions pass
from the metal into the oxide. If the latter process
is relatively rapid compared to the former, the
movement of the interface into the metal will be
sufficiently rapid to prevent marked internal oxidation. If, however, following the reasoning of Hessenbruch and of Hickman and Gulbransen, it is assumed
that the addition of rare-earth or alkaline-earth
elements causes the formation of a barrier which
prevents ready passage of metal ions into or through
the oxide, oxygen will penetrate the base metal to
a much greater depth resulting in more internal
oxidation the more restricted the outward diffusion
of metal ions. The presence of such an internal oxide
will tend to key the oxide to the metal resulting in
less spalling on intermittent heating and cooling and
correspondingly less oxidation of metal. During continuous oxidation, on the other hand, where spalling
of the oxide, because of differential contraction on
cooling, is not a factor, internal oxidation will not
greatly affect the overall rate of oxidation.
The assumption of the formation of a barrier by
the addition of certain elements serves also to explain the com~ositionof the scale. If metal ions can
pass freely into the oxide scale, the gross composition of the latter will tend to approach the base
metal composition and to consist of NiO, spinels, and
Cr,O,. If, however, the passage of metal ions into the
oxide phase is restricted by some barrier, the diffusion of oxygen into the base metal will predominate;
the inwardly diffusing oxygen will react to precipitate the oxide phase most stable in contact with
metal of the composition here used (i.e., Cr,O,), and
on continued oxidation this phase will be incorporated into the external oxide. Thus alloys showing
pronounced internal oxidation may be expected to
exhibit scales consisting predominately of Cr?O,.
It thus appears that improvement of the oxidation
behavior in intermittent tests of nickel-chromium
and iron-nickel-chromium base alloys by the addition of small amounts of certain elements is achieved
by the tendency of these elements to promote internal oxidation and thus to prevent spalling of the
protective oxide scale during cycles of heating and
cooling.
References
' W. Hessenbruch: Metalle und Legierungen fur
Hohe Temperaturen. pt. 1. Zunder-feste Legierungen
,.,.,A,
c1Y4U).
'L.Horn: The Influence of Additions on the Oxida-
tion of Nickel and Chromium-Nickel Alloys, Ztsch. f ,
Mlcde. (1948) 73-76.
5.W. Hickman and E. A. Gulbransen: Trans. AIME
(1949) 180, 519-546; Metals Tech. (June 1948) TP
2372, 2391.
996--JOURNAL OF METALS, AUGUST 1950, TRANSACTIONS AIME, VOL. 188