The susceptibility to hot corrosion of four nickel base superalloys has been studied at 900 deg and 1000 deg C in one atmosphere of slowly flowing oxygen. Hot corrosion was induced by coating the samples with known doses of NaSO4 and oxidizing the coated samples isothermally on a sensitive microbalance. In general, the order of susceptibility found was:  B-1900 is greater than 713C is greater than NASA-TRW VIA and is greater than IN738. This order corresponds to the order of decreasing molybdenum content of the alloys. Chemical evidence for B-1900 indicates that hot corrosion is instigated by acid fluxing of the protective Al2O3 coating by MoO3

Fryburg, G. C.

Kohl, F. J.

Stearns, C. A.

English

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19770003241 2020-03-22T12:13:07+00:00Z
a^  p^»Z r3^9
Nov 19^ F °^
PtE.^	 ^vn
N PS^' ST\ eRPNCN '^,^
A
NA=A TECHN ICAL 	NASA TM X-73479MEMORANDUM
o^ti
X
Q
az
('NASA — TM — X-73479)
	 HOT CORROSION STUDIES OF
	 N77-10183
FOUR NICKEL— BASF SUPERALLOYS: B-1900,
NASA —TRW VIA, 713C AND IN738 (NASA)
	 11 p HC
A n,2/MF A01
	 CSCL 11F
	 Unclas
G3/26 08915
fiHOT CORROSION STUDIES OF FOUR NICKEL-BASE SUPERALLOYS:
B-1900, NASA-TRW VIA, 713C and IN738
by George C. Fryburg, Fred J. Kohl, and Carl A. Stearns
Lewis Research Center
Cleveland, Ohio 44135
TECHNICAL PAPER to be presented at
the Symposium on Properties of High Templerature Alloys
sponsored jointly by The Electrochemical Society and
The Metallurgical Society of the American Institute
of Mining, Metallurgical and Petroleum Engineers	 M
Las Vegas, Nevada, October 17-22, 1976
^r
^Zl Z
HOT CORROSION STUDIES OF FOUR NICKEL-BASE SUPERALLOYS:
B-1900, NASA-TRW VIA. 713C and IN738
George C. Fryburg, Fred j. Kohl, and Carl A. Stearns
National Areonautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
The susceptibility to hot corrosion of four nickel-
base superalloys has been studied at 9000 and 10000C in
one atmosphere of slowly flowing oxygen. Hot corrosion
was induced by coating the samples with known doses of
Na SO and oxidizing the coated samples isothermally on
a senitive microbalance. In general, the order of sus-
ceptibility found was: B-1900 3 713C > NASA-TRW VIA
and > IN738. This order corresponds to the order of
decreasing molybdenum content of the alloys. Chemical
evidence for B-1900 indicates that hot corrosion is
instigated by acid fluxing of the protective Al 203 coat-
ing by MoO3.
INTRODUCTION
The continually increasing performance demanded from gas tur-
bine engines requires ever increasing operating temperatures,
which in turn, requires alloys with increased high temperature
strength and corrosion resistance. Higher strength has been
achieved in the nickel-base alloys used in turbine engines by in-
creasing the amount of ?forming elements, aluminum and titanium.
This trend has provided alloys with good oxidation resistance
but with decreased resistance to hot corrosion.
Most nickel-base superalloys are susceptible to hot corrosion
attack to some extent. The hot corrosion phenomenon is known to
be very complex, depending on many factors: temperature, compo-
sition of alloy, cyclic conditions, impurities in and pressure 	 .9
of air ingested into engine, and impurities in the fuel burned
by the engine. Because of the complexity of the problem, re-
search has been concerned mainly with phenomenal.ogical testing of
alloys. Research directed toward an understandipg of the reaction
2mechanisms has been less prevalent, and as a result, a complete
fundamental characterization of hot-corrosion is still lacking.
An exception to this trend has been the work of Goebel, Pettit,
and Goward (Ref. 1) and of Bornstein, DeCrescente, and Roth
(Ref. 2).
As part of a larger program aimed at understanding the chem-
ical mechanisms involved in hot corrosion, we have conducted a
laboratory-test study involving isothermal, thermogravimetric
rate determinations of the hot corrosion of four superalloys:
NASA-TRW VIA, B-1900, Alloy 713C, and Iii-738. Hot corrosion
was induced by coating the samples with measured doses of Na SO .
Tests were conducted at 900 0 and 10000C in one atmosphere of
slowly flowing oxygen. As a basis for comparison, we have also
determined the corrosion rates in simple oxidation.
Hot corrosion of superalloys usually displays an induction
period before enhanced or catastrophic oxidation begins. This
is a period of little or no weight gain; in this paper, less than
0.3 mg cm-2 . The length of the induction period is a function
of the temperature, the composition of the alloy, the concentra-
tion of Na2SO4 , and the preoxidation treatment of the sample.
We have been investigating the processes operative during and just
after the induction period for samples of the four alloys that
were pSeoxidized for 100 hours. T;ie results obtained for B-1900
at 900 C with a coating of 3 mg cm-2 Na SO4 are discussed in terms
of the "acidic fluxing" mechanism of ho corrosion (Ref. 1).
EXPER IMEP. TAL
The alloy samples were obtained from commercial sources and
were given conventional heat treatments.
The samples were 0.2 x 1.0 x ?.5 cm with a hangdown hole
in one end. The surfaces were glass-bead blasted to give uniform
matte surfaces, and were cleaned ultrasonically in detergent,
acetone, and ethanol.
Hot corrosion was induced by coating the samples with
Na2SO4 prior to oxidation as described 2by Bornstein et al. (Ref. 3).
Two doses were used: 2/3 and 3 mg cm .
Isothermal tests were performed at 9000
 and 1000°C in slowly
flowing oxygen at one atmosphere pressure on both unoxidized
Y
ar^r.w^
'-,-RE ^- 4w
3
and preoxidized samples. The direction of flow of oxygen was
downward and the speed was 20 cm min -1 . Continuous gravimetric
measurements were made using a Cahn R-100 microbalance. Corrections
in the weight change were made for the buoyancy effect. Exposure
times extended in some cases to over 400 hours.
In the induction perPd studies, hot corrosion was induced
with coatings of 3 mg cm at 9000C using samples that had been
preoxidized for 100 hours at 900 0C. Samples were run for set
periods of time, and, after cooling, were water washed, usually
for 15 minutes in warm water. The resulting solutions were
analyzed for SO4- and metal cations and anions.
The morphology and composition of all oxide scales were
examined by Y.-ray diffraction, light microscope, electron micro-
probe, SEM, and ESCA.
RESULTS AND DISCUSSION
Straight Oxidation Studies. The oxide scales formed on complex
nickel-base superalloys are a complicated mixture of several in-
dividual oxides, the nature of which depends on many factors.
However, for simplification, the alloys are often classified as
11 alumina formers" or "chromia formers." The alumina formers
are characterized by a nonparabolic, nearly logarithmic rate
law with weight gains of the order of 0.2 to 0.5 mg cm	 The
scale is composed of a thin, continuous layer of Al 20 adjacent
to the metal substrate. The chromic formers are characterized
by a parabolic, or paralinpar, rate law with weight gains of
the order of 1 to 5 mg cm c after 100 hours at 9000 or 10000C.
The scale is characterized by a predominant Cr 0 layer, usually
with a region of internal oxidation of aluminum just beneath.
Our studies have shown that, in oxygen, B-1900 and VIA are
primarily alumina formers. while 713C and 738 are chromic formers.
This is unusual for 713C as it behaves as an alumina former when
oxidized in air (Ref. 4).
Comparative Hot Corrosion Studies.)  Hot corrosion runs were con-
ducted for all four alloys at 900 and 1000 0C with two doses of
Na2SO4 : 2/3 and 3 mg cm . 3oth unoxidized and preoxidized
1tF.PRODUCIBIT,' ry 0.,
aRYGTNAL PAGE IS Y
Ir
4(100 hrs. at temperature) samples were studied.
In Figures 1 and 2 12we present the results obtained at 900°C
for the 2/3 and 3 mg cm doses, respectively. The reproducibi-
lity of the data was roughly 115% for duplicate samples. The in-
duction period is not so obvious in Figures I and 2 because of
the relatively insensitive time scales. Nevertheless. the onset
of hot corrosion in all four alloys was preceded by an induction
period. The induction period was generally followed by a period
of ra pidly acce-erating weight gain that either consumed the
entire sample: catastrophic oxidation; or transformed into a
period of decelerating weight gain resulting in final cessation
of oxidation: enhanced oxida^ion. Generally, we found enhanced
oxidation with th^ 2/3 mg cm dose and catastrophic oxidation
with the 3 mg cm dose.
The order of decreasing susceptibility to hot corrosion
shown in Figures 1 and 2 is: B-1900;> 713C > VIA > 738, and thi°
order was generally the same for all our results regardless of
dose of Na2s04 or temperature. It should be noted that 713C, a
chromia former under the conditions of our experiments, is slightly
more susceptible to hot corrosion than is VIA, an alumina former.
In fact, the order of decreasing susceptibility is not the order
of increasing chromium content, brit is the order of decreasing
molybdenum content: B-1900, 3.6; 713C, 2.4; VIA, 1.3; 738, 1.0 (in
atomic percent).
It is noteworthy that 738 appears to be hot corrosion resis-
tant for a considerable time, but after 200 hours undergoes very
rapid catastrophic oxidation for the 3 mg cm dose (Fig. 2).
In effect, it appears to have a very long induction period.
Preoxidation of the samples for 100 hours at temperature
before hot corrosion hRd the effect of increasing the induction
period markedly at 900 C, but only slightly at 1000°C. For
example, the induction period at 900°for B-1900 was extended
from 1/2 to 6 hrs. f05 the 2/3
 mg em dose and from 21t ry 7I
hrs, for the 3 mg cm dose; for 738 at 900 C and 3 mg cm dose,
the induction 8eriod was greater than 450 hrs., the length of the
test. At 1000 C, the induction period is a much less important
feature of the hot corrosion curve being only 10 to 15 minutes
long.
0
5It was found that though the induction period is increased
by preoxidizing the samples, after the onset of hot corrosion,
the rate of corrosion is the same as for nonpr.eoxidized samples;
i.e., the hot corrosion curve is merely displaced in time and the
extent of corrosion is the same for either type sample.
The oxide scales formed during hot ocrrosion usually peeled
off upon cooling to room temperature. They were composed of all
the oxides of the metals in the alloy, NiO, of course, being the
most prevalent. One combination oxide, NiMo0 4 , was especially
prominent in all four alloys, but especially in B-1900 and 713C.
Interestingly, the oxide on VIA contained considerable Na(Ta, W)O3.
Induction Period Studies. Samples of B-1900 preoxidized at 9000C
for 100 hrs._^ere subjected to hot corrosion at 900 0C by coating
with 3 mg cm Na SO	 Runs were continued for set periods of
time as depicted ^y Ke arrows on the hot corrosion curve shown
in Figure 3. The induction period in these experiments was roughly
four hours. The actual value for individual samples varied, but
all samples were normalized to a four hour induction period by
matching the weight gain with the curve depicted in Figure 3.
The analytical results of the water soluble elements and SO4
obtained from washing the samples are presented in Figure 4. We
have plotted the amount of each element in milligrams against the
length of time the spzaple underwent hot corrosion. The amount of
SO4° and Na originally applied to the 2 samples 2 is given at zero
time. The total N8 2SO4 was: 3 mg cm x 7 cm = 21 mg.
The sodium is completely recovered in all samples within +the
recovery ability of the simple water-washing process; some Na
is imbedded in scale and is not dissolved. The SO 4- disappears
rapidly, just at the: end of the induction period. Soluble alumi-
num appears just after the induction period, indicating that the
layer of Al 033 on the alloy that protects it from oxidation has
been dissolved. The soluble aluminum disappears for longer time
periods indicating reformation of Al 0 , but not as an intact
layer. Soluble chromium, which was 0 included in Figure 4 for
sake of clarity, behaves in a fashion similar to aluminum, but
reaches measurable values earlier in the induction period attain-
ing a maximum of 2.2 mg. On the other hand, soluble nickel
(also not included in Figure 4) was not observed until after the
aluminum had maximized. It attained a vali.e of 2.5 mg which
CII^ OF
n?*pRO^A  pAGE IS P^
6tailed off gradually to a value of 1 mg.
The most striking feature of the results is the large amount
of soluble molybdenum that appears just at the end of the induction
period and beyond. At first thought, one might not expect soluble
molybdenum to exceed that required to tie up the available Na
as Na MoO4 . However, it must be remembered that Moo33 is soluble
in wa er to the extent of 2 g in 100 ml water at 70 e. This does
not detract from the fact that considerable molybdenum has been
oxidized in the early stages of the hot corrosion process.
Elemental X-ray micrographs of cross-sections of th se hot-
corroded samples were made. In Figure 5, we show the micrographs
for several elements in the oxide scale of the 4 1 hr. sample;
i.e., for a time just after the induction period, at the beginning
of the period of rapidly accelerating oxidation. The protective
Al 0 layer has been dissolved and aluminum is dispersed through-
oui ^he oxide scale. The most unusual feature of these data is
the position of the molybdenum concentrated along the oxide-alloy
interface. This segregation is prominent in long time samples also
and has been reported previously by Goebel, Pettit, and Goward
(Ref. 1) and by Bourhis and St. John (Ref. 5).
Mechanistic Implications. The segregation of molybdenum along the
alloy-oxide interface observed in the samples undergoing hot
corrosion coupled with the large quantity of molybdenum detected
in our analysis indicate that B-1900 hot corrodes by an "acidic
fluxing" mechanism (Ref. 1) in contrast to the "basic fluxing"
mechanism (Ref. 6). The results of our Induction Period Studies
can be explained by the following sequence of reactions.
Initially, the Cr 203 on our preoxidized samples is oxidized
to soluble CrO4-:
Cr203 + 3/2 02 + 2SO4_—)- 2Cr04 + 2S03	(1)
This reaction continaes throughout the induction period and con-
sumes at most a few milligrams of SO4
Concomittantly, Moo in the oxide is dissolving in the
Na2SO49 consuming it by de reaction
i
7Moo3 + SO4 =--;P- moo 4	 3
	
+ SO	 (2)
In addition, the quantity of Moo increases rapidly due to further
oxidation of molybdenum carbides 3lying in the surface oxide (Ref.
1).
By the end of the induction period, the Moo has locally
lowered the oxide ion activity in the melt to th^ point where
dissolution of Al203 by "acidic fluxing" can take place:
Al203 + 3Mo0 3--^ 2A1+3 + 3MOOI+
	(3)
At this point, the molten phase can creep beneath the Al 0 layer
and spread over the sample as observed by Goebel, Pettit, nd
Goward (Ref. 1) and by Bourhis and St. John (Ref. 5).
The nature of the molten phase at this time is probably a
mixture of Moo (mp: 795 C) and Na 2MoO4
 (mp: 6870C). Our
results indica^e that little or no molten Na SO 4 is present in
contrast to the hypothesis of Cloebel, Pettit2 and ',ward (Ref. 1).
It should be noted that the atomic weight of molybdenum is the
same as the molecular weight of SO 4 	Therefore, there is suffi-
cient oxidized molybdenum to consume all the sulfate by reaction
(2): Figure 4. Actually, we were not removing all the oxidized
molybdenum by our water washing technique (as evident by the
molybdenum in the micrographs), so the amount of molybdenum availa-
able for reaction (2) was probably considerably more than the
amount reported in Figure 4.
The self-sustaining feature of the hot corrosion reaction
arises from the presence of the molten phase, Moo (1) + Na 2 MoO 4(l)'beneath the oxide scale adjacent to the alloy sur &ce. The
oxidation propa tes by a process similar to that described by
Brenner (Ref. 7)for Fe-Cr-Mo and Fe-Ni-Mo alloys.
The self-sustaining feature may be compromised by the reaction:
	
moo  (1) + NiO --> N9MoO4	(4)
This process replaces the liquid Moo with a solid phase and ac-
counts for the prominence of NiMoO in all the hod, enrrecion
scales. For the small dose of rJa„'40, 4 (:/, mg cm- ') used in this
^ 
8study, ox^dation ceased due to reaction (4). For the large dose
(3 mg cm ), oxidation continued until the sample was consumed,
probably because Na Mo0 (1) formed a larger fraction of the mol-
ten phase, and removal of MoO (1) by reaction (4) was not as
critical as with the smaller hose.
ACKNOWLEDGEMENTS
The author; would like to thank Frank Terepka for obtaining
the microprobe data, and Judson Graab and Warren Davis for per-
forming the chemical analyses.
REFEa,ENCES
1. J. A. Goebel, F. S. Pettit, and G. W. Goward, Met. Trans. 4,
261 (1973).
2. N. S. Bornstein, M. A. DeCrescente, and H . A. Roth, Met.
Trans. 1t, 1799 (1973) .
3. N. S. Bornstein and M. A. DeCrescente, Met. Trans. 2, 1971
(1971).
4. C. A. Barrett, J. Santoro, and C. E. Lowell, NASA TN D-7484,
Nov. (1973).
5. Y. Bourhis and C. St. John, Oxid. Met. .2, 507 (1975)•
6. N. S. Bornstein, M. A. DeCrescente, and H. A. Roth, Proceed-
ings of the 1972 Tri-Service Conference on Corrosion, MCIC
73-19•
7. S. S. Brenner, J. Electrochem. Soc. 102, 16 (1955).
W,
It
*A-
100[
B -1900U	 ^
V
^	 VIA
d
50
r713
3
a	 138
0	 50	 100	 150	 200
TIME, HR
Fig. 1. - Comparative hot corrosion - 9000 C,
213 mg cm -2 Na2SO4.
1W
150
C	 100r
z	 j	 T - 9000 C
d	 3.0 MG CM -2 Na2SO4
3SAMPLES PREOXIDIZED
4
0	 20	 40	 60	 80	 100
TIME, HR
•	 Fig. 3. - Typical hot corrosion curve for B-1900
preoxidized samples.
N
U
U
d
7 -11
4
200
n
0	 100	 200	 300	 400
TIME, HR
Fig. 2. - Comparative hot corrosion - 900 0 C. 3 mg cm-2
Na2SO4.
18	
—o---N— —0F END OF
INDUCTION
161– PERIOD, 4 HR
14	 i	 P
12 't	 /	 0 ALUMINUM/	 n MOIYBDENUM
SULFATE
L	 10	 /1	 0 SODIUMI
8	
/
	
_	 1
6	 J'--0--_0_-
—ti--04 
^	 1
1^
0	 10	 20	 100
W4. HR
Fiq. 4. - Water soluble elements found after various
periods of hot corrosion - B-1900 preoxidized
samples. 9000 C, 3 mg cm- 2
 Na2SO4.
1
. a
P TTRODUCIBILRY UI' t
( )JUGANAI, PAGE IS PO:
k:
.—.-.....	
.mil ^ ..
..	 ...
	
...	 .._	 a	 ..	 3 	 ^ -
	 ^.
BACK-SCATTER ELECTRONS
!i
.s`
t+
'a
CHROMIUM
I
.	 I
1	 y
SULPHUR
MOIYRUFNIiM
Is
OXYGEN
	 At ININUNI
Figure 5. - Back scattered eler ren and elemental x-ray micrographs of hot corroded sample - B-I goo, preoxidired, %00 C.I mg cm -2 Na2 504, 4,5 hrs.
NASA•L, s
'j,tlUL'^l'llil l , l'1'Y l)1'
PAGE W POO'


Hot corrosion studies of four nickel-base superalloys: B-1900, NASA-TRW VIA, 713C and IN738

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server