The effect of SO3 pressure in the gas phase on the Na2SO4 induced hot corrosion of Co-Cr, Ni-Cr, and Co-Cr-Al alloys was studied in the temperature range 700 to 750 C. The degradation of the Co-Cr and Ni-Cr alloys was found to be associated with the formation of liquid mixed sulfates (CoSO4-Na2SO4 or NiSO4-Na2SO4) which provided a selective dissolution of the Co or Ni and a subsequent sulfidation oxidation mode of attack which prevented the maintenance of a protective Cr2O3 film. A clear mechanism was not developed for the degradation of Co-Cr-Al alloys. A pitting corrosion morphology was induced by a number of different mechanisms

Chiang, K. T.

Meier, G. H.

English

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T.
I
r
I
1
r
W	 & 4^-
(NASA-Ch-159L69)	 NUT CkjhhOSiCN OF Co -Cr,
Co-Cc- A1, ANC Ni-Cr ALLCY!j iN 1NE
TEMPERATUhE 6ANGE OF 7CJ-750 DEG c
Semidnnsdl i(cporv, 1 Sep. 1 y 79 - 29 Feb.
1980 (Pittshur9h ilniv., L'.:.)	 1; E'
11
NdJ-Zo417
Uiicl
G.1/40 17du4
%mob
METALLURGICAL AND MATERIALS
ENGINEERING
University of Pittsburgh
Pittsburgh, Pennsylvania 15261
NASACR - 159689
June, 1980
.
HOT CORROSION OF Co-Cr, Co-Cr-A1,
AND Ni-Cr ALLOYS IN THE
TEMPERATURE RANGE OF 700 - 750°C
Third Semi-Annual Report
on
Grant No. NSG-3214
Prepared for
National Aeronautics and Space Administration
Lewis Research Center
by
K. T. Chiang
G. H. Meier
Department of Metallurgical and Materials Engineering
University of Pittsburgh
Pittsburgh, Pennsylvania 15261
Period Covered: September 1, 1979 - February 29, 1980
57
9
9
10
11
12f 
1.0 introduction
Coatings based on the C&-Cr-Al, system are now widely used in
wArine aircraft turbines. They are known to have excellent hot corro-
sion resistance at normal turbine operation temperatures (,400-100',°C).
However, they are subject to severe degradation at lower temperatures
(700-800•0 which can occur when the turbines are operating at low
power or atparticular locations on turbine components which normally
fall into this temperature range.
Considerable uncertainty still exists regarding the mechanisms of
this form of corrosion despite several careful investigations. The
corrosion morphology corresponding to low temperature hot corrosion
has been well characterized on components removed from marine ser-
vices tl ' 2) . The attack is primarily of a pitting type with the oxide
in the pits encriched in Cr and Al in alternating layers and depleted
in Co. A Co-rich oxide covers the pits. There is little or no sulfi-
dation in the alloy and generally no depletion of O-CoAl below the
oxide pit. A thin zone at the base of the pit is found to . contain S,
Al, and 0 in a phase (or phases) which are apparently not water soluble.
Small amounts of water soluble Co have also been observed on corroded
coatings and the oxide pits are permeated by Na2so4 which is also
water soluble. Barkalow and Pettit were able to reproduce this mor-
phology at 700°C by exposing CoCrAlY coatings with Na2so4 deposits to
oxygen containing SO  at pressures greater than 10-4 atm. The rate
of degradation was found to be proportional to the SO  pressure and
negligible in the absence of SO 3' These observations have been incor-
porated into the following mechanism by Pettit and co-workers(1,3).
The reaction of Coo and S03 forms CoSO4 which forms a low melting
point solution with Na2SO4 . The liquid salt then penetrates the
2Al203 scabs at cracks. Alternating layers of Al and Cr are formed by
selective-removal of Al from the alloy presumably by Ali-sulfite forma-
tion at the liquid/alloy interface where the p0 is low and then
2
reprecipitation as oxide where the pfl is higher. The Ai-depleted
2
alloy is converted to Cr203 to ;Form the Cr-rich region and Co which
diffuses to the liquid/gas interface forming Co,-oxide or sulfate. It
is suggested (1,3) that this mode of attack is not operative at high
temperatures because such higher 903 pressures are required to form
sulfates and sulfites.
Smeggil
(2)
v on the other hand, suggests that the corrosion morph-
_ 	 ology may be reproduced in the absence of S03 in the gas phase if the
coatings are exposed to brief thermal excursion and transient reducing
conditions and proposes this an an alternate mechanism. Laboratory hot
corrosion experiments in which specimens were periodically cycled to
1300°C for 30 seconds in carbonaceous material produced corrosion
morphologies similar to those observed in marine turbines and have pro-
duced changes in coating microstructure similar to those observed by
Smeggil in c_arine turbine components. Smeggil does not offer a mech-
anism for the degradation produced by the high temperature excursions.
Luthra and Shores (4) have studied the low temperature hot corro-
sion of Co-30Cr and Co-10A1 alloys between 600 and 750°C. They find
that Co-30Cr undergoes a pitting type of attack in the presence of
so 3' The pit contains Cr 203 and Na2so4 with a sulfur--rich band at
the alloy/scale interface and an external scale of Co304 or CoSO4,
depending on the S03 pressure. The Co-10A1 alloy showed uniform
attack rather than pitting but its corrosion morphology was otherwise
analogous to that for Co-30Cr. The mechanism proposed by Luthra and
Shores includes the formation of a liquid Na 2SO4-CoSO4 phase as a
i
introduction of SO  into gas phase results in pitting-type hot corro-
sion of both Co-27Cr and Co-18Cr-6A1 alloys giving corrosion morpholo-
gies similar to those described above. Higher SO  pressures were
required to form pits on the Co-Cr alloy and thermal cycling resulted
in pitting of both alloys at lower 803 pressures. The presence of
NaCl in the gas also produces a pitting type of attack in Na2SO4-
coated Co-Cr-AI which is somewhat similar to that produced by SO 3*
However, a major difference was a porous microstructure produced below
the pit by the formation of volatile chlorides of Al and Cr which were
transported outward through the pores and converted to oxide in
regions of higher p0 .
2
The objective of the research described in this report was to more
clearly define the effects of So 31 alloy composition, and alloy micro-
structure in the early stages of the Na2SO4-induced hot corrosion of
Co-and Ni-based alloys in the temperature range 700-750°C.
2.0 Experimental
The alloys studied were Co-27Cr, Ni-2OCr, and Co-18Cr-6A1*. The
alloys were tungsten arc melted under an argon atmosphere. Specimen
coupons were cut from the alloys, polished through 600 grit silicon
carbide and cleaned ultrasonically. Sodium sulfate coatings were applied
by spraying with aqueous solutions while the coupons were heated using
a hot plate and a 'heat lamp. Coating weights were usually 1 mglcm2
*All concentrations expressed in weight percent.
although some thinner coatings were also used.
The specimens were exposed in tube furnaces to oxygen at 1 atm.
The 803
 pressure in the gas was controlled by using 021SO2
 mixtures or
by passing 02
 through a permeation tube apparatus to introduce small
amounts of 802
 and passing the gas over a Pt catalyst to establish the
502/803 equilibrium.
The specimens` weight changes were determined by weighing them
before and after exposure. The oxidized specimens were studied using
optical and scanning electron metallography, EDAX, and X-ray diffraction .
The salt was washed from selected specimens and analysed by atomic
absorption spectroscopy at NASA Lewis Research Center.
3.0 Experimental Results and Discussion
3.1 Co - Cr and Ni - Cr Alloys
Figure 1 shows a cross-section through a pit typical of those
observed on binary Co-Cr alloys. This specimen was exposed with a 1
mg/cm2 coating of Na2SO4 for 48 hours at 150°C in oxygen in which the
PS03 was 5.8 x 10-3 atm. Table 1 indicates the presence of SO  caused
a 40-fold layer weight change for the alloy compared with 0 2 . The micro-
graph and X-ray maps indicate a thick external scale of cobalt oxide
covering a pit containing Cr203 with a sulfur-rich region at the pit
base. Some penetratior of fingerlike corrosion product is also evident
below the scale/alloy interface. Figure 2 shows more detail of these
protrusions below a similar pit. The sulfur map, Figure 2d, indicates
a significant concentration of sulfur in the protrusions. Point count
EDAX analysis indicates the matrix between the protrusions is partially
depleted of Cr. It, therefore, appears this region is depleted of Cr
by the formation of Cr203 in the pit and the formation of the protrusions
which are sulfides (and, perhaps, oxides) of Cr. (It must be pointed
as
out that the identification of the protrusions is tentative,)
The above results suggest a suifidation/oxidation mechanism to
be a major contributor to the pit formation in Co-Cr alloys, The
f
corrosion process is envisiged as follows. The high 503 pressure in
the gas phase results in the formation of a liquid Na2SO4-CoSO4 solu-
tion which locally dissolves the protective external Cr203 scale. (The
higher S03 pressure required to induce this mode of attack in Cr 203
-forming as compared with Al203-forming alloys is indicative of the lower
solubility of Cr203 in acid malts (6) .) The molten salt penetrates
below the scale. The conditions of low p
OZ	S2
and high p established
at this location result in the dissolution of Co which is transported
outward until it precipitates as Co oxide where the p0 is higher and
2
the formation of Cr sulfides. The combination of disruption produced
by the rapid dissolution of Co and localization of Cr in sulfides
prevents a continuous Cr203 scale from reforming.
ligure 3 shows severe corrosion at the corner of a Ni-20Cr specimen
exposed under the same conditions as the Co-Cr alloy in Figures 1 and 2.
The specimen has gained about ten times more weight than that exposed
in the absence of 50 3 , Table 1. In this case considerable quantities
of Cr sulfide are present at the scale-alloy interface. Figure 4 shows
a micrograph and X-ray images of the corrosion product on the same
specimen. Nickel may transport through a liquid Na 2SO4-NiSO4 melt
(There is an eutectic between the extended solid solution of NiSO4 in
Na2SO4 and NiSO4 ' Na2SO4 at about 670'C. 35% Xo/NiSO 4 .) and precipitate
near the gas interface. Chromium remains essentiallyas NiO o NiSO4 
below the initial alloy/salt interface and is oxidized in-situ. The
EDAX spectrum of Figure 3d verifies that the outer regions of the pit
are rich in Cr, Ni, and S. The result coupled with the micrograph,
Figure 3c, suggest this region consists of Cr203 and Ni-sulfide, e.g.
point A on the stability diagrams for the Cr-8-0 and Ni-S-0 systems,
Figures 5 and 6. These results suggest the sulfidation and subsequent
oxidation of Cr prevents a Cr303
 scale from forming in the attacked
region of this alloy also.
3.2 Co - Cr Al Allops
Figure 7 shows the morphology of a pit formed in Co-18Cr-6A1
exposed to 03 with ps0 0 2 x 10- etm. for 48 hours at 750'C with a
3
Na2SO4
 coating. The pit is rich in Al and Cr with a Co-rich external
layer and S-enrichment at the pit base. Water-soluble Co was found on
the surfac •
 of similar alloys, particularly after longer time under
thermal cycling conditions. An additional feature of this figure is
preferential attack of the S-CoAl phase. Figure 8 shows this feature
for an earlier stage of the pit formation . The B is preferentially
attacked and appears to provide a path for more rapid propogation of
the corrosion product into the alloy than is available through the solid
solution phase. It should t: pointed out that the round shape of the
pit in this figure may be the result of a sectioning effect through a
pit propogating in three dimensions. The more rapid attack along the
Ai-rich, Cr-free phase may be consistent with the greater solubility of
Al203 in acid melts as compared with Cr203(6).
The observations of the corrosion morphologies for the Co-Cr-A1
alloys do not lead to a clear mechanism. The sulfite mechanism pro-
posed by Pettit and co-workers (l ' 3) can explain the observed behavior.
However, thermochemical calculations for Na-sulfite indicate this com-
pound never appears as a stable compound at unit activity in the tempers-
tore range under consideration (7) . )ata are not available for A1-sulfite
and this compound has not been found in the literature. However, the
possibility of forming Al-sulfite at loss than unit activity in Na2SO4
Is a possibility and could allow the observed corrosion behavior to be
explained. More work is needed in clarifying this mechanism.
The sulfidation/oxidation mechanism proposed in the previous section
for Co Cr and Ni-Cr could also explain the behavior. However, sulfides
have not been observed at the pit bass in the present investigation.
This mechanism may be responsible for the pitting behavior observed by
Smeggil(2)
 when specimens were exposed under transient reducing condi-
tions in the absence of S0 2 . These conditions will cause the sulfur
potential in the salt to become very high and may well initiate pitting
corrosion via the sulfidstion/oxidation mechanism observed for binary
Co-Cr alloys.
Finally, it must be remarked that similar pitting morphologies may
be generated by a number of mechanisms. As noted earlier (5) , the
presence of NsC1 vapor can produce-
 of Na 2S0coated Co-Cr-A1
although additional features in the form of porosity arise in this case.
A sulfidation/oxidation mechanism is observed to operate in binary Co-Cr
alloys and pits may be produced in Co-Cr-AI both by high S0 3 pressures
and by transient reducing conditions. Also, as seen in Figure 9 a
Co-Cr-Al alloy may undergo pitting-type corrosion in the absence of a
salt dsposit{a)This specimen is a Co-16Cr-6A1-lHf alloy exposed at 1000'C
for 94 hours to a simulated coal gas in which p k 10 -15 atm. and p02	S2
k 2 x 10-6 atm. (per k 10 
15 
atm.). The major factor determining
3
whether or not pits form is the ability of the environment to continually
prevent the formation of a protective scale at localized sites. However,
the detailed features of the pits will generally be different for the
different mechanism (a.$. the thick Co-oxide outer scale did not occur
in Figure 9) and must be carefully evaluated before a mechanism can be
prepared with confidence.
of
SUMURY
The effect of S03
 pressure in the gas phase on the No2SO4-induced
hot corrosion of Co-Cr, Nip-Cr, and Co-Cr-A1 alloys has been studied in
the temperature range 700 - 750 0C. The degradation of the Co-Cr and
Ni-Cr alloys was found to be associated with the formation of liquid
mixed sulfates (CoSO4-N&2SO4
 or NiSO4-Na2SO4) which provided a selective
dissolution of the Co or Ni and a subsequent sulfidation/oxidation mode
of attack which prevented the maintenance of a protective Cr 203film.
A clear mechanism was not developed for the degradation of Co-Cr-A1
alloys. The sulfite model proposed by Pectit and coworkers or a modi-
fication of the above sulfidation/oxidation mechanism are both capable
of explaining the experimental results. Additional work is needed in
this area. Finally it was illustrated that a pitting - corrosion
morphology can be induced by a number of different mechanisms.
REFERENCES
1. R. H. Barkalow and r. S. Pettit, "On the Mechanism for Hot Corrosion
of CoCrAIY Coatings in brine Gas Turbines", paper presented at the
4th Conference on Gas Turbines in Marine Environments, Annapolis,
MD, June, 1979.
2. J. G. Smeggil, "Study of the Effects of Gaseous Environments on the
Hot Corrosion of Superalloy Materials", Quarterly Report to NASA
Lewis Research Center on Contract NAS3-21376, December, 1978.
3. C. S. Giggins and F. S. Pettit, "Hot Corrosion Degradation of Metals
and Alloys -- A Unified Theory", Pratt and Whitney Aircraft, Final
Report to Air Force Office of Scientific Research on Contract No.
F4462G-76-C-0123, June, 1979.
4. K. L. Luthra and D. A. Shores, "Morphology of Na2SO4-Induced Hot
Corrosion at 600-750'C". paper presented at 4th Conference on Gas
Turbines in Marine Environments. Annapolis, MD, June. 1979.
5. G. H. Meier and E. A. Gulbransen, "Hot Coriosivity of Coal Gasifica-
tion Products on Gas Turbine Alloys". Summary Report for the Period
15 April 1978 to 15 November 1979 on DOE Contract No. DE AC01-79-ET-
13547, Jan. 1980.
6. W. P. Stroud and R. A. Rapp, e"ft Solubilities of Cr203 and a-Al C
In Fusrd RaZ^06 at 120OK". in Lfth^e ra.. raturQ Metal t slide	 sir,
D. L. Rildenbrand and D. D. Cubricciotti, ads.. Electrochem. Soc.,
P. 374 (1978).
7. X. Z Chiang, 1. A. Gulbransen, F. S. Pettit, and C. R. War,
unpublished research.
B. C. X. Packer, Lockheed Palo Alto Research Laboratory, unpublished
research.
The authors are grateful to Prof. F. S. Pettit for many useful
discussions during the course of this study.
IQ	 3
TABLE 2: might . Chanoss (p&/c=2) After 48 sirs, at 750'C
Cis Composition 02
02 + 1000 pps $02
p50
	 6xi WO -A sts
3
02 + 1% S02
pSO	 6x10 3 atm3
Alloy NO
Salt
1 sg/cm2
Na2SO4
NO	
1
salt
	
Na 2SO4
NO
Salt
1 mg/cm2
ga2SO4
Co-20Cr 1.5 0.4 0.43
	
0.04 0.95 5.2
Co-27Cr 0.09 0.05 0	 -0.13 0.09 1.9
Cc-18Cr-6A1 0.05 -0.05 0.05	 -.14 0.04 -17.6(5)
Cc-18Cr-6A1-111f 0.1 0 0.13	 0.44 0.06 -20.95)
NI-2OCr 0.14 0.14
LO
	 0.04 0.09 2.3
(S): Stale spelled off during cooling to room temperature
1$
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Hot corrosion of Co-Cr, Co-Cr-Al, and Ni-Cr alloys in the temperature range of 700-750 deg C

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