Thin films of Na2SO4 and Na2CO3 at 1000 C lead to severe pitting of sintered alpha-SiC. These pits are important as they cause a strength reduction in this material. The growth of product layers is related to pit formation for the Na2CO3 case. The early reaction stages involve repeated oxidation and dissolution to form sodium silicate. This results in severe grain boundary attack. After this a porous silica layer forms between the sodium silicate melt and the SiC. The pores in this layer appear to act as paths for the melt to reach the SiC and create larger pits

Jacobson, N. S.

Smialek, J. L.

English

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NASA Technical Memorandum 87143
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Molten Salt Corrosion of SiC :
Pitting Mechanism
N.S. Jacobson and J.L. Smialek
Lewis Research Center
Cleveland, Ohio
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(NASA-TM-87143) MOLTEN SALT LOFFOSI ON OF
Sic: PITTING MECHANISM (WkSA)	 12 PCSCL 11G
HL A02/MF A01
N86-12310
Unclas
G3/27 04806
Prepared for the
One hundred sixty-eighth Meeting of the Electrochemical Society
Las Vegas, Nevada, October 13-18, 1985
a
i
MOLTEN SALT CORROSION OF SiC: PITTING MECHANISM
N.S. Jacobson and J.L. Smialek
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
Thin films of Na2SO 4
 and Na2CO3 at 1000 °C lead to severe pitting of sin-
tered a-SiC. These pits are important as they cause a strength reduction in
this material. The growth of product layers is related to pit formation for
the Na 2 CO3 case. The early reaction stages involve repeated oxidation and
°	 dissolution to form sodium silicate. This results in severe grain boundary
N	 attack. After this a porous silica layer forms between the sodium silicate
melt and the SiC. The pores in this layer appear to act as paths for the melt
to reach the SiC and create larger pits.
INTRODUCTION
Silicon carbide shows particular promise as a high temperature structural
material (ref. 1). In gas turbine, chemical reactor, and heat exchanger
applications, this material may be ex p osed to molten salts. Evidence has been
accumulating which indicates that SiC is quite susceptible to molten salt cor-
rosion (refs. 2 to 7). This paper summarizes some of our work in this area
and a l so presents some current ideas on corrosion pitting in SiC.
The starting material was a commercially a\3ilable sintered C,-SiC,* which
shows particular promise for future applications (ref. 8). This material con-
tains approximately 3 percent free carbon, in the form of inclusions a few
microns in diameter. Specimens were coated with Na 2CO 3 or Na 2 SO4 and placed
in a furnace at 1000 'C. Unless otherwise noted, all specimens were coated
with approximately 2.5 mg salt/cm2 . It has been shown that basic molten salts
readily attack SiC by dissolution of the protective silica scale to form sodium
silicate (ref. 4). Thin films of Na 2 CO3 with a flowing 0.01 percent CO2-02
atmosphere directly dissolve the 510 2 film and lead to corrosion. Thin films
of Na2SO 4 with flowing air behave similarly. Surprisingly, NanSO 4 with a flow-
ing 0.01 percent S0 3 -0 2 atmosphere--a more acidic system--also corrodes
this material. This is due to locally basic conditions at the Na2SO4/SiO2
interface, promoted by free carbon in the sintered a,-SiC.
In each of the three salt systems studied, the reaction occurs primarily
in the first 5 hr. Corrosion products consist of both 510 2
 and Na20•x(SiO2)
(ref. 5). The major feature of these reactions is that consumption of the SiC
substrate does not occur with a uniform material recession, but with a more
localized pitting attack. The corrosion products can be easily removed with a
10 percent HF/H 2 0 solution. This solution does not attack the SiC substrate
and thus allows examination of the corroded microstructure. Figure 1(a) is a
surface view before corrosion, figure 1(b) is a surface view after Na2CO3
*Carborundum Co., Niagara Falls, NY.
corrosion with the products removed by HF. Note the pitted structure with the
large crater-like pit in the center.
The surface and processing flaws in a ceramic have a major influence on
its strength. These flaws may be fracture origins--when the ceramic is
stressed they act like stress concentrators which result in failure strengths
inversely proportional to the size of the flaw. Thus, the surface corrosion
pits have a mayor effect on the strength of SIC (ref. 6). This is presented
in figure 2, which shows the strengths of the as-received SIC bars and the
strengths after corrosion by each of the :',ree salt systems. In order to pin
the strengtt, reductions to corrosion pi. 'ng, extensive fractography was done.
In most u ses the fracture origin was due to corrosion pits (ref. 6). Examples
are show y, in figures 3 and 4 - note the characteristic radial lines of the
fracture origin pointing toward the corrosion pit.
PITTING MECHANISM
The primary problem then is to understand how the corrosion pits form.
The focus here is on the Na 2 CO 3 /CO 2 system. This basic salt directly attacks
the S102 layer and the mechanism of Na 2CO3 corrosion of SIC is fairly well
understood (ref. 5). The goal here is to relate the mechanism of product for-
mation to mechanism of pit formation. Consider first the mechanism of product
formation. A kinetic curve at 1000 °C is shown in figure 5. This curve was
generated by chemical analysis of the corrosion products after various time
intervals. A water leach removes the silicates for analysis; an HF leach
removes the silica for analysis (ref. 4). The Na 2 CO3 is consumed in the first
0.25 hr of the reaction and is not shown in figure 5. The important point from
this kine"Ic curve is that the Na20 • x(SiO2) peaks in the first 0.25 hr. This
is very likely due to repeated oxidation and dissolution:
sic + 2 0 2 =Si02 +CO	 (1)
xS102
 + Na 2 CO 3 = Na 2 0 • x(SiO2) + CO 2	(2)
After the first 0.5 hr a stable S10 2 film forms on the SIC and a layered
sodium silicate/silica/silicon carbide structure develops. A water leach
removes the outer silicate layer and reveals the silica layer in its early
stages of development. This is shown in figure 6. The lower left hand corner
of this photomicrograph shows a region where the Si0 2 layer had spalled
revealing the pitted SIC substrate. Note the pores in the 5102 layer - which
appear to extend to the SIC. These pores probably form when CO and CO2 escape
from the oxidation of SIC and the C inclusions. Figure 5 shows that 5102
increases after 0.5 hr at a rate much faster than simple protective oxidation
(ref. 5). The pores in this layer may be oxygen paths for this nonprotective
oxidation.	 In addition, figure 5 suggests that Na 2 0•x(SiO 2 ) decomposes. This
may occur in the low oxygen regions of the melt as follows:
Na 2 0 •x(SiO 2 ) = x(Si0 2 ) + 2 Na + 2 0 2	(3)
Thus the silica forms both by nonprotective oxide growth and Na20•x(SiO2)
decomposition. After long times the lower silica layer becomes dense and quite
2
ire 7. This dense, protective silica layer seals
,..	 ....	 ..	 ...	 ...	 ck.
As the product layers become thicker, the pitting attack of the SiC sub-
strate becomes more severe. Figure 8 shows a time sequence of pit formation.
In each case the corrosion products were removed by an HF treatment. The
0.25 hr microstructure was formed by repeated oxidation and dissolution--it had
a layer of sodium silicate directly on it. The SiC substrate shows primarily
grain boundary attack, but also some intragranular attack which was very likely
initiated on crystal defect sites. This is simply an "etching type" attack--
where the higher energy sites are attacked first (ref. 9). Some of the small
pits in this structure are very likely due to oxidation of the carbon inclu-
sions in these pits. In some instances it appears the entire grain had been
pulled out. Note also the grain boundary film--this may be important in the
attack process. The 1 hr specimen had a layer of porous silica on it. This
specimen also shows extensive grain boundary attack and some intragranular
attack. However, now the pits are deeper ana have a more crater-like appear-
ance. Finally the 5 hr sample shows still larger craters. An overall view of
the specimen Shows the craters tend to be localized. These deep craters are
important because some may be large enough to act as fracture origins.
Comparing figure 6 to figures 8(b) and (c) suggest that the pores in the
lower silicon layer allow the melt to penetrate this layer and attack localized
areas. Figures 9 and 10 illustrate this pore-pit correlation more clearly.
This specimen was given a coating of 4.1 mg Na2CO3/c m 2 , which tended to produce
severe pitting along the edges after only 1 hr at 1000 °C. After corrosion,
the sample had a dense, glassy product layer. Removing the sodium silicate
with a water treatment revealed the pores shown in figures 9 and 10. Looking
down these pores indicates that they are directly above a pit in the SiC. In
both figures, the SIC grains can be seen in the bottom of a pit.
After 0.25 hr, the melt which penetrates these pores is primarily
Na?O • x(Si02 ). Nonetheless, as long as the local melt composition at the pit
bottom does not exceed the phase diagram liquidus at Na20 . 3.65(SiO2), (ref. 10)
the oxidation-dissolution process can continue to consume the SiC.	 Indeed,
thin films of Na20 . l(SiO 2 ) readily attack SiC at 1000 °C. 	 It should be noted
that not all the silica which forms is dissolved by the melt - some adds to the 	 ,.
thick silica layer shown in figure 7. Nonetheless evidence does suggest that
penetration points in this layer allow the melt to reach the carbide and create
localized, crater-like pits.
CONCLUSION
In summary, thin salt films of Na 2 CO 3 and Na 2 SO 4 cause severe corrosion
pitting of sintered o SIC at 1000 °C. These corrosion pits can act as fracture
origins and thus are responsible for corrosion attack strength decreases in
this material after corrosion. 	 In the Na 2 CO 3 case, the first 0.25 hr of reac-
tion are due to re p eated overall SIC oxidation and dissolution of the oxide to
sodium silicate. This leads to extensive grain boundary attack of the SIC sub-
strate. After 0.25 hr a porous silica layer forms between the SIC and sodium
silicate melt. This allows the melt access to the SIC in specific areas and
localized crater-like pits form.	 These larger pits are of the greatest con-
cern, because they can lead to a loss in strength of the SIC.
3
REFERENCES
1. H.B. Probst, Am. Ceram. Soc. Bull., 59, 206 (1980).
2. R.E. Tressler, M.D. Messer, aril T. Yonushonis, J. Am. Ceram. Soc., 59,
278 (1976).
3. M.K. Ferber, J. Ogle, V.J. Tennery, and T. Henson, J. Am. Ceram. Soc.,
68, 191 (1985).
4. N.S. Jacobson and J.L. Smialek, J. Am. Ceram. Soc., 68, 432 (1985).
5. N.S. Jacobson, J. Am. Ceram. Soc., in press, (1985).
6. J.L. Smialek and N.S. Jacobson, "Mechanism of Strength Degradation for Hot
Corrosion of cc-SIC," National Aeronautics and Space Administration,
Washington, D.C., NASA TM-87052 (1984).
7. N.S. Jacobson, C.A. Stearns, and J.L. Smialek, "Burner Rig Corrosion of
SIC at 1000 °C," National Aeronautics and Space Administration,
Washington, D.C., NASA 1M-87061, (1985).
B. C. Belleau, W.L. Ehlers, and F.A. Hagan, "Materials Review for Improved
Automotive Gas Turbine Engines," NASA CR-159673, 1978.
9. G.L. Kehl, "The Principles of Metallographic Laboratory Practice,"
pp. 60-79, McGraw-Hill, New York, (1949).
10. W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, "Introduction to Ceramics,"
p. 359, Wiley, New York, (1976).
4
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----^--
 
_ ^_ _-V
ORIGINAL PAGE IS
OF POOR QUALITY
Figure 1. - Sintaa a;tx- SiC tai i5pm surface finish prior to corrosion testing Ibl After 48 hrs Na 2CO31CO 2 corrosion
products removed with Hr.
w,
sal	
0
0
i 400	 0
0
300
W	 o	 0
O	 O
W
(Y O
UQ
z Ion r
-4g 	 -38
0 AS-RECEIVED	 Na2SO4 1S03	Na2CO3 1CO2	Na2SO41AIR
Figure I. - Effect of 1000 aC 148 hr hot corrosion on 4-pt bend
strength of a-SiC; samples ground to 15 w finish before
strength test.
C.
rti
tar•—
I=
V.
Figure 3. - Corrosion pit fracture origin after N8 2 SO41SO3 corrosion - -
products removed with HF (o f - 190 MPai.
Figure 4. - Corresion pit fracture origin after Na 2CG3 /CO2
 corrosion -
pfoducts removed with Hf i o f - 334 MPal.
IWI
1\
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7 ^
p
ORIGINAL PAGE 19
OF POOR QuAL4-ty
Si0 2 	 q
3	 "
2	
Na 20 x15'021J
E 1u
E ta' J_	 1-	 L	 J
n	 2n	 40	 60	 80	 l0n
Q	 TIME, hr
z
5F
i
4^K
3
0	 1	 2	 3	 4	 5
TIME, hr
Figure 5. - Product evolution for Na 2CO 33ICO 2 corroded
SiC (a' and enlargement of first 5 hr 1 1•
5
1 #'^^ t
Figure 6. -Lower silica after 0.5 hr (outer silicate layer removed with
waters.
9 n
1M
ORIGINAL P,;GE IS
'COP POOR QUALITY
0
Si MAP
Figure 17 .	 Crass section of Na ?CG31CO2 corroded SiC - 48 hrs.
^1
5 h r s !1'
	
W;
:ft
r j
err
	 ea
U
5 um
ior
s
^1
ORIGINAL PAGE I'll
OF POOR QUALITY
0.25 h^^	 l ^,-•
^,,..	 y
w4b
^5um
1 hr^
'
s
r ^
5 um
Figure 8. - lime sequence slirminy evolution of pits I corrosion products
removed by Hh.
9
'^ S
ORIGINAL PAGE E3
L' POOR QUALITY
Figure 9. - I nner silica layer over pitted SIC after 1 hr (outer silicate
layer removed with water. We pore: in silica extending to pits In SiC.
Fiqure 10. - Detail of specimen sheen in lig. 9, showing a pore In the
silica providing a path to a SIC pit.
NO
J.
722 Price,
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,t	 a; ?AV,	 41'
1 Report No.	 2 Government Accession No	 3 Recipient's Catalog No
NASA TM-87143
4. Title and Subtitle 	 5 Report Date
Molten Salt ., rosion of SiC: Pitting Mechanism
8 Performing Organization Code
533-05-12
7. Author(a)	 8. Performing Organization Report No
N.S. Jacobson and J.L. Smialek
	
E-2770
10 Work Unit No
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Centor
Cleveland, Ohio 44135
12 Sponsoring Agency Name and A Jdress
National Aeronautics and Space Administration
Washington, D.C. 20546
11. Contract or Grant No.
13 Type of Report and Period Covered
Technical Memorandum
14 Sponsoring Agency Code
15. Supplementary Notes
Prepared for the One hundred sixty-eighth Meeting of the Electrochemical Society,
Las Vegas, Nevada, October 13-18, 1985.
18. Abstract
Thin films of Na 2SO4 and Na2CO3 at 1000 °C lead to severe pitting of sintered
c,-SiC. These pits are important as they cause a strength reduction in this
material. The growth of product layers is related to pit formation for the
Na 2 CO3 case. The early reaction stages involve repeated oxidation and dis-
solution to form sodium silicate. This results in severe grain boundary attack.
After this a porous silica layer forms between the sodium silicate melt and the
SiC. The pores in this layer appear to act as paths for the melt to reach the
SiC and create larger pits.
:1
17 Key Words (Suggested by Author(s)) 	 18 Distribution Statement
	
Corrosion; Pitting; SiC; Molten salt 	 Unclassified - unlimited
i STAR Category 27
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Molten salt corrosion of SiC: Pitting mechanism

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