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United States Patent m 

Jackson et al. 



tii] 4,117,179 

[45] Sep. 26, 1978 



[54] OXIDATION CORROSION RESISTANT 
SUPERALLOYS AND COATINGS 



[75] Inventors: Melvin R. Jackson; John R. Rairden, 

III, both of Schenectady, N.Y. 



[73] Assignee: General Electric Company, 

Schenectady, N.Y. 



[21] Appl. No.: 738,649 

[22] Filed: Nov. 4, 1976 

[51] Int. CI.2 B32B 15/04; B32B 15/20 

[52] U.S. CI 427/405; 427/250; 

427/252; 427/34; 427/423; 428/652; 428/653; 
428/662; 428/663; 428/667 

[58] Field of Search 427/405, 34, 249, 250; 

29/194, 195 C, 197; 75/170, 171; 428/652, 653, 

662, 663, 667 



[56] References Cited 

U.S. PATENT DOCUMENTS 

3,573,963 4/1971 Maxwell 427/405 X 

3,741,791 6/1973 Maxwell et al 427/250 

3,873,347 3/1975 Walker et al 427/250 

4,003,765 1/1977 Davidson 75/171 X 

FOREIGN PATENT DOCUMENTS 

710,749 6/1965 Canada 427/405 

Primary Examiner — Ralph S. Kendall 

Attorney, Agent, or Firm — F. Wesley Turner; Joseph T. 

Cohen; Charles T. Watts 

[57] ABSTRACT 

An article of manufacture having improved high tem- 
perature oxidation and corrosion resistance comprising: 
(a) a superalloy substrate containing a carbide reinforc- 
ing phase, and (b) a coating consisting of chromium, 
aluminum, carbon, at least one element selected from 
iron, cobalt or nickel, and optionally an element se- 
lected from yttrium or the rare earth elements. 

8 Claims, 3 Drawing Figures 



U.S. Patent sept. 26, 1978 



Sheet 1 of 2 



4,117,179 



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U.S. Patent Sept. 26, 1978 Sheet 2 of 2 



4,117,179 



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4,117,179 



10 



OXIDATION CORROSION RESISTANT 
SUPERALLOYS AND COATINGS 

The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautic and Space Act of 1958, Public Law 85-568 
(72 Stat. 435 42 USC 2457). 

BACKGROUND OF THE INVENTION 

The present invention relates to an article of manu- 
facture having improved high temperature oxidation 
and corrosion resistance comprising: (a) a superalloy 
substrate containing a carbide reinforcing phase, and (b) IS 
a coating consisting of chromium, aluminum, carbon, at 
least one element selected from iron, cobalt or nickel, 
and optionally an element selected from yttrium or the 
rare earth elements. Another embodiment of this inven- 
tion comprises an aluminized overcoating of the coated 20 
superalloy. Still another embodiment of this invention 
comprises the method of making the article of manufac- 
ture described herein. 



DESCRIPTION OF THE PRIOR ART 



25 



Carbide reinforced superalloys well-known to the art 
are employed widely in articles of manufacture em- 
ployed in gas turbine engines including those which 
power aircraft engines. The superalloys which are car- 
bide reinforced include conventionally cast, for exam- 30 
pie, nickel-base and cobalt-base superalloys, direction- 
ally solidified nickel-base and cobalt-base superalloys 
including eutectic alloys, as well as refractory alloys, 
etc. These alloys belong to a class of superstrength 
superalloys which rely on carbides for at least a portion 35 
of their overall strength. 

To further enhance the ability of superalloys in gas 
turbine applications, surface coatings generally are used 
to protect superalloy articles from deleterious high 
temperature oxidation, corrosion and erosion effects. 40 
Especially useful coating compositions (especially with 
directionally solidified eutectic compositions which 
have an aligned carbide reinforcing fibrous phase) are 
coating compositions consisting essentially of chro- 
mium, aluminum, at least one element selected from 45 
iron, cobalt or nickel, and optionally an element se- 
lected from yttrium or rare earth elements. Aluminiza- 



tion of the coatings further enhances the oxidation and 
corrosion resistance of the coated superalloy. 

Although the above-described prior art coated super- 
alloys have improved oxidation and corrosion resis- 
tance at elevated temperatures, including service tem- 
peratures where it is highly desirable to maintain the 
integrity of the substrates at temperatures approaching 
1 100* C, the prior art coated superalloys exhibit defi- 
ciencies in the form of a carbide depletion at the inter- 
face of the coating and the substrate as a result of diffu- 
sion of carbon from the substrate into the oxidation and 
corrosion resistant coatings. This undesired diffusion of 
carbon from the solid state chemistry of the substrate 
into the oxidation and corrosion resistant coatings sig- 
nificantly and deleteriously affects the phases which 
strengthen the superalloys. 

DESCRIPTION OF THE INVENTION 

This invention embodies an article of manufacture 
having improved high temperature oxidation and corro- 
sion resistance comprising: (a) superalloy substrate con- 
taining a carbide reinforcing phase, and (b) a coating 
consisting of chromium, aluminum, carbon, at least one 
element selected from iron, cobalt or iron, and option- 
ally an element selected from yttrium or rare earth 
elements. Another embodiment of this invention com- 
prises an aluminized overcoating of the coated superal- 
loy. Still 1 another embodiment comprises methods of 
preparing the aforesaid articles of manufacture. 

Broadly, any of the superalloy compositions included 
within the Compilation of Chemical Compositions and 
Rupture Strengths of Superalloys described in the 
ASTM data series publication no. DS9E, which include 
carbon within the alloy and rely on carbides for at least 
a portion of their reinforcing strengths, e.g. (1) carbide 
reinforcement of grain boundaries in (a) monocarbide 
form, commonly referred to as MC, and (b) chromium 
carbide forms, commonly referred to as M 2i C 6 and 
M7C3, (2) refractory metal carbides, etc., in platelet or 
fiber form strengthening grain interiors, aligned or non- 
aligned in accordance with the method of casting using 
conventional or directional solidification casting tech- 
niques, are included within the scope of our invention. 
Representative generally useful superalloys include 
nickel-base alloys, iron nickel-base alloys, cobalt-base 
alloys or refractory metal alloys of the compositions 
summarized in Table I which follows: 





„ 












TABLE I 




























Nominal Composition, Weight 


%, 














Alloy(s) 


C 


Mn 


Si 


Cr 


Ni 


Co 


Mo 


w 


Cb 


Ti 


Al 


B 


Zr 


Fe 


Other 




Nickel-Base Alloys 


































IN-739 


0.17 


0.2 


0.3 


16 


Bal 


8.5 


1.75 


2.6 


.9 


3.4 


3.4 


.01 


0.10 


0.5 


1.75Ta 




MAR-M200(a) 


0.15 


— ■ 


■ — 


9.0 


Bal 


10 


■ — 


12.5 


1.0 


2.0 


5.0 


0.015 


0.05 • 


' — 


— 




NX-188(aXb) 


0.04 


— 


— 


— 


Bal 


— 


18 


— 


— ' 


T- 


8 


..■ — 


■ -*-.■ . 


— 


— 




Rene 80 


0.17 


— 


— 


14 


Bal 


9.5 


4.0 


4.0 


— 


5.0 


3.0 


0.015 


0.03 


— 


— 




Rene 95 


0.15 


— 


— 


14 


Bal 


8.0 


3.5 


3.5 


3.5 


2:5 


3.5 
0.01 


0.05 


— 


— 






TAZ-8B(aXb) 


0.125 


— 


— 


6.0 


Bal 


5.0 


.4.0 


4i0 


1.5 


— 


6.0 


0.004 


1.0 


— 


8.0Ta 




TRW VI A(a) 


0.13 


— 


— 


6 


Bal 


7.5 


2.0 


5.8 


0.5 


1.0 


5.4 


0.02 


0.13 


— 


9.0Ta,0.5Re, 
































0.43Hf 




WAZ-20(aXb) 


0.15 


— 


— 


— 


Bal 


— 


— 


18.5 


— 


— 


6.2 


— 


1.5 


— 


— 




Iron-Nickel-Base Alloys 
































Incoloy 802 


0.35 


0.75 


0.38 


21 


32.5 


— 


— 


— 


— 


— 


_ 


— 





Bal 







S-590 


0.43 


1.25 


0.40 


20.5 


20 


20 


4.0 


4.0 


4.0 


— 


— 


— 


— 


Bal 




— 


Duraloy 


































"HOM-3"(b) 


0.50 


0.80 


1.0 


25.5 


45.5 


3.25 


3.25 


3.25 


— 


— 


— 


— 


— 


Bal 




— 


Cobalt-Base Alloys 














.■:■ 


















FSX-414(a) 


0.25 


1.0(c) 


1.0(c) 


29.5 


10.5 


Bal 


— 


7.0 


- — 


— 


— 


0.012 


— 


2.0(c) 







FSX-430(a) 


0.40 


— 


— 


29.5 


10.0 


Bal 


— 


7.5 


— 


— 


' — 


0.027 


0.9 






0.5Y 


MAR-M509(a) 


0.60 


0.10(c) 


0.10(c) 


21.5 


10 


Bal 


— 


7.0 


— ■ 


0.2 


■ — 


0.010(c) 


0.50 


1.0 


: 


3,5Ta 


X-45(a) 


0.25 


1.0(c) 


— 


25.5 


10.5 


Bal 


— 


7.0. 


■ — 


— 


— 


0.010 


— 


2.0(c) 




— 



4,117,179 



TABLE I-continued 



Alloy(s) 



Nominal Composition, Weight % 



Mn 



Si 



Cr 



Ni Co Mo 



W 



Cb Ti Al 



Zr 



Fe Other 



Refractory Metal Alloys 
WC3015 0.3 

CM32M 0.1 

SU31 0.12 

TZC 0.15 



— 0.03 — — 



— 


15 


Bal 


— — 





1 


a — 


30Hf 


5 


15 


Bal 


_ — 


— 


1.5 


— 


20Ta 


— 


17 


Bal 


— — 


— 


_ 


— ■ 


3.5Hf 


Bal 


— 


— 


1.25 — 


— 


0.3 


— 


— 



(a) Cast alloy 

(b) Directionally solidified 

(c) Maximum composition 

The coating compositions consist essentially of chro- 
mium, aluminum, carbon, at least one element selected 

from iron, cobalt or nickel, and optionally an element 15 ingredients 
selected from yttrium or the rare earth elements. The 
coating compositions can be described by the formulas: 



TABLE II-cbntinued 



General 



Preferred 



More 
Preferred 



Most 
Preferred 



iron ^ 
cobalt \ 
nickel / 



Bal 



Bal 



Bal 



Bal 



MCaAlC or MCrAlCY, 



20 



in which M is base metal element, e.g. iron, cobalt or 
nickel. Any amount of base metal element, chromium, 
aluminum, and optionally yttrium or a rare earth ele- 
ment can be employed in accordance with the amounts 
well-known to those skilled in the art with regard to 25 
oxidation and corrosion resistant coatings containing 
the aforesaid elements subject to the proviso that the 
coatings contain an amount of carbon (1) sufficient to 
saturate the solid state phases of the coating composi- 
tion, (2) sufficient to essentially equilibrate the chemical 30 
potential of carbon in the coating with that in the sub- 
strate with minimum interaction, and (3) insufficient to 
form substantial quantities of carbides in the coating 
composition. The function of the carbon in the coating 
is to avoid denudation of the carbide reinforcement in 35 
the substrate which has been found to occur very rap- 
idly at service temperatures equal to or greater than 
1100° C, during periods of time in the order of magni- 
tude of 1-3 hours. Denudation will occur at lower tem- 
peratures over longer time exposures. Those skilled in 40 
the art by means of routine experimentation will be able 
to determine the amount of carbon required in the coat- 
ing composition in order to avpid any change in the 
superalloy substrate chemical structure due to diffusion 
of carbon contained within the substrate into a carbon 45 
free MCrAL or MCrAlY coating. The discovery that 
the addition of nominal amounts of carbon to prior art 
coatings generally known in the art as MCrAlY coat- 
ings as an effective means of providing carbide stabi- 
lized oxidation and corrosion resistant coating compost- 50 
tions for carbide reinforced superalloy substrates is 
unexpected since at service temperatures of about 1 100° 
C. — prior to testing of the coating of this invention — 
we believed that carbon would likely diffuuse not only 
from the substrate into the coating but also through the 55 
coating into the coating atmosphere with subsequent 
continuous oxidation of carbon at the coating atmo- 
sphere interface. 

In general, presently preferred carbon stabilized 
MCrAlY coatings are of the compositions in weight 60 
percentages set out in the following table: 

TABLE II 



Ingredients 



More Most 

General Preferred Preferred Preferred 65 



chromium 


10-50 


10-30 


15-25 


19-21 


aluminum 


0-20 


2-15 


4-11 


4-11 


carbon 


0.01-0.5 


0.01-0.2 


0.05-0.15 


0.05-0.15 


yttrium 


0-1.5 


0-1.5 


0-1.5 


0.05-0.25 



The preferred aluminum content depends strongly on 
whether a duplex aluminizing treatment is to be given to 
the coated superalloy substrate. The carbon-saturated 
MCrAlY coating of our invention can be applied to the 
superalloy substrates by any means whereby carbon 
contained within the MCrAlY coating is uniformly 
distributed throughout the coating or localized in the 
coating adjacent to the superalloy interface surface, 
subject to the proviso that the carbon content of the 
coating be sufficient to completely saturate all of the 
MCrAlY phases with carbon, however, insufficient to 
form excessive amounts of carbides within the coating 
composition which deleteriously affect the oxidation 
and corrosion resistance of the coating under superalloy 
service conditions. 

In general, the carbon saturated MCrAlY coatings 
can be applied by any means such as (1) Physical Vapor 
Deposition (subject to the proviso that the carbon be 
deposited from a separate carbon source since carbon, 
which has a very low vapor pressure, if contained in the 
MCrAlY melt source would not be transferred to the 
superalloy substrate), (2) Chemical Vapor Deposition 
wherein organometallic compounds are employed 
wherein during decomposition of the organometallic 
compounds the carbon residue incorporated into the 
coating is present in amounts sufficient to saturate all 
phases of the coating, and (3) Carburization wherein the 
MCrAlY coating is saturated with carbon by pack car- 
burizing or gas carburizing the PVD coating in an at- 
mosphere containing carbon such as an atmosphere of 
carbon monoxide or carbon dioxide, etc. A preferred 
method of preparing the coated superalloy substrates of 
our invention employs a flame spraying procedure 
wherein an alloy wire or powder of a carbon saturated 
MCrAlY composition is deposited on a superalloy sur- 
face. Flame spraying or arc plasma spray deposition 
involves projecting liquid droplets onto a superalloy 
substrate by means of a high velocity gas stream. To 
minimize the oxygen content of the coating, deposition 
is often done in an inert atmosphere such as argon or 
vacuum. In general, methods which can be employed 
are well known to those skilled in the art and are de- 
scribed in the following publications: 
Flame Spray Handbook, Volume III, by H. S. Ing- 
ham and A. P. Shepard, published by Metco, Inc., 
Westbury, Long Island, N.Y. (1965), and 
Vapor Deposition, edited by C. F. Powell, J. H. 
Oxley and J. M. Blocher, Jr., puslished by John 
Wiley & Sons, Inc., New York (1966). 



EXAMPLE I 



4,117,179 

As mentioned hereinbefore, the carbon saturated 
MCrAlY coated article of this invention can be further 

improved in oxidation and corrosion resistance by alu- Pins of NiTaC-13 were electro-discharged machined 

minizing the MCrAlY coated substrate by any method from directionally solidified NiTaC-13 ingots which 

known to those skilled in the art, including Physical 5 had been melted with a radio frequency graphite sus- 

Vapor Deposition procedures described in detail in ceptor system and solidified at 0.635 centimeters per 

Vapor Deposition, edited by C. F. Powell et al., John hour - Prior t0 deposition of the coating the pin speci- 

Wiley & Sons New York (1966). mens were centerless ground and lightly abraded with 

Our invention is more clearly understood from the alumina powder. The NiTaC-13 pin samples were 4.4 

following description taken in conjunction with the 10 centimeters long and 0.25 centimeters in diameter. The 

accompanying figures described hereafter. TaC flber direction was along the axis of the pin speci- 

FIG. 1 is a photomicrograph of a transverse section mens. . . , 
(a) and a longitudinal section (b) of a photomicrograph . In g°j, s of carbon-containing and noncarbon-contain- 
of a directionally solidified nickel-base superalloy eu- ,. m S MCrAlY coating source alloys were prepared by 
» *• , • l4 . •*• r ,, .15 mduction melting high-purity metals in a low-pressure, 
tectic havmg a melt composition on a weight percent .... 6 . B r . ..,_ , . * .. r 
basis of Ni-3 3Co-4.4Cr-3 1W-5.4Al-5.6V-6 2Re-8.lTa- ^noxidizing environment wrth subsequent casting of 
_„._ _, , ^ . , ■ . -tr i r„tuw\ the alloys in an argon atmosphere. 1 he alloys contain- 
054C. The photomicrograph section magnified (400X) . ^^ wefe ^ s d ^ a33 centime ter S diame- 
shows an ahgned monocarb.de microstructure fiber fe f wife for flame { * s Fof dectron beam 
formed dunng solidification comprising tantalum and 2Q d ition of carbo „.f r ee coatings, two 0.25 cm. diame- 
vanadium carbides <Ta,V)C which can be identified as ter pin specimens were mounted a pp roxima tely 10 cen- 
the darkest phase shown in the photomicrographs of timeters from the deposition source and were rotated at 
both the transverse and longitudinal sections. The car- approximately 10 rpm during deposition of coatings, 
bide fibers are approximately 1 urn in cross section and Specimens coated using flame-spraying techniques 
comprise 2-4 volume percent of the microstructure. A 25 wer e mounted approximately 15 centimeters from the 
face-centered-cubic ordered structure based on Ni 3 Al, carbon bearing wire spray source and were rotated at 
y', is present in the structure but cannot be seen in the approximately 200 rpm during deposition, 
unetched sample shown in FIG. 1. For purposes of The coating composition for the electron beam coat- 
brevity hereafter, the alloy melt composition described ing employed a nickel-20 chromium-10 aluminum-1 
is hereafter referred to as NiTaC-13. 30 yttrium source which deposited a composition of nick- 

FIG. 2 is a photomicrograph (200X) of a NiTaC-13 el-20 chromium-10 aluminum approximately 0.1 yttrium 

alloy which had been coated, on a weight percent basis, coating on the superalloy substrate. The flame spraying 

with a carbon free nickel-20 chromium-10 aluminum- 1.0 source alloy contained nickel-20 chromium-5 alumi- 

yttrium composition having an initial coating about 75 num-0. 1 yttrium-0. 1 carbon and was used for 

urn in thickness. FIG. 2(a) is the NiTaC-13 coated com- 35 MCrAlCY coating of the superalloy substrate. The 

position machined to remove approximately one-half of MCrAlCY coated pins were subsequently aluminized 

the coating over a section 0.3 centimeters long of the by duplex coating techniques employing pack-alumini- 

FIG. 2(6) 75 fun coating, thereby reducing it to a thick- zation in a 1% aluminum pack at 1060° C. for 3 hours in 

ness of about 25 urn. The photomicrographs illustrate dry argon. Sufficient aluminum-aluminum oxide (Al- 

that after 119 hours of cyclic oxidation exposure at 4° 2O3) mixed powder was used to produce approximately 

1 100° C. the coated regions having about a 75 urn thick- 6 milligrams per square centimeter of aluminum deposi- 

ness exhibit approximately twice the carbide fiber denu- tion during the pack cementation process, 

dation as the composition having a coating thickness of Following cyclic oxidation as described hereinbe- 

about 25 fim. This figure illustrates that the coating acts fore > the test specimens were evaluated by metallo- 

as a sink for carbon since the 75 (im thick coating shows 45 S ra P mc techniques. The results are recorded in FIGS 2 

approximately twice the fiber denudation as the 25 ^m and | described hereinbefore. As illustrated by this 

thick coatine specific example as well as the photomicrographs, car- 

FIG. 3 is a photomicrograph (600X) of a longitudinal bon saturation of oxidation and corrosbn resistant coat- 
section of the alloy of FIGS. 1 and 2 which has been <n m S s ; commonly referred to as MCrAlY coatings, effec- 
. , .., / . . , , . 50 tively substantially eliminates carbon depletion or denu- 
coated with a carbon saturated composition having a j t - r vj • r j « 1. * * -m.- 

. , , v , , . - . , dation of carbide reinforced superalloy substrates. This 

coating composition, on a weight percent basis, of mck- carbide stabiUzation effect si / mflcantly enhances the 

el-20 chromium-5 aluminum-Ol carbon-0.1 yttrium and retention rf hages m ^ ^ reS p 0nsib le for the 

subsequently aluminized. FIG. 3(a) is a longitudma h ical stre h rties which are essentia i t0 

cross-section of the as-deposited coating FIGS. 3(b), (c) 55 turbine en ^ ne articles of manufacture having service 

and (d) are longitudinal sections of cyclically oxidized tem peratures in the range of 1 100° to 1 160° C. or even 

coatings after 1000 hrs., 1500 hrs. and 2000 hrs., respec- higher In view of the signi f lcance of retaining the alloy 

tively. Cyclic oxidation consisted of one hour cycles chem istry during the expected life of the alloy sub- 

wherem the coated alloy test specimens were exposed strates, especially with regard to superailoys which are 

50 minutes at 1100° C. in a static air furnace and 10 go employed as thin-section superalloy components in jet 

minutes at 93° C. in a forced-air cooler. The cross sec- engine designs, it is anticipated that the inclusion of 

tions of the carbon containing aluminized coatings and carbon in amounts sufficient to saturate all phases of the 

substrate illustrate that there is no carbon denudation as coating may increase the service life of the superalloy 

a result of introducing a sufficient amount of carbon to substrate by as much as 100 percent over the service life 

the MCrAlY coating to provide carbon in an amount 65 which would be obtained in the absence of carbon in the 

sufficient to saturate the phases of the MCrAlY coating. coating compositions. 

Our invention is further illustrated by the following Although the above examples have illustrated various 

example: modifications and changes that can be made in carrying 



4,117,179 



8 



out our process, it will be apparent to those skilled in the 
art that other changes and modifications can be made in 
the particular embodiments of the invention described 
which are within the full intended scope of the inven- 
tion as defined by the appended claims. 
We claim: 

1. A method of improving the high temperature oxi- 
dation and corrosion resistance and preventing loss of 
strength of a carbide containing superalloy body, said 
body containing a carbide reinforcing phase, compris- 
ing steps of: (a) coating the superalloy body with a 
composition consisting essentially of chromium, alumi- 
num, carbon and at least one element selected from iron, 
cobalt, or nickel, subject to the proviso that the coatings 
contain an amount of carbon (1) sufficient to saturate 
any solid state phases of the coating composition, (2) 
sufficient to essentially equilibrate the chemical poten- 
tial of carbon in the coating with that in the substrate 
with minimum interaction, and (3) insufficient to form 
substantial quantities of carbides in the coating composi- 
tion. 

2. The claim 1 method, wherein the coating contains 
an element selected from ytrrium or the rare earth ele- 
ments. 

3. The claim 2 method, further comprising: (b) sub- 
jecting the coated body to an aluminizing overcoating 
to further increase the oxidation and corrosion resis- 
tance of the coating. 

4. The claim 1 method, wherein the superalloy body 
is selected from a wrought, conventionally cast, direc- 
tionally solidified or powder formed nickel or a cobalt- 
base superalloy body. 



10 



15 



20 



25 



30 



5. The claim 1 method, wherein the superalloy is a 
directionally solidified multivariant eutectic comprising 
a matrix of nickel or cobalt-base superalloy body, the 
matrix being an aligned eutectic carbide reinforcing 
phase. 

6. The claim 5 method, wherein the eutectic carbide 
reinforcing phase is selected from carbides of the group 
consisting of tantalum and vanadium and their alloys 
and mixture thereof embedded in the matrix. 

7. The claim 1 method wherein the superalloy body 
and the coating have initially essentially the same car- 
bon chemical potential. 

8. A method of improving the high temperature oxi- 
dation and corrosion resistance and preventing loss of 
strength of a carbide containing superalloy body, said 
body containing a carbide reinforcing phase, compris- 
ing steps of: (a) coating the superalloy body with a 
composition consisting essentially of chromium, alumi- 
num, carbon and at least one element selected from iron, 
cobalt, or nickel, subject to the proviso that the coatings 
contain an amount of carbon (1) sufficient to saturate 
any solid state phases of the coating composition, (2) 
sufficient to essentially equilibrate the chemical poten- 
tial of carbon in the coating with that in the substrate 
with minimum interaction, and (3) insufficient to form 
substantial quantities of carbides in the coating composi- 
tion, and (3) insufficient to form substantial quantities of 
carbides in the coating compositions; and (b) subjecting 
the coated body to an aluminizing overcoating to fur- 
ther increase the oxidation and corrosion resistance of 
the coating. 



35 



40 



45 



50 



55 



60 



65