Note: Descriptions are shown in the official language in which they were submitted.
1 15807S
BACKGROUND OF T~IE INVENTION
The present lnvention relates to alloys and more
particularly to nickel, cobalt or nic~el~cobalt coating
alloys having improved hot corrosion resistance.
It is known that the modern day jet engine super-
alloys are susceptible to oxidation-erosion and hot
corrosion at very high temperatures and that it is the
usual practice to coat the superalloys with a composition
different from and more oxidation-erosion and corrosion
resistant than the substrate alloy.
In general, there are two primary types of coatin~s:
(1) aluminide coatings, such as those described in the
U.S. patent to Joseph, No. 3,102,044 or the U.S. patents to
Bungardt et al, ~o. 3,677,789 and No . 3,692,554 wherein
aluminides are formed by a reaction with, or dlffusion of a
coating on, the substrate surface, and (2) overlay coatings
such as those of the MCrAlY type, e.g. NiCrAlY described in
the U.S. patent to Goward et al, No . 3,754,903, CoCrAlY des-
cribed in the U.S. patent to Evans` et al, ~o. 3,676,085,
NiCoCrAlY described in the U.S. patent to Hecht et al, No .
3,928,026 and FeCrAlY described in the U.S. patent to Talboom,
Jr. et al, No. 3,542,530. Particularly useful overlay MCrAlY
coatings are those consisting essentially of, by weight, approx-
imately 8-30 percent chromium, 5-15 percent aluminum, up to 1
percent reactive metal selected from the group consisting of
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yttrium~ scandium, thorlum and lanthanum and the other
rare earth elements, balance selected ~rom the group
consisting of nickel, cobalt and nickel-cobalt, preerably
applied to a thickness o~ approxlmately .005-.006 inch.
In contrast to the overlay coatings, the diffusion
aluminide coatings are typically provided by reacting
aluminum with the deoxidized surface of the article to
be protected -- the aluminide layer being formed as a
barrier zone of vary~ng component concentration with
consumption of the substrate components. This aluminide
layer in turn oxidizes to orm the inert barrier oxide.
In the Bungardt et al patents, a separate layer of metal
from the platinum group is applied before the aluminum
diffusion treatment. However, because of the complex
nature of most of the contemporary alloys, and because
the coating composition thereon is derived in part from
the components of the substrate alloys, it is difficult
to control the coating composition so as to cause the
formation of a suitable barrier oxide. In addition, it
is inherent in the diffusion technique that the coating
formed is nonhomogeneous and, with respect to platinum
group metal content for example, there appears a high
concentration of the platinum group metal on the surface.
The existence of such a gradient, of course, is dis-
advantageous since, with use, the coating diminishes in
effectiveness as its composition changes.
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Although the prior art coating compositions have
represented improvements over various of their predecessor
alloy compositions, the need for ~urther improvements~
particularly for example, in hot corrosion resistance,
has remained.
SU~ OF THE INVENTION
The present invention contemplates alloy compositions
and more particularly nickel, cobalt and nickel-cobalt
coating alloy compositions having improved hot corrosion
resistance. In particular, the present invention
contemplates an improved MCrAlY type allvy coating
composition consisting essentially of, by weight,
approximately 8-30 percent chromium, 5-15 percent aluminum,
up to 1 percent reactive metal selected from the group
consisting of yttrium, scandLum, thorium and the other
rare earth elements, and 3-12 percent of a noble metal
selected from the group consi~sting of platinum;or
rhodium, the balance being selected rom the group
consisting of nickel, cobalt and nickel-cobalt. As will
be appreciated, the inclusion of the noble metal as an
alloying ingredient results in a substantially uniform
dispe~sion thereo throughout the composition and thus
retains the homogeneity which is characteristic of
MCrAlY type overlay coatings.
In a preferred embodiment, the reactive metal is
yttrium and the noble metal is S-10 percent platinum.
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In anothex embodiment, the reactive metal is yttrium
and the noble me~al is 5 percent rhodium.
BRIEF DESCRIPTION OF THE DRAWINGS
An understanding of the invention will become more
apparent to those skilled in the art by reference to
the following detailed description when viewed in light
of the accompanying drawings, wherein:
Figure 1 is a graph depicting the sulfidation
behavior of various NiCr~l alloys at l,000C;
Figures 2 and 3 are graphs depicting the oxidation
behavior of various NiCrAl alloys at 1,100C and 1,200C
respectively, in air;
Figure 4 is a graph showing the hot-,,corrosion
behavior of various CoCrAlY and NiCrAlY alloys at 1,750F -
2.0 mg cm 2 Na2SO4; and
Figure 5 is a graph showing the hot corrosion
behavior of NiCrAlY alloys at l,750F - 0,5 mg cm 2 Na25O~.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
_
~The alloys of the present invention exhibit markedly
improved hot corroaion r~esistance and are considered
particularly useful as coatings on the contemporary
superalloys. The inventive alloys are in themselves
corrosion resistant and do not depend ~or their protective
effect upon a~reaction with the~substrate material.
In addition, these alloys are uniform throughout their
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thickness and thus will exhibit their protective
properties more continuously and consistently than do
the aluminide coatings.
The desired results are obtained with a basic alloy
containing approximately, by weight, 8-30 percent
chromium, 5-15 percent aluminum, 5-10 percent platinum
or rhodium, up to 1 percent reactive metal selected
from the group consisting of yttrium, scandium, thorium
and lanthanum and the other rare earth elements,
balance nickel and/or cobalt. A preferred alloy composi-
tion utilizes 0.5 percent yttrium and 5-10 percent
platinum.
It was surprising to find that the addition, as
alloying ingredients, of specified amounts of platinum
or rhodium to the MCrAlY type coatings would not only
greatly enhance sulfidation resistance but also, even
without the presence of the reactive metals (Y, Sc,~Th,
La and the other rare earths) which normally provide
oxide adherence~to the underlying substrate, would
promote additional oxide adherence.
With respect to the processes whereby the alloy
may be applied as a coating to the surface to be~pro-
~tected, the presence of platinum~or rhodium ~o the~
coating alloy,~ because of the low vapor pressure of
platinum or rhodium, generally preoludes use of the
vapor deposition technique. Other techniques are,
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however, efficacious to obtaining the properly composed
coating. It is recognized, for example, that the
coatings may be deposited by using a process involving
simultaneous vapor deposition of the MCrAlY and spu~ter
deposition of platinum or rhodium. As an alternative,
the coatings may be accomplished by plasma spraying
techniques.
A better understanding of the invention will result
when viewed in light o~ the following examples:
E~ le_l
Alloys of Ni-8Cr-6Al with alloying additions of
platinum and rhodium were made by the conventional arc
melt-drop cast technique. Specimens of the composltions
depicted in the graph of Fig. 1 had dimensions of 1 cm x
1 cm x ~ 0.2 cm and were subjected to hot corrosion
tests as follows. Specimens of the alloys were spray
coated with an aqueous solution of Na2S04, dried and
weighed. After achieving a coating of 0.5 mg cm 2 Na2S04,
they were oxidized for 20 hours at 1~000C in one
atmosphere 2 in a thermal balance. The specimen weight
was recorded continuously as a function of time with the
weight changes converted to weight gain per unit surface
area and shown in Fig. 1.
As can be seen, the addition of 2.5 weight percent
Pt did not significantly improve the performance of the
Ni-8Cr-6Al alloy in this test. However a significant
improvement in performance was obtained when 5 or 10
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weight p2rcent Pt was added. Specimens of thc ~i-8Cr-
6Al-5Rh alloy were approxlmately squivalent ~o ~hat of
the 10 Pt alloy.
Example 2
Specimens were formed as in Example 1 to the
compositions as shown in Figs. 2 and 3. The specimens
wers subjectsd to high temperature cycIic oxidation
tests and surprisingly, those containing platinum or
rhodium, were found to have improved oxide adherence of
the Al203 formed on the alloys. It can be seen that the
alloys with S or 10 weight percent Pt are superior to
the 2.5 weight percent Pt alloy which, in turn, is
signiîcantly hetter than the unmodified alloy. Oxide
adherence on a Ni-8Cr-6Al-5Rh alloy at 1,200C was
found to be~equivalent to that of the Ni-8Gr-6Al-lOPt
alloy at the same temperature.;
Example 3
Alloy specimens having dimensions;~of~l cm x 0.~8 cm~x ;~ :
0.1-0.2 cm and;compositions o~ Ni-1~7Cr-12Al-0.5Y,
Ni-17Cr-12Al-SRh-0~5Y, Ni-17Cr-12Al-lOPt-0.5Y,
Co-17Cr-llAl-0.5Y~, Co-17Cr-llAl-5Rh-O.SY and~Co-17Cr-llAl-
lOPt-oo5y`were prepared, measured and weighed,~then coated
with~0.5-2.0 mg/em2 Na2S04. They were~then~subjectèd
to up to 14 cycles, each cycle c~onsisting of oxidizing
in air for 20 hours at ~1,750F, cooling to room tempera-
ture, washing and reweighing. The~sequence~was repeated
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to failure. The resul~s obtained for one set o~
experlments at 1,750F using 2 mg/cm2 of salt ls illus~
trated in Fig. 4. Although CoCrAlY is basically more
resistant to hot corrosion than is NiCrAlY, it can be
seen that additions of either Pt or Rh to either CoCrAlY,
or NiCrAlY dramatically improve their hot corrosion
resistance.
Example 4
Erosion bars of Ni-17r-12A1 0.5Y, Ni-17Cr-12Al-5Rh-
0.5Y, Ni-17Cr-12Al-5Pt-0.5Y and Ni-17Cr-12Al-lOPt-0.5Y
were evaluated in a cyclic hot corrosion burner rig at
1,750F using 35 ppm of sea salt ingested in the fuel
prior to combustion. Severe attack of the tip of both
the NiCrAlY base composition and the rhodium modified
composition occurred after 110 hours. Hot zone failures
were observed between 300 and 400 hours for both these
bars, the rhodium modified specimen surviving for a
slightly longer time than the base composition. Although
the rhodium modified composition showed littLe improvement
over the base alloy in this test, the nature of its
failure was unusual and rendered these results ~omewhat
dubious and inconclusive. In contrast, the platinum
modified compositions were found to be dramatically more
resistant to hot corrosion than the base composition.
For these compositions no sign of failure was observed
up to 675 hours, when testing was terminated.
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What has been 9et ~or~h above i9 intended primarily
as exemplary to enable those skilled in the art in the
practice of the invention and it should ~herefore be
understood that, within the scope of the appended claims,
the invention may be practiced in other ways than as
specifically described.
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