CA1158075A

CA1158075A - Mcraly type coating alloy

Info

Publication number: CA1158075A
Application number: CA000230272A
Authority: CA
Inventors: Edward J. Felten
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1974-07-10
Filing date: 1975-06-26
Publication date: 1983-12-06
Also published as: AU8156075A; NO752355L; SE410477B; GB1500780A; IL47407A0; JPS5130530A; DE2530197C2; SE7507095L; IN144076B; NO141371B; BR7504327A; FR2277902B1; CH606455A5; NO141371C; US3918139A; JPS5842255B2; DE2530197A1; IL47407A; FR2277902A1; IT1039467B

Abstract

ABSTRACT OF THE DISCLOSURE
There is described nickel, cobalt and nickel-cobalt alloy coating compositions having improved hot corrosion resistance. In particular, an improved MCrAlY type alloy coating composition consists 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, scandium, thorium and the other rare earth elements and 3-12 percent of a noble metal selected from the group consisting of platinum or rhodium, the balance being selected from the group consisting of nickel, cobalt and nickel-cobalt. The alloys of the present invention exhibit markedly improved hot corrosion resistance and are considered particularly useful as coatings on the contemporary super-alloys. The inventive alloys are in themselves corrosion resistant and do not depend for their protective effect upon a reaction with the substrate material. In addition, these alloys are uniform throughout their thickness and thus will exhibit their protective properties more continuously and consistently than do the aluminide coatings.

Description

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 ~' ~ 158075 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.

1 1~8~7~

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.

~ 15~75 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 1 15~07~

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, 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 11~80~

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 , .: ':

1 1~8~75 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.

_g_ l 1580~5 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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a coating composition of the MCrAlY type wherein the coating composition consists essentially of, by weight, approximately 8-30 percent chromium, 5-15 percent aluminum, up to 1 percent reactive metal selected from the group consist-ing of yttrium, scandium, thorium and the other rare earth elements, balance selected from the group consisting of nickel, cobalt, and nickel-cobalt, the improvement for increasing hot corrosion resistance which comprises 3-12 percent of a noble metal selected from the group consisting of platinum and rhodium as an alloying ingredient.

2. The invention of claim 1 wherein said reactive metal is yttrium.

3. The invention of claim 2 wherein said noble metal is 5-10 percent platinum.

4. The invention of claim 2 wherein said noble metal is 5 percent rhodium.

5. A coated article comprising a nickel-base or cobalt-base superalloy, and an overlay coating thereon of the MCrAlY
type wherein the coating composition consists 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, scandium, thorium and the other rare earth elements, 3-12 percent of a noble metal selected from the group consisting of platinum and rhodium, balance selected from the group consisting of nickel, cobalt and nickel-cobalt, said platinum or rhodium being substantially uniformly dispersed throughout said overlay coating.

6. The invention of claim 5 wherein said reactive metal is yttrium.

7. The invention of claim 6 wherein said noble metal is 5-10 percent platinum.

8. A method for improving the hot corrosion resistance of a coating composition of the MCrAly type wherein the coat-ing composition consists essentially of, by weight, approxi-mately 8-30 percent chromium, 5-15 percent aluminum, up to 1 percent reactive metal selected from the group consisting of yttrium, scandium, thorium and the other rare earth elements, balance nickel and/or cobalt comprising incorporating, as an alloying ingredient to said MCrAly coating, 3-12 percent of a nobel metal selected from the group consisting of platinum and rhodium.

9. The method of claim 8 wherein said nobel metal is 5-10 percent platinum.

10. An alloy consisting of, apart from impurities:
a) 42 to 78 wt. % in total of nickel and/or cobalt in which either nickel or cobalt is present in an amount not less than 40 wt. %;
b) 8 to 23.5 wt. % chromium, c) 5 to 7 wt. % aluminum;

d) 3 to 12 wt. % of a nobel metal selected from the group con-sisting of platinum and rhodium;
e) 0 to 1 wt. % reactive metal selected from the group consist-ing of thorium and the rare earth elements.

11. The alloy of claim 10 wherein said noble metal is platinum from 5 to 10 wt. %.

12. A coated article comprising a nickel-base or cobalt-base superalloy, and an overlay coating wherein the coating composition consists essentially of the alloy of claim 10.

13. The coated article of claim 12 wherein said nobel metal is platinum from 5 to 10 wt. %.

14. A method for improving the hot corrosion resistance of a coating composition, wherein the coating composition consists essentially of 42 to 78 wt. % in total of nickel and/or cobalt in which either nickel or cobalt is present in an amount not less than 40 wt. %, 8 to 23.5 wt. % chromium, 5 to 7 wt. %
aluminum, 0 to 1 wt. % reactive metal selected from the group consisting of thorium and rare earth elements, comprising incorporating, as an alloying ingredient to said coating composition, 3 to 12 wt. % of a nobel metal selected from the group consisting of platinum and rhodium.

15. The method of claim 14 wherein said noble metal is platinum from 5 to 10 wt. %.