Use of a super alloy as a substrate for catalysts

A metal alloy in which the highest individual metal concentration is nickel and which always includes at minimum 4% by weight aluminum is used as the substrate for catalysts for purifying exhaust gases. Chromium, iron, cobalt, molybdenum and titanium are among the other metals possible.

The invention relates to the use of a nickel-based metal alloy, known per 
se, as a substrate for exhaust-gas catalysts. In the metal composition 
used, the proportion of nickel is the largest among the individual 
constituents, and it always contains at minimum 4% by weight aluminum and 
possibly other metals. The wording "possibly other metals" means small 
amounts of, i.a., iron, cobalt, molybdenum, titanium, etc., but possibly a 
larger amount of chromium. 
The preparation of a metallic exhaust-gas catalyst starts with the 
selection of a metal foil. All the metal foils used include some aluminum. 
In the annealing step this aluminum "migrates" to the surface of the foil, 
forming a thin oxide layer onto which the support material is applied. 
This oxide layer thickens during the annealing, as oxygen and metals 
diffuse through it. The chromium present in a steel foil alloy, together 
with the aluminum, protects the steel from oxidation. 
Onto the surface of the support structure thus obtained there is applied a 
thermally stable oxide layer having a large specific surface, the oxide 
generally being, for example, .gamma.-Al.sub.2 O.sub.3. 
In general, the materials used as the carrier, the metallic monolith, for 
automobile exhaust-gas catalysts are metal alloys the principal components 
of which are iron, chromium, and aluminum. 
U.S. Pat. No. 4,318,888 presents as an example of the metallic foil an 
iron-based foil containing 15% by weight chromium, 4% by weight aluminum, 
and 0.5% by weight yttrium. 
The objective to develop metal alloys which withstand heat corrosion and 
additionally have a good resistance to flow has led to the preparation of 
so-called super alloys. It is typical of these metals that they have a 
large chromium content, as well as aluminum, titanium and refractory metal 
alloying. Super alloys are typically used in industrial gas turbines and 
in airplane engine parts. Super alloys are classified into iron-based, 
nickel-based, and cobalt-based alloys. 
Super alloys are by structure austenitic (face-centered cubic crystal 
structure (p.k.k.)), having typically good mechanical properties as 
compared, for example, with body-centered cubic metals. The most important 
factor is probably the ability of austenite to dissolve other elements 
into the matrix and the possibility to precipitate, in a controlled 
manner, intermetallic compounds such as .gamma.'-Ni.sub.3 Al. 
Normally the substrates used for metal-substrate exhaust-gas catalysts are 
aluminum-containing iron-chromium alloys in which the resistance to 
oxidation is based on a protecting aluminum oxide layer. In a normal 
automobile exhaust-gas environment the said materials give the catalyst 
sufficient mechanical endurance and corrosion resistance. The advantage of 
iron-based alloys over nickel-based and cobalt-based alloys is that they 
are economical and have a lower density. 
In certain applications, for example in catalysts for power saws and in 
so-called starting catalysts to be installed very close to the exhaust 
manifold, the temperature may rise above 1000.degree. C., in which case, 
in a strongly vibrating load situation, iron-based metal alloys are not 
sufficiently durable mechanically (Example 2). This is due to the 
instability of the phase structure of iron-based alloys at high 
temperatures, especially when the alloy contains high concentrations of 
chromium and aluminum. The stability of the alloy can be improved by 
replacing part of the iron by nickel. With nickel-based alloys the 
situation is advantageous, since the p.k.k. crystal structure which 
provides good high-temperature strength properties is stable in all 
compositions. 
U.S. Pat. No. 4,601,999 discloses an iron-based metal substrate for 
catalysts, wherein the aluminum content is limited to 3% by weight owing 
to technical problems in manufacturing. In the metal substrate according 
to the invention the aluminum content is at minimum 4% by weight and the 
metal alloy is nickel-based. Likewise, in the patent DE-3 440 498 the 
metal alloy is iron-based. 
A study of various metal alloys showed that, in very thin, 0.03-0.10 mm, 
foil strips used in exhaust-gas catalysts, resistance to oxidation has a 
more significant effect on the mechanical endurance of the substrate than 
do the high-temperature strength values of the foil. The resistance of 
cobalt-chromium and nickel-chromium alloys to heat corrosion is based on a 
chromium oxide layer which forms on the surface in oxidizing conditions 
when the aluminum content in the alloys is low. The chromium oxide layer 
does not in an oxidizing atmosphere sufficiently protect the base metal at 
high temperatures, above 800.degree. C. One example of such alloys which 
can be mentioned is Hastelloy X (alloy 2), which has good hot strength 
values but not sufficient resistance to oxidation (Examples 1 and 2). For 
this reason, an oxide layer containing a large amount of aluminum can be 
regarded as necessary in order that the metal substrate endure chemically 
and mechanically under special conditions. 
The advantages of the commonly used iron-aluminum-chromium alloy are 
good resistance to oxidation at high temperatures 
strength at normal operating temperatures 
When the temperatures rises above 700.degree. C., the strength of such an 
alloy foil decreases crucially, and the remaining strength is 
approximately 30% of its strength at 20.degree. C. 
Now it has been surprisingly observed that, when a metal alloy in which the 
nickel content is higher than the content of any other metal in the alloy 
and which additionally contains aluminum at minimum 4% by weight and 
possibly other metals is used as a thin foil, this foil resists well heat 
corrosion while its resistance to oxidation is sufficient, and 
additionally the mechanical endurance of the foil at the high temperatures 
used and under strongly vibrating load situations is good. Upon oxidizing 
the material does not become brittle; its elongation values remain good. 
The nickel concentration in the metal alloy is preferably higher than 40% 
by weight and its aluminum concentration is preferably approx. 4-6% by 
weight. 
One advantageous alloy according to the invention which has been used is a 
Ni-Cr-Al alloy. When this foil is annealed, an Al.sub.2 O.sub.3 --Cr.sub.2 
O.sub.3 layer, or in certain conditions an almost pure Al.sub.2 O.sub.3 
layer, is obtained. Internal oxidation of aluminum within the wide ranges 
of partial oxygen pressure and temperature can be avoided by regulating 
the chromium concentration. 
This is important, especially if the protecting oxide layer formed in 
optimum conditions is damaged, and the damage must be self-corrected 
during operation. The adhesion and density of the aluminum oxide layer 
which protects the surface can be improved by alloying the 
aluminum-containing metals with a small amount of rare earth metals. 
It is possible to precipitation harden the nickel-chromium-aluminum alloys 
usable for the purposes according to the invention by means of 
intermetallic .gamma.' compounds; this gives these alloys unique 
mechanical properties up to a temperature of approx. T=(0.8.times.melting 
temperature), the .gamma.' precipitations dissolving in the matrix. In 
nickel-based alloys, high compatibility between the matrix and coherent 
.gamma.' precipitations ensures long-term stability of the structure. In a 
nickel-based alloy, .gamma.' is typically of the form (Ni,Co).sub.3 (Al, 
Ti), in which nickel and aluminum are dominant. In iron- and cobalt-based 
alloys, owing to the lower stability of the structures, a similar 
strengthening mechanism cannot be exploited as effectively. In an 
exhaust-gas atmosphere, in a mechanically severe load situation, and at 
high temperatures, above 900.degree. C., in long-term use, the .gamma.' 
precipitations, together with an excellent resistance to oxidation, render 
aluminum-alloyed nickel-based alloys superior to other alloys. 
The thermal expansion coefficients of nickel-based alloys are lower than 
those of iron-based alloys. This is advantageous in terms of the adhesion 
of the catalytic support layer to be sprayed onto the surface of the 
metal, since a better compatibility of the thermal expansion coefficients 
reduces thermal stresses. The adhesion of the catalytic support to the 
surface of a nickel-chromium-aluminum alloy is very good also for the 
reason that the aluminum oxide layer formed on the surface of the foil 
strip during annealing serves as an intermediate layer between the base 
metal and the support material, improving the physical adhesion of the 
aluminum-oxide-containing support to the substrate. 
Experiments have shown the excellent strength properties of the metal alloy 
foil material according to the invention in hot oxidizing conditions.

The following examples illustrate the advantages of the so-called super 
alloy according to the invention as compared with the alloys currently 
used when using in catalysts thin foils made of such alloys. 
EXAMPLE 1 
Three commercial metal alloys were selected as the alloys to be 
investigated, of which alloys 2 and 3 are so-called super alloys and alloy 
1 is a material which has been much used in catalysts. Alloy 1 is an 
iron-chromium-aluminum alloy (VDM ISE), alloy 2 is a 
nickel-chromium-iron-molybdenum alloy (Hastelloy X), and alloy 3 is a 
nickel-chromium-aluminum-iron alloy (Haynes Alloy 214). The compositions 
of the alloys are shown in Table 1. Specimens of 200.times.75.times.0.05 
mm of each alloy were annealed in an annealing furnace in an atmosphere of 
air at a temperature of 900.degree. C. for 4, 8, 16, 24, 48, 72 and 150 
hours, and at 1100.degree. C. for 4 and 8 hours. The results obtained are 
shown in FIG. 1, from which it can be observed that the weight increase of 
the aluminum-containing alloys 1 and 3 as a function of time is 
considerably less than that of the chromium-containing alloy 2. The 
chromium oxide layer formed on the surface of alloy 2 does not provide 
sufficient protection; the specimen oxidized throughout during annealing 
at 1100.degree. C. This shows clearly that aluminum alloying is 
indispensable in order that the oxide layer on the surface protect the 
base metal from oxidation at temperatures as high as these. 
TABLE 1 
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Alloys investigated (*maximum concentration) 
CONCENTRATION, % 
Component Alloy 1 Alloy 2 Alloy 3 
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Ni -- bal. bal. 
Co 0.5* 0.50-2.50 -- 
Cr 20-22 20.50-23.00 
16.0 
Mo -- 8.00-10.00 
-- 
W -- 0.20-1.00 -- 
Fe bal. 17.00-20.00 
3.0 
C 0.05* 0.05-0.15 -- 
Si 0.60* 1.00* -- 
Mn 0.40* 1.00* -- 
B -- 0.008* -- 
Ti -- 0.15* -- 
Al 4.8-5.5 0.50* 4.5 
Cu -- 0.50* -- 
P -- 0.040* -- 
S -- 0.030* -- 
Y -- -- some 
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Besides resistance to oxidation, the metal foil should also have mechanical 
endurance within high temperature ranges. This was experimented with using 
the alloy according to the invention, reference alloys also being included 
in the trials. 
In the following example, the endurance of three alloys was investigated 
when they were used as catalytic materials as the base of the metal foil. 
EXAMPLE 2 
The catalyst honeycombs of FIG. 2 were prepared from the alloys of Example 
1, and the mechanical endurance of the honeycombs was tested using a 
vibration apparatus (Ling Electronics, Inc. Model DMA 5-5/A 395). The test 
conditions were: 
acceleration 40 g 
frequency 90 Hz 
temperature 930.degree. C. 
The results obtained are presented as relative periods of endurance in 
Table 2. Only the catalytic honeycomb prepared from alloy 3 is 
sufficiently durable in conditions as demanding as these. 
TABLE 2 
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Relative endurance periods in the mechanical test. 
Alloy 1 Alloy 2 Alloy 3 
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Horizontal shaking 
1 1.2 4.4 
Vertical shaking 
1 8.0 &gt;8 
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According to the results, the alloy prepared without nickel has properties 
inferior to those of the alloy containing a large amount of nickel. 
Furthermore, according to the results the presence of aluminum is 
indispensable. 
It is observed surprisingly that when a thin foil strip made of alloy 3 is 
used as the base metal for the catalyst, a good strength is obtained at 
temperatures above 900.degree. C. The resistance to oxidation is also 
good.