Since at least 1970, much effort has been expended to develop durable catalyst supports, for use in catalytic converters, which supports can be mass produced and which achieve long converter life. In recent years, metal supports have gained favor because they can be fabricated in any shape or size and the converter is about 20% smaller than the equivalent ceramic converter. When temperatures exceed 2000.degree. F., metal converters do not melt down and plug the exhaust flow, as do ceramic converters. Furthermore, the metal supports used in honeycomb cores have approximately 90% open area, whereas ceramic cores have thicker cell walls and typically have approximately 70% open area, which generates a higher pressure drop.
Anticipated future applications will require materials that will withstand cyclic oxidation in exhaust environments in the range of 2300.degree. F. Such future applications include recuperators for gas turbines, planned for automobiles in the late-1990s, catalytic combustors for gas turbines, and converters for spark-ignited internal combustion engines in which the converter is coupled directly to the cylinder head.
Another example of the need for materials which can function at high temperatures is in the area of diesel particulate traps, where the temperature can reach 2400.degree. F. during regeneration, i.e. when carbon particles are burned off.
In addition to the need for supports which withstand high temperatures, there is a need for supports that can withstand oxidation from residual sulfur compounds found in many liquid fuels. Sulfur accelerates oxidation, and causes failure of the support, under hot cyclic conditions.
The prior art that relates to metal supports for catalytic converters culminates in U.S. Pat. No. 4,601,999, which describes a composition for a catalyst support, made by the hot-dip coating process for coating aluminum on ferritic stainless steel strip, which is subsequently rolled to foil. This composition is relatively resistant to hot cyclic corrosion, and it does not interact with the catalyst coating.
Another invention that describes the process for making an aluminium-coated base metal foil, for use as a catalyst support, is disclosed in U.S. Pat. No. 4,686,155, entitled "Oxidation Resistant Ferrous Base Foil and Method Therefore", which is also based on hot-dip coated aluminum on stainless steel.
Methods for making a metal strip into a finished catalytic converter are given in U.S. Pat. No. 4,576,800, "Catalytic Converter for an Automobile" and in U.S. Pat. No. 4,673,553, "Metal Honeycomb Catalyst Support Having a Double Taper". The metal strip is corrugated so that when it is wound into a spiral or folded back and forth upon itself, the corrugations form channels for the flow of gas. Before the strip is wound or folded it is coated with catalytic materials or heat resistant materials, depending on the application.
U.S. Pat. No. 4,711,009, "Process and Apparatus for Making Metal Substrate Catalytic Converters", describes a continuous process for producing converters made with a metal catalyst support.
Both U.S. Pat. No. 4,601,999 and U.S. Pat. No. 4,686,155 contemplate a hot-dipped aluminum coating on a ferritic stainless steel strip at least 0.020 inch thick followed by reduction to foil thickness by rolling the coated strip. This process has a number of disadvantages, which are outlined below. Also noted below are the means by which the present invention overcomes the limitations of the prior art.
1. The hot-dip process sometimes leaves the coated foil with an uneven aluminum coating along its length and across its width. Indeed, the thickness of the coating can be as low as zero, and as high as 400 microinches or more, on the same coil of 0.0025 inch thick foil. The variations in thickness occur in spots or streaks where the aluminum does not cover the underlying steel.
One of the causes for the variation in thickness is that the aluminumis softer than the steel. Because the thickness of the steel varies, the softer aluminum becomes the repository of the initial non-uniformity of both the stainless steel and the aluminum coating. Another cause of a non-uniform aluminum coating is hard spots in the stainless steel, or inclusions, such as lumps of oxide, or the like, that do not roll to foil as readily as the balance of the steel.
In the present invention, the steel is first rolled to foil thickness and then is coated with aluminum by vapor deposition. Hot-dip coating of the foil is not practical because the thin foil would dissolve, at least partly, in the aluminum. The vapor-deposited coating is uniform, even if the surface of the base metal is uneven. This uniformity is analogous to that of a snowfall, which uniformly coats both smooth and rough pavement.
2. In the the hot-dip process no more than 3% aluminum and 1% silicon can be included in the base metal. Otherwise, the base metal is not wetted by molten aluminum. This limitation of the prior art eliminates many high temperature base metals from consideration. According to the present invention, the base metals to be coated need only be (a) thin and in roll-form, (b) reasonably smooth, and (c) able to be heat treated to 1200.degree.-140020 F. to form an aluminum oxide film on the surface. Many base metals, besides ferritic stainless steel, can be used, such as austenitic stainless steel, martensitic stainless steel, superalloys, titanium, and composite materials including composites of metals and ceramics.
3. In the prior art, wherein the hot-dip coating process is used, the foil is work hardened by the many passes through a rolling mill. Intermediate annealing between rolling mill passes is not practical because the aluminum coating would develop an oxide film which is hard and abrasive and unsuitable for rerolling. When the base metal is first rolled to foil thickness and then coated with aluminum, the foil can be bright annealed (in a non-oxidizing atmosphere) before coating it with aluminum. This is important because high performance alloys can be made relatively ductile by a final annealing after they have been rolled to foil thickness.
4. In the hot-dip coating process of the prior art, annealing the aluminum-coated foil before corrugating would oxidize the aluminum coating. The aluminum oxide thus formed would crack and also would abrade the corrugating rolls. The cracks expose the underlayers of the support and thereby open sites of potential corrosion. When the foil is annealed before it is coated with aluminum, the foil is corrugated in a ductile state so that a minimum of stress centers are created in the catalyst support. Further, the aluminum surface does not abrade the corrugating rolls, but instead it serves as a lubricant.
5. In the hot-dip coating process, the thick aluminum-coated steel strip undergoes as many as 10 passes through a rolling mill. During the rolling, the rolls embed in the soft aluminum any dirt that has accumulated on the surface of the rolls. In the finished foil, this embedded dirt becomes the source of corrosion sites, just as do the stress centers mentioned earlier. Oil used in rolling is hard to remove and, if heated, tends to leave a film which lessens the adherence of the catalyst coating.
6. In the hot-dip coating process, the molten aluminum dissolves iron from the strip that is being coated. The amount of iron that is typically alloyed with the molten aluminum is 2-4%, by weight, which subsequently becomes part of the aluminum coating on the same strip. In the end product foil, this results in impurities, which reduce the corrosion resistance. Also, the iron-aluminum coating is less ductile than a pure aluminum coating and is less suitable for corrugating. In the present invention the aluminum used to coat the surfaces of the catalyst support contains no unwanted alloys or impurity.
7. According to the present invention, the base material is not limited to metals, but can include ceramics, high-temperature composite materials comprised of metals, ceramics, silicates, nitrides, borides, and refractory-metal oxides, or a combination, any of which can be coated with aluminum, heated to a temperature high enough to form an aluminum oxide coating (nominally 1400.degree. F.), and subsequently processed to make an end product that heretofore was limited to use of metal alone.
The following is a list of references dealing with the field of the present application. These items, and the patents and patent applications cited above, are incorporated by reference herein:
Oxidation of Metals & Alloys, Butterworths, by Dr. O. Kubaschewski and B. E. Hopkins, 1962.
Handbook On Thin Film Technology, McGraw Hill, Edited by L. I. Maissel & R. G. Lang, 1970.
Source Book On Materials For Elevated-Temperature Applications, American Society for Metals, by E. F. Bradley, 1975.
High-Temperature Protective Coatings, Conference Proceedings, The Metallurgical Society of AIME, Edited by Subhash C. Singhal, 1983
Ferritic Steels for High-Temperature Applications, American Society for Metals, Edited by Ashok K. Khare, 1983
Vapor Deposition, The Electrochemical Society, Inc., Edited by C. F. Powell, J. H. Oxley and J. M. Blocher, Jr., 1983
High Temperature Alloys: Theory and Design, The Metallurgical Society of AIME, Edited by J. O. Stiegler, 1984
High-Temperature Ordered Intermetallic Alloys, Materials Research Society, C. C. Koch, C. T. Liu, N. S. Stoloff, 1984
Thin Films: The Relationship of Structure to Properties, Materials Research Society, Editors Carolyn Rubin Aita and K. S. SreeHarsha, 1985
Engineers' Guide to Composite Materials, American Society for Metals, Edited by John W. Weeton, 1987