EP0480404A2

EP0480404A2 - Corrosion-resistant and heat-resistant metal composite and method of producing

Info

Publication number: EP0480404A2
Application number: EP91117218A
Authority: EP
Inventors: Shinichiro Yahagi; Hiroshi Yamada; Kikuichi Funao; Fumio Iwane
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 1990-10-09
Filing date: 1991-10-09
Publication date: 1992-04-15
Anticipated expiration: 2011-10-09
Also published as: EP0480404A3; DE69111362D1; DE69111362T2; EP0480404B1

Abstract

A metal composit is prepared by covering at least a part of the surface of a substrate, typically a sheet of an Fe-based alloy or steel with Al or an Al-alloy dirctly or with an intermediate layer of Ni, annealing the covered material under vacuum or in an inert gas atmosphere, and heating in an oxidizing atmosphere to form an internal layer of inermetallic compouds and a surface portecting layer of Al2O3. In case of direct covering with Al or Al-alloy,there will occur intermetallic compounds, Fe3Al and/or FeAl, and in case of using an Ni sheet as the intermediate layer, intermetallic compounds, Ni3Al and/or NiAl Nb may be used in place of Ni. As the substrate, Ni or a Ni-alloy containing more than 20 weight % of Ni may be used. The substrate may be in the form of wire
The product metal composit has improved high temperature corrosion resistance and chemical resistance as wee as good processability, and is paticularly useful for electric heater material and catalyst carrier of automobile exhaust gas cleaners.

Description

The present invention concerns a metal composite having both corrosion-resistant property and heat-resistant property. The invention also concerns the method of producling the composite.
As the materials for constructing equipments used in a high temperature and corrosive atmosphere, such as liners of reactors, catalyst carriers of automobile exhaust gas clearners, and electric heaters, there has been used various alloys having corrosion-resistant property and heat-resistant property required for the use.
The requirement for the corrosion-resistant property of this kind of metal materials has been getting severer. Taking the electric heater materials as an example, among the Fe-Cr alloys, 20Cr-5Al-Fe alloy or FCH1 alloy has relatively high corrosion-resistance. On the other hand, "FCH51" (product mark of Daido Steel Co.,Ltd.) containing smaller amounts of Cr and Al, or 15Cr-3Al-Fe alloy, exhibits a little lower high temperature corrosion resistance, but is less expensive and easy to process, and therefore, the alloy has wide use as the heater materials. Thus, there has been a strong demand for the material which exhibits good oxidation resistance and corrosion resistance at high temperatures, and of good processability, and therefore, is suitable for the heater materials.
In view of the facts that corrosion-resistant and heat-resistant alloys are generally hard to process and that the corrosion resistance depends on status of the surface of the material, there has been proposed a variety of composite materials made by combining a substrate having good processability and another surface material of corrosion resistance thereon, as well as materials made by surface treatment for improving the corrosion resistance.
The inventors intended to meet the above demand, and tried to form a surface protecting layer of a corrosion-resistant and heat-resistant substance such as Al₂O₃ by coating it on a heater material made of FCH51 alloy or the like. It was found that, due to the difference in the thermal expansion coefficients of the materials, the coated layer falls off from the substrace after repeated cycles of heating and cooling. Then, they formed protecting layers by vapor-deposition of Al₂O₃ and succeeded in improving durability of the layers. However, the vapor-deposition operation is of low productivity and high costs, and therefore, they concluded that this technology is difficult to be widespread.
With respect to the metal honeycomb material used in the automobile exhaust gas cleaning devices the inventors invented a material of improved corrosion resistance which is made by cladding an Al-foil on an alloy of a basic composition, 15Cr-3Al-Fe (so-called "Elemann's steel"), rolling the cladded sheet, annealing in vacuum and heating in an oxidizing atmosphere to oxidize the surface Al to Al₂O₃. The Elemann's steel has good processability and the surface corrosion resistance is improved by the formation of the Al₂O₃. Thus, it is now possible to produce corrosion-resistant and heat-resistant parts of desired shape. The material was already proposed(Japanese Patent Application Hei 2-192090).
As a matter of fact, it was sometimes observed that, when the above material is used for a long period at a high temperature, pitching occurs. It is considered that the pitching may occur due to the fact that Al₂O₃ at the surface is in the form of whiskers and therefore, though adhesion to the substrate is good, the solidity in the layer direction is not necessarily high throughout the surface.
The inventors also establish the technology for improving high temperature corrosion resistance of electric heater material made of Ni or Ni-Cr alloy by cladding an Al foin thereon and subjecting the cladded sheet to the above mentioned annealing under vacuum and heating in an oxidizing atmosphere to form the Al₂O₃. The technology was also proposed(Japanese Patent Application Hei 2-148158). The heater material exhibits high durability. The reason is considered to be that intermetallic compound, Ni₃Al (partly, NiAl) is formed between the cladded Al layer and the substrate during the heat treatment, and that the intermetallic compound solidly coats the substrate.
The general object of the present invention is to provide a metal composite with high temperature corrosion resistance given by the Al₂O₃ coating, which composit having improved durability by utilizing formation of the above mentioned intermetallic compoud such as Ni₃Al and/or NiAl, as well as to provide a method of producing a metal composite, particularly, in the form of a sheet or a wire, having such good corrosion resistance and heat resistance.
A particular object of the invention is to provide a metal composite used at a high temperature, which consists of a substrate made of an Fe-based alloy and an Al₂O₃ protecting layer solidly adhered to the substrate, and thus, the corrosion resistance, particularly, high temperature oxidation resistance is hightened. To provide a method of producing such a material at a low cost is also the particular object of the invention.
Another particular object of the inventio is to provide a metal composite used at a high temperature, which consists of a substrate made of an Fe-based alloy and an Al₂O₃ protecting layer solidly adhered to the substrate, and thus, in addition to the corrosion resistance such as high temperature oxidation resistance and chemical resistance are hightened. To provide a method of producing such a metal composite is also the particular object of the invention.
Further particular object of the invention is to provide a metal composite used at a high temperature, which consists of a substrate made of Ni or an alloy containing more than 20 weight % of Ni and an Al₂O₃ protecting layer solidly adhered to the substrate, and thus, the corrosion-resistance, particularly, high temperature oxidation resistance as well as salt water-resistance are hightened. To provide a method of producing such a metal composite is also the particular object of the invention.

Fig.1 is a conceptional cross section showing an corrosion-resistant and heat-resistant metal composite in the form of a sheet according to the present invention;
Fig.2 and Fig.3 are cross sections of the material illustrating the steps of producing the metal composite sheet of Fig.1;
Fig.4 is a conceptional cross section showing the structure of an corrosion-resistant and heat-resistant metal composite in the form of a wire according to the present invention corresponding to Fig.1;
Fig.5 and Fig.6 are cross sections of the material illustrating steps of producing the metal composite wire of Fig.4, corresponding to Fig.2 and Fig.3, respectively;
Fig.7 is a cross section showing the structure of a wire other than that of Fig.6;
Fig.8 is a cross section of the material illustrating a step of producing the metal composite sheet, corresponding to Fig.3;
Fig.9 is a cross section of the material illustrating a step of producing the metal composite wire, corresponding to Fig.6;
Fig.10 is a cross section of a metal composite according to the invention in case where the composite is a sheet, similar to Fig.1 but of the embodiment other than that of Fig.1;
Fig.11 is a cross section of a metal composite according to the invention in case where the composite is a sheet, similar to Fig.1 but of the embodiment other than that of Fig.1; and Fig.12 is a conceptional cross section corresponding to Fig.1, illustrating the structure of the metal coposit produced by the step shown in Fig.11;
Fig.13 and Fig.14 are the data of Example1 of the present invention; Fig.13 showing the results of salt water spray tests, and Fig.14 showing the weight increase by oxidation;
Fig.15 and Fig.16 are the data of Example 2 of the present invention; Fig.15 showing the weight increase at 1100^oC in air, and Fig.16 showing the results of salt water spray tests;
Fig.17 and Fig.18 are the data of Example 3 of the present invention; Fig.17 showing the weight increase at 1100^oC in air, and Fig.18 showing the results of salt water spray tests;
Fig.19 is the data of Example 5 of the present invention showing the results of salt water spray tests on the metal composite;
Fig.20 is the data of Example 6 of the present invention showing, as in Fig.19, the results of salt water spray tests;
Figs.21-23 are the data of Example 7; Fig.21 being a graph showing the results of tensile and bending tests in relation to the amounts of B added to the substrate; Fig.22 being a graph showing the results of tensile and bending tests in relation to the amounts of Fe and Mn added to the substrate; and Fig.23 being a graph showing the results of tensile and bending tests in relation to the amounts of Re added to the substrate;
Fig.24 is the data of Example 8 of the present invention, which is a graph showing the results of salt water dropping tests on the metal composite;
Fig.25 and Fig.26 are the data of Example 9 of the present invention; Fig.25 being a graph showing the results of salt water dropping tests on the metal composite; and Fig.26 being a graph showing the weight increase at 1100^oC in air; and
Fig.27 is the data of Example 10 of the present invention, which is a graph showing the hardness at the part near the surface of the metal composite.

The corrosion-resistant and heat-resistant metal composite of the present invention comprises, in the case of sheets as shown in Fig.1, and in the case of wire as shown in Fig.4, a metal substrate 1, internal layer 5 of at least one of the intermetallic compounds, NiAl, NbAl, FeAl, Ni₃Al, Nb₃Al and Fe₃Al, and surface layer 6 of Al₂O₃. The substrate may be of any metal which has heat resistance requied for the respective use, and therefore, can be chosen from a wide range as noted below.
In case where Fe-based alloy (provided that, if Ni is contained in the alloy, the content is up to 20 weight %) is used, the internal layer 5 of the intermetallic compounds will be a mixed layer of Fe₃Al and FeAl.
As the substrate of Fe-based alloy, in case where the intended composite is used as an electric heater material, Fe-Cr alloys of FCH-series, e.g., FCH51-alloy which consists of Cr 14.0 - 15.0 %, Al 2.80 - 3.30 % and the balance of Fe, C being up to 0.07 %, Si up to 0,10 % and Mn up tp 0.60%, are suitable. Other Fe-based alloys may be used as the substrate metal in accordance with the use of the metal composite. They may be Fe-based alloys in which Ni content is up to 20 weight %, for example, various structural steels, ferritic, austenitic and martensitic stainless steels.
The Fe-based alloys to be used as the substrate may further contain a component or components selected from B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr and REMs so as to improve the properties of the alloy and to cause formation of the intermetallic compounds of Al and these components.
Another embodiment of the metal composite of the present invention comprises, as explained above in regard to the first case where an Fe-based alloy is used as the substrate in reference to Fig.1 and Fig.4, a substrate 1 of Ni or an alloy containing more than 20 weight % of Ni, an internal layer 5 in which intermetallic compounds, Ni₃Al and NiAl, coexist, and a surface layer 6 of Al₂O₃.
The substrate may be pure Ni metal or an alloy containing more than 20 weight % of Ni, the latter including various Ni-based alloys and Fe-based alloys. The examples of the alloys are as follows (% is by weight):

a) Ni-Cr alloys consisting of Cr 14.0 - 23.5 %, C 0.15 %, Mn 2.5 % and the balance of Ni,
b) Ni-based alloys consisting of one of B 0.08 - 0.18 % Re 1 - 3 %, Fe 5 - 20 %, and the balance of Ni,
c) Fe-Ni-Cr alloys consisting of Ni 20 - 72 %, Cr 14 - 35 % and the balance of Fe,
d) Ni-Al alloy consisting of Al 1 - 6 % and the balance of Ni, and
e) Ni-Al-Be alloys consisting of Al 1 - 6 %, Be 0.2 - 3.0 % and the balance of Ni.

To the substrate metal of Ni or an alloy containing more than 20 weight % Ni, further addition of one or more of B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr, Ta, Nb, Sc and REMs may be made to improve the properties of the alloy or to cause formation of the inermetallic compound or compounds between these components and Al.
In the still other embodiment of the present invention, the metal composite, as illustrated in Fig.10, consists of a metal substrate 1, an internal layer 4 which is made by vapor-deposition or electroplating of Au, Ag or alloys thereof, and a coating layer 6 of Al₂O₃.
The method of producing the corrosion-resistant and heat-resistant metal composite in the form of a sheet according to the present invention comprises, as shown in Fig.2, cladding an Al sheet 3A and Ni (or Nb) sheet 2A on at least a part of the sheet of the metal substrate 1 (in the illustrated example, on one side of the sheet) in the manner that the latter is inside to form a cladded sheet shown in Fig.3, and annealing the cladded sheet under vacuum and then, heating in an oxidizing atmosphere to form, between the Al layer 3B and the Ni (or Nb) layer 2B, as shown in Fig.1, the internal layer 5 of intermetallic compound or compounds, Ni₃Al (or Nb₃Al) and/or NiAl (or NbAl), and to form the Al₂O₃ layer 6 at the surface.
It is convenient to prepare the cladded sheet of Al sheet 3A and Ni (or Nb) sheet 2B as shown in Fig.2 prior to cladding them on the metal substrate 1. In this case, suitable thickness of the cladded sheet is 0.1 - 1.0 mm, in which the thickness of the Al layer is 0.001 - 0.5 mm. Of course it is operable to separately clad Ni and Al or Nb, or to firstly clad Ni (or Nb) sheet 2A on the metal substrate 1, and then, clad Al sheet 3A.
The method of producing the corrosion-resistant and heat-resistant metal composite in the form of a wire according to the present invention comprises, as shown in Fig.5, cladding an Al tube 3E and Ni (or Nb) tube 2E on the wire of the metal substrate 1 by inserting the wire in the tubes in the manner that the latter tube is inside and wire-drawing to form a cladded wire shown in Fig.5, and annealing the cladded wire under vacuum and then, heating in an oxidizing atmosphere to form, between the Al layer 3F and the Ni (or Nb) layer 2F, as shown in Fig.4, the internal layer 5 of intermetallic compound or compounds, Ni₃Al (or Nb₃Al) and/or NiAl (or NbAl), and to form the Al₂O₃ layer 6 at the surface.
In a typical instance of producing a wire product, diameter of the metal substrate wire is around 10 mm and the thickness of the covering Al tube and Ni tube (or Nb tube) are, similar to the cases of producing a sheet, 0.1 - 1.0 mm. By wire-drawing operation for several passes of drawing at a reduction of area of 30 - 50 %, a cladded wire of a diameter of 0.5 - 3 mm will be obtained.
The wires may have any profile of section. The section may be of not only the round circle as shown in Fig.4 but also a square, rectangular and so on. For example, a cladded wire of the section profile shown in Fig.7 can be obtained by rolling using profiled rolls, and then, the cladded wire can be further rolled to get the shape near the sheet. This kind of products has a higher corrosion resistance in comparison with the products of Fig.2 and Fig.3, because not only the flat surfaces but also both the sides of the former products are covered with the protecting layers.
The method of producing a metal composite of the present invention using Ni or an alloy containing more than 20 weight % of Ni as the substrate comprises, as shown in Fig.8 and Fig.9, covering the surface of the metal substrate 1, which is made of Ni or an alloy containing more than 20 weight % of Ni, with a layer 3 of Al or Al-alloy; subjecting the covered material to annealing and heating under vecuum or in an inert gas atmosphere to form, as shown in Fig.1 and Fig.4, the layer 5 in which Ni3Al and NiAl coexist , and to form the surface protecting layer of Al2O3.
At practice of the method, in case of sheet products, it is convenient to carry out covering with Al or Al-alloy by clad-rolling. Preferable thickness of Al or Al-alloy foil to be cladded is 0.005 - 0.5 mm, and the thickness after the rolling will be 0.001 - 0.2 mm.
On the other hand, in case of wire products, covering with Al or Al-alloy may be carried out by insering a substrate wire in a tube of a covering material and wire drawing or extrusion processing. In a typical embodiment, diameter of the metal substrate is around10 mm, and thickness of the Al or Al- alloy tube for covering it may be 0.1 - 1.0 mm as in the case of sheet products. Wire drawing of several passes under a reduction of area of 30 - 50 % will give a cladded wire of a diameter of 0.5 -3 mm.
The other possible ways of covering the substrate with Al or Al-alloy are dipping the substrate in molten Al or Al-alloy, thermal-spray of Al or Al-alloy on the substrate surface, plasma powder welding, chemical plating and vapor-deposition.
In case where an Al-alloy is used for covering the substrate, the alloy may be such ones that further contain one or more of the additional components selected from B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, Zr, Ta, Nb, Sc and REMs so as to enjoy the benifit of improved properties of the Al-alloy depending on the kind of the additional components.
Annealing under vacuum or in an inert gas is carried out by heating to a temperature of 400 - 900^oC, preferably 400 - 600 ^oC for 1 - 10 hours. The subsequent heating in an oxidizing atmosphere is conducted, for example, at a temperature of 400 - 1000^oC for 1 - 36 hours.
The method of producing the metal composite of the present invention, which has particularily high corrosion-resistance, comprises vapor-depositing or plating Au or Ag or an alloy thereof on the surface of a substrate made of an Fe-based alloy (provided that the content of Ni is, if contained, up to 20 weight %), covering the deposited or plated metal with Al, annealing the covered material under vacuum, and heating in an oxidizing atmosphere to form, as shown in Fig.10, surface protecting layer of Al₂O₃ 6 on the internal layer 4 of Au, Ag or an alloy thereof. Vapor-deposition and plating of Au, Ag or an alloy thereof can be carried out in accordance with the known technology. Taking Au as an example, thickness of the layer may be, in case of vapor-deposition, some tens to some hundreds of Angstroms, but in case of plating, it is easy to get such a thicknes as one to some tens of microns.
Further embodiment of the method of producing the metal composite according to the present invention comprises, in cases of sheet products, as shown in Fig.11, compiling the first Al foil 2, Ni or Ni alloy foil 3 and the second Al foil 2' in the order from the outside to the inside on the surface of a substrate 1 in the form of sheet; cladding these foils on the sheet by rolling to obtain a cladded material; annealing the cladded material under vacuum; and then, heating in an oxidizing atmosphere to form, as shown in Fig.12, an internal layer 5 in which intermetallic compounds, NiAl and Ni₃Al and a protecting layer of Al₂O₃ on the surface.
It is recommended to prepare a laminated sheet of the first Al foil 2, Ni foil 3 and the second Al foil 2' prior to cladding them on the substrate. In this case, suitable thickness of the laminated sheet is 0.1 - 1.0 mm, and that of the Al part is 0.005 - 0.5 mm in total of both the sides. It is of course operable to clad the foils one by one, or to prepare a laminate of any two of the three foils and clad it with the remaining one foil. Anyway, sequence of cladding can be shosen arbitrarily.
The substrate 1, which is a sheet or a wire of Fe-based alloy, Ni or Ni-alloy containing more than 20 weight % of Ni, and the layer 2 of Al or Al-alloy adhere intimately during the annealing due to mutual diffusion, and the diffusion further proceeds during the subsequent heating in the oxidizing atmosphere. As the result, Ni3Al and NiAl occur at the places in the direction of depth depending on the concentrations of Ni and Al. If the covering Al layer is thick, concentrations of the formed intermetallilc compounds will have a gradient, however, if the layer is thin and the heat treatment is done thoroughly, the concentration gradient substantially disappears and a layer in which Ni₃Al and NiAl are uniformly coexist is formed. In case where the substrate is a steel such as a SUS, intermetallic compoud, Fe₃Ni, occurs at the interfece of the substrate and Ni layer.
In case where the substrate Fe-based metal, Ni or an alloy thereof contains one or more of the above mentioned alloy elements which may form an intemetallic compound, i.e., B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr, Ta, Nb, Sc and REMs, or in case where one or more of such components are contained in the cladded Al-alloy, an intermetallic compoun or compounds between these elements and Al will occur instead of the Ni-Al intermetallic compounds.
In the embodiment in which a layer of Au, Ag or an alloy thereof is formed on the substrate, diffusion occurs between the substrate metal and Au, Ag or the alloy thereof, and Au, Ag or the alloy thereof and Al or Al-alloy during the annealing under vacuum or in the oxidizing atmosphere. If a component which forms an intermetallic compound with Au or Ag, then, the intermetallic compound of this combination will occur, and no Fe-Al intermetallic compoun will occur. However, if the layer of Au, Ag or the alloy thereof is thin and the heat treatment is done at a high temperature for a long period, then reactions between the metal componets to form the intermetallic compounds through the layer are possible.
In any of the above mentione cases, the Al at the surface is oxidized by heating in an oxidizing atmosphere to form dense Al₂O₃. Crystals of this compound may grow in the form of whiskers to coat the surface. At the same time of the formation of Al₂O₃, Al diffuses into the substrate metal of Ni or an alloy containing more than 20 weight % of Ni to cause good adhesion at the interface of joint, and a portion of the Al is oxidized to form Al₂O₃. As the result, the protecting layer of Al₂O₃ exists on the substrate with ankered roots in the substrate metal, and thus, strongly coat and protect the substrate.
As explaine above, Al₂O₃ crystals grow to entwine each other on the surface of the substrate metal and coat the substrate. The coating layer is usually a dense one, but it is inevitable that there are micropores in some places. Corrosive chemical liquid such as salt water may penetrate through the micropores and attack the substrate metal. The metal composite according to the present invention generally has much better corrosion-resistance due to the Ni-Al intermetallic compounds. Particularly, in the embodiment using a vapor-deposited or plated thin layer of Au, Ag or the alloy thereof, this layer which inherently has good corrosion resistance prevents penetation of the corrosive liquid.
In the embodiment in which the substrate 1 is covered by Al-Ni(or Nb)-Al, the interfaces between the substrate and the first Al layer 2, the Al layer 2 and the Ni layer 3, and the Ni layer 3 and the second Al layer 2' adhere solidly due to the diffusions as explaine above during the annealing under vacuum, and the diffusions further proceed during the subsequent heating in an oxidizing atmosphere. Thus, at the locations in the direction of depth, layers in which Ni3Al and NiAl coexist depending on the concentraions of Ni and Al components are formed.
In case of using a substrate of Ni alloy containing more than 20 weight % of Ni, certain amounts of B and Fe, and one or more of Mn and Re, the processability of the product metal composite is very good, and therefore, the product can be easily processed even after formation of Al₂O₃ protecting layer.
The present metal composite, which comprises the substrate metal of Fe-based alloy, Ni or Ni alloy containing more than 20 weight % or Ni, and the solid coating layer of Al₂O₃ on the surface of the substrate for protection, exhibits improved corrosion resistance represented by high temperature oxidation resistance and salt water-resistance. In regard to the electric heater material, a typical use of Fe-Cr alloy, the present metal composite can be understood as the product using FCH51-alloy which is less expensive and has good processability as the substrate and improving the high temperature corrosion resistance, which is low in this alloy. The metal composit can find wide use not only the above mentione typical use, electric heater, but also various use in industrial apparatus and electric appliances. In the embodiment of using Au, Ag or an alloy thereof on the substrate, the double coating of Au, Ag or the alloy and Al₂O₃ give much higher high temperatuer oxidation resistance as well as chemical resistance.
The method of producing the metal composite of the present invention enables production of the above described products by employing the easy way of forming Al layer such as cladding and transformation of the Al to Al₂O₃. This method thus gives the product which is protected by an Al₂O₃ layer of a thickness chosen from a wide range and has roots ankered in the substrate for solid adhesion. In the embodiment of adding alloy elements forming intermetallic compounds with Al in the substrate metal or in the Al alloy, it is possible to enjoy the benefits of: improved properties such as better adhesion between the substrate and the intemetallic compound layer, and between the intermetallic compound layer and the Al₂O₃ protecting layer; better processability given by improved ductility of the intermetallic compounds of B, Mn and Al; and higher hardness and higher electric resistance given by addition of Be.
The present invention will now be explained by the following examples. It is a matter of course that, although the description concerns the products in the form of sheet and wire which are easy to produce, as long as a practical way of cladding is used, products of any other form and shape can be produced in accordance with the present invention.

EXAMPLES

Example 1

An Al/Ni laminated sheet and an Al/Nb laminated sheet both having thickness of 0.1 mm, in which Al layer is 0.04 mm thick, were prepared. The laminated sheet were rolled on both sides of thin sheets of SUS430 in the manner that the Ni or Nb was inside, to produce cladded sheets of 0.05 mm thick. Thichness of the Al layers was 0.002 mm (per one side), and that of Ni or Nb layers was 0.004 mm (per one side).
The cladded sheets were slitted to ribbons of 6 mm wide and 200 m long, which were annealed under vacuum at 600^oC for 2 minutes, and then heated in air at 600^oC for 60 minutes. For comparison, a ribbon of SUS430 alone and a ribbon of SUS430 having Al layers of 0.002 mm thick (the same as above) on both sides were prepared and heated in air at 600^oC for 60 minutes.
These materials were subjected to the following corrosion tests:

(Salt Water Spry Test)

The test pieces are kept at a temperature of 800^oC under a constant tension, and receive spray of 5%-NaCl solution once every 2 minutes, and the times of spraying until the test piece breaks is recorded. The results are shown in Fig.13. The layer structures of the test pieces of the examples and the controls were as follows:

Control 1:: SUS430
Control 2:: Al/SUS430/Al
Example 1:: Al/Ni/SUS430/Ni/Al
Example 2:: Al/Nb/SUS430/Nb/Al

(Weight Increase by Oxidation)

The test pieces are kept in air at a temperature of 1100^oC, and the weight increase by passage of time (mg/cm2) is measured. The results are as shown in Fig.14.

Example 2

Al foils of 0.1 mm thick were cladded to both the sides of a thin FCH51 alloy of 0.2 mm thick by rolling. By further rolling a cladded material with a thickness of 0.1 mm in total, in which the thickness of both the Al layers was 0.04 mm.
The cladded material was slitted to ribbons of 30 mm width, and the ribbons were heated in air to 600^oC for 1 hour so as to cause the change of Al to Al₂O₃ and to grow the Al₂O₃ layer.
The ribbon of FCH51 alloy having the Al2O3 protecting layers were heated to 1100^oC in air and the weight increase by oxidation was observed as the measure of the high temperature oxidation-resistance. For comparison, a ribbon of FCH51 alloy alone and a ribbon of FCH51 alloy on which Al layer was vapor-deposited and subjected to oxidation treatment for formation of Al₂O₃ protecting layer were also tested under the same conditions. The results are shwon in Fig.15. From the graph of Fig.15 it is seen that the Al2O3 protecting layer effectively improved the high temperature oxidation resistance of FCH51 heater material.
The corrosion resistance to the salt water was observed by repetition of the cycles below: Spray of 5%-NaCl water solution (40^oC, 4 hours) ---Wetting (50^oC, RH 100%, 2 hours) --- Drying (60^oC, 2 hours)
Also in this test, a ribbon of FCH51 alloy having no Al2O3 protecting layer was tested, and the evaluation was made by observing the state of the surface. The results are shown in Fig 16. The graph of the Figure proves improvement of corrosion resistance by Al₂O₃ layer.

Example 3

On both sides of a thin sheet of FCH51 alloy of 0.2mm thick Au was vapor-deposited to form Au layers with respective thickness of about 100 Angstroms, and Al foils with a thickness of 0.015mm were cladded by rolling. By further rolling, a cladded material was obtailned. The thickness of the cladded material is 0.05 mm in total, those of the Au layers are 30 Angstroms, and those of the Al layers are 0.002 mm.
The material was slitted to ribbons of 30 mm width, and the ribbons were annealed under vacuum at 600^oC for 1 hour, and then, heated in air to 600^oC for 1 hour to cause the change of Al to Al₂O₃ and growth of the Al₂O₃ layer.
The FCH51 alloy ribbon having the Al₂O₃ protecting layers was heated in air to 1100^oC to observe the weight increase by oxidation for determining the high temperature oxidation resistance. For comparison, the tests of the same conditions were carried out on a ribbon of FCH51 alloy alone (Control) and a ribbon of FCH51 alloy on which Al was cladded and heated for formation of Al203 (Reference). The Refernce is an embodiment of the present invention. The results are shown in Fig.17. It is seen from the graph of Fig.17 that the Al₂O₃ layer effectively hightened the high temperature oxidation resistance of the FCH51 alloy.
Then, the corrosion resistance to salt water spray was determined under the conditions noted in Example 2. The ribbons of the Control and the Reference were also tested and evaluated in view of the surface corrosion. The results are shown in the graph of Fig.18. The graphe illustrates that the corrosion resistance given by the Al₂O₃ layer is further enhanced by the Au layer.

Example 4

The Fe-based alloys of the alloy compositions (weight %, the balance being Fe) shown in Table 1 were used as the substrates. On both the sides of the sheets of 0.2 mm thickness, Al foils of 0.015 mm thickness were cladded by rolling to give the covered material in the form of sheet, the total thickness of which was 0.05 mm and those of the two Al layers, 0.002 mm.

Table 1

	C	Si	Mn	Cr	Ni	Mo	Ti	V	Al	Others
A	0.005	0.3	2.03	14.20	-	-	0.53	-	3.05	B:0.1
B	0.06	1.23	0.86	18.60	-	-	-	-	3.62	Mg:0.03
C	0.05	0.85	1.58	19.00	9.06	-	-	0.5	-	La:200ppm
D	0.08	0.63	0.88	17.59	-	0.48	-	-	-	Ca:200ppm

The above covered materials were subjected to annealing under vacuum at 600^oC for 1 hour followed by heating in air at 800^oC for 68 minutes for formation of Al₂O₃.
The product using the substrate of the alloy "A" in Table 1 was tested by bending test and tensile test. The bending test comprises repeated cycles of bending to 90^o and flattening, and the number of cycles until the test piece breaks is recorded.. The number reached to 5. The elongation was 3 %.

Example 5

On both sides of thin sheets with thickness of 0.2 mm and width of 50 mm made of pure Ni or Ni-Cr alloys having the alloy compositions shown in Table 2 (weight %) Al foils with thickness of 0.15mm were cladded on the sheets by rolling.

Table 2

No.	C	Mn	Cr	Others
1	up to 0.15%	2.0%	20.0%
2	up to 0.10%	0.5%	22.5%	Al: up to 0.40% Ti: up to 0.40% Nb + Ta: 3.15 - 4.15 %
3	up to 0.15%	0.9%	21.5%	Al: up to 0.20% Ti: 0.90%
4	-	-	-	(pure Ni)

By further rolling cladded materials having total thickness of 0.1 mm, and the thickness of both the Al layers of 0.04 mm was obtained.
The salt water spray test as described in Example 1 was conducted on these samples to determine the high temperature corrosion resistance. The results are shown in Fig.19.

Example 6

Laminated sheets of Al/Ni/Al three layers were prepared. The thickness of the Al foils was 0.008 mm (0.004 mm a side) and the total thickness of the laminate was 0.1 mm. The laminated sheets were rolled on a thin Ni sheet to produce a cladded material with 0.05 mm thickness (No.5). In both the sides, thickness of the Al layers was (both the first and the second layers) 0.002 mm, and that of the Ni layer was 0.05 mm. The material was slitted to ribbons 6 mm in width and 200 m in length, which ribbons were heated under vacuume to 600^oC for 60 minutes.
Separately, there were prepared the following two samples:
(No.7) on both the sides of the same thin Ni sheet Al foils were cladded to form the Al layers of 0.002 mm thickness, and the cladded material was then subjected to the same heat treatment as above, and,
(No.6) on one side of the same thin Ni sheet the above mentioned laminated sheet of Al/Ni/Al three layers was cladded and on the other side only an Al foil was cladded, and the cladded material was then subjected to the same heat treatment as above.
The samples, Nos. 5 - 7 were tested by the salt water spray test to determine the high temperature corrosion resistance. The results are shown in Fig.20. The data of the Figure illustrates that the product in which the Al/Ni/Al laminated sheet was cladded to the thin Ni sheet obtained higher corrosion resistance than that of the product in which simply Al foils were cladded to the thin Ni sheet.

Example 7

To the Ni or Ni-Cr alloy of the composition shown in Table 3 (weight %) different amounts of B, Fe or Mn, Re were added, and the resulting alloys were rolled to the sheets of 0.2 mm thickness and 50 mm width, which were used as the substrates.

Table 3

No	Cr	Mo	Ni	Others
8	20%	-	balance	Si: 1.13%
9	22.5%	9%	balance	Co:up to 1.0%, Al:up to 0.4% Ti:up to 0.4%, Nb+Ta:3.15-4.15%
10	21.5%	3.0%	balance	Cu: 2.5%, Al: up to 0.2% Ti: 0.9%, Sc: up tp 3.0%
11	-	-	99% or more

On both the sides of these rolled sheets an Al foil of 0.015 mm thickness was cladded, and finally, the thickness of the cladded material was rolled down to 0.1 mm, and that of the Al layers, 0.004 mm.
The cladded material was subjected to annealing under vacuum to keep at 600^oC for 30 minutes to form the Ni-Al intermetallic compound, and then, heated in air to about 800^oC for 3 hours to form the Al₂O₃ layer at the surface.
Of the obtained metal composite products having Al₂O₃ protecting layer, the product using No.8 metal as the substrate was tested by the bending test and the tensile test. The bending test was described in Example 4. The test results are shown in Figs. 21-23. The plots in the Figures are the averaged data of the bending test on 5 samples of each compositions.

Example 8

On both the sides of thin sheet of NCH2 alloy (Ni: 58 %, Cr: 16 %, Fe: the balance) of 0.4 mm thickness, Al foils of 0.1 mm thickness were rolled to obtain a cladded material of 0.05 mm thickness. In both the sides, thickness of the Al layers are 0.004 mm per layer.
The material was slitted to ribbons 6 mm wide and 200 m long, and a ribbon was heated under vacuum to 600^oC for 60 minutes (No.12). For comparison, a ribbon of NCH2 alloy of the same dimensions was prepared (No.13).
The samples were tested by salt water dropping test to detemine the high temperature corrosion resistance. The salt water dropping test comprises, as in the salt water spray test, the test piece is kept at 800^oC under a constant tension, and 5%-NaCl water solution 0.5 cc is dropped once every 2 minutes on the test piece. The times of dropping the salt water until the break of the test piece is the measure of the corrosion resistance. The test results are shown in Fig.24.

Example 9

Thin sheets with 0.2 mm thickness were prepared using Ni-Al alloys which consist of Al 2 %, 4 %, 6 %, or 8 % and the balance of Ni. On both the sides of these substrates, Al foils of 0.015 mm thick were rolled to obtain cladded materials. The thickness was 0.05 mm in total, and that of the Al layers was 0.02 mm.
The cladded materials were subjected to annealing under vacuum at 600^oC for 2 minutes followed by heating in air to 600^oC for 1 hour to form the Al₂O₃ protecting layer.
To the resulting corrosion-resistant and metal composite products (Nos. 14, 15, 16, 17) the salt water dropping test as described in example 8 was conduted. The results are shown in Fig.25, from which it was observed that the corrosion resistance increases as the addition amount of Al increases, and gradually the effect saturates.
Out of the high temperature corrosion resistant products, No.16, which uses the substrate containing Al 6 %, was subjected to the test of weight increse by oxidation comprising heating the test piece to 1100^oC in air. The results are shown in Fig.26 in comparison with the test made on the Ni sheet of the same thickness (No. 18). It is seen from the data that the product according to the present invention exhibits such a high oxidation resistance that it does not get weight even after heating for a long period.

Example 10

Using a thin sheet made of the alloy having the composition of Ni-6%Al-2%Be as the substrate, a meta composite of the present invention was produced by the same operation as those of Exampel 9.(No. 19)
With respect to the intermetallic layer which is under the Al₂O₃ surface protecting layer, hardness was measured. The vlues in HV from the surface to the point of 12 micron deep are shown in Fig.27. The Figure illustrates increase of the hardness at the parts which are 5 micors deep or shallower, and the mechanism of increasing of the abrasion resistance.

Claims

A corrosion-resistant and heat-resistant metal composite characterized in that the composit comprises a metal substrate, an internal layer of at least one of the intermetallic compounds, NiAl, NbAl, FeAl, Ni₃Al, Nb₃Al and Fe₃Al, and a surface layer of Al₂O₃.
A corrosion-resistant and heat-resistant metal composite according to claim 1, wherein the metal substrate is made of Fe-based alloy (provided that the content of Ni is, if contained, up to 20 weight %), and the internal layer is a layer in which intermetallic compouds, Fe₃Al and FeAl coexist.
A corrosion-resistant and heat-resistant metal composite characterized in that the composit comprises a metal subsrate, an internal layer of Au, Ag or an alloy thereof, and a surface layer of Al₂O₃.
A metal composite according to claim 2 or claim 3, wherein the Fe-based alloy is one of structural carbon steels and alloy steels, Fe-Cr alloys, and ferritic, austenitic and martensitic stainless steels.
A metal composite according to claim 4, wherein the Fe-based alloy is an alloy which contains one or more of the additional alloy elements selected from B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr, and REMs.
A metal composite according to claim 1, wherein the substrate metal is Ni or a Ni-alloy containing more than 20 weight % of Ni, and the internal layer is a layer in which intermetallic compouds Ni₃Al and NiAl coexist.
A metal composite according to claim 6, wherein the Ni-alloy containing more than 20 weight % of Ni is a Ni-Cr alloy consisting of Cr: 14.0 - 23.5 %, C: up to 0.15 %, Mn: up to 2.5 % and the balance of Ni.
A metal composite according to claim 6, wherein the Ni-alloy containing more than 20 weight % of Ni is a Ni-based alloy consisting one or more of B: 0.08 - 0 18 %, Re: 1 - 3 %, Fe: 5 - 20 % and Mn: 5 - 20 %, and the balance of Ni.
A metal composite according to claim 6, wherein the Ni-alloy containing more than 20 weight % of Ni is a Fe-Ni-Cr alloy consisting of Ni: 20 - 72 %, Cr: 14 - 35 % and the balance of Fe.
A metal composite according to claim 6, wherein the Ni-alloy containing more than 20 weight % of Ni is a Ni-Al alloy consisting of Al: 1 - 6 % and the balance of Ni.
A metal composite according to claim 6, wherein the Ni-alloy containing more than 20 weight % of Ni is a Ni-Al-Be alloy consisting of Al: 1 - 6 %, Be: 0.2 - 3.0 % and the balance of Ni.
A metal composite according to claim 6, wherein the Ni-alloy containing more than 20 weight % of Ni is an alloy of such a composition that one or more of B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V Zr, Ta, Nb, Sc and REMs are added to Ni or the alloys defined in claims 7 - 11.
A method of producing a corrosion-resistant and heat-resistant metal composite sheet characterized in that the method comprises the steps of cladding an Al sheet and one of Ni sheet and Nb sheet on at lease a part of a metal substrate sheet in the manner that the Ni sheet or the Nb sheet is between the Al sheet and the substrate sheet; annealing the resulting cladded material under vacuum; heating the cladded material in an oxidizing atmosphere so as to form intermetallic compounds, Ni₃Al (or Nb₃Al) and/or NiAl (or NbAl) between the Al layer and the Ni layer (or Nb layer) and Al₂O₃ at the surface.
A method of producing a corrosion-resistant and heat-resistant metal composite wire characterized in that the method comprises the steps of cladding an Al tube and one of Ni tube and Nb tube on a metal substrate wire in the manner that the Ni tube or the Nb tube is between the Al tube and the substrate wire; annealing the resulting cladded material under vacuum; heating the cladded material in an oxidizing atmosphere so as to form intermetallic compounds, Ni₃Al (or Nb₃Al) and/or NiAl (or NbAl) between the Al layer and the Ni layer (or Nb layer) and Al₂O₃ at the surface.
A method of producing a corrosion-resistant and heat-resistant metal composite characterized in that the method comprises the steps of coating a metal substrate made of an Fe-based alloy (provided that the Ni content is, if contained, up to 20 weight %) with Al or Al alloy; annealing the resulting coated material under vacuum or in an inert gas atmosphere; and heating the material in an oxidizing atmosphere so as to form a layer in which intermetallic compouds, Fe₃Al and FeAl, coexist on the substrate and to form Al₂O₃ layer at the surface.
A method of producing a corrosion-resistant and heat-resistant metal composite charaterized in that the method comprises the steps of preparing a layer of Au or Ag or an alloy thereof by vapor-deposition or plating on a metal substrate made of an Fe-based alloy (provided that the content of Ni is, if contained, up to 20 weight %); coating the layer with Al or Al-alloy; annealing the coated material under vacuum or in an inert gas atmosphere; and heating the material in an oxidizing atmosphere so as to form Al₂O₃ layer at the surface.
A method of producing according to one of claims 15 and 16, wherein the substrate of the Fe-based alloy is in the form of sheet, and coating the substrate with Al or Al-alloy is carried out by clad-rolling.
A method of producing according to one of claims 15 and 16, wherein the substrate of the Fe-based alloy is in the form of wire; and coating the substrate with Al or Al-alloy is carried out by inserting the substrate wire in a tube of the coating material and by wire drawing or extrusion.
A method of producing according to one of claims 15 and 16, wherein coating the substrate with Al or Al alloy is carried out by dipping the substrate in molten Al or Al-alloy, thermal spray of Al or Al-alloy on the substrate, plasma powder welding, chemical plating, spattering or vapor-deposition.
A method of producing according to one of claims 15 and 16, wherein the metal substrate is in a bulky shape, and coating the substrate with Al or Al-alloy is carried out by dipping the substrate in molten Al or Al-alloy, thermal spray of Al or Al- alloy on the substrate, plasma powder welding, chemical plating, spattering or vapor-deposition.
A method of producing according to one of claims 15 and 16, wherein the Al-alloy contains one or more of B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr and REMs.
A method of producing corrosion-resistant and heat resistant metal composite charaterized in that the method comprises the steps of covering the surface of a substrate made of Ni or a Ni-alloy containing more than 20 weight % or Ni with Al or Al-alloy; annealig the covered material under vacuum or in an inert gas atmosphere; and heating the material in an oxidizing atmosphere so as to form a layer in which intermetallic compounds, Ni₃Al and NiAl, coexist on the substrate, and to form an Al₂O₃ layer at the surface.
A method of producing corrosion-resistant and heat-resistant metal composite characterized in that the method comprises the steps of covering the surface of a substrate made of Ni or a Ni-alloy containing more than 20 weigh % of Ni with, in the direction from the inside to the outside, the first Al layer, a Ni or Ni-alloy layer and the second Al layer; annealing the covered material under vacuum or in an inert gas atmosphere; and heating the material in an oxidizing atmosphere so as to form a layer in which intermetallic compounds, Ni₃Al and NiAl coexist on the substrate, and to form an Al₂O₃ layer at the surface.
A method of producing according to one of claims 22 and 23, wherein the substrate made of Ni or a Ni-alloy containing more than 20 weight % of Ni is in the form of sheet, and covering the substrate with Al or Al-alloy is carried out by clad-rolling.
A method of producing according to one of claims 22 and 23, wherein the substrate made of Ni or a Ni-alloy containing more than 20 weight % or Ni is in the form of wire, and covering the substrate with Al or an Al-alloy is carried out by inserting the substrate wire into a tube of the covering material and wire-drawing or extruding.
A method of producing according to claim 22, wherein covering the substrate with Al or an Al-alloy is carried out by dipping the substrate in molten Al or Al-alloy, thermal spray of Al or Al-alloy, plasma powder welding, chemical plating, spattering or vapor-deposition.
A method of producing according to claim 22, wherein the substrate made of Ni or a Ni-alloy containing more than 20 weight % of Ni is in a bulky shape, and covering the substrate with Al or Al-alloy is carried out by dipping the substrate in molten Al or Al-alloy, thermal spray of Al or Al-aloy, plasma powder welding, chemical plating, spattering or vapor-deposition.
A method of producing according to one of claim 22 and 23, wherein the Al-alloy has such a composition that one or more of B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr, Ta, Nb, Sc and REMs are added to Al.
A method of producing according to one of claims 13, 14, 15, 16, 22 and 23, wherein the annealing under vacuum or in an inert gas atmosphere is carried out by heating at a temperature of 400 - 900^oC for 1 - 200 minutes, and the heating in the oxidizing atmosphere is carried out by heating in air at a temperature of 400 - 1000^oC for 1 minute to 36 hours.