Hydrogen-water vapour pre-treatment of Fe-Cr-Al alloys

Abstract: A method is provided for forming a precursor layer of aluminium oxide on a cold-rolled ferritic stainless steel substrate having an aluminium content of less than five weight percent, and more preferably as low as 1.5 weight percent, by exposing the stainless steel to a hydrogen atmosphere containing controlled quantities of water vapour to provide a controlled hydrogen-to-water vapour ratio of less than about 1400. The aluminium can be provided either as a constituent of the ferritic stainless steel chemistry, or it can be provided as a coating on a ferritic stainless steel which contains essentially no aluminium itself. The resulting oxidation environment is extremely oxygen-poor, having a partial pressure of oxygen between about 2.7x10⁻²³ and 1.9x10⁻¹⁸ atmospheres. In addition, the environment has a relatively high water vapour content of up to about 10,000 ppm. The water vapour serves to replenish the oxygen content of the environment, through disassociation of the water vapour, as the oxygen becomes depleted as a result of oxidation of the aluminium in the stainless steel. The above process serves as a pre-treatment of the surface of the ferritic stainless steel, enabling a subsequent growth of a fully-developed layer of aluminium oxide whiskers that provides a surface which promotes adhesion of a catalytically active material to the stainless steel substrate.

Description: The present invention generally relates to ferritic stainless steel alloys having an aluminium content which is sufficient for producing an oxidation-resistant aluminium oxide layer at elevated temperatures. In particular, this invention relates to a method for growing an aluminium oxide layer on such an alloy, as specified in the preamble of claim 1, for example, as disclosed in US-A-4,318,828. More particularly, the present invention discloses an improved method for growing an aluminium oxide layer on such an alloy wherein the aluminium content of the alloy is below 5 weight percent, and more preferably as low as 1.5 weight percent, and wherein a precursor oxidation step is performed in a H₂/H₂O. atmosphere characterised by a relatively low oxygen partial pressure. In the automotive industry, catalytic converters typically utilise a metal foil monolith structure which supports the catalytically active materials that convert the noxious exhaust gases of an internal combustion engine to harmless gases. Conventionally, the metal foil monolith has a metal foil substrate which is extremely thin, generally of the order of about 0.05 millimetres thick. The hot exhaust gas environment of an internal combustion engine is very severe, and subjects the metal foil to continued thermal cycling which is detrimental to the bond between the metal foil and the catalytically active materials adhered to its surface. To survive in this harsh environment, the metal foil must have excellent oxidation resistance at temperatures exceeding 1000°C. In addition, the metal foil must have suitable surface properties which promote adhesion of the catalytically active materials to its surface. Currently, the standard foil material used in the automotive production of these monoliths is an Fe-20Cr-5Al alloy. This alloy is extremely oxidation-resistant particularly at elevated temperatures because it forms a protective aluminium oxide scale on its surface. As is known in the art, upon exposure to an appropriate atmosphere at elevated temperatures, the relatively high level of aluminium (about five weight percent) within alloys of this type promotes the formation of a protective layer of aluminium oxide crystals, or "whiskers", on the exposed surfaces of the metal foil. Once a suitable layer of aluminium oxide whiskers is obtained, the metal foil is further coated with a catalytically-active gamma alumina washcoat material and this coating is impregnated with a noble metal catalyst. The aluminium oxide whiskers, when present in sufficient quantities to form a dense coverage over the exposed surfaces of the metal foil, provide a suitable surface to which the gamma alumina washcoat can readily adhere. Preferably, the layer of aluminium oxide whiskers is characterised by being continuous, slow growing and adherent, covering at least about 80 percent, and more preferably at least about 90 percent of the exposed surface. Furthermore, it is preferable that the whiskers are characterised as having a length of at least about 1.5 µm and possessing an aspect ratio (the ratio between the length and width of the whiskers) of at least about 2 to 1. Notably, metal foils formed from alloys having low levels of aluminium (e.g. below 5 percent weight) are generally unable to produce aluminium oxide having adequate whisker characteristics and adequate surface coverage when oxidised in an O₂-rich atmosphere. Furthermore, oxidation of Fe-Cr-Al alloys containing 3% or less of aluminium are known to cause growth of Fe-rich and Cr-rich oxides which are unsuitable for adhering a catalytic washcoat. In an effort to promote the growth of aluminium oxide whiskers on a cold-rolled Fe-20Cr-5Al alloy, U.S. Patent No. 4,318,828 to Chapman, proposed a process for forming a precursor layer of aluminium oxide in an oxygen-poor environment, and then further developing a dense layer of aluminium oxide whiskers in a subsequent oxygen-rich environment, such as atmospheric air at an elevated temperature. The oxygen-poor environment disclosed in U.S. Patent No. 4,318,828 contained oxygen at partial pressure (P-O₂) levels of between 99.99 and 199.98 Pa (0.75 and 1.5 torr) (0.1 and 0.2 volume percent O₂, or less than about 2000 parts per million (ppm)), with the balance of the environment being inert and consisting of nitrogen, hydrogen, carbon dioxide, argon or other noble gases. U.S. Patent No. 4,318,828 discloses that the precursor aluminium oxide layer could not be formed if an atmosphere containing more than about 0.2 volume percent (2000 ppm) of oxygen was present. However, in practice oxidation atmospheres having an oxygen content of 1000 ppm or more have produced little or no whisker growth. In addition, U.S. Patent No. 4,318,828 discloses that the oxygen could be introduced by the disassociation of water at the furnace temperatures taught. However, it is noted in U.S. Patent No. 4,318,828 that test results were erratic when using a pure hydrogen environment with a trace of water vapour and having a dew point of about -60°C - equivalent to a hydrogen-to-water vapour (H₂/H₂O) ratio of approximately 100,000. Such an atmosphere is extremely dry, having a water vapour content of the order of 10 ppm or less, and has an extremely low level of available oxygen for formation of the precursor layer of aluminium oxide, corresponding to a partial pressure of oxygen (P-O₂) of approximately 4.9 x 10⁻²² Pa (4.9x10⁻²⁷ atmospheres). Though U.S. Patent No. 4,318,828 is not directed to the problem of forming aluminium oxides with alloys having an aluminium content of less than about 3 weight percent, the automotive industry is continually seeking alternatives to the use of the Fe-20Cr-5Al alloy typically used, primarily in an effort to reduce costs. The high cost of this alloy is principally due to its relatively high aluminium content within the alloy, which significantly reduces the ductility of the material, making the alloy more difficult, and thus more expensive, to process than alloys having an aluminium content of less than about 5 weight percent. Because of the low ductility of the Fe-20Cr-5Al alloy, relatively inexpensive processing methods, such as continuous slab casting, cannot be used in manufacturing the alloy because thermally-induced stresses caused by-rapid cooling during the continuous casting would cause the alloy slab to crack. Therefore, to avoid cracking, the alloy is almost always ingot cast. However, this is not a preferred alternative, since the tops and bottoms of the cast ingots generally contain gross inhomogeneities and therefore must be removed prior to hot-working of the material, resulting in unacceptably poor yields. Also, the poor ductility of the material due to its high aluminium content further complicates the preparation of the cast ingot surface for subsequent hot-working. In order to remove casting defects, so as to obtain a high surface quality, surface grinding must be performed on the material. It is preferred that the surface grinding operation occur at room temperature, where ingot handling and inspection are easier, which results in a markedly better surface finish. Unfortunately, this Fe-20Cr-5Al material must be surface ground at high temperatures in order to avoid ingot cracking which would result from the thermally-induced stresses during cooling. High-temperature grinding invariably results in a surface finish of poorer quality as compared to a surface which is ground at room temperature. In addition, the poor surface quality of the cast ingot material coupled with its low ductility cause excessive strip breakage during cold rolling of the alloy. Typically, after final rolling of this material to produce the desired foil, only about 40-60 percent of the original cast ingot is retrieved. Therefore, what is needed is a method for forming a suitable layer of aluminium oxide whiskers on a metal foil characterised by having an aluminium content of substantially less than five weight percent, such that the material will be capable of providing the requisite adhesion for the catalytic washcoat. A method according to the present invention, for forming an aluminium oxide film on a substrate having an aluminium constituent, is characterised over US-A-4,318,828 by the features specified in the characterising portion of claim 1. It is an object of the present invention to provide a ferritic stainless steel material which is characterised by having a surface which is capable of promoting the adherence of a catalytically active material. It is a further object of this invention that the ferritic stainless steel should contain standard amounts of chromium and the normal steel-making impurities, though more particularly with an aluminium content of significantly less than about 5 weight percent, and preferably as low as about 1.5 weight percent. Alternatively, the ferritic stainless steel contains essentially no aluminium itself, but is coated with aluminium. It is still a further object of this invention that the aluminium content, either present within the ferritic stainless steel or in the form of an aluminium coating on the ferritic stainless steel, is oxidised in a manner that forms a layer of aluminium oxide whiskers so as to be suitable for use as a monolith catalytic converter support. It is yet another object of this invention that the aluminium content of the ferritic stainless steel is sufficient to produce a layer of aluminium oxide whiskers which will provide a suitable surface to which the catalytically active materials will adhere to the ferritic stainless steel. In accordance with a preferred embodiment of this invention, these and other objects and advantages are accomplished as follows. A cold-rolled ferritic stainless steel foil is provided which is suitable for use as an automobile monolith catalytic converter support. The ferritic stainless steel base metal foil consists essentially of, by weight, about 15-25% chromium, along with normal steelmaking impurities. The amount of aluminium within the ferritic stainless steel base metal is significantly less than about 5 weight percent, and more preferably as low as about 1.5 weight percent. Alternatively, the ferritic stainless steel base metal contains essentially no aluminium itself, but is coated with aluminium so as to obtain an average aluminium content within the composite material (which consists of the aluminium coating and the ferritic stainless steel base metal) of preferably less than about 10 weight percent. Because the base metal has either a low aluminium content or is essentially aluminium-free, the stainless steel base metal alloy of this invention is relatively ductile and can therefore be easily rolled to very thin gauges required for the formation of monolith catalytic converter supports, in contrast to the conventional Fe-20Cr-5Al alloy strip. To enable the ferritic stainless steel alloy of this invention to grow a surface oxide suitable for washcoat adhesion, the alloy is first exposed to an inert gas environment having approximately five volume percent hydrogen (H₂) with water vapour (H₂O) in sufficient quantities to provide a controlled H₂/H₂O ratio of less than about 1400. In effect, the present invention provides an extremely oxygen-poor environment, having an oxygen partial pressure of up to about 1.9 x 10⁻¹³ Pa (1.9x10⁻¹⁸ atmospheres). In addition, the environment is relatively "wet", having a significantly greater water vapour content than the prior art of up to about 10,000 ppm. The water vapour content of the environment serves as a source for oxygen (O₂) which ensures that a relatively constant but extremely low level of O₂ is available for oxidation of the aluminium to form the precursor layer of aluminium oxide. Through dissociation of the water vapour, the water vapour content of the oxygen-poor environment is capable of replenishing the oxygen content of the environment if the oxygen becomes depleted. According to a key aspect of the present invention, when exposed to high temperatures in this environment, the ferritic stainless steel alloy quickly forms a precursor layer of aluminium oxide on its exposed surfaces. The precursor layer of aluminium oxide promotes the growth of a fully-developed layer of aluminium oxide whiskers which is subsequently formed using standard whisker growth oxidation methods known in the art. The stainless steel alloy is then exposed for a longer period of time to atmospheric air at temperatures sufficient to oxidise the exposed precursor aluminium oxide to fully form the protective layer of aluminium oxide whiskers. The oxide layer is continuous, slow growing and tenaciously adherent to the base metal foil, thereby creating a surface on the stainless steel alloy which is capable of adhering a catalytically active washcoat material. The oxidation resistance of the stainless steel material of this invention, while somewhat less than that of a conventional Fe-20Cr-5Al alloy currently used within the automotive industry as the catalytic converter monolith support structure, is sufficient for light duty applications, such as diesel engines. In addition, the ductility of the stainless steel material of this invention is superior to that of stainless steels containing greater quantities of aluminium, such as the conventional Fe-20Cr-5Al alloy. As a result, processing of the stainless steel material is much easier. Therefore, an inventive feature of this material is that the ferritic stainless steel base metal foil contains aluminium in sufficiently low levels to result in a material having increased ductility as compared to the conventional alloys used to form metal foil monolith structures. As a result, the processing costs associated with the alloy of this invention are significantly reduced. Lastly, following pre-treatment in the preferred H₂/H₂O environment, the ferritic stainless steel of this invention exhibits good aluminium oxide whisker growth upon exposure to atmospheric air at elevated temperatures. This is important since it is the aluminium oxide whiskers which carry the subsequently deposited catalytically active materials when used in a catalytic converter environment for conversion of the exhaust gases. Therefore, it is desirable that the material be capable of growing high-aspect ratio oxide whiskers so as to ensure good adherence between the catalytically active materials and the foil. Other objects and advantages of this invention will be better appreciated from the following detailed description. In accordance with the method of this invention, a precursor layer of aluminium oxide is formed on a cold-rolled ferritic stainless steel substrate having an aluminium content of less than five weight percent, and more preferably as low as about 1.5 weight percent, by exposing the stainless steel to a hydrogen atmosphere containing controlled quantities of water vapour to provide a controlled H₂/H₂O ratio of less than about 1400. From this precursor layer of aluminium oxide, there can be subsequently grown a fully developed layer of aluminium oxide whiskers whose coverage and whisker length are sufficient to reliably adhere a second layer, such as a catalytically-active gamma alumina washcoat material. The aluminium can be provided either as a constituent of the ferritic stainless steel chemistry, or it can be provided as a coating on a ferritic stainless steel which contains essentially no aluminium itself. The aluminium coating is applied in sufficient quantities so as to obtain an average aluminium content within the composite material (i.e., the aluminium coating and the ferritic stainless steel) of preferably less than about 10 weight percent. At high temperatures, this aluminium coating simultaneously diffuses into the ferritic stainless steel and oxidises to form an aluminium oxide protective skin. In effect, the method of the present invention uses an extremely oxygen-poor environment, though greater than that of the prior art, characterised by a partial pressure of oxygen between about 2.7x10⁻¹⁸ and 1.9x10⁻¹³ Pa (2.7x10⁻²³ and 1.9x10⁻¹⁸ atmospheres). In addition, the environment has a relatively high water vapour content, of the order of up to about 10,000 ppm. The water vapour content of the present invention is about 1000 times greater than that known or attempted by the prior art when experimenting with a hydrogen atmosphere. At these levels, the water vapour serves as a buffer which ensures that a relatively constant but extremely low level of oxygen is available for oxidation of the aluminium to form the precursor layer of aluminium oxide. The water vapour serves to replenish the oxygen content of the environment, through dissociation of the water vapour, as the oxygen becomes depleted as a result of oxidation of the aluminium in the stainless steel. The above process serves as a pre-treatment of the surface of the ferritic stainless steel, enabling a subsequent growth of a fully developed layer of aluminium oxide whiskers that provides a surface which promotes adhesion of the catalytically active washcoat material to the stainless steel substrate. Generally, the pre-treatment process of the present invention consists of oxidising as-cold rolled foil for about 30 seconds at temperatures of between about 800°C and 1000°C in an atmosphere with a controlled H₂/H₂O ratio of less than about 1400. Experimentally, a range of preferred atmospheric compositions for the pre-treatment environment was created in a stainless steel retort. The atmospheric compositions were controlled by passing argon (Ar) with a 5 volume percent hydrogen (H₂) content through a bubbler containing distilled water. While Ar was used, it will be well understood by those skilled in the art that other inert atmospheres would also be suitable, such as nitrogen or other noble gases. The principle behind using a low hydrogen content in an inert atmosphere during testing was to reduce the hazards associated with hydrogen. The humidified Ar-5%H₂ mixture was then diluted with a dry Ar-5%H₂ mixture to obtain the desired H₂/H₂O ratios. Gas flow rates were controlled to be about 0.246 cubic metre (8.7 standard cubic feet) per hour at room temperature. Atmospheric water vapour content was monitored going into and coming out of the stainless steel retort using a chilled mirror dew-point sensor. Following pre-treatment where the precursor layer of aluminium oxide was formed, the samples were further oxidised for whisker growth in a box furnace using standard whisker growth oxidation cycles known in the art - i.e. about 16 hours in air at about 925°C and at atmospheric pressure. During experiments conducted to assess the capabilities of the pre-treatment process on 20% chromium steel sample pieces, the form in which the aluminium was provided (as a constituent of the alloy or as a coating), the aluminium content and the H₂/H₂O ratio of the pre-treatment atmosphere were varied to generate results which identify minimum atmospheric compositions for given stainless steel chemistries. Atmospheric conditions are given in terms of water vapour (H₂O) in parts per million of (ppm), the H₂/H₂O ratio, and the partial pressure of oxygen (P-O₂) in atmospheres (atm). For comparison, three conventional pre-treatments at 900°C for 30 seconds, similar to that disclosed in U.S. Patent No. 4,318,828, were also conducted. One such treatment consisted of exposing the samples to an environment containing 100 ppm O₂ in N₂. The second conventional treatment consisted of exposing the samples to an environment containing 10 ppm O₂ in N₂, while the third conventional treatment consisted of exposing the samples to an environment containing CO₂. Whisker coverage of the samples was estimated from back-scatter electron micrographs taken at a magnification of 35X, while whisker size was estimated from 5000X or 10,000X magnification micrographs taken at an angle of 25° from normal. The criterion for assessing the sufficiency of the final aluminium oxide whisker layers was a whisker length of at least about 1.5 µm and coverage of at least 90 percent of the sample surface. In addition, an aspect ratio of at least 2 is preferred for purposes of providing a surface which promotes the adhesion of the catalytically active materials to the stainless steel substrate. Whisker Coverage (%) and Size (µm) For Fe-Cr-Al Alloys Pretreatment Atmosphere %Al Content Conventional methods 4.9% 4.0% 3.0% 2.45% 2.0% 1.68% 1.5% 100 ppm O₂ in N₂ >95 >95 95 95 70 - 0 2-3 3-4 2-4 2-3 1-2 - none 10 ppm O₂ in N₂ >95 >95 95 90 70 - 0 2-3 3-4 2-4 2-4 1-2 - none CO₂ - - - - - 0 0 - - - - - (rosettes) H₂/H₂O Method H₂O Content (ppm) H₂/H₂O P-O₂(Pa) [atm] %Al Content 4.9% 4.0% 3.0% 2.45% 2.0% 1.68% 1.5% 10,000±1000 5 1.9E-13 >95 >95 >95 95 80 - 20 [1.9E-18] 3-4 2-3 2-3 2-3 1-2 - 1 1080±100 46 2.3E-15 >95 >95 95 90 90 - 50 [2.3E-20] 2-3 2-4 1-3 2-3 2-4 - 1-2 690±30 72 9.3E-16 - - - - - 90 50 [9.3E-21] - - - - - 1-2 1-2 370±160 135 2.7E-16 - - - - - 90 40 [2.7E-21] - - - - - 1-2 1 130±40 380 3.3E-17 - - - - - 90 70 [3.3E-22] - - - - - 1-2 1-2 105±10 480 2.1E-17 >95 >95 95 90 90 - 90 [2.1E-22] 2-4 3-4 2-3 2-4 2-3 - 2-3 68±20 740 9.0E-18 - - - - - 90 20 [9.0E-23] - - - - - 1-3 1 (a) 50±12 1000 4.9E-18 - - - - - 80 90 [4.9E-23] - - - - - 1-2 1-3 (b) 37±3 1350 2.7E-18 - - - - - 90 85 [2.7E-23] - - - - - 1-2 1-2 All pre-treatments were conducted at 900°C for 30 seconds, except : (a) - Pre-treatment time of 2 minutes. (b) - Pre-treatment time of 5 minutes. Whisker Coverage (%) and Size (µm) For Al-Coated Fe-Cr-Al Alloys H₂/H₂O Method H₂O Content (ppm) H₂/H₂O P-O₂ (Pa) [atm] %Al Content 6.8% 5.9% 6.3% 3000 17 1.7E-14 80 80 >95 [1.7E-19] 1-3 3-5 1-3 1050±70 48 2.1E-15 95 90 >95 [2.1E-20] 1-4 2-4 1-3 101±14 500 2.0E-17 95 90 >95 [2.0E-22] 2-4 2-4 1-3 (a) 53±6 940 5.5E-18 >95 >95 >95 [5.5E-23] 2-4 3-5 1-4 (b) 11±2 4500 2.E4E-19 70 70 0 [2.4E-24] 1-3 2-4 Coral-like All pre-treatments were conducted at 900°C for 30 seconds, except: (a) - Pre-treatment time of 5 minutes. (b) - Pre-treatment time of 30 minutes. Prior to coating : The 6.8% alloy contained 20.3% Cr and 0.015% rare earth elements. The 5.9% alloy contained 20.3% Cr and 0.015% rare earth elements. After coating: The 6.3% alloy contained 10.2% Cr, 0.2% ti and 0.12% Zr. Note: The oxygen partial pressure shown in Tables 1 and 2 is expressed in terms of a factor shown as "E-a number". This factor is used for compactness in the Tables and represents "ten raised to the power of the number". Thus, for example, in Table 1, 1.9E-18 represents 1.9x10⁻¹⁸, and, in Table 2, 5.5E-23 represents 5.5x10⁻²³. From Table 1 above, it can be seen that, for the Fe-Cr-Al stainless steel chemistries having less than 2.45 weight percent Al, the conventional pre-treatments (100 ppm and 10 ppm O₂ in N₂) exhibited whisker growth and coverage which were inadequate (i.e. less than about 1.5 µm in length and less than about 90% coverage). In addition, coarse Fe-rich and Cr-rich oxides were formed which are unsuitable for adhering the catalytically-active washcoat. Between Al contents of about 2.45 and 4.9 weight percent, the O₂ in N₂ pre-treatment methods produced good whisker growth, exhibiting high-aspect-ratio whiskers with good coverage. Results from pre-treatment of the 1.68% and 1.5% Al alloys with CO₂ produced an aluminium oxide covering roughly 80 percent of the sample surface, but the oxide was flat with sparse oxide rosettes instead of high-aspect-ratio whiskers. Accordingly, the CO₂ pre-treatment did not produce a layer of aluminium oxide upon which could be grown a fully-developed layer of aluminium oxide whiskers. For alloys containing aluminium at levels of 2.5% and higher, the Ar-5% H₂ pre-treatment method of the present invention was capable of producing a precursor layer of aluminium oxide which was comparable to that of the conventional methods. The aluminium oxide whiskers were high-aspect-ratio with good coverage. In addition, varying the H₂/H₂O ratio between 5 and 480 had only a minor effect on the coverage of the sample surfaces. In contrast to that of the conventional methods, for alloys containing aluminium at levels of 2.0% and less, the Ar-5% H₂ pre-treatment method markedly improved whisker growth. For the 2.0% Al alloy, good whisker growth was obtained with H₂/H₂O ratios between 46 and 480. Some degradation in whisker quality occurred at a H₂/H₂O ratio of 5 from formation of Fe-rich and Cr-rich oxides. At H₂/H₂O ratios between 72 and 740, the alloy containing aluminium at levels of about 1.68% exhibited good whisker growth behaviour. At higher H₂/H₂O ratios, the pre-treatment method was not as successful. However, for the H₂/H₂O ratios of 1000 and 1350, increasing the pre-treatment time from 30 seconds to 2 and 5 minutes, respectively, produced good precursor oxides. Accordingly, it is foreseeable that a good precursor oxide layer can be formed using even higher H₂/H₂O ratios if the pre-treatment time is of sufficient duration. The 1.5% aluminium alloy was more sensitive to low H₂/H₂O ratios. At a ratio of 480, this alloy showed good whisker growth. However, at ratios below 480 whisker growth quality degraded, particularly as a result from the formation of Fe-rich and Cr-rich oxides. At high H₂/H₂O ratios, the 1.5% aluminium alloy exhibited the same sensitivity as the 1.68% aluminium alloy. Results for the pre-treatment of the 1.5% and 1.68% aluminium alloys at high H₂/H₂O ratios (1000 and 1350) indicate that pre-treatment in hydrogen with a low water vapour level, as suggested in U.S. Patent No. 4,318,828 (an H₂/H₂O ratio of 100,000) would not be successful. In effect, each of the conventional pre-treatment methods (100 ppm and 10 ppm O₂ in N₂, CO₂) were inferior to the method of the present invention at aluminium contents below about 2.45 weight percent. At greater aluminium contents, the Ar-5% H₂ method of the present invention produced comparable results to the conventional pre-treatment methods. As can be seen in Table 2, the Ar-5% H₂ pre-treatment method of the present invention was also capable of producing a precursor layer of aluminium oxide on the aluminium-coated Fe-Cr alloys which was comparable to that of the conventional methods in that high-aspect-ratio aluminium oxide whiskers were grown with good coverage. In addition, varying the H₂/H₂O ratio between 48 and 940 had only a minor effect on the coverage of the sample surfaces. The aluminium-coated alloys pre-treated according to the Ar-5% H₂ method exhibited whisker growth comparable to that of Fe-Cr-Al samples which contained about 2.45 weight percent or more aluminium and which were pre-treated according to conventional methods using an O₂ in N₂ atmosphere. The 6.8 and 5.9% samples exposed to an atmosphere having a H₂/H₂O ratio of about 17 exhibited some degradation in whisker formation, while there was no apparent adverse effect on the 6.3% sample. However, a H₂/H₂O ratio of about 4500 exhibited marked degradation in all of the aluminium-coated samples tested. The 6.3% alloy appeared to be most sensitive to high H₂/H₂O ratios in that no aluminium oxide whisker growth occurred at all at an H₂/H₂O ratio of 4500. From the above, it can be generally stated that the Ar-5% H₂ method of the present invention produced comparable results to the conventional pre-treatment methods of samples with aluminium contents greater than 2.45 weight percent using an O₂ in N₂ atmosphere, as long as the H₂O content was greater than about 11 ppm. Based upon the above results, the method of the present invention can be characterised as a suitable pre-treatment for forming a precursor layer of aluminium oxide on a ferritic stainless steel containing aluminium at levels as low as about 1.5 weight percent and on a ferritic stainless steel which contains no aluminium but which is coated with aluminium. The pre-treatment method yields a precursor aluminium oxide layer from which a layer of aluminium oxide whiskers can be grown to provide a surface suitable for adhering the catalytically-active washcoat to the ferritic stainless steel. The pre-treatment method encompasses controlling the level of water vapour in a hydrogen atmosphere so as to control the H₂/H₂O ratios. The oxygen disassociated from the H₂/H₂O atmosphere reacts with the aluminium present in the ferritic stainless steel to produce a precursor aluminium oxide layer in a relatively short time at 900°C. In addition, the high water vapour content simplifies atmospheric control in that wider ranges of atmospheric compositions are capable of producing suitable results. Successful pre-treatment requires limitations on both the upper and lower H₂/H₂O ratios. The lower ratio (higher water vapour content) atmospheres are sensitive to alloy composition. Alloys containing about 5% to as low as about 2.5% aluminium can be successfully pre-treated in atmospheres with ratios as low as 5 (oxygen partial pressures of 1.9x10⁻¹³ Pa (1.9x10⁻¹⁸ atmospheres)), while alloys containing lower aluminium contents cannot be pre-treated at such high water vapour levels. Successful pre-treatment using high H₂/H₂O ratio (lower water vapour content) atmospheres is generally limited by the rate of growth of the precursor oxide. Restrictions on the upper ratio appear to be independent of the aluminium content of the alloy. Tests on the 1.5% and 1.68% aluminium alloys indicate that the maximum ratios for successful pre-treatment depend on the time and are about 480 for 30 second pre-treatments, 1000 for 2 minute pre-treatments, and 1350 for 5 minute pre-treatments. From the above, it can be seen that a significant advantage to the method of the present invention is that a suitable metal foil monolith structure can be formed from a ferritic stainless steel containing aluminium in sufficiently low levels to result in a material having increased ductility as compared to the conventional alloys used to form metal foil monolith structures. Therefore the processing costs associated with the preferred alloy of this invention are significantly reduced. Another significant advantage is that, following pre-treatment in the preferred H₂/H₂O environment, the ferritic stainless steel of this invention exhibits good aluminium oxide whisker growth upon exposure to atmospheric air at elevated temperatures. This is important since it is the aluminium oxide whiskers which carry the subsequently deposited washcoat and catalytically active materials when used in a catalytic converter environment for conversion of the exhaust gases. Therefore, it is desirable that the material be capable of growing high-aspect ratio oxide whiskers so as to ensure good adherence between the catalytically active materials and the foil. In addition, the preferred pre-treatment atmosphere has a relatively wide range of acceptable atmospheric compositions which will yield suitable precursor aluminium oxide growth on the ferritic stainless steel foil. Generally, a 30 second exposure to an environment having a H₂/H₂O ratio of up to about 500 is suitable for obtaining whisker growth on ferritic stainless steels having an aluminium content as low as about 1.68 weight percent. Over exposure times of between about 2 to 5 minutes, H₂/H₂O ratios of greater than about 500 and less than about 1400 are suitable for obtaining whisker growth on ferritic stainless steels having an aluminium content as low as about 1.5 weight percent. Whilst the present invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art, for example by modifying the processing parameters such as the temperatures or durations employed, or by substituting appropriate ferritic stainless steel materials, or by slightly altering the compositions of the pre-treatment atmospheres whilst still realising the benefit of a very low oxygen content in a H₂/H₂O atmosphere. Accordingly, the scope of the present invention is to be limited only by the sope of the following claims.

Claims: A method for forming an aluminium oxide film capable of promoting the adherence of a catalytically active material on a ferritic stainless steel substrate having an aluminium constituent, said method comprising the steps of exposing said substrate to an atmosphere having as its reactive portion a mixture consisting essentially of hydrogen and more than 10 ppm water vapour at a predetermined temperature; oxidising said exposed substrate by heating it in a predominantly inert atmosphere containing a low concentration of oxygen to promote whisker formation on a surface of said substrate during subsequent oxidation; and heating the oxidised substrate in air; characterised in that said mixture has a H₂/H₂O ratio productive of an oxygen partial pressure in the range of 2.7 x 10⁻¹⁸Pa to 1.9 x 10⁻¹³Pa (2.7 x 10⁻²³ to 1.9 x 10⁻¹⁸atm) at the surface of said substrate. A method for forming an aluminium oxide film according to claim 1, in which said predetermined temperature is between about 800°C and 1000°C. A method for forming an aluminium oxide film according to claim 1 or claim 2, in which said substrate is exposed to said atmosphere for no more than about five minutes. A method for forming an aluminium oxide film according to any one of claims 1 to 3, in which said atmosphere is substantially at atmospheric pressure. A method for forming an aluminium oxide film according to any one of claims 1 to 4, in which said aluminium constituent is no more than about five weight percent of said substrate. A method for forming an aluminium oxide film according to any one of claims 1 to 4, in which the aluminium constituent is present as an aluminium coating on said substrate, said aluminium coating being an adherent aluminium layer on said substrate, and in which said H₂/H₂O ratio is no more than about 940 and said corresponding oxygen partial pressure is as low as about 5.5 x 10⁻¹⁸Pa (5.5 x 10⁻²³ atmospheres). A method for forming an aluminium oxide film according to any one of the preceding claims, in which said substrate is a ferritic stainless steel foil. A method for forming an aluminium oxide film according to any one of claims 1 to 4, in which said predetermined length of time is up to about 30 seconds for a H₂/H₂O ratio of up to about 740 and an oxygen partial pressure of about 9.0 x 10⁻¹⁸Pa (9.0 x 10⁻²³ atmospheres). A method for forming an aluminium oxide film according to any one of claims 1 to 4, in which said predetermined length of time is up to about five minutes for a H₂/H₂O ratio of up to about 1350 and an oxygen partial pressure of as low as about 2.7 x 10⁻¹⁸Pa (2.7 x 10⁻²³ atmospheres). A method for forming an aluminium oxide film according to any one of claims 1 to 4, in which said predetermined length of time is up to about two minutes for a H₂/H₂O ratio of up to about 1000 and an oxygen partial pressure of as low as about 4.9 x 10⁻¹⁸Pa (4.9 x 10⁻²³ atmospheres).

Inventors: SIGLER DAVID RUDOLPH, SIGLER, DAVID RUDOLPH
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1993
Main classification: C23C8
IPC codes: C23C8