A New PM Process for Manufacturing of Alloyed Foams for High Temperature Applications

For many high temperature processes like combustion and reforming systems a couple of high performance alloys has been developed for semi finished products e. g. sheets, rods or tubes. Metallic foams with their high porosity are now candidates for catalyst carriers and filters in high temperature and corrosive environment. Among them Ni foam is commercial available as a mass product for battery applications mostly.Because of the poor oxidation resistance of Nickel, a powder metallurgical process was developed to transform the nickel foam into high alloyed temperature stable foam.

The biggest challenge is the alloy development with respect to the specific characteristics of the foam structure. High temperature alloys promoting impervious oxide layer formers like chromia and alumina ensure oxidation resistance. Under oxidation conditions, the alloy surface concentration of chromium and aluminium decreases triggering the diffusion of Cr and Al to the surface. For commercial sheet or foil material, a fraction of mm thick, the reservoir of alloy elements ensures the stability of the protective layer over the live time. The filligree foam struts with a wall thickness of only 10 to 15 μm requires therefore for extreme application conditions higher contents in Al and Cr.

A powder metallurgical process was developed that allows the conversion of a nickel or iron foams into high-temperature-stable alloy. The most important material parameters, namely the final composition, the specific surface area and the pore size of the foam can be varied in order to achieve optimum flexibility for the respective applications.

The nickel or iron foam is continuously unwound, coated first with a binder solution using a spraying technique and then with a high alloyed powder. Afterwards the foam is cut into sheets of the desired size. The following heat treatment includes a debindering and sintering step in which the elements from the powder diffuse into the foam struts and ensure a homogeneous alloy foam composition.

Microscopy images show a comparison of the pure Ni foam vs. alloyed foam. The high roughness of the alloyed foam offers a number of advantages, including high specific surface and good adhesion of catalytic coatings as well as good gas conversion efficiency for catalysts which are described in the sections to follow.

One promising field of application are components for exhaust aftertreatment systems e. g. diesel particle filters (DPF), diesel oxidation catalysts (DPF) and SCR catalysts (SCR — selective catalyst reaction) for the automotive and offroad industry. To examine the high temperature corrosion resistance the samples were tested in a tube furnace at 700, 800, 900 and 1000 °C in a diesel exhaust atmosphere (12%CO₂, 10%H₂O, 6%O₂, 200 ppm SO₂, balance N₂). The duration of the tests was 10, 30, and 100 hours.

Before the tests the samples were cleaned in an ethanol ultrasonic bath, dried and weighted. The furnace was heated up in argon and the samples were put into the hot furnace on a quartz crucible. After the test the samples were weighted again after cooling which indicates the oxidation rate.

Under 700 and 800°C the weight gain is marginal for the different sample qualities. Therefore the discussion will be focused on the oxidation tests at 900 and 1000°C.

The high operation temperatures up to 1000 °C and the compatibility with catalytic coatings (wash coat) needed special alloy composition and pre-treatment of the material. NiFeCrAl and FeCrAl alloys are promising candidates in terms of oxidation resistance because of its aluminium content, which enables the alloy to form an alumina layer and ensures a long-term oxidation resistance at temperatures > 950°C. Chromium supports the alumina scale formation on the surface because it prevents the internal oxidation before the alumina layer is established. Furthermore under nitrogen containing atmospheres at high temperatures without a dense alumina scale nitride formation is observed for commercially used alumina formers like FeCrAl and NiCrAl. In this case aluminium would be consumed near the surface which would result in a weak alumina scale formation and a less oxidation resistance. A further problem is the wash coat aging, which is caused by chromium migration into the catalytic material. As an effective diffusion barrier alumina is identified. Such a layer must be formed under well defined conditions, because its homogeneity and its existence prior application under operation conditions determinates the durability of the material system.

For the first based on a Ni foam substrate developed NiFe₂₂Cr₂₂Al₆ alloy a special preoxidation regime needed to evaluate. The high Ni content in that alloy is responsible for the (-structure and therefore low diffusion speed of aluminum to the surface to form a oxide layer before other metals of the alloy especially chromium is oxidized which is harmful to the catalytic layer and homogeneity of the alumina scale. Knowing the metal/metal oxide equilibriums of the different alloy components the formation of undesired oxides can be suppressed. The solution was a hydrogen atmosphere with an oxygen partial pressure p (O₂) above the equilibrium aluminium/alumina but below to that of chromium/chromia.

A further development aimed to a high temperature alloy like FeCr₂₂Al₆ alloys which could be more efficient pre-oxidized using a conventional air furnace. For that Fe foam precursors are needed for large scale production similar to that of the commercial Ni foam. This process is currently ongoing to be established. First prototypes were coated by the available coating and alloying process. The pre-oxidation regime under air was evaluated.Because of the structure of the FeCrAl the diffusion speed of the aluminium is higher than in the (matrix of NiFeCrAl and the alumina layer is established prior the oxidation of iron or chromium also under the high oxygen partial pressure of the air atmosphere. The protection of the formed oxide scale is demonstrated in the much lower weight gain of the pre-oxidized sample compared to its sintered state at 900°C.

Both alloy foams the NiFeCrAl and the FeCrAl have outstanding high temperature properties. This is shown in a diesel exhaust test at 1000°C however the weight gain of the NiFeCrAl alloy is higher compared to FeCrAl. The SEM micrographs (SEM = scanning electron microscope) and electron probe microanalysis (EPMA) in demonstrating a homogeneously formed alumina layer and no internal oxidation or nitrating of the strut material. A comparison of the distribution maps of oxygen and aluminium shows that the pre-oxidation has obviously formed a layer of alumina on the strut surface which is stable also after exposure for 100 h at 1000 °C under diesel exhaust. Looking at the NiFeCrAl micrograph on the inside strut surface a mixed oxide layer (chromia, alumina. nickel oxide, iron oxide) has formed above the alumina layer likely as a result of not fully pre-oxidation in this area. This fact can explain the higher weight gain of the sample at 1000 °C compared to FeCrAl. However the protection of the alumina scale is high enough for an outstanding durability at such high temperatures.

Source:

http://www.ifam-dd.fraunhofer.de/fhg/Images/217 _Walther_259 final_tcm260–173793. pdf