DE10116762C1 - Aluminum-titanium diffusion layers, used for improving corrosion resistance of metallic materials used in refinery for gasifying coal, petroleum coke or other residual materials, are produced by co-diffusion - Google Patents

Abstract

Aluminum-titanium diffusion layers are produced by co-diffusing the diffusion elements aluminum and titanium as pure metal powder in a weight ratio of 1: (0.1-5). Preferred Features: The layer is produced using a powder packing process. Activators, preferably ammonium halides, especially aluminum or titanium halides are used to produce the diffusion layers.

Description

The invention relates to the production of diffusion layers with high Contained in aluminum and titanium and their use as corrosion protection for metallic materials in reducing, sulfidizing environments with high carbon activities at temperatures up to at least 700 ° C.

In various industrial processes in the field of refinery technology nik, the generation of energy by gasification of coal and petroleum coke or other residues, thermal (special) waste disposal and Processing refinery residues can be strongly reducing sulfi dying atmospheres occur that lead to a high corrosion load for metallic materials (K. Natesan: Corrosion-Nace Vol. 41, No. 11 (1985) 646-655; B. Glaser, M. Schütze, F. Vollhardt: Materials and Corrosion 42 (1991) 374-376). Conventional apparatus building materials can not be used in these environments at high temperatures because they are no longer able to apply stable protective oxide layers build the corrosion protection mechanism of this plant fabrics based. This also applies to high-alloy special materials that form slowly growing Al and / or Si oxide layers, as the permissible Quantity of these alloying elements for a few per mechanical reasons is limited, which is not sufficient in such atmospheres to dense oxide layers permanently with a sufficient crack to develop healing potential.

With very high sulfur partial pressures in reducing environments with correspondingly low oxygen partial pressures inevitably occur also to sulfide formation, so that only materials can be used besides slow-growing oxides, sulfides with low wakefulness train rate rates.

The intermetallic phase TiAl has proven to be stable under reducing, sulfidizing conditions at sulfur partial pressures of ≧ 10 ^-6 bar and oxygen partial pressures of ≦ 10 ^-25 bar at temperatures of 700 ° C (M. Schütze, M. Nöth: 13 ^th ICC, Paper 296, Melbourne Nov. 1996, ACA Inc.) and was registered for this use as a European patent (EP 0 716 154 B1). The applicants applied TiAl as a thermally sprayed layer on ferritic apparatus structural steels as a protective layer against corrosion. The almost identical coefficients of thermal expansion of TiAl and the ferritic substrates meant that the layers remained free of cracks and flaking even under thermocyclic loads (M. Schütze, T. Weber, EUROCORR'99, EFC Event No. 227, G. Schmitt, M. Schütze (eds.), Dechema eV, 1999). However, the use of thermally sprayed TiAl layers is not possible on austenitic materials due to their high thermal expansion coefficients.

This is where the invention comes into play, which provides for simultaneous diffusion of Al and Ti in a powder packing process. Here, however, no TiAl alloy is used as claimed in DE 15 21 269, but the pure elements Al and Ti are simultaneously subjected to a diffusion process. Likewise, no TiAl alloy is produced on the substrate material, but mixed phases of Al, Cr, Fe, Ni and Ti are formed depending on the composition of the substrate. NH ₄ Cl can be used as an activator, but the use of other ammonium halides or a mixture of several ammonium halides or Al and Ti halides is also possible. With the help of thermodynamic calculations, the proportions within the diffusion powder mixture are set in such a way that the vapor pressures of the resulting gaseous Al and Ti halides are at an optimal order of magnitude at the diffusion temperatures of about 950-1050 ° C in question.

Other processes can also be used instead of the powder packing process chemical vapor deposition (CVD) can be used (over pack, out of pack, etc.), which are known from the literature (C. Duret, R. Pi choir: Coatings for High Temperature Application, E. Lang (ed.), 33-78, Applied Science Publishers, London & New York, 1983).

Likewise, the use of a physical gas phase method divorce (PUD) conceivable (D.G. Tar: Coatings for High Temperature Appli cation, E. Lang (ed.), 78-120, Applied Science Publishers, London & New York, 1983).

example

The Al-Ti diffusion layers were produced in a powder pack diffusion process in the codiffusion process, wherein Al powder was mixed with Ti powder and NH ₄ Cl powder as activator in a ratio of 1: (2-4): (1-3) in parts by weight , Al ₂ O ₃ powder with a total content of 75-90% (wt.) Was used as the inert filler. At annealing temperatures of 900-1050 ° C in a reducing atmosphere (Ar-10% H ₂ ) after 10 h diffusion layers of 30-40 µm thickness were achieved on substrates made of steel No. 1.4876.

The typical structure and composition of a diffusion layer are shown in element distribution images in Fig. 1 and in a linescan in Fig. 2. At the upper edge of the diffusion layer there is an Al, Ti, Cr and Fe-rich zone, followed by an area enriched with Ni-Al phases, which ends with a Ti carbide layer. Below that is an area with Ni-Al precipitates within a phase rich in Fe and Cr.

This layer was tested under reducing, carburizing sulfidizing conditions at temperatures of 700 ° C. CH ₄ , CO, CO ₂ , H ₂ and H ₂ S were contained in the test atmosphere used, a sulfur partial pressure of 10 ^-6 bar, an oxygen partial pressure of 10 ^-25 bar and a theoretical carbon activity of 270 being found at 700 ° C , A very good corrosion resistance could be achieved. Even after a test period of 800 hours, there was only a 10-15 µm thick crack-free and well-adhering top layer made of oxides and sulfides. Cross-section element distribution images and a linescan are shown in Fig. 3 and Fig. 4. Fe sulfides have grown on the outside, but they do not form a continuous layer. Below is a layered structure consisting of continuous Ti- over Al-oxide bands, to which a thin Al-sulfide and presumably a Ti-carbide band is connected, which indicates an only minimal degree of inward diffusion of S and C. This structure can offer effective corrosion protection, which is also evident from the Fe sulfides, which have a very slow growth, from which a greatly reduced Fe outward diffusion can be concluded.

In addition, the Al-Ti diffusion layer has a relatively small Brittleness, as a result of which cracks and flaking of the layer largely can be avoided. In addition, there is a good crack healing agent like given by fast growing Ti oxides.

The relative increases in mass, which showed a parabolic course, were correspondingly small. A value of k _p = 2.4 × 10 ^-13 g ² cm ^-4 s ^{-1 was} calculated for the growth constant. A course of the relative mass increases is shown in Fig. 5 compared to a pure Al diffusion layer. Al diffusion layers are only partially resistant in such environments because they are highly brittle and do not offer sufficient crack healing potential at these temperatures.

Claims (4)

1. aluminum-titanium diffusion layers to improve the corrosion resistance of metallic materials at high temperatures in reducing, sulfiding and / or carburizing atmospheres, characterized in that the layer in the codiffusion process is manufactured using the diffusion elements Al and Ti as pure metal powder in a weight ratio of 1: (0.1-5). 2. Diffusion layer according to claim 1, characterized in that the Layer is made via a powder packing process. 3. Diffusion layer according to claim 1 and 2, characterized in that ammonium halides can be used as activators. 4. Diffusion layer according to claim 3, characterized in that as Activator additional Al and / or Ti halides can be used.