Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C14/00—Alloys based on titanium

Definitions

• the invention relates to a high-temperature-resistant, corrosion-resistant material, which mainly consists of aluminum and titanium.
• Such a material is described, for example, in DE-A-42 15 017 as a material for, for example, turbine blades.
• the material When used at high temperatures up to 900 ° C, the material should become resistant to oxidation and corrosion by forming a slowly growing Al2O3 layer instead of a rapidly growing TiO2 layer.
• EP-A-04 95 454 also describes a material made of aluminum and titanium, in which a protective layer made of Al2O3 is to prevent corrosion of the material.
• the proposed aluminum-titanium alloy is intended as a material for engine parts, in particular valves and piston pins.
• atmospheres that the materials used so far are not able to withstand sufficiently.
• Such atmospheres are characterized by low oxygen partial pressures, for example in the range from 1x10 ⁇ 34 to 1x10 ⁇ 27 bar and, for example, H2S contents of up to 1% by volume and higher (Ullmann's Encyclopedia of Industrial Chemistry, volumes A12, see 169-306 and A18, pp. 52 - 99, VCH Verlag, Weinheim 1989 and 1991).
• processes in which such atmospheres exist are the distillation and gasification of tar and heavy oil residues, gasification coal and the gasification of (special) waste (B. Glaser, M. Schütze, F.
• intermetallic titanium aluminide materials are suitable for use due to their properties lying between the metallic and the ceramic materials under these conditions without exhibiting the toughness problems mentioned.
• the object underlying the invention was to develop a material that can be used in the environments mentioned at significantly higher temperatures without massive corrosion attack than was possible with the technical materials previously used.
• Such a material has a high resistance to corrosion attack both under isothermal and under thermal cycling.
• the basic idea of the invention is that a material is only stable in a corrosive environment at high temperatures if, when reacting with the surrounding atmosphere, very thin, dense and extremely slow-growing corrosion product layers are formed, which act as a barrier between the external environment and develop the metal. With the materials previously used in technology in the environments mentioned, this is not the case at temperatures above 400 ° C. and generally also at temperatures below it, since rapidly growing sulfides of the elements Fe, Cr, Ni or mixed sulfides result from these Form elements, cf. S. Mrowec, K. Przybylski: High Temp. Mat.
• Titanium aluminide alloys can be manufactured for structural applications in the form of precision cast parts, powder metallurgy parts, forged parts, extruded parts and rolled sheets, cf. Report volume BMFT Symposium Materials Research - New Materials, PLR Jülich 1994. Plasma spraying processes are available.
• Porosity in the layer which may not be entirely avoidable during the spraying process, is closed by the formation of corrosion products from the reaction with the environment under operating conditions. In this way, the coating becomes impervious to the passage of the gas atmosphere from the surroundings to the substrate metal.
• the invention is illustrated by the examples below.
• an alloy with 51 atom% aluminum and 49 atom% titanium is exposed to a sulfidating gas atmosphere with 1 vol.% Hydrogen sulfide in a carrier gas made of argon and with 5 vol.% Hydrogen.
• the reactor is electrically heated by a tube furnace surrounding it.
• the measurable area-specific mass increase by reaction in the gas atmosphere is only 0.13 mg / cm2, while the technically used alloys X10CrNiTi18 9 (material no. 1.4541), alloy 800H (X10NiCrAlTi32 20 , Material no. 1.4876) and HK40 (G-X10CrNiSi25 20, material no. 1.4848) under the same conditions have area-specific mass increases due to sulfidation of over 160 mg / cm2.
• the alloy therefore has a corrosion rate which is reduced by up to three orders of magnitude under conditions in which commercially used, high-alloy steels are severely damaged.
• titanium-aluminum alloys of different compositions are exposed to a hydrogen sulfide-containing gas atmosphere at elevated temperature.
• thermocyclic test with rapid cooling processes is carried out with the help of movable half-shells that open and close under computer control.
• T 500 ° C
• the reaction gas is cooled to 350 ° C in 24-hour cycle intervals and then reheated. This procedure continues for a total of 504 hours.
• the area-specific mass changes of the titanium-aluminum alloys amount to 0.03 - 0.04 mg / cm2.
• the values lie between 21 mg / cm2 (St37, material no. 1.0212) and 10 mg / cm2 (X10CrNiTi18 9, material no. 1.4541).
• Metallographic follow-up examinations of the samples taken from commercially available heat exchanger tubes show that, under the reaction conditions, sulfidic cover layers have formed which have detached from the substrate several times as a result of the temperature changes and have therefore lost any diffusion barrier effect. No external corrosion products can be found on the surfaces of the titanium-aluminum alloys in the scanning electron microscope, even with a magnification factor of 4500.
• the tested alloys based on titanium and aluminum thus have excellent corrosion resistance in a sulphidating gas atmosphere even under temperature changes.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Coating By Spraying Or Casting (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)

Applications Claiming Priority (2)

Publications (3)

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Cited By (3)

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• 1994
  • 1994-12-05 DE DE4443147A patent/DE4443147A1/de not_active Ceased
• 1995
  • 1995-11-29 EP EP95118740A patent/EP0716154B1/fr not_active Expired - Lifetime
  • 1995-11-29 DE DE59507645T patent/DE59507645D1/de not_active Expired - Fee Related
  • 1995-12-01 JP JP7314280A patent/JPH08218138A/ja active Pending

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