High Temperature Materials Research Group

High temperature corrosion - Titanium Aluminides - Ceramic Thermal Barrier Coatings - Metal Dusting - Diffusion Coatings - Chemical Process Environments - Investigation Methods - Research for Industry

The development of high temperature materials has been stimulated by the demand for processes that protect natural resources and the environment, while at the same time increasing their efficiency. The group's objective is on the one hand to determine the limits of the applicability of existing and well established high temperature materials, even under extreme service conditions, and on the other hand to extend the existing limits by developing new materials systems for applications at high temperatures and in specific environments. This is achieved by basic and applied research on materials properties (high temperature strength, creep resistance, corrosion resistance, etc.) and the active mechanisms in a temperature range of 400 to 1800 °C under various environmental conditions. In this research the main areas are new intermetallic materials (titanium aluminides, silicides), ceramic thermal barrier coatings and various high temperature corrosion protection coating systems and the field of extremely aggressive high temperature environments (chlorine corrosion, sulfidation, metal dusting, etc.). Furthermore, extensive efforts are being made to develop new investigation methods.

Titanium Aluminides

Intermetallic titanium aluminides are an interesting group of new light-weight materials (specific weight about 4.0 g/cm³) for applications as moving components at high temperature. The temperature range of these materials is limited to about 750°C because of their poor oxidation resistance. Low amounts of halogens in the surface zone of the materials can improve the oxidation resistance dramatically and allow temperatures up to 1000°C (halogen effect). Besides the characterisation of the high temperature properties and especially the oxidation behaviour, several projects deal with the development of new procedures for applying different halogens or halogen compounds to the material surface in order to improve the oxidation resistance by several orders of magnitude. The procedures include ion implantation, dipping, spraying, painting, etc. Thermodynamic calculations are used for optimized tailoring of the halogen effect.

Fig. 1: Improvement of Oxidation Resistance by Ion Implantation

Literature

• C. Lang, M. Schütze, Oxidation of Metals 46 (1996), 255-285
• M. Schmitz-Niederau, M. Schütze, Oxidation of Metals 52 (1999), 225-276
• A. Zeller, F. Dettenwanger, M. Schütze, Intermetallics 10 (2002) 33-72
• M. Schütze, G. Schumacher, F. Dettenwanger, U. Hornauer, E. Richter, E. Wieser, W. Möller, Corrosion Science 44 (2002) 303-318
• A. Donchev, B. Gleeson, M. Schütze, Intermetallics 11 (2003) 5, 387-398
• A. Donchev, M. Schütze, Materials Science Forum 461-464 (2004), 447-454
• A. Donchev, H.-E. Zschau, M. Schütze, Materials at High Temperatures 22 (2005), 309-314

Ceramic Thermal Barrier Coatings

An increase in efficiency and a reduction in the CO₂ emission of gas turbines can be achieved by increasing the inlet temperatures by means of ceramic thermal barrier coatings (TBC) on Ni-base super alloys. For these materials systems there is an urgent need for a reliable life time assessment. Therefore in this field research activities in the group concentrate on the development of efficient life time models based on fracture mechanics concepts including time- and strain-dependent kinetics of micro-damage accumulation. The work covers thermal barrier systems which are manufactured by atmospheric plasma spraying (APS) and electron beam - physical vapour deposition (EB-PVD) processes. Quantification of the damage kinetics is made by in situ acoustic emission measurements and post-test optical and electron microscopy microstructural investigations. Furthermore, the development of new ceramic thermal barrier coatings on intermetallic TiAl alloys has been started.
As an entirely new approach another method is under development in which a combined system of a foam-like ceramic TBC and an aluminium-rich intermetallic diffusion bond coat on a nickel base alloy is formed in a single step using nano-scale metal powder.
Fundamental investigations are performed to elucidate the so-called desktop effect, where the ceramic TBC spalls with a delay (often of several days) after cooling from high to ambient temperatures.

Fig. 2: a) Turbine blade with a ceramic TBC b) Lifetime modeling

Literature

• D. Renusch, H. Echsler, M. Schütze, in "Lifetime modelling of high temperature corrosion processes", Hrsg. M. Schütze, W.J. Quadakkers, J.R. Nicholls, Maney Publ., London 2001, 324-336
• V. Gauthier, F. Dettenwanger, M. Schütze, Intermetallics 10 (2002) 667-674
• H. Echsler, D. Renusch, M. Schütze, Materials Science and Technology 20 (2004) 307-317
• H. Echsler, D. Renusch, M. Schütze, Materials Science and Technology 20 (2004) 869-876,
• H. Echsler, W. Przybilla, M. Schütze, Proc. EUROCORR 2000, IoM Communications LTD, London, (2000)
• D. Renusch, H. Echsler, M. Schütze, Materials at High Temperatures 21 (2004) 2, 65-76
• D. Renusch, H. Echsler, M. Schütze, Materials at High Temperatures 21 (2004) 2, 1-13
• D. Renusch, M. Schütze, Materials Science Forum 595 (2008) 598, 151-158
• M. Rudolphi, D. Renusch, M. Schütze, Scripta Materialia 59 (2008) 2, 255-257

Metal Dusting

Metal dusting is a catastrophic corrosion phenomenon occurring at high temperatures (400-900 °C) under strongly carburizing and reducing atmospheres such as those from coal gasification, petrochemical processes, coal liquefaction, in synthesis gas reactors and in ammonia and methanol production. Besides basic investigation on materials behaviour under these environments the research group is engaged in the development of new protective coating systems to counteract metal dusting attack. These coatings (complex 2-step and co-diffusion coatings as well as overlay coatings) with high amounts of strong oxide formers, such as Si, Ti, Cr and Al, are able to form protective oxide scales even under highly reducing metal dusting atmospheres.
In addition to the development of coatings the group also investigates concepts of preventing carbon deposition on the material surface and thus metal dusting attack by catalytic poisoning of the material surface. Tin has turned out to be a particularly effective poisoning element for this purpose.

Fig. 3: a) coated and b) uncoated sample under exposure in highly carburizing and reducing atmosphere

Literature

• C. Rosado, F. Dettenwanger, M. Schütze. Proc. Stainless Steel World 2001. KCI Publishing, Zutphen (NL), 306-314
• T. Weber, C. Rosado, M. Schütze. Corrosion 2002, NACE, Houston 2002, Paper 02376, 1-12
• C. Rosado, M. Schütze, Materials and Corrosion 54 (2003) 11, 831-854
• D. Röhnert, F. Phillipp, H. Reuther, T. Weber, E. Wessel, M. Schütze. Oxidation of Metals 68 (2007), 271-293
• D. Röhnert, M. Schütze, T. Weber. NACE Corrosion 2007, Paper 074175/1-16
• C. Geers, T. Weber, M. Schütze. Proc. EUROCORR 2008 Edinburgh/UK, (2008)
  

Diffusion Coatings

Diffusion coatings aim at protecting high temperature components by increasing their oxidation/corrosion resistance in the chemical industry, power plants and the aerospace industry. They are typically deposited by a pack cementation process, which consists of introducing the substrate material into a powder mixture containing the desired alloying elements to be deposited. Heated at high temperature, in an inert atmosphere, diffusion of protective elements into the substrate occurs, leading to the formation of the coating.

Materials und Coatings for Chemical Process Environments

The corrosion resistance of conventional structural alloys is being tested and the corrosion mechanisms investigated for different industrial process environments. Metallic coatings have been developed to increase the range of applicability of these materials under extreme conditions and to extend the service intervals. These coatings contain high quantities of the elements Al, Si and Ti, which are stable oxide formers even under very low oxygen pressures. A prerequisite for the optimal performance of coating systems is the similar thermal expansion behavior of coating and underlying substrate. Besides diffusion coatings, thermal spray coatings, consisting for example of the intermetallic γ-TiAl, can also be applied. The latter proved to be resistant to extremely sulfidizing reducing conditions. Atmospheric plasma spraying (APS) and high velocity oxy-fuel (HVOF) methods were used to apply TiAl as a coating on conventional ferritic steels (α_TiAl ≈ α_ferrite ). Very low porosity within the coatings could be achieved by the HVOF technique. Metallic coating systems were also developed for high chlorine containing atmospheres, and the corrosion resistance of alloys in such environments was characterized quantitatively as a function of chlorine and oxygen partial pressures.
In this context the group developed the concept of a quasi-stability diagram, which allows a prediction of material consumption by high temperature chlorine corrosion as a function of temperature, chlorine and oxygen partial pressures as well as gas flow conditions. Based on this concept it was possible to develop highly Cl-resistant protective coating systems with a Ni-Mo-Al base.

Literatur

• C. Schwalm, M. Schütze, Materials and Corrosion 51 (2000) 34-49, 73-79, 161-172
• R. Bender, M. Schütze, CORROSION 2002, Paper No. 239, NACE, Houston 2000
• B. Aumüller, T. Weber, M. Schütze, Proc. Int. Thermal Spray Conference 2002, Ed. E. Lugscheider, DVS-Verlag, Düsseldorf 2002
• M. Schütze, M. Malessa, Materials and Corrosion 57 (2006) 5-13
• M. Schütze. Corrosion 63 (2007) 4-18
• H.J. Grabke, M. Schütze (Eds.): Corrosion by carbon and nitrogen, metal dusting, carburisation and nitridation, Woodhead Publishing, Cambridge 2007
• H. Latreche, S. Doublet, M. Schütze. Oxidation of Metals 72 (2009)

Development of Investigation Methods

Cyclic oxidation testing

The simulation of temperature cycles to which materials are exposed in industrial applications can be performed by cyclic oxidation testing. Investigations in a joint European project with partners from industry and academia, coordinated by the research group, aim to further develop this testing method and to compile a code of practice for CEN and ISO.

Acoustic emission

Recent developments in acoustic emission technique allow long term in situ investigation of several high temperature oxidation mechanisms. For example, the technique is applied to quantify the evolution of damage in thermal barrier coatings as a function of temperature and time changes. The experimental results serve as a basis for the development of life time prediction models. Additionally the "bad actor" effect is investigated by means of acoustic emission: many materials are resistant to high temperature oxidation in a dry atmosphere (e.g. 9%Cr steel shows no breakaway oxidation in 10000 h of exposure at 650 °C). The addition of, for example, water vapour, SO₂ or CO₂ to the hot environment can, however, lead to early spallation of the oxide scales (some 9%Cr steels exhibit breakaway oxidation in the presence of water vapour in less than 24 h). Acoustic emission analysis has also been developed for use in four-point bending experiments (measurement of adherence of layers to the substrates). This allows insight into the internal stress situation within a material composite, which leads to fracture of the protective oxide scales when a critical limit is exceeded. One example that is being investigated is the failure of thermal barrier coatings based on ZrO₂ . The critical loads for delamination as well as for segmentation cracking can be assessed. Both parameters are recorded as a function of thermal and mechanical load.

Metallographic and micro-analytical Methods

The specific problems encountered in high temperature corrosion require special post-experimental investigation methods. One exemplary method developed in the group is water-free sample preparation which allows the detection of halogenides even in low concentrations by microprobe investigations.
On behalf of the American MTI the group compiled an Atlas of Microstructures for modern centrifugally cast alloys. This reference book exemplifies the changes that occur in material microstructure during operation of up to 100,000 hours with approx. 1,200 images.

Literature

• S.R.J. Saunders, M.M. Nagl, M. Schütze, Mater. High Temp. 12 (1994) 103-109
• K. Rahts, M. Schorr, Ch. Schwalm, M. Schütze, Praktische Metallographie 36 (1999) 86-97
• C. Bruns, M. Schütze, Oxidation of Metals 55 (2001) 35-68
• M. Schütze, S. Ito, W. Przybilla, H. Echsler, C. Bruns, Materials at High Temperature 18 (2001) 39-50
• M. Schütze, M. Malessa (Eds.): Standardisation of thermal cycling exposure testing, Woodhead Publishing, Cambridge 2007
• E. Berghof-Hasselbächer, P. Gawenda, M. Schorr, M. Schütze, J.J. Hoffman: Atlas of Microstructures. Materials Technology Institute, St. Louis 2008
  

Research for Industry

The work group also offers its methods and experience to industry in the form of research commissions. This includes work on materials selection for complex conditions, characterization of high temperature corrosion resistance, development and testing of high temperature corrosion protection measures and investigation of damage cases. Particularly for the latter a highly skilled metallography and electron microscopy unit completes the group.

General Reading

• M. Schütze, Protective Oxide Scales and Their Breakdown, Series on Corrosion and Protection, Vol. 1, Hrsg. D.R. Holmes, John Wiley and Sons, Chichester 1997
• H.J. Grabke, M. Schütze (Eds.), Oxidation of Intermetallics, Wiley VCH, Weinheim 1997
• M. Schütze, W.J. Quadakkers (Eds.), Cyclic Oxidation of High Temperature Materials, London 1999, EFC-Publication No. 27, IoM Communications
• M. Schütze (Ed.), Corrosion and Environmental Degradation, Vol. 19 in "Materials Science and Technology", Wiley-VCH, Weinheim 2000
• M. Schütze, W.J. Quadakkers, J.R. Nicholls (Eds.): Lifetime Modelling of High Temperature Corrosion Processes, Maney Publishing, London 2001
• M. Schütze, W.J. Quadakkers (Eds.): Novel approaches to improving high temperature corrosion resistance. EFC no. 47, Woodhead, Cambridge 2008