High temperature corrosion

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High temperature sulfur corrosion on a 12 CrMo 19 5 PI nozzle

High-temperature corrosion is the chemical reaction between an environment ( medium ) and a material surface ( component ) at temperatures that are too high for aqueous corrosion media ( electrolytes ). The result is the formation of solid, liquid or gaseous corrosion products, which i. d. Usually leads to a weakening of the load-bearing cross section of a component or to an impairment of its function. Examples of high-temperature corrosion processes can be found in all technical applications that work at temperatures well above 100 ° C, from scaling processes in vehicle exhaust systems to high-temperature corrosion processes inthermal power plants (firing with coal , oil , gas , biomass , waste materials) to the corrosion in aircraft engines and marine diesel engines as well as in chemical and petrochemical processes. Suitable corrosion protection measures (high chromium or aluminum alloys or protective layers) can reduce high temperature corrosion to an uncritical level in most applications, but in many cases high temperature corrosion determines the service life of the relevant technical components. The time course of high-temperature corrosion is modeled often described by a parabolic law when diffusion- controlled processes determine the corrosion process:

ΔA 2 = k p • t

where ΔA represents the corrosion-damaged part of the component cross-section, k p the parabolic reaction constant and t the time. What is feared, however, is the transition from the controllable parabolic corrosion behavior to the “breakaway effect” with strongly accelerated, often linear material damage after a certain incubation period, whereby the latter can take several thousand hours under technical conditions.

High temperature corrosive environments

In addition to oxygen, important components of high-temperature corrosion environments are :

This leads to the formation of “soft” or brittle metal sulfide or metal sulfate phases, which can also be molten at high temperatures. The cause is often the burning of fuels with a higher sulfur content (coal, oil, waste ).

This usually leads to carburization via the formation of internal metal carbides in the material, which causes the component to harden or become brittle . One extreme form is “metal dusting”, which occurs when the carbon content in the environment is extremely high, which leads to carbon deposition on the surface ( coke , soot ). As a result, the metallic component disintegrates into "dust" made of graphite and metal or metal carbide particles. Carburization can be done in a number of industrial combustion or gasification atmospheres.

This has a similar effect to carbon, only that instead of metal carbides, inner or outer metal nitride phases are formed. Even the nitrogen in air can lead to nitriding of materials at high temperatures.

At high temperatures, these form gaseous metal halides as corrosion products, so that the metal "evaporates" and metal is removed very quickly. Halogen-influenced corrosion is often observed in the incineration of chlorine-containing biomass or in waste incineration .

Contrary to what might be expected, water vapor at high temperatures can accelerate the oxidation process. This is often found in moist fuel combustion processes.

research

Intensive research work in the field of high temperature corrosion and high temperature corrosion protection is ongoing in Germany. B. at the DECHEMA Research Institute (DFI).

literature

  • Ralf Bürgel: Handbook high temperature materials technology. Vieweg + Teubner, Braunschweig 1998, ISBN 978-3528031077 .
  • Michael Schütze (Ed.): Corrosion and Environmental Degradation, Vol. 1 and 2. Wiley-VCH, Weinheim 2000, ISBN 978-3527299713 .

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