High-temperature corrosion

High temperature corrosion is a chemical reaction between an environment ( medium) and a material surface ( member) at temperatures for aqueous corrosion media ( electrolyte ) are too high. The consequence is the formation of solid, liquid or gaseous corrosion products, which usually leads to a weakening of the load-bearing cross -section of a component or to an impairment of the function. Examples of high-temperature corrosion processes can be found in all technical applications that operate at temperatures well above 100 ° C, ranging from Verzunderungsprozessen in exhaust systems of vehicles on the high temperature corrosion processes in thermal 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. By a suitable corrosion protection measures (high chrome or aluminum-containing alloys or protective layers ) can be the high-temperature corrosion in most applications reduce to a non-critical level, in many cases, high temperature corrosion, however, lifetime determines the appropriate technical components. The time course of high-temperature corrosion is a model often described by a parabolic law when diffusion-controlled processes determine the corrosion process:

ΔA2 = kp • t

? A represents wherein the corrosion- damaged portion of the component cross section kp parabolic rate constant and t is time. However Feared is the transition from controllable parabolic corrosion behavior for " Breakaway " effect with highly accelerated, often linear material damage after a certain incubation period, the latter being under industrial conditions can take several thousand hours.

High temperature corrosion environments

Important components of high-temperature corrosion environments are in addition to oxygen:

• Sulfur

This leads to the formation of "soft" or brittle metal sulfide or metal sulfate phases, which may be molten at high temperatures as well. The cause is often in the burning of higher sulfur fuels (coal, oil, waste).

• Carbon

This generally leads to cementation of the formation of internal metal carbides in the material, thereby forming a hardening or embrittlement of the component takes place. An extreme form is the " metal dusting " that takes place at very high carbon contents in the environment that lead to carbon deposition on the surface (coke, carbon black). As a result, the metal component is divided to " dust " of graphite and metal or Metallkarbidpartikeln. Carburization can be done in a number of industrial combustion or gasification atmospheres.

• Nitrogen

This shows similar effect as carbon, only that instead of metal carbides inner or outer Metallnitridphasen be formed. The nitrogen in the air itself at elevated temperatures may lead to the nitration of materials.

• Halogens (Cl, F, I, Br)

These form gaseous metal halides at high temperatures as corrosion products, so that the metal is " evaporated " and a very rapid removal of metal takes place. Halogen Influenced Corrosion is commonly observed during the combustion of chlorine-containing biomass, or waste incineration.

• Water vapor

Other than perhaps expected, even water vapor at high temperatures can lead to an acceleration of the oxidation process. This is often found with wet fuels in combustion processes.

Research

Intensive research in the field of high-temperature corrosion and high-temperature corrosion protection run in Germany, for example, at DECHEMA Research Institute ( DFI).