What Effects Do Complex Gases
Have on Components?
- Accelerate Oxidation.
- Break down the adherent oxide layer
by adding other compounds (sulfides, carbonates).
- Promote complex liquid phases, which further
accelerate reaction rates (vanadium in fuels).
- Sufficient sulfur changes oxidation to sulfidation --
(sulfide films are not adherent).
- Chromium and aluminum in structural steels promote
BOTH oxidation and sulfidation resistance by the presence
of their oxides.
- Oxygen (or OH ions) must have access to Fe ions
(or Cr, Ni, etc. in the steel). This happens by diffusion
of oxygen ions (or OH) through the oxide film or outward
diffusion of metal ions -- usually the case for Fe, Ni, Cr,
and Co. Film thickness growth follows parabolic kinetics.
- Cr is added to steels to promote corrosion resistance.
If it is used up by oxidation or forming nitrides or carbides
nitridation or carburization, corrosion resistance deteriorates.
Al also forms an adherent oxide.
- Both Al and Cr increase corrosion resistance, including
resistance to hot corrosion. (Big problem in burning "dirty"
fuels in gas turbines.
- Erosion-Corrosion can be caused by the presence of fly ash
and other solid particles in hot, fast moving gases. The solid
particles attack and erode the growing protective film, which
increase the formation rate of oxide or other protective film.
- As oxides are Ceramics, it is optimistic, at best, to
expect them to adhere to a metal surface.