Corrosive gases

	Air, oxygen: The reaction between air and a material is normally called oxidation and is quite often observed as a separate discipline in regard to corrosion science. In most industrial cases where metal is in contact with air, the oxidation processes are not as much determined by plain oxidation by oxygen but one has to consider all possible contaminants of the air which can have disastrous consequences for the oxidation resistance.
	Steam: Steam in contact with steel can effect the carbon level of the steel can also oxidize the iron.
	Carbon, carbon oxides and methane: Compounds of carbon like carbon dioxide, carbon monoxide and methane can change the carbon level in the steel and subsequently influence the mechanical properties of the metal.
	Sulfur containing Gases: Even in small amounts, sulfur in various forms can accelerate corrosion at high temperatures.
	Hydrogen: Hydrogen gas is a reducing agent and in contact with steel at high temperatures can result in decarburization and the subsequent formation of hydrogen carbons; C(Fe) + 2H2 <==> CH4
	Nitrogen: Nitrogen plays in most cases no role in oxidation phenomena as it is overshadowed to the large effect of oxygen and most nitrides are only formed at high temperatures. However, active nitrogen produced from ammonia can form nitrides below 540oC.
	Combustion gases: The gas mixture arising from combustion of fuels contains for a large part carbon oxides and water vapor together with nitrogen. In situations with incomplete combustion hydrogen, carbon monoxide, and several hydrocarbons are present as well as oxygen. Most fuels contain sulfur compounds so that sulfur oxides and even hydrogen sulfide will be present in the combustion gases resulting in more severe corrosive conditions.
	Chlorine and hydrogen chloride: Dry chlorine and hydrogen chloride do not cause major problems as far as corrosion is concerned however, accidental or deliberate increases of the moisture in the gas result in rapid localized attack.

Ash: Vanadium pentoxide and sodium sulfate present in fuel ash can attack stainless steels due to the formation of a molten oxide phase that fluxes the protective oxide scale.

Molten salts: Molten salts generally act as fluxes, removing possible protective scales of corrosion products. Combined with the high temperature and the high ionic conductivity of molten salts this will result in high corrosion rates.

Molten metals: Contact between molten metals and condensed metals frequently results in severe corrosion attack, which in some cases can be due to temperature differences or concentration gradients in the system used.