Dry corrosion of metals — Industrial cases: sulfurizing, nitriding

Many industrial processes produce complex gaseous environments consisting of mixtures of sulfurous (H ₂ S , SO ₂ , COS) and oxidizing (O ₂ , H ₂ O , CO ₂ ) species. Sulfur appears to be one of the most widespread corrosive contaminant elements in high-temperature industrial environments. It is generally present in fuel oils. Sulfurous corrosion attacks are often quite localized and can lead to tube or wall perforations. Such problems are encountered in mineral calcination, petrochemical processes, oil refining, coal gasification, waste incinerators or fluidized-bed coal combustion.

Nitriding, like carburizing, can be considered as a metallurgical surface treatment, in this case hardening, or unfortunately as a corrosion process. We will confine ourselves to the latter phenomenon, which mainly concerns furnace components used in nitriding or carbonitriding treatments, or parts used in chemical processes for the production of ammonia, nitric acid or nylon. Corrosion by molecular nitrogen (N ₂ ) is generally not a problem, despite its content of around 80% in air. The relatively high solubility and slow kinetics of nitride formation in refractory alloys mean that, in the presence of oxygen, oxidation prevails and the oxide protects the metal.

Nitriding can become a problem at high temperatures if the amount of oxygen in the atmosphere is insufficient, or especially when nitrogen is present in the more reactive atomic state. Both of these conditions are present in high-temperature pure nitrogen or in cracked ammonia.