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Important Issues in High Temperature Corrosion - Nitridation
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Nitridation is a process that results in the formation of nitrides in a material. It results from exposure to reducing, high temperature environments with high nitrogen activity. Since nitrides are inherently hard, brittle phases, nitidation can produce local or widespread loss of material strength and possible metal wastage. A beneficial form of this phenomenon, nitriding, is a controlled metallurgical process whereby the nitrides are formed on the surface of a material to increase hardness and improve wear resistance.
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Nitridation is a process that results in the formation of nitrides in a material. It
results from exposure to reducing, high temperature environments with high
nitrogen activity. Since nitrides are inherently hard, brittle phases,
nitidation can produce local or widespread loss of material strength and
possible metal wastage. A beneficial form of this phenomenon, nitriding, is a
controlled metallurgical process whereby the nitrides are formed on the surface
of a material to increase hardness and improve wear resistance. Commercial
processes where nitridation is encountered are those involving reducing process
environments which contain ammonia or other reactive nitrogen-containing
species. Examples of these processes are found in the production of nitric acid
and nylon. Since these environments are typically reducing and consequently do
not have high oxygen activity, Cr content is not important. The most effective
alloying element for resistance to nitridation is Ni. Up to 600 C, it is common
to utilize stabilized stainless steels such as AISI 347. However, for more
severe conditions involving higher temperatures, Ni and Co base alloys are
employed. These include alloys B through B-3 and N which utilize large
additions of Ni, Mo and/or W to promote resistance to nitride formation. In
some cases, alloying with Al to over 4 percent has been reported to improve
corrosion performance. In
some chemical processes involving exposure to very high temperature and/or very
severe nitriding conditions, nitrogen based ceramic material may be an
attractive alternative. Such materials include silicon nitride and other highly
stable nitrogen containing refractory materials. They are inherently more
resistant to nitridation than metallic materials, but are also less ductile and
require special fabrication methods. These ceramic materials can also be
applied as coatings to metal substrates in some cases to promote resistance to
particular components.
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