Page 754 - Corrosion Engineering Principles and Practice

704 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 705

(b)
FIGURE 15.28 (continued)

steels and heat-resistant alloys can be attacked and that the rate of
damage can be extremely high. The mechanisms of metal dusting
attack are not understood. One remedial measure is adjusting the gas
composition by reducing the CO partial pressure [8].

15.4.5 Nitridation
Nitridation usually occurs when carbon, low-alloy, and stainless
steels are exposed to an ammonia-bearing environment at elevated
temperatures. The production of ammonia, nitric acid, melamine,
and nylon generate such conditions. Nitridation can also result from
nitrogen atmospheres, especially under reducing conditions and high
temperatures. There are many parallels to carburization; nitridation
occurs when chromium and other elements combine with nitrogen to
form embrittling nitrides in the microstructure.
Although stainless steels may have adequate resistance, high-
nickel alloys tend to be more resistant. Increasing nickel and cobalt
contents are also considered to be beneficial. However, pure nickel
has shown poor resistance. Alloy 600, with 72 percent nickel, is often
used in the heat-treating industry and occasionally in refining and
petrochemical applications involving ammonia at temperatures
above 340°C. Economics and its lower strength, compared with Alloy
800H and cast-modified HP, have limited its applications in the latter
industries.