CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
healing chromium oxide film on the surface [34]. Even though, austenitic stainless steels are
having good passivation property and corrosion resistance in nitric acid environment, they are not
without problems and failures. In fact, these alloys are not immune to corrosion attack, and
different types of corrosion may occur in the reprocessing environment of nitric acid [1-7, 42-46].
In general, the corrosion resistance decreases with increase in concentration and temperature, and
high corrosion rate at elevated temperature has been realized in actual plant condition. It has also
been observed that in more sever condition such as 65 % concentration in boiling condition, these
material fail severely leading to grain loss [1,42]. The type of corrosion attack in austenitic
stainless steels in nitric acid medium depends upon the chromium content of the alloy, presence of
other minor alloying elements, metallurgical condition, fabrication process, and the
electrochemical environment. Major problems associated with these materials in nitric acid are (a)
intergranular corrosion, (b) end-grain attack, (c) transpassive corrosion, (d) pitting corrosion, and
(e) crevice corrosion as summarized below. The occurrence of these problems act as life-limiting
factor of the structural components, and have direct effect on the performance of the plant.
2.10.1 Intergranular corrosion
Austenitic stainless steel, although protected by a passive layer rich in chromium oxide can
suffer from intergranular corrosion due to selective attack at the grain boundaries. The localized
attack on the grain boundaries in corrosive media results in loss of strength and ductility. The
usual form of intergranular corrosion occurs due to sensitization i.e. depletion of chromium and
formation of chromium carbide precipitate adjacent to grain boundary. This happens when
austenitic stainless steel is heated in the temperature range from 450 °C to 800 °C. The alloy
becomes sensitized and susceptible to intergranular corrosion. Apart from this, intergranular
corrosion can occur in non-sensitized austenitic stainless steel as a result of segregation of certain
impurity elements to grain boundaries. However, this is known to occur in highly oxidizing