CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 1
corrosion resistance. The corrosion resistance of austenitic stainless steel depends on the
concentration of nitric acid, for lower concentration (< 50 %) corrosion susceptibility is
determined by chromium content in the alloy, and for higher concentration containing Cr 6+ ions it
is determined by impurity elements in the alloy. Mainly, AISI type 304L SS (18 % Cr-8 % Ni-
0.03% C) finds large scale application because of good passivity, corrosion resistance, and
appropriate practical aspects such as working, welding, cost competitiveness etc. Despite many
beneficial effects, type 304L SS fails in the oxidizing environment of nitric acid leading to
intergranular corrosion, end-grain attack, transpassivity etc [2]. To overcome these problems
different grades of austenitic stainless steels have been evolved over time. Fe-18Cr-15Ni-4Si-
0.01C austenitic stainless steel was developed to keep the intergranular and uniform corrosion low
in the process environment of hot nitric acid. The addition of titanium and niobium to these alloys
gives still improved property, however due to high silicon, and low carbon content these materials
tend to form crack in the welded region. Similarly, special nitric acid grade (NAG) austenitic
stainless steel such as URANS-16 (18 Cr-12 Ni) and URANS-65 (25 Cr-20 Ni) are designed with
sharp control of carbon, silicon, sulphur and phosphorus. Because of high chromium content and
tighter control of impurities, nitric acid grade stainless steel performs satisfactorily up to 65 %
concentration in boiling condition. Nevertheless, these materials fail in nitric acid solution
containing hexavalent chromium (Cr 6+ ) at elevated temperature due to transpassivity [3]. Thus, use
of these materials in these extreme conditions is restricted. Moreover, large scale use is also not
possible due to extra cost in the manufacturing process because of controlling the impurity level.
1.4.2 Titanium
Titanium is a unique material, as strong as stainless steel with less than 60 % of its density,
and possessing excellent corrosion resistance. Titanium is used in critical components such as
spent nuclear fuel dissolvers handling hot concentrated nitric acid where austenitic stainless steel