CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 1
choice of zirconium is due to its high corrosion resistance as compared to titanium and austenitic
stainless steel with or without the presence of oxidizing species. As compared to austenitic
stainless steel, zirconium does not undergo intergranular corrosion, and compared to titanium it
does not suffer from vapour and condensate phase corrosion problems in boiling nitric acid.
Comparative analysis on the corrosion resistance of 20 Cr-20 Ni austenitic stainless steel, Ti, and
Zr in 17-70 % nitric acid concentration in boiling condition in presence of hexavalent chromium
(Cr 6+ ) showed high corrosion resistance for zirconium as compared to others [29]. The high
corrosion resistance is attributed to natural formation of dense, stable and self healing zirconium
oxide (ZrO 2 ) film on the surface which protects the material from corrosive environments [1].
Moreover, it does not form non-adherent oxide with poor corrosion resistance under condensing
condition as seen in case of titanium. Recent corrosion study [30] on zirconium, and its welds
showed superior corrosion resistance as compared to other candidate material like Ti, Ti-5% Ta,
and Ti- 5% Ta-1.8 % Nb. No corrosion attack was observed in liquid, vapor and condensate zones
of boiling 11.5 M nitric acid. Due to high corrosion resistance, inspection, and maintenance is also
a less concern for zirconium.
Even though, zirconium shows outstanding corrosion resistance in nitric acid medium,
certain factors require thorough investigation, such as fluoride content in the process medium and
stress corrosion cracking. The presence of fluoride ions as complexing agent is detrimental to
passive film stability, and increases the dissolution rate of the material even at low concentration
of fluoride ion [31]. However, the effect of fluoride ion can be reduced by adding corrosion
inhibitor such as zirconium sponge and zirconium nitrate to convert to non-corrosive compound.
Stress corrosion cracking (SCC), can occur in Zr exposed to hot boiling nitric acid (> 70 %) [29].
Particularly, heat effected zones of zirconium welds are considered to be sensitive to SCC.
However, recent study shows that zirconium has excellent corrosion resistance, and do not show