CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
transpassive dissolution [39]. According to another mechanism [40], the active sites in the passive
film are rapidly blocked by molybdenum oxyhydroxide or molybdate salts, thereby inhibiting the
localized corrosion.
2. 9 Corrosion mechanism of austenitic stainless steel in nitric acid
Nitric acid is a clear colorless liquid having boiling point 107 °C, melting point -42 °C and
specific gravity 1.4, respectively. It is completely soluble in water, and is stable in ambient
condition. It is highly ionized acid, and has all the properties of strong acid. Apart from being a
strong acid it is a powerful oxidizing agent even in very dilute solution. The reduction mechanism
of nitric acid is widely investigated due to large industrial applications, and in relation to the
corrosion of stainless steel as well as disposal of radiolytic waste containing nitric acid from spent
nuclear fuel reprocessing plant. Reduction mechanism of nitric acid is dependent on the
concentration of nitric acid, and the auto-catalytic activity of nitrous acid produced as a result of
reduction [3-7, 20-22, 41,42]. Being a strong acid, nitric acid completely ionizes to hydrogen ions
(H + ), and nitrate ions (NO - 3 ) as shown below.
HNO 3 H + + NO 3
-
(1)
In the ionized state NO - 3 ions are more oxidizing as compared to that of H + ions. Thus, in most
acidic media nitrate ions get reduced to the nitrous acid as given below.
-
NO 3 + 3 H + + 2e - (HNO 2 ) aq + H 2 O (2)
Nitrous acid produced maintains the auto-catalytic activity by a heterogeneous electron transfer
reaction followed by a chemical reaction leading to its regeneration as presented below.
(HNO 2 ) aq + H + + e - (NO) g + H 2 O (3)
HNO 3 + (NO) g (HNO 2 ) aq + (NO 2 ) g (4)
In the overall reduction process the nature of the product formed depends upon the
concentration of nitric acid. Thermodynamic study for the equilibrium between gaseous and liquid