CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
It is well known that the reduction of nitric acid is auto-catalytic in nature with the
generation of aqueous HNO 2 , and gaseous species such as NO and NO 2 depending up on the
concentration used [3-7, 20-22, 41,42]. The sequences of reduction process of nitric acid are as
given below.
HNO 3 H + + NO 3
-
(1)
NO 3 - + 3H + +2e - HNO 2 + H 2 O (2)
HNO 2 + H + + e - NO + H 2 O (3)
HNO 3 + NO HNO 2 + NO 2 (4)
The species which imposes redox potential to the electrochemical environment is nitrous acid
(HNO 2 ). However, nitrous acid undergoes reduction to nitrogen monoxide (NO) by a
heterogeneous charge transfer reaction (step.3) at electrode-electrolyte interface. Nitrous acid is
then again regenerated along with nitrogen dioxide (NO 2 ) by heterogeneous chemical reaction of
nitric acid and nitrogen monoxide, which is auto-catalytic in nature (step.4). Hence, with increase
in nitric acid concentration the reduction rate, thus the oxidizing power also increases as the ratio
of nitrous acid to nitric acid increases [20]. Consequently from kinetics point of view, the
generation of nitrous acid which maintains the auto-catalytic nature, accelerates the corrosion rate
due to oxidation of alloying elements such as Fe, and Cr. As a result, chromium which is key to
passive film stability depletes from the surface leading to increase in passive current density, and
corrosion current density. Similarly, owing to higher catalytic activity at higher concentration, the
transpassive dissolution of passive film also becomes faster leading to decrease in transpassive
potential [20].