CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
gets lost. Metallic ions such as Cr 6+ , V 2 O 5
2-
present in nitric acid solution in general greatly
increases the transpassive corrosion of austenite stainless steel.
2.10.4 Pitting corrosion
Pitting corrosion is one of the localized corrosion where small area on the surface corrodes
preferentially leading to formation of cavities or holes leaving the bulk surface unattacked. In
general austenitic stainless steel undergoes pitting corrosion, and passivation property decreases
when nitric acid is contaminated with halide ions [47,48].
Pitting corrosion is one of the major concerns for austenitic grade stainless steels used in
spent nuclear fuel reprocessing plant as these plants are mostly located in the saline atmosphere of
coastal region. The combined action of both powerful oxidizing (HNO 3 ) and de-passivating (Cl - )
agents on the corrosion properties of austenitic stainless steel depends upon the alloy composition,
solution composition and pH of the solution.
Fig. 2.10: Pitting corrosion austenitic stainless steel in nitric acid medium containing chloride ions
[48].
The general effect of the chloride ions is that they inhibit the cathodic reaction by
adsorption on the passivated surface resulting in the easy breakdown of passive film. Apart from
this the surface dissolution in the combined action of nitric acid and chloride ions is auto-catalytic