CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
is intimately related to the stability of the protective passive film on the material, electrochemical
environment, presence of other oxidizing and reducing species, and rate of cathodic and anodic
process. Austenitic stainless steels, even though are protected by passive film on the surface,
undergo transpassive corrosion when their corrosion potential shifted to the transpassive domain
in some particular nitric acid media especially in presence of hexavalent chromium ion (Cr 6+ ). The
characteristic feature of transpassive dissolution is mainly intergranular corrosion with opening up
of grain boundaries and observed intergranular ditches [1-7]. The schematic of transpasive
corrosion of austenitic stainless steel in nitric acid medium containing hexavalent chromium ion is
shown in Fig. 2.9 [45].
Fig. 2.9: Transpassive dissolution of austenitic stainless steel in nitric acid [45].
If the conditions are sufficiently reducing austenitic stainless steel under goes uniform
dissolution in the active state. If the medium is moderately oxidizing, austenitic stainless steel are
in the passive state. In passive state also austenitic stainless steels are characterized by uniform
passive film but with low surface dissolution. If the medium becomes excessively oxidizing, the
dissolution of passive film initiates specially with oxidation of insoluble Cr 2 O 3 to soluble Cr 2 O - 7 .
The dissolution of passive film results in accelerated transpassive dissolution of oxide covered
material. In the more sever oxidizing medium, grain losses occur, and successive rows of material