CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
Intensity (Arbitrary units)
O1s (1 M HNO 3
)
Fe 2
O 3
Cr 2
O 3
Cr(OH) 3
535 534 533 532 531 530 529 528 527
Binding energy (eV)
Fig. 4.13b: XPS profile of O1s region of 304L SS passivated in 1 M HNO 3 .
Overall, X-ray photoelectron spectroscopy investigation in the aforementioned
concentration of nitric acid revealed the generation, and decay of a duplex passive film consisting
of hydroxide rich layer and oxide layer at lower concentration (0.1 M) which mostly transferred to
oxide layer with increase in concentration (0.6 M, 1 M). The observed hydroxide-oxide synergism
is accredited to the formation of adsorbed hydrated species of chromium, and their oxidation in the
oxidizing environment of nitric acid with increase in concentration [127]. According to the
passivation process of stainless steel [127-129], chromium mainly forms chromous (Cr 2+ ) species
in the pre-passivation stage at lower potential, and thereby forms the hydrated species of Cr(OH) 2 .
Chromous hydroxide is electro-inactive in nature unless aggressive ions are present in
electrochemical environment [130]. However, with increase in potential it transfers to chromic
hydroxide [Cr(OH) 3 ], which is also a stable hydrated species of chromium. Moreover, towards
higher potential, chromous ions (Cr 2+ ) also convert to chromic ions (Cr 3+ ), which subsequently
forms Cr(OH) 3 . The sequences of passivation process are as summarized below.