CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
Cr Cr 2+ + 2e - (1)
Cr 2+ + 2H 2 O Cr(OH) 2 + 2H + (2)
Cr 2+ Cr 3+ + e - (3)
Cr 3+ + 3H 2 O Cr(OH) 3 + 3H + (4)
Chromic hydroxide is stable in neutral and alkaline solution. However, in acidic
environment with increase in oxidizing power it converts to chromium oxide by a solid state
reaction as given below [128].
Cr(OH) 3 + Cr Cr 2 O 3 + 3H + + 3e - (5)
Chromium oxide is an intermediate oxide, and has the properties between network forming
(Cr-O-Cr) and modifying oxide [131]. With increasing oxidizing power of the electrochemical
environment, the oxidation state of chromium increases, and the oxides formed are in general
soluble in nitric acid [17-18]. Thus Cr 2 O 3 which is key to passive film stability starts dissolving in
acidic environment at structural heterogeneous areas and oxide layer depletes from the surface,
thereby passive film disintegration initiates. The depletion of chromium oxide at weaker points
such as inclusions and grain boundaries exposes the surface to aggressive medium where localized
corrosion initiates. In extreme situation when potential reaches the transpassive domain most of
chromium (III) is incorporated as chromium (VI) in the passive film, and these oxy-compounds
dissolves faster in acidic solutions [14].