CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
in nature because the dissolved ions also act as oxidizing agent, and increase the dissolution rate.
Alloying composition and solution composition play greater role on the susceptibility of austenitic
stainless steel to pitting corrosion. Alloying elements such as Cr, Mo, N, etc increase the passive
film breakdown potential, and C, S, P reduce the breakdown potential [15,34]. Solution
composition also has effect on the pitting susceptibility of austenitic stainless steel, and the
inhibiting action to pitting corrosion decreases in the order, OH - >NO - 3 >acetate>SO 2- 4 >ClO - 4 . In
acidic range of pH, the pitting potential remains unaffected, in alkaline region the pitting potential
shifts in noble direction. The induction time for pitting depends upon the chloride concentration,
and in general decreases with increase in chloride concentration.
2.10.5 Stress corrosion cracking
Austenitic stainless steel in general possess inferior stress corrosion cracking resistance.
The contributing factors for the stress corrosion cracking of austenitic stainless steel are presence
of aggressive media, temperature, hydrogen ion concentration, and residual, applied or thermal
stress. Nitric acid is a strong oxidizing agent, and generates hydrogen as a result of ionisation. The
generated hydrogens at the alloy/solution interface in combination with residual or thermal stress
in the material can cause hydrogen-embrittlement initiating stress corrosion cracking [15,48].
Moreover, the liberated atomic hydrogens are readily absorbed by non-metallic inclusions or at
grain boundaries producing high pressure area within the matrix. The adsorbtion of hydrogen in
the intermetallic inclusions leads to formation of metal hydrides which are known to be brittle. In
the transition states such as active to passive, and passive to transpassive the susceptibility to
stress corrosion cracking is generally higher [34]. These transition regions provide active surface
which is necessary for passive film rupture to initiate the stress corrosion cracking. Apart from
this, solute (phosphorus, silicon) segregation brings compositional changes in the grain boundary
region as compared to the matrix which provides active anodic areas for stress corrosion cracking.