Page 377 - Failure Analysis Case Studies II

362

Fig. 8 SEM photograph of d speclinen with Corr d = 2, showing the shallow dimples. x 1000

Fig. 9. SEM photograph of a specimen with Corr. d. = 5, showing the brittle nature of the fracture surface.
x 1000.

the H + and C1- ions regenerate. The H + ions increase the acidity in the pit (the pH value reached
is 3.5) [6]. Due to the lack of oxygen as a depolarizer at the cathode, H+ ions act as depolarizers in
the pit :

However, the reaction [eqn. (3)] does not take place in its entirety. At favourable locations the
adsorbed atomic hydrogen migrates into the crystal lattice where, due to various mechanisms [7, 8,
91, it causes hydrogen embrittlement. This results in uncorroded regions which are the forerunners