Environmental Engineering Reference
In-Depth Information
Figure 3.7
Focused transducer for pit depth measurement. The curvature of the pit
collimates the diverging beam in the steel plate resulting in a strong back surface reflection.
3.4.3 Mechanism
Any mechanism proposed for pitting should explain its discrete initiation and
autocatalytic propagation. Again, since pitting is most prevalent in the presence
of chloride ions and occurs mostly in active-passive metals, the mechanism de-
scribed should involve these systems.
It has been observed that pitting initiates at a critical pitting potential,
E
pit
,
which is illustrated in Fig. 3.8. The anodic polarization curve of an active-passive
alloy, say 304 stainless steel, shifts to higher current density values with chloride
in the medium and the passivity breakdown with accompanying large increase
in current density is observed at some intermediate potential in the passive region.
This potential is referred to as
E
pit
. In a potentiodynamic procedure, if the direc-
tion of polarization is reversed after some degree of anodic polarization above
E
pit
, the return polarization curve follows an active path and a hysteresis is ob-
served. Its intersection with the passive region is referred to as protection poten-
tial,
E
prot
, below which existing pits cannot grow. At intermediate potentials be-
tween
E
pit
and
E
prot
, a new pit cannot initiate, but a pit initiated above
E
pit
can
grow. A more positive value of
E
pit
implies more resistance to pitting.
Chloride ions are held responsible for breakdown of passivity and initiation
of pitting. Chloride ions adsorb on the outer side of the passive film [4] and
the adsorption intensifies at potentials near
E
pit
, apparently because of a stronger
electrostatic attraction between the negatively charged chloride ions and an in-
