Environmental Engineering Reference
In-Depth Information
Figure 3.44
Schematic anodic polarization curve of iron-base alloys showing zones
of susceptibility to SCC.
potential of some mild steels lies at the boundary of cracking range in hydroxide,
and SCC occurs with the addition of a small amount of lead oxide that causes
a potential shift into the cracking range.
The SCC of brass has also been shown to be potential-dependent (Fig. 3.45)
with cracking mode changing over from intergranular to transgranular beyond a
certain value of anodic potential [22]. The figure also indicates the effect of bulk
solution composition (copper concentration) on cracking time as well as on the
susceptible potential range. The potential and pH conditions inside the crack are
not the same as those in the bulk of the solution or at the surface of the alloy,
and so a correlation may not always be possible with the potential-pH diagram.
However, it is reasonable to believe that the conditions prevailing inside the crack
essentially meet the electrochemical requirements as presented above for the SCC
to occur.
If electrochemical dissolution at the crack tip accounts for the propagation of
