Environmental Engineering Reference
In-Depth Information
environment that is itself partially passivating. Carbon steels stress-corrode in
the solutions containing nitrates, hydroxides, or carbonates, which are anodic
inhibitors for steel. SCC of magnesium-base alloys is encountered in the presence
of both chromate (CrO
4
2
) and chloride ions, the former providing passivity and
the latter destroying it locally. The specificity of the environment is explained
by these considerations.
The dependence of cracking time of alpha brass on the solution pH is illus-
trated in Figure 3.43. The most susceptible pH range is also the stability range
for Cu
2
O in the potential-pH diagram for Cu-H
2
O-NH
3
system [20]. For SCC of
carbon steels in various environments, potential-pH conditions for severe crack-
ing susceptibility have been identified which also coincide with the regions of
thermodynamically stable protective films [21].
The susceptible ranges for iron-base alloys with respect to the schematic an-
odic polarization curve are shown in Fig. 3.44. The upper zone corresponds to
the breakdown potential range for stainless steels initiating pitting in chloride-
containing solutions. At more negative (active) potentials, pitting is absent and
so is the SCC originating from pits. The lower zone represents the borderline
passivity condition for carbon steels in nitrates, hydroxides, or carbonates. The
addition of small amounts of nitrates to concentrated NaOH solution shifts the
corrosion potential significantly in the positive direction beyond the cracking
potential range in alkali and the SCC stops. On the other hand, the free corrosion
Figure 3.43
Time to cracking as a function of pH for brass in ammoniacal copper
sulfate solution.
