Environmental Engineering Reference
In-Depth Information
corrosion potential outside the range of cracking. The addition of small amounts
of nitrates to concentrated NaOH prevents SCC of steel, as discussed in the sec-
tion ''Electrochemical Aspects of SCC.'' Substances like H
3
PO
2
,Na
2
O
4
, and
CO(NH
2
)
2
, which may be expected to form insoluble products with iron, retard
or prevent cracking in nitrates. Addition of traces of NO to N
2
O
4
or 1-2% H
2
O
to red fuming nitric acid or CH
3
OH/HCl mixtures prevent SCC of titanium alloys
in these media [45]. The addition of water causes anodic inhibition by shifting
the potential to the safe passive potential range. However, there are some practical
limitations for the use of inhibitors. Many failures occur in steam or under con-
densation conditions, and in both cases the transport of inhibitors to sites of crack
initiation is not feasible.
Increasing the corrosion rate to reduce SCC might appear to be a ridiculous
proposition. But because SCC is a form of highly localized corrosion, extending
corrosion over the whole of the surface will usually lessen the probability of such
failures. It has been employed in making up mixtures containing HCl to clean
austenitic stainless steel parts in chemical plants; the corrosion rate is maintained
10 mpy [46].
Electrochemical Protection.
Cathodic protection will control SCC in alloys that
crack by anodic dissolution mechanism, but is likely to accelerate hydrogen-
induced cracking, particularly in high-strength alloys. SCC failure of Kraft con-
tinuous digesters in the pulp and paper industry in an NaOH-Na
2
S environment
at 140
C has been reported [47] to have been mitigated by the application of
anodic protection.
Material Selection.
Choosing a different alloy resistant to the particular envi-
ronment is a popular option to prevent SCC. An alloy having the lowest plateau
velocity, as discussed in the section ''Testing Methods,'' can be chosen from
among a number of susceptible alloys. Mitigation of SCC by alloy development
has been a rare endeavor. However, it is reported [42] that a relatively inexpensive
stainless steel has been developed that resists SCC up to 140
°
°
C in crevice tests
with 20% NaCl in which 304 stainless steel will fail at 60
C. The addition of
copper raises the lower critical potential for SCC. Minimization of phosphorus
in austenitic stainless steels is also a key approach to making them resistant to
chloride-induced SCC.
The cladding of high-strength aluminum alloys with pure aluminum has been
successfully employed to mitigate SCC in aircraft components. The relatively
low susceptibility of pure metals to SCC has been utilized in this preventive
measure.
°
Practical Examples
Practical examples of SCC in engineering alloys are abundant and varied. Figure
3.53 shows intensive SCC in a high-pressure autoclave made of 18-8 stainless
