Environmental Engineering Reference
In-Depth Information
Figure 3.41
Effect of nickel content on SCC of iron-chromium-nickel alloys in boiling
42% MgCl
2
solution.
chloride environments. The beneficial effect of high nickel content is clearly visi-
ble. However, a ferritic stainless steel devoid of nickel, which is generally resis-
tant to SCC in most common service environments that attack austenitic stainless
steels, becomes susceptible with the addition of 1.5% to 2% nickel. Duplex stain-
less steels with high chromium and low nickel content are more resistant to SCC
than austenitic stainless steels. Pure copper, which is almost immune to SCC in
ammoniacal environments, becomes readily susceptible with the alloying of 1%
silicon or 0.2% phosphorus. Brasses with a zinc content below 15% are resistant
to SCC. Addition of a third element like Si, Al, to 70Cu-30Zn brass changes the
mode of cracking from intergranular to transgranular.
Grain boundary precipitation and grain boundary segregation play a major
role in intergranular SCC. Austenitic stainless steels, heated in the temperature
range of 500-850
C, develop a chromium-depleted zone along the grain bound-
ary through precipitation of chromium carbide, a phenomenon known as sensiti-
zation (Section 3.6.1). Susceptibility to intergranular SCC and the crack growth
rate are related to the degree of sensitization. Intergranular SCC of sensitized
austenitic stainless steels is a recurring problem in cooling-water piping of boil-
ing-water nuclear power plants. The Ni-Cr-Fe alloy 600 also gets sensitized
through chromium depletion at the grain boundary and has been reported to fail
by intergranular SCC in high-temperature pressurized water. Grain boundary pre-
°
