Environmental Engineering Reference
In-Depth Information
cipitation has been identified as a contributing factor in the intergranular SCC
of aluminum alloys. The 7000 series alloys (Al-Zn-Mg-Cu) are most susceptible
in the peak-aged condition; the susceptibility is reduced with overaging.
Grain boundary segregation of alloying elements or impurities has been identi-
fied as the causative factor of intergranular SCC in many alloys. Grain boundary
enrichment of magnesium in Al-Mg alloys accounts for the increased anodic
activity or possible formation of magnesium hydride along the grain boundaries.
Grain boundary enrichment of impurities such as phosphorus, sulfur, carbon, and
silicon contributes to the intergranular SCC of iron-base alloys, austenitic stain-
less steels, and nickel-base alloys. The enrichment of impurities in the grain
boundaries can be as high as 50% within a region 1-2 nm thick, facilitating the
propagation of a stress corrosion crack along the grain boundary.
The effect of variation of carbon on the SCC of very low carbon steels is
shown in Fig. 3.42. Carbon segregation at the grain boundary is considered to
provide suitable imperfection sites for adsorption of nitrates to promote SCC,
and at a very low level of bulk carbon content such segregation is not attainable.
It has been shown experimentally [11] that about 0.01% carbon was required to
cause SCC in nitrate or caustic environments, which is above the room tempera-
ture solubility limit. Phosphorus segregation has been shown by several authors
to promote intergranular SCC of low-alloy steels (Cr-Mo or Ni-Cr-Mo-V) in
caustic or water environments at relatively oxidizing potentials [12-14]. Phos-
phorus segregation has also been identified as a contributory factor in the inter-
granular SCC of austenitic stainless steels containing less than 0.002% carbon
and also in nickel alloy 600 [15]. Grain boundary segregation of impurities is
deemed responsible for the intergranular SCC of pure iron [16].
In transgranular SCC, alloying effect on slip planarity is a major factor. A
number of crack growth models have been proposed based on the planar slip-
localized corrosion processes. Planar slip occurs in alloys with low stacking-fault
energy, alloys containing ordered phases, or alloys exhibiting short- or long-range
ordering. The additions of nickel to stainless steels [17] and manganese to copper
[18] have been reported to develop planar array of dislocations due to the low-
ering of stacking-fault energy and consequently a susceptibility to transgranular
SCC. Alloys, on the other hand, are prone to dealloying. It has been suggested
[19] that the dealloyed layer acts as a cleavage crack initiator in brass, copper,
gold, and stainless steels.
Electrochemical Aspects of SCC
A number of environmental parameters influence SCC susceptibility, in terms of
both crack initiation and crack growth, and the cracking mode in an alloy. Impor-
tant among these parameters are temperature, nature of solute species and its
concentration, pH, and the electrochemical potential. Although the ''specificity''
of an environment is not strictly valid, still only a few environments can induce
