Page 27 - Handbook of Materials Failure Analysis

3 Case Studies 19

Elemental microanalysis showed the presence of mixed Fe/Cr oxides and scale
products in the interior of the cracks, suggestive for the corrosive action of the flow-
ing medium.
Deterioration of the material has been associated also with the formation of pits
(voids) of average diameter 30 μm(Figure 1.15). As it is mentioned above, cracks
emanated from pits and further crack growth and “linking” of individual pits lead
to severe metal detachment and final failure. Microstructure features at the vicinity
of these voids (as-polished condition) are consistent to twinning and slip band for-
mation (Figure 1.15). These characteristics could be attributed to stress fields gen-
erated due to the thermomechanical loading deformation imposed by the operation.
Slip bands signify the presence of strain fields which constitute also another indicator
of SCC likelihood. In relevant studies, similar “slip band”-type formations are cor-
related with the precipitation of Cr-carbides. In case of AISI 310 stainless steel, acic-
ular chromium carbide precipitations were reported [25], while in case of AISI 304
stainless steel, the presence of Cr-carbides favored at slip bands and twin boundaries
[9]. This effect could be considered as a secondary degradation mechanism since Cr-
impoverishment renders the matrix vulnerable and less corrosion resistant. In addi-
tion, strain-induced austenite destabilization and martensite transformation at the

20 µm

FIGURE 1.15
Optical micrograph showing the presence of pronounced slip bands close to the location of
corrosion pits.