Stainless steels 325Transverse
sectionRolling direction
,"llk'i'- I ~ ,
(a)Plate
thickness(b)(c)Figure 4.16 Schematic illustration of end grain attack from outcropping inclusions:
(a) elongated inclusions outcropping on transverse sections; (b) outcropping inclusions
dissolved in nitric acid; (c) heavy intergranular attack from crevices
In addition to segregation and precipitation effects at grain boundaries, the
intergranular corrosion behaviour of austenitic stainless steels can also be
influenced very markedly by the presence of elongated particles or clusters of
second phases. These can take the form of sulphides or other plastic inclusions
but adverse effects can also be induced in niobium-stabilized Type 347 steel due
to the presence of stringers of coarse niobium carbonitrides. The mechanism is
shown schematically in Figure 4.16. In the Huey test, outcropping inclusions
appear to dissolve quickly, creating long narrow passages from the end faces
into the body of the samples. These passages do not provide ready access for
the ingress of new acid and it is thought that this gives rise to the formation
and concentration of hexavalent Cr e+ ions. This leads to particularly aggressive
corrosion conditions within the passages and intergranular attack proceeds very
quickly along the adjacent grain boundaries. The effect is known as end grain
attack and can lead to weight losses far in excess of that experienced in
cleaner steels or those containing shorter inclusions. In chemical plant, end grain
corrosion can be experienced in forgings or in tube-to-tube welds where end
faces are exposed due to ovality effects or differences in tube diameter. In such
cases, the exposed end grain faces should be covered by capping welds.
Because carbon has a very low solid solubility in ferritic stainless steels, it is
extremely difficult to prevent the formation of chromium carbides in these grades.
Thus when carbon is taken into solution by heating to temperatures above 925~