Environmental Engineering Reference
In-Depth Information
Active-passive cells of appreciable potential difference are set up between the
chromium-depleted alloy at the grain boundary and the chromium-rich grain
body. An unfavorable large cathode/small anode ratio leads to the rapid attack
of the grain boundary in corrosive environments. The chromium carbide precipi-
tated at the grain boundary, which has a leaf-like structure, is not attacked; rather,
as a cathodic constituent it aggravates the attack in the adjacent areas. The af-
fected volume of the alloy extends over a small distance on either side of the
grain boundary and the boundary thus appears to be broadened when viewed
under the microscope.
In alloys of low carbon content (below 0.02%), sensitization is absent as a
sufficient amount of carbon is not available to produce a continuous chromium-
depleted grain boundary. The precipitated carbides are sparse and remain dis-
persed without affecting the corrosion resistance of the alloy. In the normal grades
of austenitic stainless steels having 0.06-0.08% C, sensitization can be avoided
if the alloy is rapidly cooled through the sensitizing range as carbon does not
get time to reach the grain boundaries or react there with chromium.
Austenitic stainless steels, quenched from 1050
C, undergo slow intergranular
attack in strongly oxidizing media like boiling 5 N HNO 3 with added oxidizing
ions such as Cr 6 or Mn 7 . The attack is attributed to the segregation of specific
impurities to the grain boundary during quenching.
°
Weld Decay and Knife-Line Attack
The sensitization of austenitic stainless steels during welding is known as weld
decay. The affected zone is usually a band in the parent plate somewhat removed
from the weld bead. When exposed to corrosive environments, intergranular cor-
rosion takes place in this zone and the attack gives a granular appearance (Fig.
3.19). During welding, this area remains in the sensitizing temperature range for
a sufficient length of time resulting in chromium carbide precipitation, whereas
at weld pool and its adjacent areas the temperature is high and time insufficient
for the precipitation to take place. Areas further remote from the weld decay zone
do not attain the sensitizing temperature and remain unsensitized.
The exact location of the affected zone depends on the metallurgical history,
plate thickness, and rate of heat input and cooling. Since temperature and time
combinedly contribute to sensitization, a prolonged welding operation or slow
cooling after welding induces susceptibility. Arc welding is therefore preferred
to gas welding for stainless steels. Spot welding is still preferable as the heating
is intense and rapid, followed by a rapid cooling.
Knife-line attack (KLA) is yet another type of intergranular corrosion encoun-
tered in stabilized stainless steels (see section ''Remedial measures'') on welding.
A narrow band adjacent to the weld is the zone of attack. The thermal history
of the material for KLA is different from that for weld decay.
Titanium carbide and niobium carbide precipitate at temperatures higher than
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