Environmental Engineering Reference
In-Depth Information
ments normally unaggressive toward these stainless steels. This deterioration of
corrosion resistance of stainless steels on welding has been termed ''weld de-
cay.'' However, welding is not the only causative factor for intergranular corro-
sion. Austenitic stainless steels become susceptible to intergranular corrosion
when heated in the temperature range of about 500
C. The material is
then said to be
sensitized.
The extent of sensitization effect is a function of both
time and temperature. Exposure to temperatures near the middle of this range
for a few minutes is equivalent to several hours near the upper and lower limits.
However, the limits are not fixed and they are influenced by the composition,
particularly the carbon content, of the steel. Figure 3.18 shows the sensitization
diagram for type 304 stainless steel for various carbon compositions. The steel
does not sensitize if the carbon content is below 0.02%. The higher the nickel
content of the alloy, the shorter is the time for sensitization to occur at a given
temperature. Molybdenum has the opposite effect.
°
C to 800
°
Mechanism
Depletion of chromium in the grain boundary areas is usually the cause of inter-
granular corrosion in austenitic stainless steels. In the sensitizing range, carbon
that is almost completely dispersed throughout the alloy rapidly diffuses to the
grain boundaries where it combines preferentially with chromium to form chro-
mium carbide, Cr
23
C
6
. The adjusting areas thus get depleted of chromium. As
chromium diffuses much more slowly at these temperatures, the loss is not made
up and the chromium content at the grain boundaries drops to a value much lower
than 12%, which is the minimum required to maintain passivity. In these areas,
the stainless steel behaves no better than ordinary steel.
Figure 3.18
Sensitization diagram for 18Cr-8Ni stainless steels of varying carbon
content.
