MATERIALS Q&A BYDONBUSH
Q: Why do refineries tend to
specify type 321 and type 347
stainless steels? Why can’t
they just use standard 316
constructions?
A: Many of the hydrocarbon streams in
a refinery contain 0.5 to 5 weight percent sulfur or sulfur compounds. Under
certain circumstances, corrosion causes the formation of a predominantly
iron sulfide scale on the internal surfaces of piping, vessels, valves, etc.
This form of corrosion is commonly
encountered in desulfurizing, hydrocracking, hydrotreating, crude distillation and fluid catalytic cracking units. 1
POLYTHIONIC ACID
When these iron sulfide scales are
exposed to liquid water and oxygen,
the three can react to form polythionic
acid. Polythionic acid is actually a
family of acids with a chemical
formula of:
H2SxOy
x may be any number from 1 to 6
y may be any number from < 1 to 6
Oxygen and liquid water are generally not available to support the formation of polythionic acid during normal
refinery operations. However, during
shutdowns, both can be introduced into
the system.
The most common source for that
oxygen is from air that enters the system when equipment is opened for
maintenance. However, oxygen also
can be introduced in cleaning solutions
or in impure nitrogen used for equipment purging.
Liquid water is typically introduced
by condensation of steam used for
removing hydrocarbons from the equip-
ment or from water washing, rainfall,
high humidity that reaches dew-point
conditions, hydrotesting during shut-
down inspections and similar
conditions.
STRESS CORROSION
CRACKING
Austenitic materials, such as the 300-
series stainless steels and the solid
solution nickel alloys, are susceptible if
they are in a sensitized condition to
intergranular stress corrosion cracking
when exposed to polythionic acid.
Sensitization can occur in two ways:
1. During welding, the heat-affected
zone can be subjected to temperatures that can cause the formation of chromium carbides, thereby reducing the amount of
chromium in the surrounding
matrix phase and reducing its
corrosion resistance.
2. In elevated-temperature service,
the material could become sensitized due to the long-term exposure to the service temperature.
In either case, two approaches are
used to avoid sensitization:
; Low-Carbon Content: The first
approach is to use a stainless-steel grade with low-carbon content. By holding the carbon content to 0.03% maximum, not
enough carbon is available to
cause a significant reduction in
corrosion resistance even when
the alloy is exposed to sensitizing
temperatures.
; Chemical Stabilization: The stabi-
lization method entails alloying
the stainless steel with an ele-
ment that has more affinity than
chromium for carbon in the sen-
sitizing temperature range.
Stainless steels are usually stabi-
lized with either titanium or nio-
bium (often called columbium).
For example, type 321 is essen-
tially type 304 with titanium
added, and type 347 is type 304
with niobium added. These two
elements will form carbides more
readily than chromium does, and
thus will consume the carbon and
leave the chromium intact in the
matrix phase. The stabilization
mechanism can be further
enhanced by performing a “sta-
bilization heat treatment” that
intentionally causes the forma-
tion of the titanium or niobium
carbides. Note there is no such
thing as a cast version of type
321 stainless steel because the
titanium tends to oxidize readily
while being melted, transferred
and poured. The cast version of
type 347 is grade CF8C.
For a more complete discussion of
sensitization and its avoidance, please
refer to the Materials Q&A column in
the Fall 2001 issue of Valve Magazine.
In long-term exposure at elevated
temperatures, such as many refinery
applications encounter, sensitization
can occur even in those alloys designed
to resist it. The low-carbon stainless
steels and the stabilized grades have
been found to sensitize when exposed
for long periods of time at temperatures from 750-1500° F (400° C and
up). However, a number of variables
other than temperature and time affect
sensitization in service, and refinery
operational experience indicates these
sensitization-resistant alloys can be
successfully used at temperatures
exceeding the minimum sensitization
temperatures mentioned above. 2
In most cases, refineries tend to
favor the use of stabilized stainless-steel grades rather than low-carbon
grades. Two possible reasons include
the lower pressure capabilities of the
low-carbon grades and the maximum
