90 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
corrosion, and the formation of aluminum oxide pushes
the metal apart in a direction perpendicular to the
rolling direction. This is shown in Figure 5.35 , which
shows exfoliation (the loss of leaves) intergranular cor-
rosion of an aluminum guard rail in a coastal marine
environment. Note how the galvanized bolt has rubbed
against the bolt hole on the aluminum rail and caused
corrosion where the bare metal was exposed after the
protective aluminum oxide passive fi lm was removed.
This picture is also an example of the problems associ-
ated with improper design for thermal expansion
and contraction, which caused the relative motion
between the expanding and contracting aluminum and
the fi xed bolt location. This problem of expansion
and contraction can be expected to occur on helidecks
and other sunlight - exposed structures. Aluminum is not
welded in most fi eld applications, so exposure of
that the newly recrystallized grains are located approxi-
mately halfway through the HAZ.
This corrosion can be minimized by using austenitic
stainless steels with lower carbon contents, 304L or
316L (UNS S30403 or UNS S 31603), or by using stabi-
lized grades of stainless steel, types 321 or 347 (UNS
S32100 or S34700), which have titanium or niobium
additions. The titanium or niobium additions are added
as “ carbide getters ” that preferentially form titanium or
niobium carbides, thus tying up the carbon and mini-
mizing the formation of chromium carbides. Both of the
above approaches are similarly effective in minimizing
sensitization and the resulting intergranular weld decay
in HAZs of austenitic stainless steels. North American
steel producers tend to prefer the low - carbon approach,
and European producers have traditionally preferred
the stabilizing addition approach.
Manufacturing operations can undo sensitization by
post - weld heat treatment, but this is very diffi cult in
fi eld operations.
Once again, the use of carefully approved welding
procedures is very important.
Refi neries and other high - temperature operations
can sometimes operate at temperatures above 500 ° C
(950 ° F) where sensitization can occur. Most down-
stream operations are at temperatures too low for this
to happen, but it is possible for manufacturers to deliver
sensitized stainless steel components due to improper
manufacturing procedures.
Corrosion Parallel to Forming Directions
Many metal objects are more prone to corrosion on
surfaces perpendicular to the metal forming direction
(the rolling or drawing direction). This is shown in
Figure 5.34 , which shows a stainless steel nut that cor-
roded along grain boundaries in a marine exposure. The
nut was machined from hexagonal bar stock, and sulfi de
stringers parallel to the original bar stock ’ s longitudinal
direction allowed intergranular crevice corrosion on the
facing side (to the right in the picture) of the nut. Most
machining - grade metals have deliberate additions of a
soft second phase to expedite machining processes.
Sheet, plate, and tubing are also more corrosion suscep-
tible in the through - thickness direction. This is why cor-
rosion rates are faster on cut surfaces, for example,
where tapping is necessary for instrumentation connec-
tions. No inappropriate heating was involved in causing
the corrosion shown in Figure 5.34 .
Aluminum
Aluminum alloys get their strength from alloying. This
makes the grain boundaries of aluminum susceptible to
Figure 5.34 Intergranular corrosion along the forming direc-
tion on a stainless steel nut.
Figure 5.35 Exfoliation of aluminum at a coastal location.
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