FORMS OF CORROSION 81
in the 1940s and early 1950s. The metal in the HAZ of
oilfi eld production tubing sometimes corrodes near the
upset head or welded tool joint. In upset tubing, the
metal needs to be heated to the austenite stable region
(above approximately 750 ° C (1382 ° F) so that it can be
deformed. Welded connections have similar HAZs that
can lead to the same problem.
9,16
The problem was
solved in the 1950s by introducing the practice of full -
length normalizing — heat - treating the entire joint to a
suitable (austenizing) temperature and then air - cooling
(normal cooling procedure) so that the entire joint has
the same microstructure and corrosion resistance. This
problem has reappeared worldwide in recent years,
because engineers and purchasing organizations have
not learned the lessons of decades past.
L ü ders Band Corrosion L ü ders bands (also called
stretcher marks, Hartmann lines, or Piobert lines) are
localized bands of plastic deformation that can occur on
carbon steels and other materials in regions of localized
plastic deformation.
17
They form in carbon steel and
other materials when the initial resistance to deforma-
tion is overcome and localized yielding (plastic defor-
mation) occurs. This localized deformation is usually at
approximately 45 degrees to the primary stress axis and
may form ripples, L ü ders bands, when the deformation
reaches the metal surface. These deformations are an
indication that part of the metal has been stressed more
than the other regions, which are in a lower energy state
and are less susceptible to corrosion. If the differences
in stress are not removed, they can lead to a corrosion
pattern termed “ Luders band ” corrosion. This is shown
in Figures 5.14 and 5.15 . Once again, full - length normal-
izing is the recommended solution to this problem.
While Figures 5.14 and 5.15 show downhole tubing; this
is also a potential problem on pipeline steel.
While the corrosion patterns shown in Figures 5.14
and 5.15 are due to plastic deformation during pipe -
mill processing, this problem can also occur as a
result of deformation in the fi eld. It is important that
piping, especially for large - diameter pipelines, be
handled and bent very carefully to avoid localized
regions of high stress and the formation of Luders
bands.
18 – 20
Environmentally Induced Galvanic Corrosion
Changes in electrolytes produce galvanic corrosion
cells. Typical examples are the differences between the
potentials in deep water, which is usually colder and has
less oxygen, and surface water, which is high in oxygen.
Galvanic differences also occur offshore from major
rivers, where the surface water may be fresh and have
low salinity for several meters before the lower, denser
saltwater becomes prevalent. Temperature gradients
fi ller metals should always be cathodic to the base metal
being joined has already been discussed in the discus-
sion of the area effect. Unfortunately, improper welding
procedures can produce situations where the HAZs
become anodic to the surrounding metal. This is shown
in Figure 5.12 , which shows HAZ corrosion in a crude
oil pipeline. Note how the corrosion is located parallel
to the fi eld - installed girth weld and no corrosion is asso-
ciated with the longitudinal weld that was made under
controlled conditions in the pipe mill.
Problems with girth welds are common, and this is
why pipeline inspections, for example, internal pig
inspections, concentrate on these areas as potential cor-
rosion sites. Many organizations have also decided not
to use electrical - resistance - welded (ERW) oil country
tubular goods (OCTGs) because of corrosion problems
along the longitudinal welds.
Ringworm Corrosion Figure 5.13 shows a phenome-
non called “ ringworm corrosion, ” which was a major
corrosion concern in the Permian Basin of West Texas
Figure 5.12 Heat - affected - zone corrosion on carbon steel
crude oil pipeline.
Figure 5.13 Ringworm corrosion in oilfi eld production
tubing. Note the internal corrosion in the heat - affected zone
a short distance from the welded connection.
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