68 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
There have been limited attempts to develop and
market aluminum drill pipe. Drill pipe is usually made
from 2xxx alloys containing copper as the main alloying
addition. The 2xxx alloys have some of the highest
fatigue strengths of any aluminum alloys, and this pro-
perty, combined with their low weight, makes them
acceptable for many applications in other industries.
They were the fi rst high - strength aluminum alloys
developed. Their corrosion resistance, while adequate
for many aerospace applications, is not suffi cient for
oilfi eld operations, except in circumstances like drill
pipe, where they are only exposed to downhole fl uids
for limited periods of time and can be inspected for
corrosion and other damage, typically wear and fatigue
cracking, between uses.
80,81
Additional Considerations with CRA s
CRAs are used whenever the increased capital cost is
justifi ed by reduced maintenance and inspection costs
or when increased reliability is necessary. Many high -
volume gas wells are so corrosive that carbon steels are
not considered, and the question becomes which CRAs
should be used. While some gas fi elds have high - H
2
S
levels, most gas well corrosion is due to CO
2
.
Oil fi elds are generally not corrosive until the water
cut increases and/or the system “ sours ” due to the
increased production of H
2
S. For this reason, it is
common to specify that all equipment be constructed
from materials considered resistant to H
2
S - related
cracking in accordance with the appropriate guidelines
in NACE MR0176/ISO 15156.
10 – 12
These precautions do
not prevent other forms of environmental degradation,
and corrosion can be the unfortunate result.
The increased alloying content of CRAs means that
they cost more than carbon steels. A way of reducing
costs is to use CRA cladding or lining on carbon steel
components. This long - standing practice for wellhead
and process equipment has been extended to downhole
equipment and tubular goods. The CRA can be applied
by a number of processes. If CRAs are mechanically
bonded using either explosive bonding or thermal
shrinking, the composite structures are referred to as a
lined pipe instead of clad pipe.
82
Pitting Resistance Equivalent Number ( PREN ) Many
organizations use the PREN formula in NACE MR
0176/ISO 15156 as a basis for determining the relative
corrosion resistance of oilfi eld alloys. The PREN
formula in this standard is:
12
PREN w w w w
CR Mo W N
=+ +
()
+33 05 16..
where:
alloys, for example, Aluminum Association alloy 5052 -
H32 would be UNS A95052H32.
Note that the aluminum alloys fall into two
groupings — those that can be thermally treated for
strength and those that can only be strain - hardened
(work - hardened). Sheet and plate aluminum alloy com-
ponents are typically the work - hardened 3xxx and 5xxx
alloys, while extrusions for structural members and
tubular products are typically produced from the ther-
mally treated (age - hardened) 6xxx alloys.
Typical Applications for Various Aluminum Alloy
Groups Commercially pure aluminum alloys, the 1xxx
group, are the most corrosion - resistant alloys. They are
so soft that they are not used in oilfi eld applications
except as corrosion - resistant surfaces on clad structural -
alloy plate.
Most users consider the 5xxx and 6xxx alloys to be
the most suitable for marine applications, and their use
in nonmarine applications is also widespread. A typical
workboat or helideck would have plate components of
5xxx alloys and the supporting structure/frame made
from 6xxx alloys. Aluminum fasteners could also be
made from 6xxx alloys, but aluminum has serious galling
problems, and it is common to use specially designed
connections when mechanically joining aluminum struc-
tural components.
Clad aluminum alloys are available. They were origi-
nally developed with commercially pure aluminum sur-
faces for corrosion control over stronger structural
aluminum alloy plate. Many complex aluminum heat
exchangers, to include those used in oilfi eld heat
exchangers, have low - melting temperature exteriors
with structural alloys in the center. These clad products
are used to manufacture brazed aluminum structures
such as heat exchangers.
Brazed aluminum heat exchangers are frequently
used offshore for gas processing. The parting sheets,
which separate the fl uids, are usually made from 3xxx
alloys, and they are brazed together using a low - melting
point aluminum - silicon alloy from the 4xxx series.
79
The
aluminum - silicon alloys have very low - melting temper-
atures in comparison with other aluminum alloys, and
they are usually used as castings. They are seldom used
for structural purposes in oilfi eld applications, and this
is the reason why they are not listed in Tables 4.16
and 4.17 .
Welding of aluminum is possible, but it is more dif-
fi cult than for carbon steel and other iron - based alloys.
Complex machinery and process vessels are factory -
welded, but it is common to use mechanical connections
in oilfi eld construction and assembly, for example, of
helidecks, while similar structures from carbon steel
would be welded.
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