Gerhard Schiroky, Swagelok Co.
Anibal Dam, BP Exploration & Production Inc.
Akinyemi Okeremi, Shell International Exploration & Production
Charlie Speed, Consultant
Oil and gas platforms regularly use stainless steel tubing in process instrumentation
and sensing, as well as in chemical inhibition, hydraulic lines, impulse lines,
and utility applications, over a wide range of temperatures, flows, and
pressures. Corrosion of 316 stainless steel tubing has been observed in
offshore applications around the world. Corrosion is a serious development that
can lead to perforations of the tubing wall and the escape, under pressure, of
highly flammable chemicals.
The two prevalent forms of localized corrosion are pitting, often readily
recognizable, and crevice, which can be more difficult to see. Many factors
contribute to the onset of localized corrosion. Inadequate tubing alloy and
suboptimal installation practices can lead to deterioration of tubing surfaces
in a matter of months. It is speculated that today's minimally alloyed 316
stainless steel tubing, with about 10% nickel, 2% molybdenum, and 16% chromium,
may more readily corrode than the more generously alloyed 316 tubing products
produced decades ago.
Contamination is another leading cause for surface degradation. Such
contamination may be caused by iron particles from welding and grinding
operations; surface deposits from handling, drilling, and blasting; and from
sulfur-rich diesel exhaust. Periodic testing of seawater deluge systems, especially
in combination with insufficient freshwater cleansing, may leave undesirable
chloride-laden deposits behind.
Pitting and crevice corrosion
Pitting corrosion of tubing usually is readily recognized. Individual
shallow pits, and in later stages, deep and sometimes connected pits can be
seen with the unaided eye. Pitting corrosion starts when the chromium-rich
passive oxide film on 316 tubing breaks down in a chloride-rich environment.
The higher the chloride concentration and the more elevated the temperature,
the more likely the breakdown of this passive film.
(Above) Corrosion of 316 stainless
steel tubing. (Below) Pitting often can be seen with the unaided eye.
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Once the passive film is breached, an
electrochemical cell becomes active. Iron goes into solution in the more anodic
bottom of the pit, diffuses toward the top, and oxidizes to iron oxide. The
concentration of the iron chloride solution in a pit can increase as the pit
deepens. The consequences are accelerated pitting, perforation of tubing walls
and leaks. Pitting can penetrate deep into the tubing walls, creating a
situation where tubing could fail.
Crevices are difficult, or even
impossible, to avoid in tubing installations. They exist between tubing and
tube supports, in tubing clamps, between adjacent tubing runs, and underneath
contamination and deposits that may accumulate on tubing surfaces. Relatively
tight crevices pose the greatest danger. General corrosion of tubing in a tight
crevice causes the oxygen concentration in the fluid that is contained within a
crevice to drop. A lower oxygen concentration increases the likelihood for
breakdown of the passive surface oxide film. The result is a shallow pit.
Iron goes into solution in the more
anodic bottom of a pit, diffuses toward the top, and oxidizes to iron oxide
(rust).
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Unlike in pitting corrosion, formation
of a pit on tubing surrounded by a crevice leads to an increase of the Fe++
concentration in the fluid contained in the gap. Because of the strong
interaction of the Fe++ ions with the OH ions, the pH value drops. Chloride
ions also will diffuse into the gap, being attracted by the Fe++ ions. The
result is an acidic ferric chloride solution that can accelerate corrosion of
tubing within the crevice.
Ideally, tubing should resist all
forms of corrosion, including general, localized (pitting and crevice),
galvanic, microbiological, chloride-induced stress corrosion cracking, and sour
gas cracking. The tubing also should have adequate mechanical properties,
especially when fluid pressures are high. Resistance to erosion comes into play
when fluids contain potentially erosive particles. The environmental impact of
the tubing also should be a concern; aquatic life can be harmed by small
concentrations of copper ions that can be released by copper-zinc alloys.
The resistance of an alloy to
localized tubing corrosion can be estimated from its chemical composition by
calculating the alloy's pitting resistance equivalent number (PREN). The most
frequently used relationship is: PREN = %Cr + 3.3 %Mo + 16 %N. The higher the
PREN value of an alloy, the higher its resistance to localized corrosion, i.e.,
the higher its critical pitting temperature (CPT) and critical crevice
corrosion temperature (CCT). These critical temperatures can be determined by
common testing procedures such as ASTM G48 and ASTM G150.
Alloy selection
The importance of selecting the
optimal alloy is demonstrated when austenitic 316 stainless steel tubing shows
heavy corrosion while no signs of corrosion were detected on alloy 2507
superduplex tubing installed side by side. In a Gulf of Mexico installation of
alloy 2507 tubing, only a few instances of external chloride crevice corrosion
damage were identified. No perforations leading to the loss of containment of
system fluids were observed. The only instances where crevice corrosion damage
occurred involved the use of plastic support strips and neoprene gaskets.
Numerous alloys have been used or have
presented themselves as candidates for use in installations that require
resistance to seawater corrosion. The most frequently used alloys have been the
300-series austenitic stainless steels, mainly 316 and in some cases 317.
Alloys with at least 6% molybdenum, the so-called "6-moly" alloys,
perform well offshore. Typical 6-moly alloys include 254SMO, AL6XN, and 25-6Mo.
More recently, alloys with slightly
more than 6% molybdenum have been introduced: 654SMO, AL6XN Plus, 27-7Mo, and
31. The published properties of these alloys suggest that they would perform
well in chloride environments.
Nickel alloys such as 825, 625, and
C-276 are more frequently used in sour gas applications. Of these alloys, 625
and C-276 demonstrate excellent resistance to localized corrosion. Ferritic
alloys like Sea-Cure and AL29-4C resist attack by aqueous chloride solutions
and are primarily used as heat exchanger tubing.
Mechanical Properties for Duplex Alloys
Tubing alloys are available that offer a combination of attractive
properties for even unique applications in global construction projects. It is
good practice to select an alloy with a critical pitting temperature above
operating temperature. Depending on the application, it may be just as
important to select an alloy with a critical crevice corrosion temperature
above operating temperature.
Even highly corrosion-resistant tubing can be sacrificed when tubing
surfaces are not kept clean. If possible, tubing should be installed following
heavy construction activities that would otherwise allow weld splatter and
grinding debris to accumulate on tubing. Where adjustments of construction
sequences are not possible, tubing should be shielded from contamination, and
if contaminated, should be thoroughly cleaned.
The growing number of duplex alloys reflects the increasing use of this
promising class of materials. The workhorse 2205 duplex alloy was introduced
decades ago. Now there is superduplex alloy 2507, which has performed very well
in recent years in more demanding applications that require PREN values of 40
and above. More recently, the hyperduplex alloy 3207 was introduced with an
even higher PREN value.
At the low end of alloy content, several lean duplex alloys such as 2101,
2304, and 2003 are candidates for less demanding applications.
A graph plot of the critical pitting temperature and critical crevice
temperature shows the increase in chromium, molybdenum, and nitrogen leads to
an increase in the CPT and CCT values of austenitic and duplex stainless
steels. That also illustrates the economic advantage of duplex alloys. Despite
an overall lower content of costly nickel and molybdenum, they offer
performance similar to that of highly alloyed austenitic stainless steels.
Not only do duplex alloys offer satisfactory resistance to localized
corrosion, they also have high mechanical properties, which make them prime
candidates for high-pressure applications. Note that 2507 has a yield strength
more than three times that of 316L.
Jacketed tubing
For applications in seawater, a tubing alloy that is highly resistant to
localized corrosion is not the only option. Alternatively, one may select a
less resistant alloy and then shield or protect the tubing.
Adequate protection appears to be offered by a thermoplastic polyurethane
jacket that can be cost-effectively extruded onto continuous tubing. Recent
installations in the Gulf of Mexico combine this clamping concept with
superduplex tubing, and will generate valuable performance data.
An alternate approach uses jacketed tubing. The extrusion of a
thermoplastic coating onto tubing is an economically attractive solution.
Tubing is typically 316 or 317 stainless steel, and the preferred coating is
polyurethane. Limited installations that use urethane jacketed 316 tubing
report satisfactory results.
While the jacket must offer reliable protection from corrosive fluids, it
must fulfill additional requirements. The jacket must resist impact, abrasion,
and degradation by UV-radiation. It must allow tubing to bend, and must allow
for cost-effective tubing installation, i.e., removal of the jacket and make-up
of tubing connections. Once made up, the connections typically have to be
protected from the environment using shrink tubing or tape. Without this
protection, seawater access could cause pitting corrosion of exposed tubing or
crevice corrosion in the gap between the tubing and the jacket.
Appropriate tubing clamps must be selected and care taken to prevent them
from cutting into jackets and sacrificing their protective character. Jacketed
tubing also can insulate, or heat and insulate, tubing when system fluids must
be kept above ambient temperature.
Tubing supports and clamps
Many types of tubing supports and clamps have been used. Some have led to
significant crevice corrosion, especially when tight crevices with large
crevice surface areas result in depletion of oxygen so the alloy cannot reform
the passive oxide layer. In particular, plastic tubing clamps are prone to
inducing crevice corrosion because the plastic deforms around the tubing to
create tighter crevices that limit oxygen ingress.
One early approach to preventing or mitigating crevice corrosion was the
use of marine aluminum alloys in tubing supports and clamps. The tubing rests
on a thin strip of aluminum alloy contained within a fiber-reinforced plastic
tray. The tubing is held in place with an aluminum alloy bar.
Tubing support structures that use aluminum alloys appear to perform well.
Galvanic corrosion between aluminum alloy and stainless steel may occur, but
the aluminum alloy is more anodic than stainless steel, which means aluminum
will corrode preferentially. Once sufficient corrosion has taken place over a
number of years, affected aluminum supports and clamps can be replaced while
the stainless steel tubing remains in place.
An alternate design originally developed for piping supports has recently
been adopted for the installation of stainless steel tubing. The tubing is
sandwiched between two half-round rods of a thermoplastic material. With the
round tubing running perpendicular to the round support rod surface, the
crevice contact area is minimized. Theoretically, there should be only one
point of contact; however, some plastic deformation of the support rod takes
place that results in a finite contact (crevice) area. A benefit of this design
is that the supports/clamps allow for differential expansion of tubing and
support structure.
Industry standards
The recently published industry
standard, NACE SP0108-2008 "Corrosion Control of Offshore Structures by
Protective Coatings," provides guidance for more effective corrosion
protection for offshore structures. Another industry standard, API RP 552
"Transmission Systems," contains a section on installation practices.
Those described practices do not address the avoidance of crevice corrosion.
Source: http://www.offshore-mag.com/articles/print/volume-73/issue-5/productions-operations/pitting-and-crevice-corrosion-of-offshore-stainless-steel-tubing.html. Accessed by 24-1-2014
Source: http://www.offshore-mag.com/articles/print/volume-73/issue-5/productions-operations/pitting-and-crevice-corrosion-of-offshore-stainless-steel-tubing.html. Accessed by 24-1-2014
nice post on ASTM A213 TP 317 Stainless Steel Seamless Pipe can you write more on shims
BalasHapus