High integrity alloys:
Selection issues for corrosion protection
Alan Robinson
Arc Energy Resources
Arc Energy Resources
Consider the
problems. Hydrogen sulfide (H2S), dissolved carbon dioxide (CO2) and
various chlorides are all present in the hydrocarbons delivered from subsea
fields, and they can be accompanied by high pressures and high temperatures. And
sour service at high temperature is more corrosive, while the same service at
high pressure is more erosive. A combination of the two is a potentially
expensive and hazardous situation that impacts materials selection, in terms of
protecting low-cost carbon steels or manufacturing in high-cost corrosion
resistant alloys.
Corrosion and
corrosion prevention cost the subsea oil and gas industry billions of dollars
every year, so the decisions taken are vital. The selection of the materials
and the preventative processes used to extend the operating life of materials
is essential to the cost-effective manufacture and safe long-term operation of
equipment such as pipelines and valves, especially in deepwater operations.
When assessing corrosion
protection for any production system pipeline, process engineers have numerous
options. The effectiveness of each will vary according to factors such as the
aggressiveness of the product; pressure and temperature; the size and
complexity of the system; projected life expectancy of the well; the
development period available; and, perhaps most important, overall budget
constraints.
So how can welding
engineers help the oil and gas industry to resist these attacks?
Protection, where
risk of attack is low and life cycle relatively short, may be as simple as
using an injected inhibitor with conventional high-strength carbon or low-alloy
steel. Where greater protection is needed, corrosion-resistant alloys (CRAs)
must be considered. These include austenitic (300 series) stainless steels,
ferritic/martensitic (400 series) stainless steels, duplex stainless steels, or
the more complex high nickel chromium alloys.
Duplex steels and
nickel-based alloys, such as alloy 625, are the only materials in general
production which, when welded, achieve suitable levels of protection. However,
there are constraints on the use of these materials in their solid form –
namely cost, availability, and the need for very tightly controlled welding
procedures.
Cost is particularly
relevant where large quantities of pipe and fittings are needed or when large
forgings or castings are used. Typical examples are wellhead valve systems and
pipe bundle bulkheads.
The use of carbon
and low-alloy steels clad with a corrosion-resistant alloy is common practice
for some years now. It is a well-proven, economical, and technical alternative
to solid alloys. It offers the benefits of strength and/or availability of base
materials combined with corrosion resistance, when applied in selected areas.
Weld overlay
cladding presents the materials engineer with a choice of processes and more
flexibility. An almost infinite range of component shapes and sizes can be
protected, with an equally wide range of base material/cladding alloy
alternatives. Weld procedures are normally qualified to ASME IX, as are the
welding operators.
Additional testing
to prove conformity with API 6A and NACE MR01-75 also is essential. Selection
of the most appropriate welding process largely depends on factors such as the
size and geometry of the clad area; access to the area to be clad; alloy type;
specified clad thickness; chemical composition limits; welding position; and
NDT acceptance standards.
There are many
common welding processes but given that the process used must be practical,
viable, and provide the mechanical and chemical conditions to achieve service
requirements, economics dictate that the higher deposition rate processes
should prevail.
GTAW (gas tungsten
arc welding) provides excellent control and a high quality result. It can be
used in bores as small as 20 mm (0.78-in.), and is suited for components of
varied geometry, where the position of the welding head requires frequent
adjustment. These could range from a simple flange that needs to be clad
through the bore and across the sealing face, to a complex valve body with
several interconnecting bores. Utilizing twin wire, hot wire, and multi-head
configurations increases the deposition rates.
Often such equipment
also needs cladding to RTJ (ring-type joint flange) grooves. The control
available with GTAW means cladding can follow the profile of the groove rather
than filling it completely. This not only saves time and material during
cladding, it also reduces the cost of subsequent finish machining.
Using this process
the chemical composition of the welding consumable can be achieved at <2.0
mm (0.08-in.) from the base material/cladding interface (this can be reduced to
<1.0 mm (0.40-in.), in the case of 300 series stainless steels, where
over-alloyed wires are available).
Plasma-transferred
arc is another option. The process equipment costs are higher and the process
variables slightly more complex than GTAW, but the increased control available
on the arc makes it more amenable to CNC control. When combined with oscillation,
dilution levels down to 3% have been achieved at 1 mm from the interface.
Arc's development
engineers have been working with the new breeds of GMAW (gas metal arc welding)
to improve control of the arc, and the resulting process likely will supplant
some current GTAW applications.
For more open access
applications, the electroslag process is economically attractive. It does
employ a large weld pool that requires substantial base metal backing
(generally a minimum of 20 mm) in order to prevent excessive dilution. The
deposit thickness is nominally 5 mm (0.2 in.) with the strip widths discussed
here. With 60-mm (2.4-in.) strip, deposition rates of up to 22 kg/hr (48.5
lb/hr) can be achieved.
To enable the
chemical composition of the deposit to match that of the consumable
specification within the first layer (3 mm, or 0.12 in., from the interface),
over-alloyed strip and "loaded" metal containing fluxes are
available.
Problems associated
with electroslag strip cladding involve the limited availability of strip,
which tends to increase the cost of the material; and the difficulty of feeding
the strip when cladding within bores of pipe. Arc Energy Resources is
developing a multi-wire electroslag configuration for pipe cladding. This
should solve both problems and provide a combination of high deposition,
excellent profile, and good quality.
Submerged arc
welding using a solid wire consumable, while not as fast, is a useful
"halfway house" between strip cladding and the slower GTAW and pulsed
GMAW. The welding heads are not as large as strip heads, and the consumable
delivery method is more flexible. Hence, the capability to use this in smaller
bore diameters. Traditionally larger-diameter (2.4 mm+, or 0.09-in.+)
consumables have been used for this process, again resulting in the need for
fairly thick substrates to accept the high heat inputs and large weld deposits.
11/01/2011
http://www.offshore-mag.com/articles/print/volume-71/issue-11/equipment-__engineering/high-integrity-alloys-selection-issues-for-corrosion-protection.html
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