High integrity alloys: Selection issues for corrosion protection

Rotating head.

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.