In order to maintain the efficient and reliable operation of pipelines and other equipment in the LNG industry, engineers need to know what techniques are available to extend the life of new components or refurbish worn or corroded ones and, just as important, what are their cost benefits compared to the use of expensive base materials or replacing the part?

There are a number of options available to protect flanges and the internal surfaces of equipment but the final choice will normally be based on an evaluation of factors such as the application, required service life, operational priorities, installation deadlines and, of course, budget restraints.

So, how can engineers ensure the long-term integrity of internal surfaces of pipelines, flanges and other equipment against aggressive corrosion caused by hazardous and corrosive LNG environments?

Where budget is not a constraint, engineers can simply specify components in corrosion or wear resistant alloys known to withstand the conditions. However, this is rarely the case and other, more cost-effective options are usually sought.

Where pipe, flanges or fittings and other components such as valves and pumps, require protection, weld overlay cladding is certainly a versatile option, providing the assurance of a heavy-duty metallurgically-bonded protective layer that will not be degraded in hostile environments.

The use of carbon and low alloy steels clad with a corrosion resistant alloy has been common practice for some years and is a well proven, economical and technical alternative to solid alloys.

For use in aggressive applications, weld overlay cladding should be considered the default option because it will provide excellent protection and extend service life for new equipment; as well as refurbishing worn or older components that are already badly corroded or eroded.

Even pipeline equipment in enclosed areas would benefit from weld overlay cladding.  While internal surfaces may need to be fully protected, a conventional and inexpensive stainless steel deposit may be adequate for the flange seal faces of pipes, valves and pumps that may suffer corrosion during their normal service life.

For the most corrosive applications, the use of a higher grade stainless steel, complex nickel chromium or hardfacing alloys is normally recommended. Whilst these tend to be expensive if used in solid form, a 3mm thick layer applied to the affected surface will offer the same operational performance and could lead to significant savings from the extended life expectancy of the equipment.

Protective materials

These include austenitic (300 series) stainless steels, ferritic/martensitic (400 series) stainless steels, duplex stainless steels or the more complex high nickel chromium alloys. With apologies to the manufacturers of austenitic stainless steels, it is unlikely they would have the resistance required for the very worst conditions.

Inconel alloys’ oxidation and corrosion-resistant properties make them the material of choice for many severe service applications. They are invaluable in natural gas applications likely to involve sour, corrosive gases such as hydrogen sulphide. However, whilst smaller valves can be cast in Inconel, the cost is frequently prohibitive for larger valves, which is where weld overlay cladding proves its cost effectiveness.

When repairing equipment, the affected areas can in many cases be pre-machined and, using automated weld overlay cladding or specialised manual welding, rebuilt with a corrosion resistant alloy (CRA) such as complex nickel aluminium bronze. Typically, the repair will be superior to the original metal!

Welding processes

After first identifying the surfaces that need to be protected, engineers can choose from a number of welding processes and a wide range of cladding alloys. Weld overlay cladding technology presents the materials engineer with a wide choice of welding processes that offer immense flexibility. An almost infinite range of component shapes and sizes can be protected, with an equally wide range of base material/cladding alloy alternatives.

The GTAW (TIG) process can be used in bores as small as 20mm, and is ideally 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. This flexibility also lends itself to the cladding of irregular shaped components, such as pump and valve internals.

GMAW (MIG), submerged arc and electroslag welding processes are used where large areas and thicker deposits are required.  Fast deposition rates mean these methods also offer cost savings.  A wider selection of consumable materials, which may not be produced in the standard solid wire form, is also available.

Selection of the most appropriate welding process is largely dependent on factors such as the size 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.

Automated or mechanised processes generally offer the best deposition rates and provide the most consistent quality of deposit, which enables adherence to the results provided during procedure qualification testing. Mechanised equipment can be designed to access areas that simply cannot be reached by manual methods.

Using this process, the chemical composition of the welding consumable can be achieved at 2.5mm from the base material/cladding interface (this can be reduced to 1.5mm in the case of 300 series stainless steels, where over alloyed wires are available).

Submerged arc welding is used where larger surfaces are clad and access is easy. Traditionally larger diameter (2.4mm ) 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.

Recently, procedures have been developed using 1.2mm wires, allowing use on thinner section components and giving more controlled thickness of deposit, while maintaining deposition rates in the region of 5kg per hour. There are consumable/flux combinations available that make single layer deposits viable. This is particularly true with duplex and ferritic/martensitic stainless steels.

When weld overlay cladding was first employed, re-machining after cladding was the norm, but as techniques and equipment have improved the ‘as welded’ finish has become much smoother, to the extent that many areas of clad equipment are now left as clad. This would not apply to the sealing/gasket areas, which have to be produced to the very finest of tolerances.

The fact is that weld overlay clad parts are now widely used in the oil and gas, power generation, chemical and marine industries because the process has proved to be a fast, flexible and cost effective remedy to the effects of corrosion and wear.

The choice of coating chemistry is legion; and the processes available extend from the lowly manual metal arc to multi head hot wire TIG to laser and beyond! Many previously ‘difficult to weld’ materials are now commonly welded with consistent success. And the development of the cladding process is such that the market acceptance standards for the cladding material (a cast structure) are now identical to the acceptance levels for the base material.

The acceptance and viability of weld overlay cladding in oil & gas environments has been proven extensively by Arc Energy. Managing Director Alan Robinson has been at the forefront of weld overlay cladding development for over 25 years, and the company is experienced in cladding bores ranging from 20mm up to four metres diameter, and up to six metres in length.

Arc Energy has developed a wide range of base material/cladding combinations, qualified to standards including ASME, NACE, DNV and ISO. The weld overlay cladding techniques have been tried and tested, and are widely implemented by a wide variety of market sectors.

To summarise, in many environments weld overlay cladding offers clear advantages because protection can be applied specifically to the areas under attack, eliminating the need to produce a whole component or item of plant from an expensive corrosion resistant material. 

However, the overwhelming advantage is its versatility.  Whatever the shape or size of the equipment or component there is a process that can be applied and an alloy to counteract the different levels of corrosive attack.  Weld overlay cladding is a proven and recognised cost saving technology that is already well established and engineers in extreme environments are sharing the significant benefits, both practical and financial, of increased life expectancy.

And finally a caveat: the development of today’s cladding technology is so tightly defined that we believe it is important to use a cladding provider that maintains the best quality structure and technical support. At the very least, the provider should hold ISO 3834-2 (Comprehensive Quality Requirements for Fusion Welding of Metallic Materials) and employ the services of an International Welding Engineer.