Corrosion resistant alloys in pipelines

The corrosion resistance of nickel-containing alloys is useful in a wide range of energy applications – from renewables to fossil fuels. Even as society makes the switch to renewable energy sources, there is currently a continued need for oil and gas. 

Oil and gas engineers use corrosion resistant alloys (CRAs) for pipelines based on mechanically lined pipe (MLP) and clad pipe, as well as flexible pipes and risers.

Clad and MLP are both supplied as standard 12-meter lengths of solid pipe that are welded or bolted together. In both cases, a thin layer of CRA provides corrosion resistance and a carbon steel shell gives strength. However, there’s a big difference between the two.

In clad pipe, the CRA and carbon steel are joined with a metallurgical bond that is formed under intense force at a rolling mill or through explosion bonding or weld overlay. Specialist pipe mills will then purchase this clad plate to form into pipes and fittings for Engineering Procurement and Construction (EPC) contractors that serve oil companies.

In comparison, MLP is made by inserting a corrosion resistant liner into a carbon steel pipe. A mechanical bond is formed with hydraulic or mechanical force and the liner is then welded into place at both ends to seal the layers together and prevent ingress.

While MLP is less costly, the metallurgical bond in clad pipe provides assurance of performance, which is particularly important in high-pressure pipelines and sensitive environments.

For offshore pipelines, flexible pipe can also be used and has the benefit that it is supplied to EPCs in long lengths that need less jointing work offshore. Flexible pipes are used as flowlines on the seabed, as well as risers, and can handle wave and tidal movements.

Flexible pipe manufacture is highly specialized. Original Equipment Manufacturers (OEMs) each take their own approach to the design and construction, but all use multiple layers to provide strength and flexibility.

Strips of CRAs are used to make the internal carcass in flexible pipes. They are formed into special profiles that interlock in a spiral to create a pipe and outer layers are added to provide strength and flexibility.

Certifications are the key to supplying material to the oil and gas industry. That is particularly true for flexible pipe as CRA profiles must interlock perfectly, so the material must meet extremely high specifications for mechanical performance, dimensional tolerances, quality and consistency.

In addition, Outokumpu’s mills often must meet the in-house materials specifications and standards set by oil companies and pipe OEMs. This can create complex combinations of specifications that require a stringent approach to project management, which we have developed over 20 years.

Today’s oil and gas operators want to avoid over-engineering their lines. Their engineers will consider a wider range of alloys to achieve the right level of corrosion resistance at the right price or with a lower carbon footprint. They will consider options on a project-by-project basis.

For MLP, clad pipe and flexible pipe in shallow seas, high strength is not vital. Stainless steel grades such as 316L (S31603) or the European variant with 2.5% minimum molybdenum are widely used. Depending on the oil field conditions and the reservoir, 6%Mo super austenitic grades, such as Ultra 6XN (N08367) and Ultra 254 SMO (S31254) may be suitable. And alloys such as Ultra 904L (N08904) and Ultra Alloy 825 (N08825), with relatively high nickel content are used in sour gas applications.

Choice of CRA for flexible pipes depends on the sea depth and required corrosion resistance. Duplex grades and super duplex grades are often used. They have high mechanical strength, which is essential to resist water pressure in deep water projects and their nickel content supports corrosion resistance.

_{The views and opinions expressed in this article are those of the author and do not necessarily reflect those of the Nickel Institute.}