Production • Processing • Handling
‘Fit and forget’ corrosion-resistant tubing
Exotic alloys hold the key to long-lasting tubing, explains Brian Mercer.
– ever more critical. Every operator knows that checking for, and replacing, corroded tubes is time consuming and costly, not only in terms of the replacement operation itself but also in terms of lost production during the downtime. Te pursuit of this ‘holy grail’ has led manufacturers such as the UK-based Fine Tubes and its US-based alliance partner, Superior Tube, to develop high-performance exotic alloy tubing solutions for their customers. Te move is away from standard 316L type grades towards more corrosion-resistant duplex and super duplex stainless steels, as well as titanium grades and nickel alloys. UNS NO8904, commonly known as 904L, is a
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low carbon, high alloy austenitic stainless steel that is widely used in applications where the corrosion properties of conventional chrome nickel stainless steels are not adequate. Te addition of copper to this grade gives it resistance – in particular, to sulphuric, phosphoric and acetic acids. 6 Mo (UNS S31254), on the other hand, is a super austenitic stainless steel with a high level of molybdenum and nitrogen, providing high resistance to pitting and crevice corrosion as well as high strength compared with conventional austenitic stainless steels. Duplex stainless steels, combining approximately 50% austenite and 50% ferrite, are resistant to the cracking caused by chloride stress corrosion in austenitic grades and have improved resistance to localised corrosion. As such, typical applications for this alloy group include heat exchanger tubes, umbilicals, structural tanks, pipes, fittings and seawater handling systems.
Nickel alloys One of the great advantages of nickel-based alloys is their excellent corrosion resistance in both aqueous and high temperature applications. One example, Alloy 625, is a nickel-chromium alloy used for its high strength, excellent fabricability and outstanding corrosion resistance in environments of unusual severity as well as its resistance to
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he drive to exploit oil and gas fields in increasingly hostile, deepwater environments makes the job of producing ‘fit and forget’ tubing – tubing that will last for 30+ years
high-temperature effects such as oxidation and carburisation. Similarly, Alloy 825, an austenitic nickel-iron-chromium alloy with additions of molybdenum, copper and titanium, provides exceptional corrosion resistance in both oxidising and reducing environments and is resistant to chloride stress-corrosion cracking and pitting.
Fig. 1. Corrosion-resistant tubing is the key to ‘fit and forget’ solutions.
Titanium, because of its strength, unique density and corrosion resistance, has found applications in many industries. With a tensile strength greater than that of austenitic or ferritic stainless steels, it is also exceptionally corrosion resistant, making it a highly desirable material for high performance applications such as offshore drilling rig components and subsea equipment. It has to be said, of course, that advanced materials call for advanced manufacturing processes – which is why Fine Tubes and Superior Tube have invested in production and quality control facilities that include:
l Cold pilger mills l Cold draw benches l Tube welding mills – in-line weld mills l Controlled atmosphere heat treatment l Pickling and passivation plant l NDT ultrasonic and eddy current testing l Hydrostatic testing l Radiographic examination
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