For many years, Tungum UNS C69100 tubing has been considered a superior material for installations in the harsh environments experienced by the oil and gas, dive, transport and marine industries. There was a need to re-assess and bring up-to-date the performance of Tungum UNS C69100 to determine its performance against competitor products and stainless steels.

To undertake a study of this type and provide robust data, the nC2 Engineering Consultancy of the University of Southampton was commissioned to conduct rigorous performance testing of Tungum UNC C69100 against other alloys specified for the same use. Tungum UNS C69100 is marketed as a viable, cost effective, more durable material in place of UNS S31603 stainless steel, UNS S31254 and UNS S32750 super duplex alloys. Tungum UNS C69100’s economic benefits offer more than just reduced capital costs as with superior durability operational expenditure is also less. With UNS S31603 stainless steel predicted to last on average four years and super duplex alloys 20 years at best, Tungum UNS C69100 is expected to last more than 30 years without failure. Indeed, the company can boast 381 installations worldwide, spanning 80 years, with no recorded failures.

This endurance in safety critical installations and hostile environments is a great benefit given the difficulties in maintenance for offshore vessels, and the huge costs of downtime while repairs are made.

Scope

In order to assess the corrosive resistance of Tungum UNS C69100, any testing had to be replicated on competitor products and alloys to enable quantification of performance. Six other materials commonly used in similar environments were therefore subjected to the same range of tests and conditions. The reference names for the materials tested were:

• UNS C69100: Tungum alloy; an aluminium-nickel-silicon brass alloy.
• UNS S31603: High alloy stainless steel – 316
• UNS S31254: High alloy stainless steel – 6Mo
• UNS S32750: High alloy stainless steel – 2507
• UNS NO8904: High alloy stainless steel – 904
• UNS NO440: Nickel copper alloy – Alloy 400
• UNS C36000: Brass – a free cutting Brass

Corrosion is an electrochemical reaction, and electrochemical tests offer some of the most powerful results in gaining an understanding of corrosion behaviour. They offer rapid results, real-time corrosion rate measurements and can be set up to simulate a variety of environmental factors.

The measurements and tests carried out on the samples were:

• Potentiodynamic Polarisation
• Galvanic Coupling (Tungum UNS C69100 and UNS S31603 stainless steel)
• Corrosion monitoring using Electrochemical Impedance Spectroscopy (EIS)
• Scanning Electron Microscopy (SEM)

These are recognised methods for testing corrosion and were chosen to mirror industry accepted standard testing for materials used in the marine environment.

The measurements were taken over a 30-90-day timescale, and involved accelerated tests as well as galvanic reaction testing.

Results and analysis

The report states that in the timeframe involved, Tungum alloy performs as expected, standing up to scrutiny against competitor materials and showing strong indications to support the benefits provided by the quick-forming oxide layer.

It is widely accepted that UNS S31603 stainless steel can no longer offer the life span or level of integrity required in system designs. However it is generally accepted that this limitation applies to UNS S31603 stainless tube, whereas UNS S31603 fittings still offer good service life at a reasonable cost. Alternative alloys such as UNS S31254 and UNS S32750 offer greater protection against corrosion than UNS S31603 stainless steel, but at a significant increase in cost. Often these materials require a more expensive fitting in the same alloy grade, which adds even more cost to the total system price. Although these alloys offer a greater service life they are still affected by pitting and crevice corrosion and require more lengthy and complicated installation processes.

Tungum UNS C69100 can be used with standard UNS S31603 stainless steel compression fittings without any adverse galvanic corrosion, to offer an easy to install and cost effective system.  The galvanic testing conducted proved this combination as no deleterious effect was noted on either material when coupled.

The oxide layer that forms on Tungum was also evidenced in the experimental procedure. All copper based alloys generate a coating to some extent, but in many cases it isn’t durable and simply rubs off on contact. The layer that was observed on the Tungum samples was much more robust and measurements suggested that over time its durability increased substantially, providing an added multiplier to the corrosion resistance offered by the alloy.

Conclusions

Tungum’s own comparison data which considers options for a project undertaken in 2015, shows that the material costs for using Tungum UNS C69100 tube with UNS S31603 stainless steel fittings were less than 30% of the capital costs of using UNS S32750 tube and fittings. The predicted time to failure when using Tungum UNS C69100 was at least 30 years, whereas the predicted failure timescale of using UNS S32750 was 20 years.

Generating test data to measure the performance and durability of Tungum will form the next stage of this research. By testing ‘weathered’ samples from installations that have been in-situ for 20 plus years will verify the enhanced durability of Tungum UNS C69100.

In summary, the testing procedures and results have provided external, independent verification and validation of Tungum’s long-held beliefs that the alloy out performs competitor materials in certain applications.

There is no doubt that the second phase of this research testing of weathered samples, will provide similar validation of Tungum’s durability claims; evidencing that the corrosion resistance experienced with Tungum over other alloys, endures for decades. Using Tungum UNS C69100 reduces operating and maintenance costs, makes systems safer and outlasts competitor materials.