A growing number of Outokumpu’s customers in the oil and gas industry are turning to Ultra Alloy 825 for its superior performance in the most challenging applications, especially in “sour” sulfur-containing environments. Rodrigo Signorelli, Outokumpu Lead Technical Manager – Marine & Energy, explains the background.
Ultra Alloy 825 is a titanium-stabilized nickel base alloy with an addition of copper. It contains about 40% nickel and has an austenitic microstructure.
The importance of the titanium stabilization is that, together with the low carbon content, it makes the alloy less sensitive to intergranular corrosion at elevated temperatures – which can be an issue for some stainless steel grades. Furthermore, a primary advantage of Ultra Alloy 825 is its good resistance to sulfur containing environments, where it has superior resistance to the Sulfide Stress Cracking (SSC) that can occur when steel is exposed to hydrogen sulfide (H2S).
Its resistance to SSC is one of the main reasons why the oil and gas industry is adopting Ultra Alloy 825, since a large proportion of the world’s natural gas reserves is “sour” gas. This means the gas contains significant levels of H2S and carbon dioxide (CO2). In fact, in the Middle East, sour gas comprises around 60% of the total gas reserves. Historically, the presence of H2S has made it challenging to exploit sour gas resources because it can cause corrosion and SSC of steels, especially in pipelines.
In addition to creating a corrosive environment, sour gas is usually found at temperatures up to 260C and high pressures up to 1,700 bar. That is why it is vital to specify a material that offers the ideal combination of corrosion resistance and strength for cost-effective operation over a typical 30-year service life.
A portfolio of useful properties
There is much more to Ultra Alloy 825 than its performance in sour environments. It shows good mechanical properties at moderately high temperatures – up to 540 °C and down to cryogenic temperatures. It also shows very good resistance in many acids such as sulfuric acid, phosphoric acid, nitric acid as well as organic acids. Added to this, the material also shows good resistance in alkaline environments like sodium and potassium hydroxide solutions.
When it comes to pitting and crevice corrosion, Ultra Alloy 825 outperforms stainless steels such as Supra 316L/4404, although for the most demanding conditions we would recommend Ultra 254 SMO. The high nickel content of Ultra Alloy 825 also contributes to its very high resistance to stress corrosion cracking, both chloride-induced as well as in alkaline environments.
In fabrication, Ultra Alloy 825 is similar to other types of nickel base alloys. It has good ductility and can be formed using conventional methods. Conventional machining methods can be used as well as a range of welding techniques.
Consistent delivery makes the difference
While the consistent quality and high performance of Ultra Alloy 825 is critical, one area that has really helped Outokumpu build a solid, and growing, reputation with the oil and gas industry is our attention to supply chain management.
This starts with the cost advantage we offer through our capability to produce Ultra Alloy 825 by continuous casting, while competitors are primarily limited to more expensive ingot casting methods. It is then enhanced by the flexibility of format we offer, with coils up to 1,500 mm wide, both hot and cold rolled. This enables pipe and tube manufacturers to improve their productivity, while products produced from coil also have improved tolerances. But we do recognize that some customers need plate. That is why Ultra Alloy 825 is also available in quarto plate format in thicknesses up to 40 mm. The benefit for customers is that they can meet both their coil and plate needs from a single supplier.
We have also paid particular attention to logistics such as creating buffer stocks in our plate mills. This helps to ensure on-time deliveries of Ultra Alloy 825 with minimal delays, which is a key element in building strong relationships with the major players in the oil and gas industry.
From upstream to downstream
Ultra Alloy 825 is being adopted in a wide variety of applications such as clad and lined subsea pipelines with a thin layer of the corrosion resistant alloy (CRA) on the inside diameter, while mechanical strength is provided by an outer shell of carbon steel.
Some customers use Ultra Alloy 825 for weld overlay cladding, a technique where a thin layer of the Ultra Alloy 825 is welded onto components made of carbon steel or another material. This avoids the long lead time and expense of ordering special components in a high specification material for the construction of large vessels or metallic components such as pipes, valves, flanges, connectors and elbows, as well as pipework assemblies.
One of the first practical applications for Ultra Alloy 825 was the fractionating trays in the distillation columns that separate crude oil into its component fractions. It is also used in the manufacture of vane inlet devices for sour gas processing applications.
A fast-growing application is for air cooled heat exchangers, sometimes known as air fin coolers, that play a vital role in refineries in helping to remove process heat. In demanding applications where other materials are failing prematurely, a switch to Ultra Alloy 825 is the answer.
Hitting the sweet spot
Ultra Alloy 825 is now established as a “go-to” alternative to stainless steel for safety, reliability and long-life in demanding oil and gas applications. However, we always recommend thorough analysis of the specific conditions and conducting tests before making a final selection of materials. This is where Outokumpu’s team of experts can offer vital guidance and advice.
Learn more about Outokumpu's solutions for offshore applications