Engineering designers have traditionally turned to high-priced nickel alloys such as Alloy 625 as their default choice for demanding applications exposed to corrosive fluids such as seawater and chemical solutions. Rodrigo Signorelli explains how a high nitrogen-alloyed grade can offer a cost-effective alternative with much higher corrosion resistance
Quality and certification have always driven the choice of materials for systems such as plate heat exchangers (PHEs), tubing, pipes and pumps in the downstream oil and gas industry. Technical specifications ensure that assets provide process continuity over a long life, while protecting quality, safety and the environment. That is why many operators have embedded nickel alloys such as Alloy 625 in their technical specifications and standards.
However, engineers are currently facing pressure to control CAPEX and nickel alloys are both costly and subject to price volatility. This was highlighted in March 2022, when the nickel prices hit the headlines when they doubled within the space of a week in response to market trading. While a high price means that using nickel alloy is costly, this variability creates challenges for project engineers to manage as sudden price changes could suddenly impact profitability.
Therefore, while they know they can rely on its quality, many design engineers are now open to replacing Alloy 625 with an alternative. The key is identifying a suitable alloy with the right level of corrosion resistance for seawater systems, as well as providing a match for the mechanical properties.
One material that fits the bill is the grade EN 1.4652, which is also known as Outokumpu’s Ultra 654 SMO. It is regarded as the world’s most corrosion-resistant stainless steel.
Nickel-alloy 625 contains at least 58% nickel, while Ultra 654 contains 22%. Both have broadly similar levels of chromium and molybdenum. Meanwhile, Ultra 654 SMO also contains small quantities of nitrogen, manganese and copper; and Alloy 625 contains Niobium and Titanium, which are far more expensive than nickel.
Meanwhile, it provides a significant step-up from stainless steel grade 316L, which is typically regarded as the starting point for high-performance stainless steel.
Superior seawater performance
In terms of performance, the alloy provides very good resistance to uniform corrosion, exceptionally high resistance to pitting and crevice corrosion and good resistance to stress corrosion cracking. However, when it comes to seawater systems, the stainless steel alloy has an advantage over Alloy 625 as it has superior resistance to chloride environments.
Seawater is extremely corrosive due to the presence of 18,000 – 30,000 parts per million of chloride ions from salt. Chlorides create a risk of chemical corrosion for many steel grades. However, living organisms in seawater can also lead to the creation of biofilms that produce electrochemical reactions and can also affect performance.
Cost saving of 30-40%
With its low nickel and molybdenum content, the alloying mix of Ultra 654 SMO makes it significantly less costly than the traditional high-spec Alloy 625, while providing the right level of performance. It offers a typical cost saving of 30-40%.
In addition, by cutting content of high-value alloying elements, the stainless steel also cuts out exposure to the volatility of the nickel market. As a result, a manufacturer can have greater confidence in the accuracy of their project bids and price quotations.
A material’s mechanical properties are another essential consideration for an engineer. Pipework, heat exchangers and other systems need to contain high pressure, variable temperature and often mechanical vibration or shock. Ultra 654 SMO has good capabilities in this space. It has similar high strength to Alloy 625 and significantly higher strength than other stainless steels.
At the same time, manufacturers need materials that are formable and weldable enough to ensure straightforward production, and that can be readily obtained in the required product forms.
The alloy is a good choice in this regard as it retains the good formability of a conventional austenitic grade and good elongation that makes it ideal for designing strong and lightweight heat exchanger plates, for example.
Meanwhile it also has good weldability and is available in a wide range of formats, including coil and sheets up to 1,000 mm in width and thickness 0.5 to 3 mm or 4 to 6 mm.
A further cost advantage is that the alloy has a lower density than Alloy 625 (8.0 compared to 8.5 kg/dm3). While this difference may seem marginal, it reduce tonnage by 6%, which adds up to major savings when buying in large quantities for a project such as a long pipeline.
Building on this, the lower density means that the finished structure will be lighter, making logistics, lifting and installation easier. This will be particularly beneficial for subsea and offshore structures, where heavy systems are more challenging to handle.
Bright future for stainless steel alloy
When considering all the properties and benefits of Ultra 654 SMO – high corrosion resistance and mechanical strength, price stability, and enabling accurate planning – it has clear potential to be a more competitive alternative to nickel alloys.
Rodrigo Signorelli is Outokumpu’s Technical Manager