A practical example from the world of heavy salts and geothermal brines. At Dutch Filtration, we have specialized in filtering heavy salts and chloride-containing liquids, also known as brines, for many years. We filter these heavy liquids with our high-quality filter elements down to 0.5 microns (Beta 5000) and, depending on the application, down to several tens of microns.
When choosing the right filter vessel, it is essential to have a thorough understanding of the properties of the liquid. How salty is the medium? What is the temperature? Are there any gases present? These are crucial factors, which is precisely the kind of complexity we specialize in.
Recently, a geothermal customer asked us why stainless steel 316Ti is not suitable for his application. His system worked with stainless steel 316Ti, an operating temperature of 60C to 90C degrees Celsius, a theoretically oxygen-free environment, and geothermal water that is about five times saltier than seawater. The existing filter vessel was already clearly corroded and needed to be replaced. Below, we explain why this happens.
Why oxygen-poor geothermal water still causes corrosion:
Even when a system is theoretically oxygen-free, in practice you can never completely rule out the presence of oxygen. Rinsing with nitrogen never removes all residual oxygen; there are always dead zones and small air pockets, and there are moments when oxygen can re-enter the system.
Without oxygen, some forms of corrosion, such as pitting corrosion and general corrosion, are greatly slowed down. However, at high chloride loads, such as five times that of seawater and temperatures around or above 60°C to 90°C, geothermal water remains very aggressive to stainless steel 316 and stainless steel 316Ti. Geothermal water also often contains CO₂, H₂S, and various salts that further accelerate corrosion.
In practice, oxygen is almost always present. This occurs, for example, during start-up and filling of the system, during shutdown, due to small leaks or venting, and during maintenance work. It is precisely at these moments that stainless steel 316Ti can be quickly corroded.
PREN value and why 316Ti falls short
The PREN value, or Pitting Resistance Equivalent Number, indicates a material’s resistance to pitting corrosion. Stainless steel 316Ti has a PREN of approximately 24. For geothermal water that is five times saltier than seawater, a PREN of at least 40 is desirable. Based on this comparison, it is clear that stainless steel 316Ti does not offer sufficient protection against pitting corrosion under these conditions.
Our advice therefore remains that stainless steel 316Ti should only be used in components that do not have a primary pressure-bearing or critical function, in systems that can be kept completely closed and oxygen-free, and where regular inspection and monitoring is possible.
Which materials are more suitable:
For this specific geothermal brine, materials such as duplex with a PREN around 35, superduplex such as 2205 or 2507 with a PREN around 42, and titanium with a PREN around 69 are significantly more resistant to corrosion. Duplex is often just not sufficient at extremely high salt concentrations. Super duplex is usually sufficient, depending on the exact composition of the brine. Titanium is the best choice for highly aggressive brines.
The disadvantage of these materials is that they are considerably more expensive. Depending on the configuration, a filter vessel can be one to ten times more expensive than a filter vessel made of 316Ti stainless steel. As a result, 316Ti is sometimes chosen again, but without any guarantee of a predictable service life.
In summary:
Even when a geothermal fluid is almost oxygen-free, a brine that is five times saltier than seawater and has a temperature of around 60°C to 90°C remains too aggressive for stainless steel 316Ti. For long-term and reliable use, 316Ti is therefore not a suitable material for critical parts of a geothermal system.
Materials such as duplex, super duplex, and titanium offer the corrosion resistance and reliability required for these harsh conditions, even though they are considerably more expensive to purchase.
Source: 2016.03.15_tno 2015 r10160_corrosion in dutch geothermal systems_public.pdf
