Filtration of seawater is a necessity across a diverse range of industries – such as intake for a desalination plant, ballast filtration for an oil tanker, or to maintain pressure and the production rate. When running a complex operation there are plenty of unexpected expenses to deal with on a regular basis, and the last thing anyone wants is a preventable failure of their filtration equipment. This is especially pertinent when dealing with seawater because it is filled with corrosive elements that can lead to material failure like corrosion or strength degradation. Therefore it’s important to have a quality, durable filtration system.
As part of the search for the best system, an often underappreciated but critical component to its success is dependent on which alloy the screen filter, the heart of the system, is made of. The filter screen is the physical barrier that prevents particles (organic and inorganic alike) that are larger that its pore size from passing through with the water. Corrosion to the screen can damage its ability to be cleaned, clog its open area and inhibit water flow, or degrade the screen’s structural integrity, letting particles larger than its pore size to get through and compromise the integrity of the system. Various seawater filtration industries struggle with the same decision as to which is the best.
Seawater filters have unique and specific requirements: they need to be non-corrosive, as well as effective at removing debris of various sizes, which can harm down-stream processes. This is extra important in industries that rely on very fine filtration (100-5 microns), such as in desalination and ballast water filtration. In these cases, the screens must also be highly effective at removing phytoplankton and zooplankton from the water. Any seawater processing application requires a durable pre-filtration protection system so that time and money isn’t wasted over the years to repair poorly protected systems. With this in mind, let us look at the most common alloys used in seawater filtration and determine which alloy best suits your system.
316L stainless steel, also known as “marine grade stainless steel,” is the second-most common stainless steel used in manufacturing today (after 304 stainless steel). This alloy is made up of iron, chromium (about 16%), nickel (10%) and molybdenum (2%), as well as trace amounts of quantities of silicon, phosphorus, and sulfur. Although it is a popular choice, the latest advice from the International Stainless Steel Forum is that AISI 316 and its derivatives “are no longer recommended for permanent contact with seawater.” We’ll see some of the reasons behind this decision in the Cons section.
316L stainless steel is a popular choice for seawater filtration. This is in part due to the alloy’s widespread use throughout many industries, making it widely available, as well as the benefits described below.
Like all of the alloys discussed here, 316L stainless steel contains chromium. This element is an important part of the chemical makeup; it creates a thin film over the alloy when exposed to oxygen, which helps prevent the steel from becoming corroded over time.
The chromium in the alloy does provide a degree of protection that prolongs the steel’s – and by extension, the filtration system’s – lifespan. However, it is important to note that 316L does not provide complete protection from corrosion. While the alloy can offer reasonable protection in the short term, its particularly susceptible to pitting corrosion caused by inconsistent salinity levels in the seawater.
Tolerant to Higher Temperatures
Temperature fluctuations are most acutely felt in the shipping industry, as seawater can vary wildly in temperature between locations, seasons or even times of the day. 316L can tolerate water temperatures up to Celsius (68 degrees Fahrenheit), which means it is able to successfully stand up to most seawater.
One of the biggest benefits of 316L stainless steel is its relatively low cost and wide availability. Manufacturers can purchase 316L scrap relatively inexpensively, which makes it a highly accessible material for filtration system engineering.
Despite 316L’s popularity, there are some industry experts who believe that this stainless-steel alloy is inappropriate for use specifically in ballast water treatment systems. These professionals cite issues with the welding process as a major drawback – and the consequences of these issues certainly warrant a second look in other seawater applications as well.
High Ferrite Content
One of the biggest drawbacks in using 316L stainless steel for seawater filtration can occur during the welding process. 316L must be welded under very strict conditions, or else it might develop a high ferrite content in the weld seam. Ferrite can diminish an alloy’s ability to resist corrosion. Even a small amount (around 2%) of ferrite content in a weld seam can result in a less effective finished product.
Since 316L stainless steel is prone to developing excess ferrite during production, it can make some filtration experts leery of using this alloy in their systems. In fact, some argue that the potential for high ferrite content in 316L stainless steel can lead to greater incidence of pitting corrosion or crevice corrosion. This kind of damage can require significant repair or even a completely new filtration system – effectively counteracting the reduced costs of using the 316L alloy.
Duplex 2205 is a stainless steel characterized by its strength. With a makeup of 22% chromium, 3% molybdenum, and about 5% nickel, it is about on par with 904L in terms of corrosion resistance. However, duplex 2205 is (as the name suggests) a duplex stainless steel; this means that it contains both austenite and ferrite phases in its metallurgical structure, giving the alloy a greater overall durability.
Duplex 2205 is the most popular of the duplex stainless steels, and it is often used in pipework systems for offshore oil and gas. This alloy’s strength and chemical composition, as well as its durability against
seawater in offshore uses, makes it a logical choice on paper.
Good Corrosion Resistance
Duplex 2205 has a PREN average of 35.9 – just shy of 904L stainless steels average of 36.7. This indicates that this alloy would be able to fair just as well as 904L when dealing with pitting corrosion, crevice corrosion, or any other type of destructive wear and tear within the filtration system. Theoretically, duplex 2205 would be a reasonable alloy for a seawater filter.
While duplex 2205 has had plenty of success within the oil and gas industry when used as a solid structure such as piping, it’s not used often in complex manufacturing procedures such as a weaved screen mesh used in seawater filtration. This is likely due to two main issues: There are challenges when working with it and its lower temperature threshold for corrosion.
Hard to Work With
One of Duplex 2205’s assets, it’s strength, is actually a double-edged sword. Since the alloy is so strong, it makes it difficult to work with, and requires special heavy machinery that is not commonplace. In addition, it makes it very challenging to mold the steel into the very tiny and delicate structures, such as ballast water filters or desalination filters that require filtration down to 10 microns.
Corrodes Faster than 904L
While Duplex 2205 has better corrosion resistance than 316L, it still corrodes at a lower temperature than 904L (50C vs 55C for 904L).
Another popular alloy used in seawater filtration is 904L stainless steel. This alloy contains many of the same elements as 316L steel, but it is comprised of higher levels of chromium (about 19%), nickel (23%), and molybdenum (4%).
904L stainless steel is most famously used in luxury watches (Rolex, for example, swears by this alloy), but it is also a popular metal for seawater filtration. This is because the high amounts of nickel in this alloy
makes it particularly corrosion resistant, protecting your seawater filter from damage.
Choosing the right alloy is essential for a durable and effective seawater filtration system. When alloys have similar compositions, like 316L and 904L, the decision may seem insignificant; however, 904L stainless steel has some major benefits that can make a big difference for a seawater filter.
As we mentioned above, 904L is highly resistant to corrosion due to the high levels of nickel in its composition. This means that seawater filtration systems made with 904L tend to have a longer lifespan than their competitors.
In fact, 904L stainless steel has an average pitting resistance equivalent number (PREN) of 36.7 – more than 10 points higher than 316L’s PREN average (26.1). This is an important parameter to consider when
designing seawater filters, as it will help determine how effective the filter’s corrosion resistance and durability will be over its life.
Another benefit 904L has to its credit is its availability in the market. This alloy is readily available from most suppliers, which means that engineers and manufacturers can procure the metal and produce filtration systems much more easily than they might with other metals.
While 904L stainless steel offers both practical and logistical benefits, it is not a perfect alloy. Some engineers or manufacturers may opt for another metal due to the one major flaw that accompanies using this metal: the cost to use it.
The costs of most alloys vary based on the amount of chromium and molybdenum in the metal. As we’ve mentioned previously, 904L stainless steel has higher levels of both these elements compared to 316L – which means that it tends to be about 1.3 times more expensive.
This price difference might drive some budget-conscious manufacturers to a lower-quality alloy. However, it is important to mention that the greater strength and durability of 904L stainless steel means that filters made from this alloy are less likely to need repairs during their lifespan. The cost savings you will earn from this lack of repairs more than offsets the cost of the alloy itself.
Like the two alloys we’ve already mentioned, 254 SMO is an austenitic stainless steel. However, this alloy was originally developed for use in chloride-heavy environments – such as seawater cooling pipes, heat exchangers, pulp and paper plants and more. The metal contains a similar amount of chromium as 904L, but it has a greater amount of molybdenum (6% vs. 4% in 904L).
With this composition in mind (not to mention the alloy’s intended use), it is no surprise that 254 SMO is an alloy that generates lots of interest among seawater filtration engineers. There are unquestionable benefits that come with using this alloy – but there are also a few drawbacks that can complicate its use.
The greatest benefit of using 254 SMO for seawater filtration is its corrosion resistance. The chemical composition of this alloy makes it highly resistant to both microbiologically induced corrosion and chemical corrosion – making it incredibly durable against seawater even after prolonged exposure.
Excellent Corrosion Resistance
254 SMO has a PREN average of 43.8, which means it has the greatest protection against corrosion of all the alloys studied here. It also has a low carbon content (around .02%), which means that there is little risk of ferrite development during production. These two facts mean that 254 SMO is one of the best alloys for protecting your seawater filtration system from the corrosive effects of seawater.
The excellent protective qualities of 254 SMO might make it seem like the gold standard in seawater filtration materials. However, at least in their ballast water treatment systems, few developers use it – why? 254 SMO is often passed over due to two flaws in its practicality.
As we mentioned earlier, alloy costs vary depending on the amount of chromium and molybdenum in the steel. 254 SMO contains higher levels of these elements than both 316L and 904L stainless steel – which means that it tends to cost much more than its competitor alloys.
254 SMO can cost as much as three times more than 316L, and many manufacturers are unwilling to spend more to produce their filtration systems. This cost is above the threshold deemed “acceptable” by most industry professionals (unlike 904L stainless steel, which has a slightly higher cost but offsets the cost of maintenance).
One of the hinderances that limits 254 SMO from being suggested for more seawater filters is that this alloy is not produced regularly due to its higher costs.
Which Alloy is Best?
Due to the nature of seawater filtration and its association with heavy industry, engineers of all applications endeavor to build filtration systems standing up to the ravages of seawater — all without costing you too much money. At the same time, the cost of using a lower-quality alloy will corrode more easily and lead to expensive problems – specifically more frequent maintenance and reduced filtration capabilities as corrosion overtakes the screen. These outcomes will, in the long run, increase operating costs, as equipment owners will have to pay for more frequent servicing, and where applicable such as in regulated industries like ballast water management systems (BWMS), potential fines for non-compliance of malfunctioning systems.
The solution? Invest in a filtration system that’s made from a durable, yet cost-efficient material. And when it comes to choosing the best alloys for seawater filtration without consideration of cost – SMO 254 is definitely the favored alloy. When dealing with a high end application where even the smallest downtime can be very costly such as in the oil & gas or desalination sector, the initial investment in using a Filter with an SMO 254 screen pays off in the long run. However, when a cost-effective solution is required, there is one option that stands head and shoulders above the competition: 904L stainless steel.
904L stainless steel is the ideal marriage of strength, corrosion resistance, and accessibility – all at a reasonable cost to manufacturers. Seawater filters made from this alloy will stand up to corrosion and keep your filtration system running effectively for many years, saving you thousands in fees and maintenance costs.
For a deeper investigation into the alloys discussed above for use in seawater applications please download our peer-reviewed white paper on the topic – Overcoming Corrosion of Stainless Steel Screens in Seawater Applications.