The global maritime industry has been shifting towards greater environmental protection efforts in recent decades. The International Maritime Organization’s Marine Environment Protection Committee (IMO MEPC) works to address issues that affect and threaten our oceans, including air and water pollution, disaster preparedness, and ship recycling.
However, this organization also studies an environmental issue hardly known to the public: ballast water management.
Ballast water tanks keep a ship balanced as she transports cargo, but it must be fully filtered and treated before being released at her port of call. The IMO guidelines require that ships only discharge ballast water, that is:
- < 10 viable organisms/m3 that are ≥ to 50 micrometers in minimum dimension
- < 10 viable organisms between 10 micrometers and 50 micrometers in minimum dimension per ml
- < 1 colony-forming unit (CFU) per 100 milliliters of Toxicogenic Vibrio Cholerae
- < 250 CFU per 100 ml of Escherichia coli
- < 100 CFU per 100 ml of Intestinal Enterococci
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These guidelines have solidified the importance of ballast water management systems (BWMS), making them a vital part of every ship. It is also more important than ever to maintain (and in some cases, upgrade) these systems – mainly because, as Filtersafe Head of Marine Mark Riggio recently said, “[the] global maritime industry [is] shifting its focus to operational compliance.”
The Best Alloy for Ballast Water Management Systems
Seawater filtration systems have a unique challenge to overcome. They must effectively filter debris and microorganisms while also withstanding the high chloride levels in the water, which constantly threaten to corrode the filter and its filtration screen. To achieve this goal, manufacturers often design BWMS with a filter that has a filtration screen made from a durable stainless steel alloy.
For many years, the industry-standard alloy of choice for filter screens has been 316L stainless steel. But due to the rising need for operational compliance, some of its shortcomings in seawater environments are coming to light. This has led some manufacturers to opt for the higher-grade 904L stainless steel when producing filtrations screens for seawater applications. Both these alloys can be applied in seawater filtration, but what are the differences between these two types of steel? And ultimately, as many shipowners are left to wonder, which is the right alloy for their BWMS, and their bottom line?
What is 316L Steel?
316L stainless steel is an austenitic alloy commonly known as “marine grade stainless steel” because it can be used for nearly 90% of marine applications – including filtration. In addition to metals like iron and nickel, 316L contains 16-18% chromium and 2-3% of molybdenum. These elements are important because they increase the alloy’s corrosion resistance; the chromium interacts with oxygen in the seawater to create a protective layer of chromium oxide, and molybdenum improves the metal’s ability to resist pitting corrosion. Additionally, 316L has lower levels of carbon (hence the “L” in its name), which gives it greater protection against corrosion.
What is 904L steel?
While 316L has long been the primary alloy ballast water filtration manufacturers use for their filter screens, it is not the only alloy available on the market. In fact, Filtersafe has been examining the value of 316L for many years now. As global leaders in the seawater filtration industry, our engineers sought to improve filtration standards by improving our materials. Our studies concluded that 316L stainless steel was not durable enough to meet our requirements – and so we opted instead to manufacture filter screens from 904L stainless steel In fact, Filtersafe – the global leader in seawater filtration – has been examining the value of 316L for many years now.
Like 316L, 904L stainless steel is a low-carbon austenitic stainless steel. However, its chemical composition includes greater numbers of chromium (19-23%) and molybdenum (4-5%), which gives the alloy greater corrosion resistance than 316L. Some people are familiar with 904L stainless steel as a metal commonly used to produce Rolex watches, which speaks to its high quality and durability. However, the same benefits that make it a must-use for Rolex also make it hugely beneficial for seawater applications.
316L vs. 904L: Durability
Seawater is highly unpredictable. It can have dramatic variances in temperature, chloride level, and the presence of microorganisms or debris. Therefore, a BWMS filter must be durable enough to withstand these changes.
Both 316L and 904L stainless steel contain chromium, which reacts with oxygen in the air to form a thin, protective layer of chromium oxide on the surface of the alloy. However, it is important to note that 904L contains more chromium than 316L. This means it is likely to provide greater protection over a longer period of time.
904L vs. 316L: Corrosion Resistance
Perhaps the most important feature of any alloy used in marine applications is its ability to handle constant exposure to corrosive substances. Seawater filters are prone to several different types of corrosion attacks, including from chloride, microbiologically induced corrosion, and crevice corrosion. The filter is particularly susceptible to these three corrosion types during the 1-3 week period between ballasting, when the filter is immersed in stagnant water, which can allow the development of a biofilm inside the filter and on the screen, and lead to corrosion. Therefore, a ship owner needs a filter with as much corrosion resistance as possible.
Shipowners and manufacturers can measure corrosion resistance by looking at an alloy’s PREN value. This formula looks at the amount of chromium (Cr), molybdenum (Mo), and nitrogen (N) in an alloy to determine just how well it will hold up against corrosion. In this case, 904L stainless steel is the clear victor over 316L; with an average PREN value of 36.7, it is much stronger than 316L (which only has a PREN of 26.1). Therefore, 904L is more likely to withstand the corrosive power of seawater.
904 vs. 316: Steel Hardness
To further understand the differences between 904L and 316L stainless steel, we must examine the hardness of both metals. The best way to do this is to look at the Rockwell Scale, which measures the indentation hardness of a material. Metals that score highly on the Rockwell scale are harder, which implies that they will be strong and withstand any bumps or bruises.
Both 904L and 316L stainless steel have a Rockwell hardness value below 95, which is typical for most stainless steel. This means that they will be able to withstand most forces, which is important for BWMS. However, it is far more important that seawater filter screens withstand the internal dangers like chloride and other corrosion attacks. 904L offers greater protection overall, which sets it above 316L.
904 vs. 316: Cost
904L stainless steel and 316L stainless steel are both readily available for purchase. Many industries and companies use 904L stainless steel, and the alloy has some brand recognition as it is famously used by watch manufacturers like Rolex and OMEGA. 316L stainless steel is commonly used for exhaust manifolds, heat exchangers, jet engine parts, and much more. As a result, BWMS manufacturers can introduce either alloy into their supply chains without suffering a delay in supply or a product bottleneck.
But which alloys offer the most “bang for your buck”?
If you look solely at the numbers, 316L may seem like the more attractive choice. Because of its widespread use across so many industries, this metal is both easy to purchase and available for a lower price. 904L tends to cost about 1.3 times more than 316L, which can make some manufacturers balk at the idea of producing all their filter screens from this material.
However, it is important to remember that filter screens made from 904L stainless steel will last longer and require fewer repairs than filters made from less durable materials. The enhanced corrosion resistance capabilities of 904L stainless steel make it less likely to wear down and break from corrosion, and therefore it will require fewer repairs over its lifespan. This is ideal for shipowners who do not want to incur additional costs to keep their ship in compliance with industry environmental and operating standards.
What is the Difference Between 316 and 904L Stainless Steel?
When ship owners are deciding between filters for their BWMS, they must consider a variety of factors before making their choice: cost, availability, value, etc. On the surface, the differences between 316L and 904L stainless steel may seem negligible, but in actuality, they have huge impacts on the long-term value of the filter.
To the Filtersafe team, the choice is clear: 904L stainless steel is the best choice for BWMS filters. 904L stainless steel offers greater durability, which makes it more effective and less costly over time (despite a higher initial cost). This alloy will help boost standards across the seawater filtration industry – and, in turn, improve environmental factors in oceans across the globe.
To learn more, download our white paper on Overcoming Corrosion of Stainless Steel in Seawater Applications for more extensive research on the common alloys used in seawater.