For many years the industry standard alloy used for strainers has been 316L stainless steel. However, due to the increasing demand for operational compliance, some of its drawbacks in a seawater environment are coming to light. This has led some manufacturers to opt for higher grade 904L stainless steel when producing strainers for seawater applications. Both alloys can be used for seawater filtration applications, but what is the difference between these two sheets of steel? Ultimately, many ship owners want to know which alloy is right for their ballast water management system, and what is their bottom line.
316L stainless steel is an austenitic alloy, often referred to as "marine grade stainless steel" because it is used in nearly 90% of marine applications - including filtration. In addition to metals such as iron and nickel, 316L also contains 16-18% chromium and 2-3% molybdenum. These elements are important because they increase the corrosion resistance of the alloy; chromium interacts with oxygen in seawater to form a protective layer of chromium oxide, while molybdenum increases the metal's resistance to pitting. In addition, 316L has a lower carbon content (hence the "L" in its name), which gives it better protection against corrosion.
Super Austenitic Stainless Steel 904L is a high-alloy austenitic stainless steel with low carbon content. It has good corrosion resistance in dilute sulfuric acid and is specially designed for harsh corrosion conditions. It has high chromium content and sufficient nickel content. The addition of copper makes it highly resistant to acid, especially to chloride crevice corrosion and stress corrosion cracking. It is not easy to appear corrosion spots and cracks, and it is resistant to spots. Corrosion ability is slightly better than other steel grades, has good workability and weldability, and can be used in pressure vessels.
Although 316L has long been the primary alloy used by manufacturers for ballast water filters, it is not the only alloy available on the market. Research has concluded that 316L stainless steel is not durable enough to meet our requirements, so we have chosen to use 904L stainless steel for our strainers.
904 904L Stainless Steel Mesh
Like 316L and 904L stainless steel, it is a low-carbon austenitic stainless steel. However, its chemical composition includes higher amounts of chromium (19-23%) and molybdenum (4-5%), which gives the alloy a higher corrosion resistance than 316L. 904L stainless steel is a common metal used in Rolex watches and is familiar to some, which speaks to its high quality and durability. However, the same benefits that make it a must for Rolex also make it highly beneficial for seawater applications.
Seawater is highly unpredictable. It can vary enormously in terms of temperature, chloride levels, and the presence of microorganisms or debris. Therefore, BWMS filters must be durable enough to withstand these variations.
Both 316L and 904L stainless steels 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 that it may provide greater protection over a longer period of time.
904 904L Stainless Steel Mesh
Probably the most important characteristic of any alloy used in marine applications is its ability to handle constant exposure to corrosive substances. Marine filters are susceptible to several different types of corrosion attacks, including chlorides, corrosion caused by micro-organisms, and crevice corrosion. Filters are particularly susceptible to these three types of corrosion when submerged in stagnant water during the 1-3 week period between ballast, which can lead to the formation of biofilm inside the filter and on the screen and result in corrosion. Shipowners, therefore, need a filter that is as resistant to corrosion as possible.
Shipowners and manufacturers can measure corrosion resistance by looking at the PREN value of an alloy. This formula looks at the amount of chromium (Cr), molybdenum (Mo) and nitrogen (N) in the alloy to determine its corrosion resistance. In this case, 904L stainless steel clearly outperforms 316L; with an average PREN value of 36.7, it is much stronger than 316L (which has a PREN of only 26.1). Thus, 904L is more resistant to the corrosive forces of seawater.
To learn more about the differences between 904L and 316L stainless steels, we must check the hardness of both metals. The best way to do this is to check the Rockwell hardness scale, which measures the indentation hardness of the material. Metals that score high on the Rockwell scale are harder, which means they are strong and can withstand any bumps or scrapes.
Both 904L and 316L stainless steels have Rockwell hardness values below 95, which is typical of most stainless steels. This means that they will be able to withstand most forces, which is important for BWMS. However, it is more important that seawater screens are able to withstand internal hazards such as chlorides and other corrosion attacks. 904L offers better overall protection than 316L.
For further technical information talk with our specialist or visit our website https://www.apwiremeshfilter.com. We can help you to identify the best solution for your application.
