--- title: "Stainless Steel Flanges For Desalination Plants: Grade Selection And Chloride Corrosion Challenges" date: 2026-06-09T03:37:54Z modified: 2026-06-26T01:07:08Z permalink: "https://www.longanflange.com/blog/stainless-steel-flanges-desalination-corrosion/" type: post status: publish excerpt: "" wpid: 3706 categories: - Blog featured_image: "https://www.longanflange.com/wp-content/uploads/2026/06/图片8-1.webp" featured_image_alt: Stainless Steel Flanges For Desalination Plants timestamp: 2026-06-26T01:07:08Z tags: - Blog --- A corrosion flange problem in a desalination plant usually starts with the wrong grade choice. Many buyers think 316L is enough because it is stainless steel. In some areas, it can work. In other areas, it can fail too early. Desalination is not one simple water line. The chloride level, temperature, and pressure change from one section to another. Raw seawater, RO high-pressure lines, and brine discharge lines should not always use the same flange grade. So the better question is not “What grade of stainless steel is corrosion resistant?” The better question is “Where will this flange be installed, and what chloride level will it face?” For desalination buyers, stainless steel flange material selection should be checked by process stage, not by a general rule. ## **Why Desalination Is One Of The Toughest Environments For Stainless Steel Flanges** Desalination plants are hard on stainless steel because they combine salt, heat, and moisture. Seawater has about 19,000 ppm chloride ions. In hot areas like the Middle East, inlet water can reach 35°C or higher. That mix can cause pitting and crevice corrosion. Pitting looks small at first. It may only show as tiny holes on the flange face. But those holes can grow deeper over time. Crevice corrosion is more hidden. It happens in tight spaces where water sits, and oxygen cannot move freely. Flange joints have many of these spaces. The risky areas are easy to picture: the flange face under the gasket, bolt holes, and small gaps around the joint. If the grade is too weak for the service, the flange can become the weak point of the whole line. In real projects, the pipe body may still look fine after years of service, while the flange sealing face already shows corrosion marks. ![Understanding The Desalination Process](https://www.longanflange.com/wp-content/uploads/2026/06/图片9-1-1024x512.webp) ## **Understanding The Desalination Process — Where Flanges Are Used** Flanges are used in several parts of a desalination plant. The two common systems are SWRO, or seawater reverse osmosis, and MSF/MED, which are thermal desalination processes. In an SWRO plant, flanges may appear in the raw seawater intake line, pretreatment system, high-pressure pump area, membrane module connection, brine discharge line, and product water line. Each area has a different level of risk. The raw seawater intake line handles normal seawater, so the chloride level is already high. The pretreatment line is similar, but the pressure is usually lower. The RO high-pressure section is more demanding. It has seawater, pressure, and often a higher temperature. The brine discharge line is usually the harshest section. After water is removed, the chloride level may rise to about 38,000 to 70,000 ppm. Product water is different. It has much lower chloride, often below 500 ppm, so the corrosion risk is much lower. This is why one stainless grade cannot cover the whole plant safely. ## **Grade Selection By Process Stage — A Practical Guide** The flange grade should follow the actual service area. A grade that works in product water may not last in brine discharge. | **Process Stage** | **Chloride Level** | **Temperature** | **Suggested Grade** | **Practical Note** | | --- | --- | --- | --- | --- | | Raw seawater intake | ~19,000 ppm | Ambient | 316L / Duplex 2205 | 316L may work, but service life may be shorter | | Pretreatment system | ~19,000 ppm | Ambient | 316L | Often acceptable because pressure is lower | | RO high-pressure stage | ~19,000 ppm | 25–40°C | Duplex 2205 / Super Duplex 2507 | 316L is risky in many cases | | Brine discharge line | 38,000–70,000 ppm | 30–45°C | Super Duplex 2507 / 904L | This is one of the toughest sections | | Product water line | <500 ppm | Ambient | 304L / 316L | Lower corrosion risk and cost can be considered | For intake and pretreatment lines, 316L may be acceptable if the temperature is moderate and the expected life is not too demanding. For RO high-pressure lines, Duplex 2205 is often safer because a standard 316L flange faces both high chloride and pressure, making it highly susceptible to becoming a [corrosion flange](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/post/flange-corrosionprevention.md) bottleneck. For brine discharge, Super Duplex 2507 or 904L is usually a better choice. This section carries concentrated saltwater, so using 316L can be a costly mistake. ## **Why 316L Alone Is Not Enough — The Pitting Resistance Equivalent Number** 316L alone is not enough in many desalination areas because chloride attacks stainless steel over time. A simple way to compare grades is PREN. PREN means Pitting Resistance Equivalent Number. The formula is: PREN = %Cr + 3.3 × %Mo + 16 × %N You do not need to treat this like a classroom formula. For buyers, the basic idea is simple: a higher PREN means better resistance to pitting corrosion. | **Grade** | **Approx. PREN** | **What It Means** | | --- | --- | --- | | 304L | 18 | Not suitable for seawater | | 316L | 24 | Limited use in lower-risk areas | | Duplex 2205 | 34 | Good for many seawater areas | | Super Duplex 2507 | 42 | Better for brine and harsh seawater service | A useful rule is this: seawater usually needs PREN above 30. Concentrated brine often needs a PREN above 40. This is why 316L can be a problem. It is a good stainless grade, but desalination service can push it past its comfort zone. ## **Crevice Corrosion At Flange Joints — The Hidden Risk** Crevice corrosion at flange joints is easy to miss during design. The drawing may look correct, but the joint still creates small spaces where seawater can sit. These spaces form between the gasket and flange face. They also form around bolts and uneven contact areas. Inside the gap, water does not move much. Oxygen becomes low. The stainless steel cannot repair its passive layer properly. Local corrosion then starts to speed up. A typical issue is a 316L flange used in seawater service for two or three years. The pipe may still be fine, but the flange face starts to show pits. Once the sealing face is damaged, the joint can leak or fail inspection. To reduce this risk, buyers should look at more than the grade name. They should consider upgrading from 316L to 2205 or 2507 in high-risk sections. The gasket material should also match the service. Carbon steel parts should not be used where they can contaminate stainless surfaces. Pickling, passivation, and proper surface roughness also matter. A clean, smooth sealing face gives the gasket a better chance to work. ![Socket weld flange](https://www.longanflange.com/wp-content/uploads/2024/08/39635332bbea8ed9dcdf.png) Socket weld flange Our socket weld flanges offer a reliable and robust solution for connecting pipes in high-pressure and high-temperature systems. [Inquiry Now](https://www.longanflange.com/products/socket-weld-flange/) ## **Fabrication And Quality Considerations For Desalination Flanges** Good material choice can still fail if fabrication is poor. Desalination flanges need careful handling from machining to final packing. After welding, stainless steel parts may need pickling and passivation. Without this step, the heat-affected area can corrode faster than the rest of the part. The [flange sealing face](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/post/cnc-flange-sealing-surface.md) also needs attention. The Ra value, or surface roughness, affects gasket sealing. A damaged or rough face can trap liquid and raise the risk of crevice corrosion. Cross-contamination is another common problem. If carbon steel dust or scraps touch the stainless steel surface, rust marks and local corrosion can appear later. This is why a pure stainless steel production environment is useful for desalination projects. Inspection should include PMI material verification, PT surface checks, and dimensional inspection. For large flanges, bolt hole alignment, flatness, and sealing face quality need extra attention. ## **Choosing The Right Flange Supplier For Your Desalination Project** Not every stainless steel flange supplier understands how to prevent a severe corrosion flange issue in high-salinity applications. The project may need duplex grades, clean processing, large-diameter machining, and full material documents. Before placing an order, ask whether the supplier has real experience with 2205 and 2507. These grades are common in seawater and brine service, but they need proper forging and machining control. Ask for [EN 10204 3.1](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/post/en-10204-certificate-types-ped-4-3-valve-flange.md) certificates and full material data. If PREN is important to the project, the chemical composition should be clear enough to check it. Large-diameter capacity also matters. Desalination main lines are often DN600 or larger. For these sizes, the supplier must control machining, flatness, bolt holes, and packing. The supplier should also understand how to prevent carbon steel contamination during production. This small detail can matter a lot in seawater service. Custom Large Diameter Flanges Manufacturer We specialize in manufacturing large diameter stainless steel flanges—custom-made to your specs, forged in-house, and machined with high precision. [Start Your Custom Project with Longan](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/page/custom-large-diameter-flange-manufacturer-factory.md) ## **Choose The Right Grade Before Production** Before production starts, confirm the flange grade for each part of the desalination system. Raw seawater, RO high-pressure lines, and brine discharge do not carry the same corrosion risk. If you are looking to eliminate corrosion flange risks in your piping design, Longan Flange provides full material validation. Whether your project needs 316L, Duplex 2205, [Super Duplex](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/post/stainless-steel-super-duplex-flange-benefits-features.md) 2507, or custom [large stainless steel flanges](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/post/applications-of-large-diameter-stainless-steel-flanges.md) up to DN4000, Longan Flange can help check your drawings and material requirements before quoting. [Get a Quote](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/page/contact.md)