Choosing the wrong metal for a seawater pipeline is an expensive mistake. It leads to early valve failure, unplanned shutdowns, and high repair costs. Two of the most common materials used in marine and desalination systems are 316 stainless steel and Duplex 2205. Both are good metals — but they perform very differently in saltwater environments.
This guide explains the key differences in simple terms, so you can choose the right material before you order.
The core weakness of 316 stainless steel in seawater is its low resistance to chloride pitting. It scores only 24–26 on the PREN scale, well below the 40+ recommended for full seawater service.
For many years, the 316 stainless steel ball valve was the standard choice for seawater systems. It contains 2–3% molybdenum, which gives it better rust resistance than regular 304 stainless steel. It works well in many industrial applications.
But in seawater, it has a serious problem: chloride corrosion.
Seawater contains 18,000–35,000 ppm of chlorides (salt). At these levels, 316 stainless steel develops small holes called pits. These pits form in hidden areas — under valve seats, behind gaskets, and inside threaded joints. They are hard to see, but they grow quickly. This type of damage is called pitting corrosion.
The critical pitting temperature (CPT) for 316 SS in seawater is about 15–20°C. This means even in normal seawater temperatures, 316 SS can begin to pit.
Engineers measure corrosion resistance using a number called PREN (Pitting Resistance Equivalent Number), calculated as:
PREN = %Cr + 3.3 × %Mo + 16 × %N
This formula, referenced in ASTM and ISO corrosion standards, weights molybdenum and nitrogen content heavily because these elements most directly block chloride attack at the metal surface. A higher PREN means better resistance to saltwater. 316 SS has a PREN of 24–26. For full seawater exposure, engineers recommend a PREN above 40.
There is another problem called stress corrosion cracking (SCC). This happens when the metal is under stress and exposed to warm, salty water above 60°C. SCC creates invisible cracks near valve stems and flanges. These cracks grow slowly — until the valve suddenly leaks or breaks.
Real-world results are clear. Desalination plants in the Middle East and Southeast Asia report that 316 SS valves and fittings need replacement every 3–5 years when in direct seawater contact. This is a short service life for an industrial valve.
Duplex 2205 is a different type of stainless steel. Its structure is made of two phases — austenite and ferrite — in roughly equal amounts. This two-phase structure gives it much better strength and corrosion resistance than single-phase alloys like 316 SS.
The PREN of Duplex 2205 is 34–43. Its critical pitting temperature in seawater is above 35°C. This means a duplex stainless steel valve made from 2205 can handle continuous seawater exposure without pitting in conditions that would damage 316 SS within one season.
The ferritic phase in 2205 blocks the paths that chloride ions use to cause stress corrosion cracking. Tests and field data from reverse osmosis (RO) plants show that 2205 components stay crack-free at temperatures up to 150°C in salty environments. 316 SS cannot match this performance.
Duplex 2205 is approximately twice as strong as 316 SS:
This higher strength means a corrosion-resistant valve body made from 2205 can handle the same pressure with thinner walls. This partly reduces the extra cost of using 2205.
In desalination plants, Duplex 2205 valves and fittings regularly last 15–25 years in high-pressure seawater feed lines. Operators who switched from 316 SS to 2205 saw big improvements. Inspections went from once a year to once every three years. Corrosion issues did not increase.
Duplex 2205 costs about 20–40% more than 316 stainless steel in raw material price. For a small valve (DN50 / 2”), the extra cost is small. For a large valve (DN300 / 12”) or a full seawater valve system, the difference is bigger.
But the upfront price is only part of the story. Here are four questions to ask:
Systems in constant contact with seawater, brine, or salty cooling water benefit most from 2205. If the system only touches seawater occasionally and is flushed with fresh water, 316 SS may still be acceptable.
Above 25°C, the pitting risk for 316 SS increases. Above 60°C, the cracking risk becomes serious. At these temperatures, 316 SS needs more frequent maintenance. Duplex 2205 stays stable.
Replacing a failed valve on an offshore platform or remote desalination plant is expensive. Labor, scaffolding, shutdown time — these costs add up fast. In many cases, the labor cost alone is several times higher than the price difference between 316 SS and 2205.
If your project has a 20-year design life, 316 SS in aggressive seawater service may not reach that target. 2205 can.
Based on procurement data from a mid-scale seawater cooling system retrofit (DN50 valves, continuous saltwater service, Gulf region, 2022), actual replacement records showed the following:
Over a 20-year project, a 316 SS valve costs roughly $400 upfront—but requires four replacements, bringing material spend to ~$2,000. Add four labor events at ~$250 each, and the total reaches ~$3,000.
A Duplex 2205 valve costs ~$560 once. With no corrosion-related replacements, you pay labor only at installation. Total: ~$810.
The 2205 valve delivered over 70% total cost savings. It also eliminated four unplanned shutdowns over the project’s life. Similar outcomes have been documented in RO plant case studies from Southeast Asia, where operators report that switching to 2205 on high-pressure feed lines reduced annual maintenance costs by 40–60%.
Discover our strict quality control and state-of-the-art facilities ensuring the highest standard for every valve.
Explore ProcessThe choice between 316 SS and Duplex 2205 comes down to one question: How long must your system survive in seawater — and what does failure cost?
For systems in continuous seawater contact, temperatures above 25°C, or projects with a 10+ year design life, Duplex 2205 nearly always delivers lower total cost. 316 SS is still a good choice for lower-risk applications, short service life, or projects with tight budgets where regular maintenance is planned.
Lianke Valve manufactures stainless steel ball valves in both 316 SS and Duplex 2205, built to ANSI/DIN/JIS standards with factory-direct pricing and 2–3 day delivery on stocked sizes.
Choosing the right material from the start is the most important decision in seawater pipeline design.
A: Yes, for short-term or occasional contact. For continuous seawater exposure — especially above 25°C—316 SS is at risk of pitting and cracking. Duplex 2205 is the safer long-term choice.
A: PREN stands for Pitting Resistance Equivalent Number. It measures how well a metal resists chloride pitting. Higher is better. 316 SS scores 24–26; Duplex 2205 scores 34–43. For full seawater service, a PREN above 40 is recommended.
A: In most cases, yes. Duplex 2205 valves come in the same sizes, pressure ratings, and end connections as 316 SS valves. Just check that the seals and gaskets are also compatible.
A: Check these four things: chloride levels above 1,000 ppm, temperatures above 25°C, design life longer than 10 years, and high downtime costs if a valve fails. If two or more apply, 2205 is likely the better investment.
A: Yes. 2205 welding requires careful heat control to keep the correct two-phase structure. Proper welding procedures and inspection are important. For many seawater valve installations, flanged connections are preferred to avoid welding in the field.
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