Looking for corrosion resistant materials for salt water? See the comparison of most corrosion resistant metals
Exposure to chlorides makes materials susceptible to SCC or stress corrosion cracking. SCC can be dangerous as it causes sudden, massive failure even when the operating load is within limits. Here, stress transforms pits into cracks, advancing the damage rapidly. The capacity to tackle chlorine is crucial for chemical processing industries and marine applications.
The best metal for salt water is duplex steel, its unique microstructure (austenitic and ferritic) perfect for highly corrosive environments. For instance, Alloy 2507 (super duplex) contains nitrogen and manganese with chromium, nickel, and molybdenum. These acid resistant materials perform optimally in sour gas applications (NACE), subsea systems, and offshore refineries.
Grade 316 stainless steel contains nickel, chromium, and molybdenum (2-3%). It can tackle general and intergranular corrosion and hydrogen embrittlement. High-quality, extensively tested Grade 316 can deliver sustainable performance as chlorine resistant tubing. This 18-8 austenitic stainless steel is also weldable, formable, and adaptable.
See which grade is the most corrosion resistant stainless steel and durable metals
Rust refers to the formation of iron oxide – an orange-brown layer – that forms upon exposure to air and water. It can lead to expansion, stress, and eventual weakening. Rust is a continual process since the layer is porous. Resistant materials should have negligible iron. Notably, corrosion is an umbrella term for metallic damage, including rust and other chemical agents.
Aluminum gets viewed as the least corrosive metal. An aluminium-oxide layer that develops upon exposure to oxygen protects it from damage.
Other corrosion resistant metals are:
- Stainless steel*
*Galvanised steel (zinc coating) is also effective for rust resistance. Some manufacturers exceed minimum ASTM specifications to produce rust resistant metal.
What is the chloride limit for duplex stainless steel?
The chloride limit lets you select durable metals for your application, determining the concentration it can handle without facing corrosion. The chloride limit for duplex grades is around 9,000 to 35,000 ppm. However, setting this limit can be challenging because of other factors. In seawater, the pH level and aquatic species affect the capacity of acid resistant metals.
The corrosion resistance can also get estimated from its Pitting Resistance Equivalent Number or PREN. A high value denotes better pitting resistance for chlorine resistant tubing.
Is duplex SS better than SS 316L?
316L, a low-carbon austenitic version of Grade 316 of stainless steel, can attain properties of corrosive metals – close to duplex grades upon cold working. Its low carbon content makes it more corrosion resistant than Grade 316.
However, duplex steels offer greater strength, robustness, and corrosion resistance than Grade 316L. Duplex grades have both ferritic and austenitic microstructure. Among metals that don’t rust, ferritic and martensitic grades are known to handle SCC much better than austenitic (the microstructure of 316L). Therefore, it makes a more capable choice for corrosive conditions like desalination plants and sour-gas applications.
Duplex steels may seem expensive, but their lower nickel concentrations and long-term use can prove cost-effective. Your final choice of rust resistant metals will depend on the application requirements, budget, and compliance needs.
Are duplex stainless steel and 316L suitable for seawater?
As a rule, super duplex grades can face chlorides at higher concentrations than duplex ones. Their resistance to chlorides is satisfactory even at elevated temperatures and exceptional at lower temperatures, for instance, in water desalination plants. So, duplex steels get preferred as a rust free metal for water treatment facilities with lower salinity than seawater.
316L can work optimally in seawater filtration units. But it is likely to undergo corrosion upon prolonged exposure to stagnant seawater. The temperature, tidal influence, and flow rates are also important considerations. As a rule, Grade 316L can handle up to 500 ppm at about 30°C. The SCC risk rises with residual chlorine concentrations of over 0.5 ppm. It is best suited to intermittent exposure, over 1 metre/second flow, and moderate temperatures.
Confused between duplex 2205 and 316L? How to choose non corrosive metal or rust free metal? Read more about the metals that don’t rust
Both these materials get widely used in corrosive environments containing chlorine. Duplex 2205 contains more chromium (22%) than 316L and has 5-6% nitrogen.
However, duplex 2205 and 316L materials have some significant differences:
|Resisting SCC||Excellent, even in marine environments||Good, but not as competent|
|Energy absorption||More capacity to handle impact||Average|
|Heat handling||Less (up to 300°C)||High heat resistant materials (up to 925 °C)|