How to Select Stainless Steel? A Complete Grade & Application Guide | Shenlan Nickel

   Stainless steel is the workhorse of corrosion-resistant design. It is an iron-based alloy family containing a minimum of 10.5% chromium, protected by a self-healing chromium oxide (Cr₂O₃) passive film only a few nanometres thick. This passive layer enables stainless steel to perform reliably in oxidising, chloride-containing and high-temperature environments where carbon steel fails completely. Its applications range from domestic kitchen panels to large-scale Red Sea desalination facilities.

Part 1 — Austenitic: The Industrial Mainstay

304 / 304L UNS S30400 / S30403

Composition: Cr 18–20 %, Ni 8–10.5 %, C ≤0.08 % (0.030 % for the L grade).
Character: The classic 18‑8 austenitic grade. Good general atmospheric corrosion resistance, excellent formability and weldability. Low‑carbon 304L prevents intergranular sensitisation after welding. It is the economical general-purpose option for mild environments.
Typical use: medical devices, chemical piping and tanks, desalination equipment, coastal architectural components.

316L UNS S31603

Composition: Cr 16.0–18.0 %, Ni 10.0–14.0 %, Mo 2.0–3.0 %, C ≤0.030 %, Si ≤1.00 %, Mn ≤2.00 %, P ≤0.045 %, S ≤0.030 %.
Character: the molybdenum addition lifts pitting and crevice corrosion resistance well above 304, and the low carbon avoids weld-decay sensitisation. The default upgrade grade for chloride duty.
Typical use: medical devices, chemical piping and tanks, desalination equipment, coastal architectural components.

316Ti UNS S31635

Composition: Cr 16.5–18.5 %, Ni 10.5–13.5 %, Mo 2.0–2.5 %, Ti ≥5 × C and ≤0.70 %, C ≤0.08 %.
Character: Titanium stabilisation forms TiC preferentially, so the grade resists intergranular corrosion after long exposure within the 450–850°C sensitisation range. Ideal for welded components requiring post-weld heat treatment.
Typical use: heat exchangers, automotive exhaust systems, fuel heaters, hot flue ducts.

317L UNS S31703

Composition: Cr 18.0–20.0 %, Ni 11.0–15.0 %, Mo 3.0–4.0 %, C ≤0.030 %.
Character: Higher molybdenum content than 316L delivers a higher PREN value, offering better resistance to dilute sulphuric acid, phosphoric acid and organic acids, while retaining good weldability.
Typical use: pulp and paper equipment, chemical scrubbers, tanks for aggressive liquids.

310S UNS S31008

Composition: Cr 24.0–26.0 %, Ni 19.0–22.0 %, C ≤0.08 %, Si ≤1.50 %.
Character: Higher molybdenum content than 316L delivers a higher PREN value, offering better resistance to dilute sulphuric acid, phosphoric acid and organic acids, while retaining good weldability.
Typical use: Industrial furnace tubes, heat-treatment retorts and muffles, incinerator parts, radiant heater components.

Part 2 — High-Performance Austenitic and Heat-Resisting Grades

253MA UNS S30815

Composition: Cr 21 %, Ni 11 %, Si 1.5 %, N 0.17 %, plus a controlled rare-earth (cerium) addition.
Character: designed for oxidation and carburisation service. Silicon strengthens the oxide scale, nitrogen raises strength, and the rare earths improve scale adhesion under cycling. Creep strength and thermal-fatigue resistance are excellent in the 900–1100 °C band.
Typical use: boiler superheaters, waste incinerator parts, heat-treatment furnace fixtures.

RA330 UNS N08330

Composition: Ni 34–37 %, Cr 19–21 %, Si 1.2 %.
Character: Strictly speaking, this is a nickel‑iron‑chromium heat‑resistant alloy rather than standard stainless steel. High nickel stabilises the austenite phase and oxide scale up to 1200°C, with excellent thermal shock resistance and good resistance to carburising & nitriding atmospheres.
Typical use: furnace rolls, heat-treatment baskets and fixtures, glass furnace components.

Nitronic 60 UNS S21800

Composition: Cr 16–18 %, Ni 8–9 %, Mn 7–9 %, Si 3.5–4.5 %, N 0.08–0.18 %.
Character: a nitrogen-strengthened austenitic grade whose selling point is wear, not PREN. It work-hardens rapidly and resists galling and seizing even under dry sliding — a problem that conventional 304/316 fasteners handle badly.
Typical use: high-pressure valve trim, pump shafts, marine fasteners, bearing sleeves.

Part 3 — Duplex: Strength and Corrosion Resistance in Balance

Duplex grades carry roughly 50 % austenite and 50 % ferrite. The payoff is a yield strength about twice that of austenitic stainless, far better chloride stress-corrosion cracking resistance, and — importantly for project economics — the ability to replace nickel alloys at a fraction of the cost.

2205 UNS S32205 / S31803

Composition: Cr 22 %, Ni 5–6 %, Mo 3 %, N 0.14–0.20 %.
Character: the workhorse duplex. PREN about 35, excellent strength and corrosion balance, cost far below nickel alloys of comparable performance.
Typical use: oil and gas flowlines, chemical pressure vessels, desalination systems, structural components.

2507 UNS S32750

Composition: Cr 25 %, Ni 7 %, Mo 4 %, N 0.27 %.
Character: super duplex. PREN above 40, top-tier pitting, crevice and SCC resistance within the duplex family, with a further strength increase over 2205.
Typical use: deepwater oil and gas platforms, high-salinity wastewater plant, high-end chemical reactors, seawater heat exchangers.

Part 4 — Super Austenitic: The Last Line of Defence

254SMO UNS S31254

Composition: Cr 19.5–20.5 %, Ni 17.5–18.5 %, Mo 6.0–6.5 %, N 0.18–0.22 %, Cu 0.5–1.0 %, C ≤0.02 %.
Character: the reference super austenitic. Very high chromium, molybdenum and nitrogen push PREN to about 45, giving extremely low corrosion rates in chlorides and strong acids — among the most pitting-resistant austenitic stainless steels available. Welding requires a matching high-alloy filler (typically ERNiCrMo-3) and disciplined heat input.
Typical use: FGD systems, desalination plant, pharmaceutical equipment, heat exchangers in aggressive chloride service.

904L UNS N08904

Composition: Cr 20 %, Ni 25 %, Mo 4.5 %, Cu 1.5 %.
Character: High‑nickel copper‑bearing austenitic grade. Copper enhances resistance to sulphuric, phosphoric and acetic acids via a different corrosion mechanism. It has a mid‑range PREN and offers good weldability and formability at a lower cost than 254SMO.
Typical use: chemical reactors, pulp bleaching equipment, large-scale wastewater treatment.

Quick Selection Reference

Three Mistakes We See Most Often

1. Relying solely on PREN value PREN only evaluates resistance to chloride pitting corrosion. For reducing acids, high-temperature oxidation, abrasive wear and stress corrosion cracking (SCC), other material properties dominate. A grade with a higher PREN is not always the correct choice.
2. Applying duplex grades for high-temperature service Duplex grades 2205 and 2507 will suffer toughness degradation caused by σ-phase precipitation when operating above approximately 300°C. For higher temperature conditions, select austenitic stainless steel or nickel-based alloys instead.
3. Overestimating 316L chloride tolerance 316L works well under mild chloride conditions. It is not suitable for warm seawater or concentrated brine. When chloride media exceeds ~60°C, the risk of stress corrosion cracking increases rapidly. Duplex or super austenitic stainless steel shall be considered.