How do I select the right stainless steel grade?

Grade selection follows a systematic process: (1) Define the corrosive environment — chloride concentration, temperature, pH; (2) Calculate minimum PREN required using our PREN Calculator; (3) Check mechanical property requirements — duplex grades offer 2× higher yield than austenitic at similar corrosion resistance; (4) Consider weldability — high-Mo grades require controlled heat input and preheat; (5) Check availability and cost — 316L is often the cost-optimum for mild chloride service.

What does PREN mean and what value do I need?

PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3×%Mo + 16×%N. It ranks stainless steels for chloride pitting resistance. General guidance: PREN <20 = indoor/mild environments only; PREN 25-35 = industrial and mild chloride service; PREN ≥40 = seawater service at ambient temperature; PREN ≥45 = hot seawater and aggressive chemical service. Use our free PREN Calculator to evaluate any composition.

Can stainless steel be welded without special precautions?

Yes, but specific precautions apply by grade: Austenitic 304/316 — use low-H consumables (308L/316L), control heat input <2.5 kJ/mm to limit sensitisation risk; Duplex 2205 — control heat input 0.5-2.0 kJ/mm, ensure correct shielding gas (Ar+2%N₂), verify ferrite number (3-8 FN) in weld metal; Ferritic grades — maintain interpass temperature <150°C, use austenitic filler or matching ferritic filler. PWHT is generally not required for austenitic grades.

PREN Calculator — Free Online Pitting

Q: Can stainless steel be welded without special precautions?

Yes, but specific precautions apply by grade: Austenitic 304/316 — use low-H consumables (308L/316L), control heat input <2.5 kJ/mm to limit sensitisation risk; Duplex 2205 — control heat input 0.5-2.0 kJ/mm, ensure correct shielding gas (Ar+2%N₂), verify ferrite number (3-8 FN) in weld metal; Ferritic grades — maintain interpass temperature <150°C, use austenitic filler or matching ferritic filler. PWHT is generally not required for austenitic grades.

The Pitting Resistance Equivalent Number (PREN) is the primary metric for ranking the resistance of stainless steels and nickel alloys to chloride-induced pitting and crevice corrosion. This calculator computes both PREN (standard formula) and PRENw (extended formula including tungsten — required for super duplex and some super austenitic grades) from alloy composition.

PREN Calculator — Pitting Resistance Equivalent Number

%Cr — Chromium

%Mo — Molybdenum

%N — Nitrogen

%W — Tungsten (for PRENw, optional)

PREN = %Cr + 3.3×%Mo + 16×%N—

PRENw = %Cr + 3.3×(%Mo + 0.5×%W) + 16×%N—

Corrosion Service Rating—

Closest Standard Grade—

PREN = %Cr + 3.3×%Mo + 16×%N (ISO 10272, ASTM A923 Annex)
PRENw = %Cr + 3.3×(%Mo + 0.5×%W) + 16×%N (includes tungsten contribution)
Critical Pitting Temperature (CPT) in 6% FeCl₃ ≈ PREN − 20°C (approximate)

Figure: Duplex stainless steel — schematic microstructure showing austenite islands (green) in ferrite matrix (blue), and property comparison with 304L austenitic stainless. © metallurgyzone.com/

PREN Reference Values for Common Grades

Grade (UNS)	Cr%	Mo%	N%	PREN	Service Limit
304L (S30403)	18.5	—	0.06	~19	Mild, non-chloride
316L (S31603)	17.5	2.2	0.06	~25	Cooling water, mild Cl⁻
317L (S31703)	19.0	3.3	0.06	~30	Chemical plant
2205 Duplex (S31803)	22.0	3.1	0.17	~35	Seawater ambient
SAF 2507 (S32750)	25.0	4.0	0.28	~42	Hot seawater to 40°C
254 SMO (S31254)	20.0	6.1	0.20	~43	Seawater heat exchangers
Alloy 625 (N06625)	21.5	9.0	0.02	~51	Aggressive acids and brine

References

ISO 10272-3 Corrosion of metals and alloys — Pitting resistance equivalent numbers.
ASTM A923 Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels.
Sedriks, A.J., Corrosion of Stainless Steels. 2nd ed. Wiley-Interscience, 1996.

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