Good. Better. Premium. Chloride ladder.

PREN = Pitting Resistance Equivalent Number = %Cr + 3.3×%Mo + 16×%N. Rough measure of chloride pitting resistance. 304: PREN 19 (not seawater-rated). 316L: PREN 26 (chloride splash OK). Duplex 2205: PREN 35 (seawater immersion). Super-duplex 2507: PREN 42 (severe chloride). Higher PREN = better chloride performance. Use as quick screening — if your service chloride level exceeds material PREN capability, step up.

Duplex has mixed austenitic + ferritic microstructure. The ferrite phase provides higher strength than pure austenitic (304/316L). Yield strength 450 MPa vs 215 MPa for 304L — 2× stronger. Enables thinner sections, lighter designs. Also better SCC resistance (less austenite, less susceptibility). Trade-offs: lower ductility, more difficult to weld, lower cost-effectiveness at thin sections.

"L" designation means low carbon (<0.03%). Regular 304 has 0.08% carbon. In welding, carbon combines with chromium forming chromium carbides (sensitization) — depletes chromium near grain boundaries, causing intergranular corrosion. Low-carbon L grades minimize this. For welded parts: always specify L grade. For machined-only parts: either acceptable but L is widely available and costs similar.

Duplex is harder to machine than 304L/316L. Higher strength = more cutting force. Work hardens aggressively. Low thermal conductivity. Machinability rating ~25% of mild steel. Solutions: sharp tools (replace often), slow speeds (40-60 m/min), heavy feed to stay ahead of work hardening, flood coolant. Plan 30-50% longer machining time vs 316L. For production duplex parts, specify appropriate tooling and realistic cycle times.

Super-duplex 2507: PREN 42, 550 MPa yield. Used when: (1) Seawater service at elevated temperature (above 40°C). (2) High-velocity seawater (crevice corrosion risk). (3) Severe sour oil and gas service. (4) Process chemistry with chloride + elevated temperature. Significant cost premium over 2205 duplex — justify with actual service conditions, not "just in case."

Super-austenitic (254 SMO, AL-6XN): high-nickel austenitic with molybdenum 6% and nitrogen. PREN 43-45. Austenitic structure (easier to weld than duplex), excellent chloride resistance. Cost similar to duplex. Used when austenitic properties + chloride resistance both needed. For chemical processing: often specified. For offshore: duplex usually cheaper for equivalent performance.