Chromium + nickel. Higher chromium + higher PREN. Chloride defeated.

PREN = Pitting Resistance Equivalent Number = %Cr + 3.3×%Mo + 16×%N. Higher PREN = better chloride pitting resistance. Minimum PREN for severe service: seawater splash zone 32, seawater immersion 40, high-chloride hot brine 50. 2205 (PREN 35) works for moderate chloride service. 2507 (PREN 42) works for severe chloride service. Hastelloy C-276 (PREN 65) works for extreme. PREN is a screening tool — always validate for specific service conditions.

Nitrogen is a strong austenite stabilizer and dramatically improves pitting resistance. Super Duplex 2507 has 0.3% nitrogen vs 2205 at 0.17%. The 16× multiplier in PREN formula shows nitrogen's strong effect. Nitrogen also: (1) Stabilizes austenite phase during cooling. (2) Increases yield strength. (3) Improves fatigue resistance. Nitrogen is why Super Duplex has higher PREN without proportional Cr/Mo increase.

Duplex stainless requires specific welding procedures to maintain 50/50 austenite-ferrite balance. Improper welding: heat-affected zone becomes all-ferrite (brittle) or all-austenite (reduced properties). Controlled heat input, matching filler metal (higher Ni to re-balance phases), nitrogen backing gas essential. Super Duplex more sensitive than standard duplex. For NACE-qualified welding, specific procedures required. We coordinate welding with qualified shops for duplex stainless assemblies.

Super Duplex costs 40-60% more than Duplex 2205. Justified for severe service where 2205 would fail. Specifically: offshore platforms where replacement is expensive, deep sour wells with high H2S, hot seawater circulation systems, pulp mill bleaching. For moderate service, 2205 provides adequate corrosion resistance at lower cost. Don't over-specify — 2205 handles 80% of duplex applications.

316L: austenitic stainless, ductile, weldable, susceptible to chloride stress corrosion cracking. PREN ~26. Duplex 2205: higher strength, better chloride resistance, PREN ~35. Cost: 2205 is 2-3× 316L. For general stainless service (food, indoor chemical): 316L adequate and cheaper. For marine/chloride/pressure: duplex 2205 or better worth cost premium. For severe chloride + pressure + temperature: Super Duplex 2507.

Duplex 2205: NACE MR0175 qualified with hardness control (typically <31 HRC for wetted surfaces). Standard for oil and gas production equipment in moderately sour service (<3% H2S partial pressure). Super Duplex 2507: NACE MR0175 qualified with hardness control, extended service envelope (higher H2S partial pressure, chloride concentration). For deep sour service and high-chloride brine service, Super Duplex often required.