316 contains 2-3% molybdenum which gives it better chloride pitting resistance. For seawater, brines, or chloride > 200 ppm, use 316. For general atmospheric, fresh water, food-grade, or low-temperature service, 304 is sufficient and ~15-20% cheaper. The PREN (Pitting Resistance Equivalent) for 304 is 18-19; for 316 it is 24-26.

"L" means low-carbon — the carbon content is 0.030% max instead of 0.08% max. Low-carbon prevents sensitization during welding (chromium carbide precipitation at the grain boundary). For welded assemblies thicker than 5 mm, 304L or 316L is required. For non-welded service, 304 and 304L are interchangeable.

Use 321 for service temperatures in the 425-870°C range continuous (where 304 would suffer chromium carbide precipitation), and for welded assemblies that cannot be solution-annealed after welding. The titanium stabilization in 321 prevents sensitization. Do NOT use 321 for chloride service — use 316 instead.

321 is stabilized with titanium (Ti ≥ 5×C); 347 is stabilized with niobium (Nb ≥ 10×C). For most services they are interchangeable. For refinery FCC (fluid catalytic cracking) service, 347 has better polythionic acid resistance. For nuclear (ASME Section III), 347 is the approved grade.

Mixing 304 and 316 in the same system is generally not recommended because of galvanic corrosion at the joint — the more noble alloy (316) is protected while the less noble (304) corrodes preferentially. If mixing is unavoidable (e.g., during transition phases), use dielectric unions or insulating gaskets to break the galvanic path.

Use duplex (typically 2205 / F51) for seawater piping, hot brine, chemical tankers, and any service where chloride pitting is the primary failure mode. Duplex has PREN 35-36 (vs. 24-26 for 316), much higher strength (2× austenitic), but limited temperature range (-50°C to 300°C) and requires more careful welding procedures. For severe service beyond 2205, consider super-duplex 2507 or super-austenitic 254 SMO.