Both corrosion resistant. 316 handles seawater. 304 handles everything else.

316: standard molybdenum-bearing stainless, carbon content up to 0.08%. 316L: "Low carbon" variant, carbon content limited to 0.03%. The lower carbon prevents chromium carbide precipitation at grain boundaries during welding — critical for welded assemblies. For welded 316 applications, always specify 316L. For non-welded parts, either 316 or 316L works. Most mill stock is 316L now — the small premium over 316 is worth the versatility.

Significantly — measured in orders of magnitude. ASTM B117 salt spray testing: 304 starts pitting after 100–500 hours; 316L typically passes 1000+ hours without pitting. In actual saltwater service: 304 may pit or tea-stain in weeks to months; 316L lasts 10+ years. The molybdenum addition specifically stabilizes the passive layer against chloride attack.

316L material is typically 15–25% more expensive than 304 per kg. For finished CNC parts, the total cost difference is usually 10–20% since machining cost is similar. For a $50 machined part, 316L version costs $55–60. Over 1000-part production: 304 saves $5,000–10,000. When marine or chemical service actually requires 316L, the savings from specifying 304 disappear in warranty/replacement costs.

Yes, in non-coastal, low-chloride environments. 304 handles outdoor rain and humidity fine for 10–20 years. Problem arises near oceans (salt spray), on roads treated with salt (winter de-icing), in industrial pollution areas, or in chlorinated pools. For coastal or road-salt exposure, 316L is worth the premium. For inland, suburban, non-industrial outdoor use, 304 is adequate.

304 is austenitic (FCC crystal structure — non-magnetic) in annealed condition. Heavy cold working (bending, deep drawing) can partially transform to martensite (BCT structure — magnetic). Slight magnetism in 304 sheet that's been bent is normal and doesn't indicate 430 (which is ferritic and fully magnetic). For non-magnetic applications (MRI compatibility, sensor housings), specify "annealed" or use 316 (more stable austenitic due to higher nickel).

Both benefit from passivation per ASTM A967. Passivation removes free iron from the surface (contamination from machining tools), allowing full chromium oxide passive layer to form. Nitric acid passivation standard. For medical or food-contact parts, passivation is typically required. We include ASTM A967 passivation at no additional cost on most stainless orders.