What is the difference between 304 and 316 stainless steel?

Chemically the two grades are close: both are chromium-nickel austenitic stainless steels around 18% Cr. The decisive difference is molybdenum — 316 and 316L carry 2.0 to 3.0%, 304 and 304L carry none. Molybdenum raises resistance to pitting and crevice corrosion in chloride-bearing water, which is the failure mode that destroys 304 in marine and coastal service.

In practice: specify 304 / 304L for fresh water, food and dairy, architectural trim, dry process vessels and general fabrication. Specify 316 / 316L for seawater-adjacent plant, brine, chlorinated water, pharmaceutical and chemical process duty, and for anything installed within reach of Gulf coastal air, where salt deposition alone is enough to pit 304.

The 'L' suffix on either grade means low carbon (0.030% max). Low carbon suppresses chromium carbide precipitation at the grain boundaries during welding, which is what causes weld decay. For any welded section above roughly 5 mm, specify the L variant or a stabilised grade such as 321.

Neither grade is a substitute for duplex where chlorides are high and strength matters: 2205 duplex carries roughly twice the yield strength of 316L with far higher pitting resistance (PREN about 35 against 24). Where 316L is being specified thicker and thicker to survive, duplex is usually the cheaper installed answer.