Grades differ by molybdenum, and that is what buys chloride resistance. Grades 304 and 304L are the general-purpose austenitic stainless steels, with good performance in clean atmospheric and freshwater conditions but no molybdenum, which leaves chloride pitting resistance limited. Grades 316 and 316L are distinguished from them by a molybdenum addition of roughly two to three percent, and that addition is the reason 316 is the default specification for coastal structures, food and beverage plants and swimming pool environments. Duplex grades use a mixed austenitic-ferritic microstructure to combine higher strength with better chloride resistance, and the strength advantage matters structurally because a lighter member partly offsets the higher material price. Super duplex grades exist for the most severe service, including seawater handling and offshore work, but they are expensive and harder to fabricate and are rarely justified for building structures. Where grades need ranking against each other, the Pitting Resistance Equivalent Number is the usual index, weighting chromium, molybdenum and nitrogen content — useful for comparison, but an index rather than a design value, since it accounts for none of surface finish, weld quality, crevice geometry or service temperature.