304 vs 316 stainless near the sea: what am I actually paying for?
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TL;DR: On paper the strength is the same (205 MPa yield for both, annealed). The extra money for 316 buys molybdenum, which helps against chloride pitting. It does not make 316 immune to seawater, and for permanent immersion I would look at duplex. The situation: you are speccing stainless for something near the coast or on a boat, and every supplier says "use 316, it is marine grade." Is it really worth the premium over 304? Here is how I think about it. Strength is not the difference In the annealed condition the datasheets in our archive put them on top of each other. 304 plate: 205 MPa minimum yield, 515 MPa tensile, 40 % elongation, about 92 HRB (Outokumpu). 316: 205 MPa and 515 MPa as well (ATI, UpMet), 40 % elongation, 95 HRB on one sheet. Density is about 8 g/cm3 for both. So you are not buying a tougher steel. You are buying chemistry. What the premium actually buys 316 contains about 2 to 3 % molybdenum and 304 contains none. Molybdenum helps stainless resist pitting and crevice corrosion in chlorides, and seawater is chloride soup. That is the whole reputation of 316 in marine use. Where "marine grade" goes wrong It is not a force field. In warm, stagnant seawater, under deposits, or in tight crevices under washers and gaskets, 316 can pit and crevice-corrode too. Brown staining on a 316 rail is common and usually cosmetic; a pit under a fastener is not. Design details, cleaning, and whether water can sit in a gap often matter as much as the grade. My rough guide (opinion, not a code) - Indoor, fresh water, food, inland architecture: 304 is usually fine. Do not pay the premium out of habit. - Coastal air and splash: 316, with crevices designed out. - Permanently immersed, warm, under deposits, or costly to repair: look at duplex. Our archive lists super duplex 2507 at 550 MPa yield at 20 C (Alleima tube), more than double annealed 316, with a PREN of about 42.5 on the same sheet. It is a different decision, not a free upgrade: more cost, tighter welding practice, temperature limits. If you do use 316 near the sea Avoid crevices where you can. Rinse salt off with fresh water now and then, because dried chloride keeps working. Clean and passivate welds, and keep carbon-steel grinding dust off the stainless, because embedded iron rusts and makes the steel look like it is failing. Where I might be wrong: datasheets give mechanical properties, not corrosion rates. Temperature, chloride level, surface finish and welds change everything, so treat the grade as one input among several. Has anyone watched 316 fail in seawater? Was it the grade or the detail around it?