SS 316 stainless steel for salt, chlorides and chemicals

Stainless steel survives because of a thin chromium oxide film on its surface. Chloride ions attack that film at weak points and start pits. Once a pit forms, the chemistry inside it turns acidic and the pit grows downward faster than the surface around it.

Molybdenum strengthens the passive film and helps it heal, so pits are harder to start and slower to grow. Engineers compare grades using PREN (pitting resistance equivalent number), roughly Cr + 3.3 × Mo + 16 × N. Typical 304 lands around 18 to 20. Typical 316 lands around 24. That gap is the difference between staining on a sea-facing railing and a railing that stays bright.

For atmospheric marine exposure, coastal buildings, boat fittings above the waterline and handrails at the seafront, 316 is a sound choice and clearly better than 304. Regular rinsing keeps it looking good.

Fully immersed in warm seawater, especially in crevices under gaskets, bolts or marine growth, 316 can still pit and crevice-corrode. For permanent seawater immersion, pumps and offshore pipework, engineers often move up to duplex or super-duplex steels. See our duplex steel page for that step. Our marine and offshore page covers material choice for coastal and shipboard work.

316 costs more than 304 because of the molybdenum and extra nickel. It earns that back wherever 304 would fail early. In South India that often means coastal plants near Mangaluru, Karwar or Chennai, sugar mills and distilleries handling acidic juice and spent wash, salt-laden food processing, pharmaceutical plants with aggressive cleaning agents, and chemical plants dealing with dilute sulphuric acid or chloride-bearing streams.

• Pitting and crevice corrosion resistance well above 304
• Better resistance to dilute sulphuric and phosphoric acids
• Somewhat higher creep strength than 304 at high temperature
• Same ease of welding, forming and polishing as 304

Straight 316 allows up to 0.08% carbon, while 316L is limited to 0.030%. For welded equipment going into corrosive service, 316L is the safer choice because it resists sensitisation in the heat-affected zone. Standard 316 keeps a higher minimum strength and is preferred for some bar, fastener and higher-temperature uses. As with 304, a lot of today's material is dual-certified 316/316L.

The molybdenum only helps if the surface is clean and the welds are done properly. Most early failures we hear about in 316 equipment start at a weld or at a spot contaminated with carbon steel, not in the parent metal.

Use 316 or 316L filler so the weld metal keeps its molybdenum. Welding 316 with 308 filler leaves a weld bead with lower pitting resistance than the plate around it. Remove heat tint after welding by pickling paste or bath, then passivate. Use stainless-only wire brushes and grinding discs, and do not store 316 sheet on bare carbon steel racks where iron particles can transfer to it. These steps cost little and add years of service in chloride conditions.

State the product standard on your enquiry, not just the grade. ASTM A240 covers 316 plate, sheet and strip. ASTM A312 covers pipe, while A269 and A213 cover tube. ASTM A276 and A479 cover bar, and A182 covers forged flanges and fittings. Chemistry limits are close across them, but test requirements and tolerances differ, and your inspector will check the certificate against the standard named on the purchase order.

We supply 316 as seamless and ERW pipe and tube, capillary tube, round, square and hex bar, wire, sheet, strip, angles, channels, flanges and pipe fittings. Shim, perforated and corrugated sheet are also available in 316. Sizes are confirmed at enquiry, and each consignment comes with a test certificate through a government-recognised test house. For a cost-benefit view against 304, read our 304 vs 316 guide.