SS 304: the 18/8 stainless that does most of the work

Under ASTM A240, 304 needs 17.5–19.5% chromium and 8.0–10.5% nickel, with carbon held to 0.07% maximum. Nickel is the expensive element, and it is the one that gets cut when someone wants to sell cheaper material under the 304 name.

The usual substitute in the local market is 200-series stainless, such as 202, which replaces part of the nickel with manganese and nitrogen. It looks identical when new. The difference shows up months later as rust spots in a kitchen, a dairy or a coastal site. A proper mill or lab test certificate, matched to the heat number stamped on the material, is your protection. We supply every lot with a certificate through a government-recognised test house.

304 suits a very wide range of jobs because it balances cost, corrosion resistance and fabrication. Food and dairy equipment, brewery and beverage tanks, kitchen fittings, water lines, architectural trim and general chemical storage are all typical homes for it.

It is also comfortable at moderate heat. In air, 304 resists scaling in continuous service up to roughly 925 °C, though prolonged exposure in the 425–860 °C range can cause carbide precipitation that weakens corrosion resistance at grain boundaries. For hot welded work, see 304L.

Chlorides are 304's weak point. Salt water, coastal spray, brine, swimming pool air and some cleaning chemicals can cause pitting and crevice corrosion. In warm chloride solutions, roughly above 60 °C, 304 is also at risk of stress corrosion cracking, which can split a stressed component with little warning.

For those conditions the usual step up is SS 316, which adds molybdenum. If you are weighing the two for a project, our 304 vs 316 comparison goes through cost and service life in more detail.

• Sea water and marine splash zones
• Hot chloride brines and some bleaches
• Strong reducing acids such as hydrochloric acid
• Sustained heating in the sensitising range when the steel is welded and not low-carbon

Annealed 304 is austenitic and essentially non-magnetic. Many buyers use a magnet as a quick check, and it is a useful first filter, but it is not proof. Cold working such as bending, drawing or roll-forming converts some austenite to martensite, so cold-drawn 304 wire, bent pipe and spun sheet often pick up a slight magnetic pull.

Equally, some 200-series grades are non-magnetic too, so a magnet cannot tell 202 from 304. The only reliable checks are the chemical analysis on the test certificate or a PMI (positive material identification) spectrometer reading.

The same 304 chemistry is sold in several surface finishes, and choosing the right one saves polishing cost later. 2B is the standard cold-rolled, annealed and pickled sheet finish, smooth and slightly dull, and it is what most fabricators use for tanks and equipment. BA, or bright annealed, has a more reflective surface straight from the mill and is common on kitchen and appliance work. No. 4 is a brushed, directional finish for visible panels and food counters. Hot-rolled plate comes with a rougher, pickled surface for heavier structural and process work.

Pipe and tube are normally supplied with a mill or pickled finish, with polished or bright annealed options for hygienic lines. Tell us the finish when you ask for a quote, because it changes the price and sometimes the lead time.

304 welds readily by TIG, MIG and stick processes, normally with 308 or 308L filler. Thin sheet rarely gives trouble. For thick sections or multi-pass welds that will later see acids or chlorides, low-carbon 304L is the safer specification. After welding, remove heat tint and any embedded carbon steel by pickling or passivation, otherwise rust spots can appear along the weld line within weeks. Keep stainless work away from grinding dust from mild steel in the same shop.

304 is available in more forms than any other grade on our list. That includes seamless and ERW pipe and tube, capillary tube, round, square and hex bar, wire, sheet, strip, angles, channels, flanges and pipe fittings, plus shim, perforated and corrugated sheet. Sizes are confirmed at enquiry. Tell us the form, size and quantity and the standard you need on the certificate, for example ASTM A312 for pipe or ASTM A240 for sheet.