Chemical composition compared

Full limits for each product standard, and the low-carbon L grades, are on the SS 304 and SS 316 grade pages.

Corrosion resistance

Both grades resist atmospheric corrosion, fresh water, most foods and many chemicals. The difference shows in chlorides and acids. Chlorides – from seawater, salt spray, de-icing salts, brines and chlorinated cleaning agents – break down the passive film locally, causing pitting and crevice corrosion. Molybdenum strengthens the film, so 316 resists pitting and crevice corrosion much better than 304.

A common measure is the pitting resistance equivalent number (PREN = %Cr + 3.3 × %Mo + 16 × %N). 304 has a PREN of about 18–20 and 316 about 23–26. In practice, 304 is suitable for inland, indoor and fresh-water service, while 316 is chosen for coastal and marine atmospheres, chlorinated water, and chemical and pharmaceutical processes. 316 also resists sulphuric, phosphoric and acetic acids better than 304.

Neither grade is suitable for warm seawater or hot concentrated chlorides, where both can pit and suffer stress corrosion cracking; duplex, super duplex or nickel alloys are used instead.

A practical example: handrails and cladding on a building a few kilometres from the sea may show brown staining within a year or two in 304, while 316 with a smooth finish stays bright with periodic washing. In a dairy plant, 304 tanks may be fine for milk, but pipework carrying brine or cleaned with chlorinated sanitisers is better in 316L. In a chemical plant, the process fluid, its temperature and impurities decide the grade – and sometimes neither 304 nor 316 is suitable, and duplex or nickel alloys are needed. If in doubt, share the operating conditions with your supplier or a corrosion engineer before ordering.

Mechanical and physical properties

304 and 316 have the same minimum strength in ASTM A240 – 515 MPa tensile and 205 MPa yield – and similar ductility and toughness, including at cryogenic temperatures. Both are non-magnetic when annealed and can become slightly magnetic after cold work. Density is about 7.9 g/cm³ for 304 and 8.0 g/cm³ for 316. Both are formed and welded in the same way, with 308L filler for 304 and 316L filler for 316. At elevated temperatures, 316 has slightly better creep strength.

Cost

316 costs more than 304 because molybdenum and the extra nickel are expensive alloying elements. The difference varies with metal markets and product form. In equipment where only a small part is exposed to chlorides, it can make sense to use 316 only for those parts. Where corrosion would lead to failure, contamination or frequent replacement, the extra material cost of 316 is usually small compared with the cost of downtime.

When to use 304

• Indoor and inland outdoor environments, away from the coast and road salt
• Food and dairy equipment cleaned without aggressive chlorinated agents
• Fresh-water piping and tanks with low chloride content
• Kitchen equipment, architectural interiors, lifts and furniture
• Cryogenic vessels and general fabrication

When to use 316

• Coastal and marine atmospheres, splash zones and offshore utilities
• Chemical and pharmaceutical process equipment and piping
• Dairy, brewery and food plants using chlorinated or acidic cleaning-in-place chemicals
• Water treatment with chlorinated or brackish water
• Pulp and paper equipment, and surgical and medical devices (316L)

304L and 316L

The L grades have carbon limited to 0.030%, which prevents chromium carbides forming during welding and protects the heat-affected zone from intergranular corrosion. For welded equipment and piping, 304L or 316L – or dual-certified 304/304L and 316/316L, which meet the strength of the standard grade and the low carbon of the L grade – are normally specified.

How to tell 304 and 316 apart

The two grades look identical. Check the marking on the product and the mill test certificate, which states the grade and the molybdenum content. In the workshop, a portable XRF analyser – positive material identification – shows the molybdenum content in seconds, and chemical spot-test kits for molybdenum are also available. Magnet tests do not distinguish the two.

Heat resistance

Both grades resist oxidation in continuous service up to about 870–925 °C. For long service in the range of about 425–860 °C, the standard grades can become sensitised; the L grades or stabilised grades such as 321 and 347 are used instead. For higher temperatures, heat-resistant grades such as 310S are chosen. The H versions – 304H and 316H – have controlled carbon for guaranteed creep strength in pressure equipment at elevated temperature.

Food, pharmaceutical and hygienic use

Both 304 and 316 are accepted for food contact and hygienic equipment. 304 is the usual choice for dairy, brewery and food processing where cleaning chemicals are mild. 316L is preferred where products are salty or acidic – sauces, pickles, fruit juices – and in pharmaceutical and biotech plants, where equipment is cleaned and sanitised with aggressive chemicals and steam, and where low carbon and controlled sulphur help achieve smooth electropolished surfaces.

Common mistakes

Using 304 near the coast is the most common cause of tea staining and pitting on architectural stainless steel; 316 with a smooth finish and regular cleaning performs far better. Mixing 304 and 316 parts in one assembly can cause the 304 parts to fail first. Welding standard-carbon grades without considering sensitisation can lead to corrosion along the weld. Contamination by carbon steel particles from tools and grinding can cause rust spots on either grade, so fabrication should use dedicated tools and finish with pickling and passivation.

Quick comparison

Products in 304 and 316

We supply both grades in every product form – see SS 304 pipes, SS 316 pipes, SS 316 sheets and plates and the grade pages for SS 304 and SS 316. More guides and charts are on the technical resources page.