304 vs 316 Stainless Steel: Differences, Uses and Which to Choose

The difference between 304 and 316 stainless steel is molybdenum. Grade 304 contains about 18% chromium and 8% nickel; grade 316 contains slightly less chromium, more nickel and 2 to 3% molybdenum, which gives it much better resistance to pitting from salt, chlorides and acids. 304 is cheaper and suits most indoor and food applications; 316 is the choice for coastal and marine sites, chlorinated cleaning and chemical exposure.

Both grades belong to the austenitic family, one of the five families of stainless steel alongside ferritic, martensitic, duplex and precipitation-hardening grades. Austenitic steels are non-magnetic when annealed, cannot be hardened by heat treatment and are easy to form and weld, which is why they dominate food, pharmaceutical and industrial equipment. The two grades look and machine almost identically; the difference is in the chemistry.

The pitting resistance equivalent number (PREN) is a rough guide to how well a stainless steel resists pitting in chloride environments; the higher the number, the better. Molybdenum counts heavily in the calculation, which is why 316 scores several points above 304 despite containing less chromium.

Stainless steel resists corrosion because chromium forms a thin, invisible oxide layer on the surface that repairs itself in air. Chlorides attack that layer at weak points, starting small pits that grow underneath the surface. Salt water, sea spray, road salt, brines and chlorine-based sanitisers all supply chlorides.

Molybdenum makes the passive layer far more resistant to that attack, and helps it recover when it is damaged. That is why 316 survives coastal air and daily chlorinated cleaning where 304 shows rust spots and pitting within months. It also improves resistance to some acids, such as the sulphuric and phosphoric acids used in descaling.

304 is the default for most equipment and the most widely used stainless steel in the world. Specify 316 when chlorides, acids or aggressive cleaning are part of the environment.

The extra cost of 316 is small against the life of the equipment when chlorides are present: a 304 housing that pits in a salty environment has to be replaced, and pitted surfaces are harder to keep clean.

The “L” grades are low-carbon versions: 304L and 316L contain no more than 0.03% carbon, against 0.08% for standard 304 and 316. Low carbon matters when the steel is welded. In standard grades, the heat of welding can draw chromium out of solution next to the weld, a problem called sensitisation, which leaves the area open to intergranular corrosion.

For welded enclosures and fabricated housings, 304L and 316L avoid that risk with no need for heat treatment after welding. Corrosion resistance is otherwise the same as the standard grade, and strength is slightly lower. Much stainless sheet is now dual-certified, for example as 316/316L, meeting both specifications at once.

“Marine grade” and “food grade” are descriptions rather than grades. Marine grade stainless steel almost always means 316 or 316L, chosen for its resistance to salt. Food grade stainless steel usually means 304 or 316, chosen because stainless is non-porous, corrosion resistant and easy to clean; compliance for food contact comes from the regulations of your market, not the label.

For fasteners, the same grades are sold as A2 (304) and A4 (316). A 316 enclosure held together with A2 bolts will show rust at the fixings first, so specify matching fasteners.

Even the best grade can appear to rust if its surface is contaminated. Tools, wire brushes and grinding belts used on carbon steel leave iron particles embedded in stainless, and those particles rust within weeks, staining the surface as if the stainless itself were corroding. Good fabricators keep separate tools for stainless steel for this reason.

Passivation restores the protective layer after fabrication. The surface is treated with a mild acid, usually nitric or citric, that removes free iron and lets the chromium oxide layer re-form evenly. In service, rinse off salt and cleaning chemicals, use non-abrasive pads rather than steel wool, and follow the chemical concentrations your supplier recommends.

The finish on a stainless surface affects how easily it cleans and how well it resists corrosion, whichever grade is used. 2B is the smooth, slightly reflective mill finish on cold-rolled sheet. No. 4 is the familiar brushed, satin finish on most enclosures and appliances, which hides handling marks. Electropolishing removes a thin surface layer electrochemically, leaving a brighter, smoother surface with fewer places for soil and bacteria to settle.

For hygienic equipment, a surface roughness of around Ra 0.8 µm or smoother is widely used as a target for easy cleaning. Whatever the finish, the grain on brushed surfaces should run so that water drains off rather than collecting in the lines.

For panel PCs and displays in food plants, pharmaceutical sites and marine environments, the stainless housing is only part of the specification. The bezel, fasteners, seals and connectors all need to match the environment, and the unit needs an ingress rating, usually IP66 or IP69K, that survives the cleaning regime. Our guide to hygienic design and IP rating chart cover those choices.

Our stainless steel panel PCs and stainless steel monitors are built in 304 or 316 depending on the model. If the environment calls for 316, tell us and we will confirm the grade before recommending a unit.