The short answer
Use 304 indoors and inland: kitchens, food equipment, frames and balustrades in Johannesburg, Pretoria and the rest of the Highveld. Use 316 near the sea, around swimming pools, and wherever salt, brine or process chemicals reach the steel.
What is in them
| Element | 304 | 316 |
|---|---|---|
| Chromium | 17.5 – 19.5 | 16.5 – 18.5 |
| Nickel | 8.0 – 10.5 | 10.0 – 13.0 |
| Molybdenum | – | 2.0 – 2.5 |
| Carbon, max | 0.07 | 0.07 |
What the molybdenum does
Stainless steel resists rust because of a thin chromium oxide film on its surface. Chlorides, found in sea air, pool water, bleach and brine, break that film down at small points and eat pits into the steel. Molybdenum makes the film much harder to break.
The usual way to compare grades is the pitting resistance number, PREN. 304 scores about 19 and 316 about 24. Higher is better.
Where each grade belongs
| Use | Grade |
|---|---|
| Commercial kitchens, splashbacks and food equipment | 304 |
| Balustrades, handrails and frames inland | 304 |
| Anything within reach of sea air: Durban, Cape Town, Gqeberha, East London, Richards Bay | 316 |
| Swimming pools and pool surrounds | 316 |
| Boat and marine fittings | 316 |
| Chemical, pharmaceutical and brine handling | 316 |
| Food processing with salty or acidic products | 316 |
304L and 316L
The L grades limit carbon to 0.03%. Welding heats the steel beside the weld, and with more carbon present, chromium carbides can form there and leave that zone open to corrosion. On thin sheet the difference is small. For welded plate and pipe in corrosive service, ask for the L grade.
Much of the material sold today is dual-certified, meeting both 304 and 304L, or 316 and 316L.
Telling them apart
You cannot tell 304 from 316 by eye or with a magnet. Both are non-magnetic when annealed, and both can turn slightly magnetic after bending or cold work. The reliable checks are the mill certificate, a molybdenum spot test, or a handheld XRF analyser.
Cost
316 costs noticeably more than 304, mainly because of the molybdenum and the extra nickel. Using it where 304 would do is money spent for nothing. Using 304 at the coast usually shows up as brown tea staining on the surface, and in time as pitting.
We supply both grades in sheet and plate, tube, bar and schedule pipe. Tell us the grade with the sizes and we confirm what is in stock.