The essential difference between 304 and 316 is molybdenum: grade 316 contains it, grade 304 does not. Molybdenum markedly improves resistance to pitting in environments containing chlorides.
In practical terms: where there is sea water, salt or contact with chlorinated chemicals, 316 is chosen. Where there is not, 304 is both sufficient and the more economical option.
The two grades side by side
| Molybdenum | Not present in 304, present in 316 |
|---|---|
| Chloride resistance | Moderate in 304, high in 316 |
| Typical use | 304 for kitchens and architecture, 316 for marine, chemical and food processing |
| Magnetism | Neither is magnetic when annealed; cold forming can produce slight magnetism |
| Welding | Both can be welded with the appropriate filler |
Is stainless steel really stain-free
No. The corrosion resistance of stainless steel comes from a thin chromium oxide film that forms on the surface by itself. When the film is damaged it repairs itself in the presence of oxygen.
Where conditions prevent that repair - for example when carbon steel particles are left on the surface - rust spots appear on stainless surfaces too. This does not mean the material is defective; it is usually contamination.
Preventing contamination
- Do not work stainless on the same bench as carbon steel
- Keep cutting and grinding tools for stainless separate
- Avoid direct contact with carbon steel in storage
- Use suitable slings rather than steel chain when lifting
Surface finishes
Stainless sheet is supplied with a range of surface finishes. The choice is not only about appearance: the rougher the surface, the more readily dirt and bacteria adhere, which matters in food and pharmaceutical work.
Architectural work tends to prioritise appearance, industrial work cleanability and durability.
Three questions before choosing
- Will the material meet chlorides - salt, sea water, chlorinated cleaning products?
- How often and with what will the surface be cleaned?
- Will the product touch food or pharmaceuticals?