The main difference is molybdenum. Alloy 316 contains around 2% molybdenum, which Alloy 304 does not, giving 316 markedly better pitting corrosion resistance at a higher cost.
Alloy 304
Alloy 304 (UNS S30400, 1.4301) is perhaps the most widely used austenitic stainless steel, based on 18% chromium and 8% nickel additions, with smaller amounts of manganese, on a base of iron. It can contain a small amount of carbon, which can limit corrosion resistance after welding, so the lower-carbon grade 304L is widely used. Its lower alloy content makes it cost-competitive for general use - utensils, tubing and sanitary ware, and a wide range of other domestic and industrial applications.
Alloy 316
Alloy 316 (UNS S31600, 1.4401) contains 16% chromium, 10% nickel and 2% molybdenum. The molybdenum greatly improves pitting corrosion resistance, albeit at extra cost. As with 304, the lower-carbon variant 316L (UNS S31603, 1.4404) is more widely specified. The improved corrosion performance extends its use into more aggressive industrial and coastal applications.
Comparing them by PREN
The most common form of corrosion in stainless steels is pitting. The Pitting Resistance Equivalent Number (PREN) indicates resistance to pitting from the alloy's composition, where PREN = %Cr + 3.3 %Mo + 16 %N. The PREN for Alloy 316 is around 25, which is 4-5 points higher than Alloy 304.
| Grade | Chromium | Nickel | Molybdenum | PREN |
|---|---|---|---|---|
| Alloy 304 | 18% | 8% | none | 4-5 points lower than 316 |
| Alloy 316 | 16% | 10% | ~2% | ~25 |