These two Stainless Steels are very similar in their mechanical properties and are among most widely used austenitic stainless steels (304 edging out 316 for most prevalent). Their chemical makeups are slightly different:
- SAE 304 SS Also known as A2 Stainless or 18/8 for its composition of 18% chromium and 8% nickel.
- SAE 316 SS Also Known as A4 Stainless or 18/10 for its composition of 18% chromium and 10% nickel.
Along with the increased nickel content, 316SS also has an additional element 'Mo' (molybdenum) in its chemical makeup (2%-3%). This makes it generally more corrosion resistant than the 304SS, and more expensive. Although this makes the 316SS more desirable than the 304SS in some cases, circumstances may require that you use the 316SS instead.
Both are inherently resistant to oxidation (rusting) due to their chromium content that passivates the surface with a chromium oxide layer, but 304SS is more susceptible to corrosion from chloride solutions than 316SS (notably saline environments with high amounts of sodium chloride or salt) due to its lack of molybdenum content. Solutions with as little as 100 ppm of sodium chloride can begin to have a corrosive effect on 304SS and create localized areas of corrosion called "pitting," which can spread beneath protective chromium barriers to compromise internal structures. However, 316SS (with its molybdenum content) can handle up to 2000 ppm of sodium chloride before it begins to have a corrosive effect. Because of this 316SS is also most preferred in marine environments.
Chlorine also has an effect on the degradation of 304 SS and 316 SS. Again, 316SS is more resilient as it is less susceptible to corrosion from chlorine solutions than 304SS, which shows signs of corrosion at 2 ppm of chlorine and 316SS showing signs of corrosion at 4 ppm of chlorine.
304SS and 316SS can also come in different grades depending on the qualities needed for various other applications:
304 Stainless Steel
- 304 - General Grade for Kitchen equipment and appliances, chemical containers including for transport, and food processing equipment particularly in beer brewing, milk processing, and wine making.
- 304L - Low Carbon grade for Kitchen equipment and appliances, chemical containers including for transport, and food processing equipment particularly in beer brewing, milk processing, and wine making.
- 304H - High Carbon grade for use in petroleum refineries, boilers, heat exchange's and condensers, pipelines, cooling towers, steam exhausts, and electric generation plants.
316 Stainless Steel
- 316 - General grade for food processing, chemical storage and transport, textile dying equipment, cladding of nuclear fuel, and oil refining equipment as well as some medical implants.
- 316L - Low-carbon grade for handling paper pulp as well as the production of rayon, rubber, textile bleaches, and high-temperature industrial equipment. This is the preferred grade for medical implants as it is resistant to sensitization.
- 316F - Free-machining grade for automatic machine screw parts as well as surgical implants and pharmaceutical processing equipment.
- 316N - High-nitrogen grade with increased resistance to pitting and to corrosion in crevices. Used for chemical handling accessories.
304SS vs. 316SS in Chloride Solutions
Marine Grade Stainless Steel
Stainless Steel Grades and Common Uses