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Stainless steel (SS) does not readily corrode, rust or stain with water as ordinary steel does. However, it is not fully stain-proof in low-oxygen, high-salinity, or poor air-circulation environments. Stainless steel is mainly used where both the properties of steel and corrosion resistance are required.
There is a series of stainless steels that ranges from SAE 100 - 600. However, for a stainless steel to be food contact substances (FCS) approved by the US Food and Drug Administration (FDA), American National Standards Institute (ANSI) and National Science Foundation (NSF), it must have a minimum chromium content of 16%. Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface. It also blocks corrosion from spreading into the metal's internal structure. Passivation occurs only if the proportion of chromium is high enough AND oxygen is present, hence the mandated 16% chromium for FCS applications and the need for good air circulation environments and abundant oxygen.
Stainless steel with a chromium content of less than 16% may be used for cutlery, blades, and similar applications requiring a sharp edge, but the alloy must be hardened or tempered by an appropriate post-weld heat treatment process.
To be approved by the FDA, ANSI and NSF, Stainless Steel that is to be used in food equipment must be of a type in the SAE 200 series (chromium-nickel-manganese alloys), SAE 300 series (chromium-nickel alloys), or SAE 400 series (chromium alloys). The composition of these stainless steels varies in chemical makeup and concentrations, but they provide the necessary chromium content making them acceptable materials for food contact. The series designations however only refer to the 'SAE Steel Grade Standard' and can be labeled something different depending on the standardizing body (such as the 'EN-Standard' or 'UNS Standard').
SAE 200 Series
- Austenitic crystalline structure, chromium-nickel-manganese alloy
- Contain a maximum of 0.15% carbon, a minimum of 16% chromium
- 200 Series stainless steels replace most their nickel content with manganese to reduce cost but results in weaker corrosion resistance
SAE 300 Series
- Austenitic crystalline structure, chromium-nickel alloy
- Contain a maximum of 0.15% carbon, a minimum of 16% chromium
- Tend to be the more expensive of the 3 SAE Series but the higher nickel content gives them the best corrosion resistance out of the 3
- The most widely used austenite steel is the SAE 304 SS (also known as A2 Stainless or 18/8 for its composition of 18% chromium and 8% nickel)
- The second most common austenite steel is the SAE 316 SS (also Known as A4 Stainless or 18/10for its composition of 18% chromium and 10% nickel)
SAE 400 Series
- Ferritic and Martensitic crystalline structures, chromium alloys
- Some of the alloys in this group fall below the required 16% chromium and therefore will not be FCS (i.e. most of the martensitic alloys as they tend to have a chromium content of 12–14%)
- The ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance due to lower chromium and nickel content but are usually less expensive due to that.
- The ferritic stainless steels contain between 10.5% and 27% chromium with very little nickel, if any, but some types can contain lead. However, the only the alloys with 16%+ chromium will be considered FCS
- These ferritic alloys can be degraded by the presence of chromium, an intermetallic phase which can precipitate upon welding.
Additional examples of Stainless steel grades and their general properties along with common uses.
Austenitic Stainless Steels generally exhibit the best corrosion resistance of the three crystalline structures. As such, the 304 and 316 SS are the most commonly used stainless steel even though they are often not the cost leader.
References:
ANSI/NSF - Food equipment materials
Stainless Steel grades and qualities
