In metallurgy, stainless steel, also known as inox steel or inox from French “inoxydable”, is defined as a steel alloy with a minimum of 10.5% to 11% chromium content by mass.
Stainless steel does not corrode, rust or stain with water as ordinary steel does, but despite the name it is not fully stain-proof, most notably under low oxygen, high salinity, or poor circulation environments. It is also called corrosion-resistant steel or CRES when the alloy type and grade are not detailed, particularly in the aviation industry. There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and resistance to corrosion are required.
Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide, and due to the dissimilar size of the iron and iron oxide molecules (iron oxide is larger) these tend to flake and fall away. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal’s internal structure, and due to the similar size of the steel and oxide molecules they bond very strongly and remain attached to the surface.
Passivization only occurs if the proportion of chromium is high enough and in the presence of oxygen.
Stainless steel Material Notes
Type 304 stainless steel is a T 300 Series Stainless Steel austenitic. It has a minimum of 18% chromium and 8% nickel, combined with a maximum of 0.08% carbon. It is defined as a Chromium-Nickel austenitic alloy.
Grade 304 is the standard “18/8” stainless that you will probably see in your pans and cookery tools.
These are some of its characteristics:
– Forming and welding properties
– Corrosion/ oxidation resistance thanks to the chromium content
– Deep drawing quality
– Excellent toughness, even down to cryogenic temperatures which are defined as very
– Low temperature properties responding well to hardening by cold working
– Ease of cleaning, ease of fabrication, beauty of appearance
Grade 304L is the low carbon version of 304. It does not require post-weld annealing and so is extensively used in heavy gauge components (over about 6mm).
Grade 304H with its higher carbon content finds application at elevated temperatures.
It is used for a wide variety of home and commercial applications, this is one of the most familiar and most frequently used alloys in the stainless steel family.
Typical applications include tanks and containers for a large variety of liquids and solids
Food processing equipment, particularly in beer brewing, milk processing & wine making.
For example it is highly suitable and applied in dairy equipment such as milking machines, containers, homogenizers, sterilizers, and storage and hauling tanks, including piping, valves, milk trucks and railroad cars.
Very common in the brewing industry where it is used in pipelines, yeast pans, fermentation vats, storage and railway cars, etc.
The citrus and fruit juice industry also uses Type 304 for all their handling, crushing, preparation, storage and hauling equipment.
– Domestic tools industry
Because of its ability to withstand the corrosive action of various acids found in fruits, meats, milk, and vegetables, Type 304 is used for sinks, tabletops, coffee urns, stoves, refrigerators, milk and cream dispensers, and steam tables. It is also used in numerous other utensils such as cooking appliances, pots, pans, and flatware.
– Architectural paneling, railings & trim
– Chemical containers, including for transport
– Heat Exchangers
– Woven or welded screens for mining, quarrying & water filtration
– Dyeing industry
In the marine environment, because of it slightly higher strength and wear resistance than type 316 it is also used for nuts, bolts, screws, and other fasteners.
Stainless steel finishes
Standard mill finishes can be applied to flat rolled stainless steel directly by the rollers and by mechanical abrasives. Steel is first rolled to size and thickness and then annealed to change the properties of the final material. Any oxidation that forms on the surface (mill scale) is removed by pickling, and a passivation layer is created on the surface. A final finish can then be applied to achieve the desired aesthetic appearance.
No. 0: Hot rolled, annealed, thicker plates
No. 1: Hot rolled, annealed and passivated
No. 2D: Cold rolled, annealed, pickled and passivated
No. 2B: Same as above with additional pass-through highly polished rollers
No. 2BA: Bright annealed (BA or 2R) same as above then bright annealed under
oxygen-free atmospheric condition
No. 3: Coarse abrasive finish applied mechanically
No. 4: Brushed finish
No. 5: Satin finish
No. 6: Matte finish (brushed but smoother than #4)
No. 7: Reflective finish
No. 8: Mirror finish
No. 9: Bead blast finish
No. 10: Heat colored finish-wide range of electropolished and heat colored surfaces