The simple answer is 304 contains 18% chromium and 8% nickel while 316 contains 16% chromium, 10% nickel and 2% molybdenum. The molybdenum is added to help resist corrosion to chlorides (like sea water and de-icing salts).
Firstly though, lets define Stainless Steel: Also known as inox steel from the French “inoxydable” is a steel alloy with a minimum of 10.5 % chromium. The chromium helps the alloy to resist staining and corrosion. The main point to make here is that it helps resist corrosion, it does not prevent it. Perhaps we could say “A highly corrosion-resistant grade of steel”
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) accelerates corrosion by forming more iron oxide. 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.
Passivation only occurs if the mixture of chromium is high enough.
Other alloying elements are added to enhance the structure and properties such as formability, strength and cryogenic toughness. These include metals such as:
Non-metal additions are also made, the main ones being:
A2 Stainless Steel
A2 stainless steel is often referred to as 304 or 18/8 Stainless. 18/8 actually refers to the amount of chromium and nickel in the alloy – 18% chromium and 8% nickel.
A2 (304, 18/8) is an austenitic steel and is non-magnetic. The chromium provides a corrosion and oxidation resistance, however it can tarnish. It is immune to foodstuffs, sterilizing solutions, most organic chemicals and dyestuffs, also a wide variety of inorganic chemicals. As such it is used extensively for sinks, tabletops, stoves, refrigerators , pots, pans dairy equipment, brewing industry, fruit industry, food processing plants, dye tanks, pipelines, and more.
What is Type 304 stainless steel and what’s it used for?
Type 304, with its chromium-nickel content and low carbon, is the most versatile and widely used of the austenitic stainless steels. Its alloys are all modifications of the 18% chromium, 8% nickel austenitic alloy. Type 304 proves to be resistant to oxidation, corrosion, and durability. All provide ease of fabrication and cleaning, prevention of product contamination offer a variety of finishes and appearances.
A4 Stainless Steel
A4 Stainless is often referred to as 316 or 18/10 stainless. As in A2 above, the numbers 18/10 refer to the chromium and nickel content- 18% chromium and 10% nickel.
A4 grade then is also austenitic, non-magnetic and suitable for all the situations as A2 BUT has the added advantage of being suitable for marine solutions. Often called Marine Grade stainless steel. The molybdenum increases the corrosion resistance to withstand attack from many industrial chemicals and solvents and of course, chlorides. Used in the production of inks, photographic chemicals, surgical implants, and the marine environment.
What is Type 316 stainless steel and what’s it used for?
Type 316 stainless steel is an austenitic chromium-nickel stainless and heat-resisting steel with superior corrosion resistance as compared to other chromium-nickel steels when exposed to many types of chemical corrodents such as sea water, brine solutions, and the like. Since Type 316 stainless steel alloy contains molybdenum bearing it has a greater resistance to chemical attack than 304. Type 316 is durable, easy-to-fabricate, clean, weld and finish. It is considerably more resistant to solutions of sulfuric acid, chlorides, bromides, iodides and fatty acids at high temperature.
Both A2 and A4 grades are classed as Austenitic stainless steel (sometimes called the 300 series). This means that they have high chemical resistance but cannot be hardened by heat treatment. They can be hardened by cold working.
Both A2 and A4 stainless steels are mostly non-magnetic. Some magnetism can be induced by the cold working process and also by welding.