Types Of Stainless Steel: A2 vs A4 or 304 vs 316

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:

• Nickel
• Molybdenum
• Titanium
• Copper

Non-metal additions are also made, the main ones being:

• Carbon
• Nitrogen

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. 304 stainless steel is the most common form of stainless steel used around the world, due to its excellent corrosion resistance and value. It contains between 16 and 24 percent chromium and up to 35 percent nickel, as well as small amounts of carbon and manganese. 304 stainless steel does have one weakness: it is susceptible to corrosion from chloride solutions, or from saline environments like the coast. Chloride ions can create localized areas of corrosion, called “pitting,” which can spread beneath protective chromium barriers to compromise internal structures. Solutions with as little as 25 ppm of sodium chloride can begin to have a corrosive effect.

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. 

COMMON USES FOR 304 STAINLESS STEEL:

• Storage tanks
• Fasteners and finishing hardware (screws, nuts, bolts, plates, handles)
• Pots and pans
• Residential sinks and sink parts
• Indoor architectural/decorative hardware (panels, sculptures, sconces)
• Equipment tubing
• Residential appliances

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. 316 stainless steel is commonly used in many industrial applications involving processing chemicals, as well as high-saline environments such as coastal regions and outdoor areas where de-icing salts are common. Due to its non-reactive qualities, 316 stainless steel is also used in the manufacture of medical surgical instruments.

COMMON USES FOR 316 STAINLESS STEEL:

• Pharmaceutical manufacturing
• Chemical manufacturing
• Industrial and chemical transportation
• Pressure vessels
• Cisterns and pipes for chemical applications
• Medical equipment where non-surgical steel
• Marine equipment
• Outdoor site furnishings
• Commercial kitchens
• Food production and processing in saline environments
• Commercial appliances

Common Properties

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. Austenitic steels ensure a versatile balance of strength, workability, and corrosion resistance, making them ideal for outdoor architectural features, surgical instrumentation, and food processing equipment.

Both A2 and A4 stainless steels are mostly non-magnetic. Some magnetism can be induced by the cold working process and also by welding. The ultimate benefits of stainless steel include a long service life that will retain an attractive, clean finish. Properly cared for and cleaned stainless steels present a low maintenance cost.