Chemical Properties of Stainless Steel

Stainless steel is a metal alloy which is preferred for making kitchen utensils, because it does not affect the flavor of food. The surfaces of stainless steel utensils are easy to clean. Minimal maintenance and total recycling of stainless steel utensils also contribute to their popularity. Stainless steel is the universal name for a metal alloy, that is made up of chromium and iron. It is called stainless as it is highly resistant to stains (rusting).

Chemical Properties of Stainless Steel

Pure iron is the main element of stainless steel. Pure iron is prone to rusting and is highly unstable, as it is extracted from iron ore. Rusting of iron is due to its reaction with oxygen, in the presence of water. Chromium is a metal that stalls the oxidization of iron. Chromium forms a transparent and passive layer of chromium oxide, which prevents mechanical and chemical damage. The other minor constituents of steel are nickel, nitrogen and molybdenum. Small contents of nickel increase the corrosion resistance further, and protect stainless steel from rough usage and harsh environmental conditions. Pitting or scarring is avoided by adding molybdenum to steel.
Chemical properties and structure of the stainless steel is improved using other alloys. Titanium, vanadium and copper are the alloys which make stainless steel more suitable for specific uses. Not only metals, but also non-metals like nitrogen, carbon and silicon are used to make stainless steel.

Classes Depending on Chemical Properties of Stainless Steel

The five classes of stainless steel are austenitic, martensitic, duplex, ferritic and precipitation-hardening. The chemical composition of these five classes are as follows:

Austenitic Stainless Steel: This steel is called austenitic because it is made from austenitizing elements. Iron, nickel and chromium are the basic austenitizing constituents of this type of stainless steel. This steel has high ductility and relatively high tensile strength. Approximately, 16-26% chromium and less than 35% nickel are the typical contents of this steel. Applications are:

• Petrochemical industries
• Food processing industries
• Kitchen sinks
• Chemical plants

Martensitic Stainless Steel: This steel is called martensitic as it possesses a martensitic crystal structure in hardened condition. Chromium and carbon are the main contents of martensitic stainless steel. This type of stainless steel are less resistant to corrosion. Around 18% chromium and 1% carbon make the martensitic stainless steel. Applications are:

• Surgical instruments
• Knives and blades
• Shafts and spindles

Ferritic stainless steel: This type of stainless steel is ferromagnetic in nature. This steel has relatively good ductility and is usually used to make kitchen utensils. Approximately 30% chromium is present along with iron. Applications are:

• Automotive exhaust
• Automotive trims
• Computer floppy disk hubs

Duplex stainless steel: This type of steel is used in chloride and sulphide environments and is least corrosive. It is a mixture of ferritic and austenitic stainless steel. Primary constituents are chromium and nickel. Applications of duplex stainless steel are

• Oil and gas explorations and off-shore rigs
• Chemical processing, transport and storage
• Pulp and paper manufacturing

Precipitation hardening stainless steel: This type of steel is made from chromium and nickel. Precipitation hardening stainless steel is made from annealed martensitic or annealed austenitic stainless steel. Annealing of stainless steel is the process of heating steel to change the chemical and physical properties of stainless steel.

Chemical properties are responsible for the corrosion resistance and mechanical structures of stainless steel which are important for selecting the perfect grade for the required application. Stainless steel has a basic property of corrosion-resistance. The factors affecting this property are chemical composition of the corrosive medium, chemical composition of metals used, temperature and temperature variations, oxygen content and aeration of the medium. Thus, slight variations in chemical compositions can be used to make wide variety of stainless steel. Stainless steel is 100% recyclable and is therefore environment friendly.