Alloys are a combination of various metals which then help the resulting metal combination to add to the physical and other properties of the resulting combination. Stainless steel is one such alloy in which iron is combined with a minimum of 10.5 % of chromium.
Under normal usage, iron forms a layer of ferric oxide on the surface of the item made of iron, to form rust that can further erode and help in its degradation. The addition of chromium allows the alloy so formed to add a thin layer of chromium oxide on the surface of the item, to act as a passive layer that helps in preventing any further corrosion. Varying the amount of chromium can help in adding to resistance to corrosion. Stainless steel also has a content of carbon, manganese, and silicon, which help to give it better physical properties. Other useful properties can be given to the stainless steel through the addition of other elements like molybdenum and nickel.
Stainless steel is under normal conditions free from stains, but harsh environments can still affect it. Under normal environments or water-based conditions, stainless steel will not corrode and this has led to its extensive use in cutlery, work surfaces, sinks and many other items where corrosion can present hygiene and other problems. Where conditions are more aggressive, the alloys of stainless steel can be modified to resist these conditions.
There are stainless steels of many types, and austenitic steels are those that are the most common. These steels are easy to form and weld, giving them a versatility that allows for wider use. They can have their microstructures formed because of the manganese, nitrogen, and nickel that is added to the alloys. The corrosion resistance of these austenitic steels is enhanced with the addition of molybdenum and chromium. They are normally non-magnetic and resistant to hardening by heat treatment. Ferritic stainless steel has low amounts of carbon, cannot be hardened by heat treatment, but has a high resistance to stress corrosion cracking. It is magnetic and generally used in thin sections that can be formed and do not need to be welded. Martensitic stainless steel is similar to the ferritic type but has higher carbon content that allows it to be hardened and tempered. This steel offers high strength though corrosion resistance is moderate. It is magnetic.
Other forms of stainless steel can combine the properties of ferritic and austenitic steels to form various varieties of duplex steel. High strength stainless steels are formed by the addition of elements like aluminium, niobium, and copper to the alloy. This makes for steel with very fine particles that greatly add to the strength of the steel. These steels, called the PH or precipitation hardening type of stainless steel can even be machined to good tolerances and further given ageing treatment that can cause the minimum of distortion.
Decisions on the type of steel that is required for any function or use will depend on various factors, and it is possible to engineer the alloy to accommodate any requirement of strength, resistance to corrosion, formability, weldability, and any other required properties. The corrosion present in the environment in which the steel item will be placed, the temperatures under which it is required to work or function, the required strength, the need for welding, forming requirements, and the product form that is required can all play a part in deciding the right stainless steel type. Aesthetics can also play a part when such stainless steel items are used for architectural purposes. Costs can vary and influence the choice of the alloy composition, but it is always advantageous to use life cycle costing to arrive at the right decision.