Stainless Steel grades

Stainless steel is not a single material but the name for a family of corrosion resistant steels. Like many scientific discoveries the origins of stainless steel lies in a serendipitous accident. In 1913 Sheffield , England , Harry Brearley was investigating the development of new steel alloys for use in gun barrels. He noticed that some of his samples didn't rust and were difficult to etch. These alloys contained around 13% chromium.

The first application of these steels was in cutlery for which Sheffield subsequently became world famous. Simultaneous work in France led to the development of the first austenitic stainless steels.

Worldwide demand for stainless steel is increasing at a rate of about 5% per annum. Annual consumption is now well over 20 million tonnes and is rising in areas such as the construction industry and household appliances. New uses are being continuously found for the attractive appearance, corrosion resistance, low maintenance and strength of stainless steel. Stainless steel is more expensive than standard grades of steel but it has greater resistance to corrosion, needs low maintenance and has no need for painting or other protective coatings. These factors mean stainless steel can be more econom ically viable once service life and life-cycle costs are considered.

All stainless steel are iron-based alloys that contain a minimum of around 10.5% Chromium. The Chromium in the alloy forms a self-healing protective clear oxide layer. This oxide layer gives stainless steel their corrosion resistance. The self healing nature of the oxide layer means the corrosion resistance remains intact regardless of fabrication methods. Even if the material surface is cut or damaged, it will self heal and corrosion resistance will be maintained.

Conversely, normal carbon steels may be protected from corrosion by painting or other coatings like galvanising. Any modification of the surface exposes the underlying steel and corrosion can occur.

The corrosion of different grades of stainless steel will differ with various environments. Suitable grades will depend upon the service environment. Even trace amounts of some elements can markedly alter the corrosion resistance. Chlorides in particular can have an adverse effect on the corrosion resistance of stainless steel.