Stainless Steel
In many situations galvanic protection from painting a mild steel surface is impractical. This is where the special oxide film formed on Chromium alloyed stainless steels become useful.
Many types of stainless steels have been developed to resist different corrosion environments and working conditions for example ensuring that workings are safe, things last longer and our food is hygienic. Stainless steel is even used to clean up the exhaust gases from cars and power stations.
Stainless steel is also recyclable: when scrapped, it can be re-melted and reused.
Stainless steels are steel alloys containing chromium. The minimum chromium content of the standardised stainless steels is 10.5 %. Chromium makes the steel 'stainless' -- this means improved corrosion resistance. The better corrosion resistance is due to a chromium oxide film that is formed on the steel surface. This extremely thin layer, under the right conditions, is also self-repairing.
Besides chromium, typical alloying elements are molybdenum, nickel and nitrogen. Nickel is mostly alloyed to improve the formability and ductility of stainless steel. Alloying these elements brings out different crystal structures to give different properties in machining, forming, welding etc.
The major types of stainless steel are:
- Austenitic stainless steels, which can be hardened by cold working, meet a wide range of design criteria. They are essentially non-magnetic although they may become slightly magnetic due to cold working.
- Martensitic stainless steels are straight-chromium types that can be hardened by heat treatment. They are magnetic.
- Ferritic stainless steels, like martensitic stainless steels, are straight-chromium types and they are magnetic. Ferritic stainless steel, however, cannot be hardened by heat treating and only moderately hardened by cold working.