General

Stainless steel, rust-resistant steel, rust-free steel, SS - all synonyms for the same thing but what exactly is stainless steel and what process is used to increase its corrosion resistance?

The precise definition of rust-resistant steel is “an iron alloy to which a minimum 10.5% chromium is added and which contains a maximum 1.2% carbon”. These figures represent the turning point at which steel starts to form a self-healing oxide layer (also called an oxide skin or passivation layer) which protects the underlying metal and greatly improves the corrosion resistance. The oxidation layer (the name says it all) forms spontaneously in contact with air (oxygen) and creates a layer which is passive and therefore more corrosion resistant. The correct technical term, therefore, is “rust-resistant” steel, not “rust-free”.

If the conditions damage the surface of the steel, the first thing to happen is that the iron alloy is released but without the oxide skin. The material then becomes more vulnerable but the oxide layer generally re-establishes itself spontaneously, once again in contact with air.

Of course there are very many different iron alloys that meet the definition of “rest-resistant steel”, some more corrosion resistant than others. Aside from iron, chromium and carbon plus materials such as nickel, molybdenum, titanium, etc. are also added. The most used alloy, SS 304, consists of 18% chromium and 8% nickel.

History

Stainless steel is an alloy that was only “discovered” at the beginning of the last century, when it was used in the arms industry. This particular alloy had better resistance to corrosion and also offered advantages such as wear and heat resistance, making it ideal for gun barrels. But the breakthrough into daily use only came after the Second World War. Its many advantages made it very popular in a short time.