Stainless steel: Steel that will not rust

“Stainless steel is a ‘steel that does not rust’” is a common misconception. Stainless steel is an alloy composed of iron, chromium, carbon, and many other elements, with iron as the major component element and chromium as the primary alloy element. Stainless steel does not rust easily because chromium forms a delicate chromium oxide protective film on the steel surface, acting like a bullet-proof vest. By blocking corrosive gases or liquids from corroding the interior under general atmospheric or moist environments, chromium protects the steel against corrosion. When this thin film is consistently damaged for some reason, oxygen atoms in the air or liquids will keep infiltrating into the metal or the iron atoms in the metal will keep releasing to form loosened iron oxides, known as brownish yellow rust.

Such a type of surface coating can be damaged in many ways, and the common ways of damage in daily life are as follows:

The stainless-steel surface is covered by dust containing other metal elements or attachment of particles of heterogeneous metals. In a moist environment, the condensate between these attachments and stainless steel or a micro cell formed by them trigger an electrochemical reaction to damage the protective film. This process is called electrochemical corrosion.

➤When organic juices (e.g. kitchen leftovers, soup, and sputum) attach on the stainless-steel surface, organic acid will form in the presence of water oxygen. After a longperiod of contact, this organic acid will corrode the metal surface.

➤Acidic, alkaline, or saline substances (e.g. detergents, lime water sputtering) attach on the stainless-steel surface and cause local corrosion.

➤Chemical corrosion caused by sulfuric acid, nitric acid, or acetic acid liquid points formed by contaminated air (e.g. containing high intensity of SOx, COx, NOx) come into contact with condensate.