Stainless Steel Passivation
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Stainless steel passivation is a method of treating the material so as to make it less affected by environmental factors such as water and air (the stainless steel is made more “passive”). Though stainless steels are naturally corrosion-resistant, they are not completely invulnerable to rust, which may appear on stainless steel through a process called “rouging.” In rouging, small spots on the surface of the stainless steel begin to rust due to embedded bits of foreign matter, grinding swarf, etc., or grain boundaries that allow water molecules to oxidize iron in these areas despite the alloying chromium in the material. Some grades of stainless steel are more resistant to rouging than others—these materials often do not need to be passivated. To passivate stainless steel, a shielding outer layer a material such as metal oxide is applied as a micro-coating to protect against corrosion. The resulting layer A typical method of cleaning and restoring passivated stainless steel involves cleaning the surface of the material with sodium hydroxide and citric acid, followed by up to 20% nitric acid in water at 120°F. The cleaning agents and acids are then washed away with water. Passivated stainless steel is commonly the standard construction material for commercial airliners and other aerospace crafts.