Stainless steel is made stainless by adding chrome to the steel
High oxidation resistance in air at ambient temperature is normally achieved with addition of a minimum of 13% (by weight) chromium, and up to 26% is used for harsh environments.[4] The chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, and the metal remains lustrous and smooth. The layer is impervious to water and air, protecting the metal beneath, and this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is seen in other metals, such as aluminium and titanium. Corrosion resistance can be adversely affected if the component is used in a non-oxygenated environment, a typical example being underwater keel bolts buried in timber.When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off, allowing the parts to weld together. When forcibly disassembled, the welded material may be torn and pitted, an effect known as galling. This destructive galling can be avoided by the use of dissimilar materials for the parts forced together, for example bronze and stainless steel, or even different types of stainless steels (martensitic against austenitic). However, two different alloys electrically connected in a humid environment may act as a voltaic pile and corrode faster. Nitronic alloys made by selective alloying with manganese and nitrogen may have a reduced tendency to gall. Additionally, threaded joints may be lubricated to prevent galling. Low-temperature carburizing is another option that virtually eliminates galling and allows the use of similar materials without the risk of corrosion and the need for lubrication.
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