Steel Bolt

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Steel Bolt
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Contrary to popular belief, it is possible for stainless steel to corrode (rust). Stainless steels are deemed corrosion resistant due to the high percentage of chromium – normally 16 to 36% Cr. The higher the Chromium content of the steel, the less likely it is to rust. Chromium is primarily responsible for the formation of an oxide layer that prevents corrosion, when the steel is exposed to oxidizing media such as acids, water, or air for example. The chromium forms a thin, tightly adhering and impervious oxide layer (predominantly CrO) that prevents rust from forming.

However, it is still possible for rust to form on the surface of stainless steel if a condition develops in which the metal molecules at the steel’s surface are not sufficiently alloyed with its chromium content, meaning the necessary oxide layer is unable to form or be maintained. The simplest condition under which rusting can occur on stainless steel is when a piece of ordinary steel is rubbed against the surface of an otherwise corrosion-resistant piece of stainless steel. In such a condition, the iron from the ordinary steel will rub off onto the stainless steel surface as a film of unalloyed steel. Once such a film is exposed to atmospheric moisture this unalloyed steel film will form rust. This happens because the unalloyed steel film on the stainless steel has little or no chromium, so the film transforms to ordinary ‘red rust’ – as seen on our cross section. Interestingly, once this shallow surface film of unalloyed steel has been oxidized, the corrosion process stops. The chromium in the stainless steel under the film of rust forms a suitable corrosion-resistant oxide layer, corrosion is halted once the surface film of unalloyed steel turns to rust. This can be seen by observing the rust-free inside of our steel bolt.

Sample ID: 148


Donated by
King's College London
Bolt | Corrosion | Metal | Rust | Shiny | Stainless Steel

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