Bluing (steel)

<p>Bluing, sometimes spelled as blueing, is a <a href="/facts/Passivation_(chemistry)/tPADJj5G">passivation</a> process in which <a href="/facts/Steel/aN6qWo4H">steel</a> is partially protected against <a href="/facts/Rust/sajUmaz8">rust</a> using a <a href="/facts/Black_oxide/nLx3A1Bv">black oxide</a> coating. It is named after the blue-black appearance of the resulting protective finish. Bluing involves an electrochemical <a href="/facts/Conversion_coating/Bz85cGHd">conversion coating</a> resulting from an oxidizing chemical reaction with iron on the surface selectively forming <a href="/facts/Magnetite/SVxoGUs2">magnetite</a> (Fe3O4), the black <a href="/facts/Iron_oxide/KDo8hLWn">oxide of iron</a>. In comparison, <a href="/facts/Rust/sajUmaz8">rust</a>, the <a href="/facts/Iron(III)_oxide/vam0XoC5">red oxide of iron</a> (Fe2O3), undergoes an extremely large volume change upon hydration; as a result, the oxide easily flakes off, causing the typical reddish rusting away of iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and <a href="/facts/Galvanic_corrosion/MRDFNBJm">galvanic</a> action. In colloquial use, thin coatings of black oxide are often termed "gun bluing", while heavier coatings are termed "black oxide". Both refer to the same chemical process for providing true gun bluing.
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Bluing (steel) open-in-new

Bluing (steel)