Blackening

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Blackening

 

a type of oxide coating for iron and steel products, used for corrosion protection and for decorative purposes. The most common method of blackening is to immerse the products in a concentrated alkaline solution containing sodium nitrate or sodium nitrite as an oxidizer. As the concentration of the alkali is increased (other conditions being equal), the thickness of the oxide film also increases. A thin coating (20 to 40 angstroms) does not alter the external appearance of the surface nor does it provide practical protection for the metal against corrosion. Thick coatings (2.5 micrometers), which are black or grayish black, have poor mechanical strength. The best thickness is 0.6 to 0.8 micrometer. Blackening is a special case of blueing.

References in periodicals archive ?
Pointing to a photograph of her own feet-washing/blackening in the 1940s, Millie explained to me how shocked she was at the combination of products used to blacken her granddaughter:
344), the ancients were exploiting them to blacken hair and probably wool.
England's skies blacken as coal makes the industrial revolution possible.
The groups offered a reward of $2,201 to anyone who would blacken Rushdie's face.