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 classic literature ?
I think," said Father Brown, "he would blacken his face.
But this time she kept away too long, and stayed beyond the half-hour; so she had not time to take off her fine dress, and threw her fur mantle over it, and in her haste did not blacken herself all over with soot, but left one of her fingers white.
But the consequence is, that the whole profession in Middlemarch have set themselves tooth and nail against the Hospital, and not only refuse to cooperate themselves, but try to blacken the whole affair and hinder subscriptions.
When Rokoff commenced to blacken about the face Tarzan released his hold and shoved the fellow back into his chair.
The Fuller replied, "The arrangement is impossible as far as I am concerned, for whatever I should whiten, you would immediately blacken again with your charcoal.