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The NaOH (caustic soda) and elevated temperature cause Fe 3 O 4 (black oxide) to form on the surface of the metal instead of Fe 2 O 3 (red oxide; rust). While it is physically denser than red oxide, the fresh black oxide is porous, so oil is then applied as post treatment to the heated part, which seals it by "sinking" into it.
Tannic acid chemically converts the reddish iron oxides into bluish-black ferric tannate, a more stable material. [2] The second active ingredient is an organic solvent such as 2-butoxyethanol (ethylene glycol monobutyl ether, trade name butyl cellosolve) that acts as a wetting agent and provides a protective primer layer in conjunction with an ...
Mill scale is a complex oxide that contains around 70% iron with traces of nonferrous metals and alkaline compounds. Reduced iron powder may be obtained by conversion of mill scale into a single highest oxide i.e. hematite (Fe 2 O 3) followed by reduction with hydrogen.
Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic 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. [1]
An additional step can be added to the nitrocarburizing process called post-oxidation. When properly performed, post-oxidation creates a layer of black oxide (Fe 3 O 4), that greatly increases the corrosion resistance of the treated substrate while leaving an aesthetically attractive black color. [25]
Heat-shrinkable sleeves are applied onto the cutback at the field weld or "field joint" during the construction of a pipeline. As described above, the heat-shrinkable sleeves have an adhesive that sticks the sleeve to the cutback and the factory applied mainline coating and also acts as a corrosion protective layer.