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Letter 39030
Definition of blackening stainless steel
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Is the process by SS is darkened to a black or grey called "black
oxide", or is it called other things, such as "acid-washed"? Thank
you.
Chris Lawrence
job supervision - Boston, MA
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Black oxide is just that - chemical conversion of IRON to the
black oxide form of rust (as opposed to the all to familiar red
form). It has an appealling black color, fairly well adherent to the
basis metal, and adsorbs post treat preservatives such as oil quite
well. This should be considered versus a black etch smut that forms
upon immersion of certain stainless steels to various acid media such
as hot sulfuric or hot muriatic (hydrochloric). This black smut is
non-adherent and non-uniform in appearance. They are obvioulsy not
the same!
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Black oxide conversion coatings for steel are formed in alkaline
solutions containing strong oxidizers (nitrite/nitrate).
The chemical reaction for the formation of magnetite, black oxide, is
as follows: The iron for the blackening process is contributed by the
base metal. Byproducts of the blackening reaction are colloidal iron
and carbonates, which are removed from the bath by desludging and
skimming.
Black oxide coatings do not change the dimensions of the parts
blackened. The coating will not chip or peel and acts as an absorbent
to hold oils or waxes.
The black oxide bath is simple to operate. A rolling boil should be
maintained at 150°C with additions of water or blackening salts.
Cleaned parts are immersed from 5-20 min in the bath and then rinsed
thoroughly. Finally, a rust preventive coating is applied.
The black oxide coating itself does not afford much corrosion
protection (only 1-2 hr salt spray exposure); although the rust
preventive top coat is absorbed by the black oxide and can achieve up
to 100 hr of salt spray exposure depending on the product used.
V. G. Rajendran
- Trichy, Tamilnadu, India
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