Letter 39030

Definition of blackening stainless steel [Massachusetts] 

January 17, 2006

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

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!

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

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, Team Leader Manufacturing,
- Surface Treatment specialist - Trichy, Tamilnadu, India

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