Burnishing is a procedure where steel parts get immerged into a hot strongly alkaline solution until they get a dark brown or black colour.

This discoloration renders a thin layer of Fe3O4. There is no particular corrosion protection applied, it just serves visual purposes.

Since black passivation on zinc has been developed we can offer an optically even more appealing surface with better corrosion protection at the same time. We can meet the demanded trueness-to-dimension in slight-interference-fits with extremely thin zinc layers within the range of 2-3µm.

Hence we don't offer burnishing any more.

Zinc
Phosphate