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Burnishing

As a rule, burnishing produces an even, black to black-brown surface coating on objects made from ferrous materials (cast iron, unalloyed and alloyed steel) in order to give the workpieces an attractive appearance and increase their resistance to corrosion, while largely maintaining their dimensional accuracy.

The objects are dipped in a boiling (130-140 °C) alkali-oxidising solution. The process is carried out in the Rohde factory in Göttingen.

Characteristics and Composition of Burnishing
Preferred applications	To increase resistance to corrosion while preponderant maintaining dimensional stability in the steel machine parts
Approximate chemical composition	Iron (II-III)-oxide
Weight per unit area	approx. 4.5-6.5 g/m2
Inherent colour of the protective layer	brown-black – black, depends among other things on the quality of the material and surface treatment
Adhesion	very good, deformation stability
Resistance to corrosion, subsequent treatment	approx. 24-48 h in condensation water constant atmosphere; not recommendable for parts with high exposure to corrosive factors.If there is no additional lacquer or similar coating, the items are usually oiled as subsequent treatment.
See also DIN 50 938 for further details