Black oxidiging of steel

1. Characteristics and properties

Burnished layers are deep black and very even in colour. The thoroughly cleaned workpieces are dipped into the burnishing solution at temperatures above 130°C, in which a chemical reaction takes place immediately on the surface of the workpiece. A burnishing layer consists mainly of various iron oxides and has a layer thickness of approx. 1µm, whereby the dimensional accuracy of the workpieces remains virtually unaffected, as these are conversion layers. The ferromagnetic properties of the base substrate are practically unaffected. As this is a chemical process, the workpieces are also uniformly coated in bores and recesses, except for blind holes and recesses with a diameter of less than 1 mm.

The layers produced are very ductile, non-slip and temperature resistant up to approx. 300°C without discolouring.

The corrosion protection of a burnished steel part is temporary, but can be considerably improved by subsequent oiling, greasing or waxing of the workpieces. Burnished workpieces are resistant to alkaline media – even in concentrated form. It is also resistant to most organic solvents. They have limited resistance to weakly acidic media, but are dissolved by strongly acidic media.  Burnishing layers are standardised according to DIN 50938.

2. Benefits and areas of application

The burnishing is suitable for both decorative and technical applications. In technical applications it is mainly used in machine and tool construction. Especially in technical applications, burnishing is used for temporary corrosion protection in order to increase the shelf life of workpieces. In the decorative sector, burnishing is used in a wide variety of applications, e.g. for screws, springs, buttons or weapon parts.

3. Basic material

All types of steel can be provided with this treatment.

4. Required delivery condition

The goods provided must be largely metallically bright. Residual oils or residual impurities must be removable with conventional aqueous alkaline degreasers. Oils containing silicone, alkali/alkaline earth and high-pressure additives must be avoided at all costs.