Bluing (steel)

Browning is the formation of a weak protective layer on the ferrous surfaces in order to reduce corrosion. By immersing the workpieces in acid or alkaline solutions (eg sodium hydroxide ) or molten salt to form black mixed oxide FeO and Fe2O3 ( patina ). The bluing is not a coating.

Due to the low thickness of the conversion layer of about 1 micron, the burnished workpieces remain largely true to size. Because of the porosity of the Brünierschicht they have a little corrosion protection, but can be significantly improved by oiling or greasing. These layers are largely resistant to bending and abrasion-resistant and temperature resistant up to 300 ° C. The field of application is in the machine and tool. Furthermore, the bluing is used as a primer for further surface treatments such as painting. A special application is the browning of handguns.

The method is standardized in Germany in DIN 50938. There is also a distinction between one-, two -, and Dreibadbrünieren.

In addition to protecting the bluing often, for example, an "antique " appearance to produce, as in furniture fittings and screws made of brass is also used for visual enhancement of objects. There, the burnishing with vinegar (acetic acid) can be done.

Many methods for blackening of iron are complicated, toxic or flammable.

Blackening

A much easier realizable method used especially in the past is the burning of linseed oil on the iron surface. For this, the metal is heated between hot coals and then quenched with linseed oil. Alternatively, the oil applied first and the article are then heated. Usually multiple passes are required in order to achieve a good result. Also suitable is old engine oil. The temperature should be about 400 ° C ( steel brown) to 700 ° C (dark red ) amount. The oil smokes then during application from visible.