Blacking

Brunning is a surface process for producing a black protective layer on iron-containing surfaces. The aim of the bronification is primarily to protect against corrosion, whereby a decorative, antique appearance is frequently also achieved.
The Production method Breasting is defined in the standard DIN 50983 and is usually based on the immersion of the workpiece to be treated into an acidic or alkaline solution, the broaching agent. This process forms a black mixed oxide layer on the surface of the workpiece, which is also referred to as “stainless grate”. Black-oxideating is therefore not a coating, but a conversion of material.

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Technical Basics for Black-oxidemination

The method of brewing is not limited to the actual brewing of the workpiece, but also comprises a pretreatment and an aftertreatment.

Brushed metal rods from KNEISSLER Brüniertechnik GmbH

In the metalworking industry, the following procedure has proved successful:

Pretreatment:

Cleaning of the metal surface:
In order to achieve optimal results during bronification, the workpiece is first made of residues, fats,
dirt and other impurities freed.
In turn, the methods degreasing, rinsing, pickling and rinsing again are used.

Breathing:

Replacement of the broach salt solution:
While stirring continuously, the broaching salt is added to cold water,
the temperature rises sharply due to the exothermic process.
Browns/Fears:
The workpieces to be treated are immersed in frames, sieves or drums in the broaching bath.
The diving time is between 5 and 30 minutes depending on the material, application and workpiece.
The workpieces are removed from the bath as soon as they have assumed a deep black color.
Temperature control:
During the entire process, care must be taken that the broaching solution is always at the boiling point.
Restoration/Restarting:
After the brewing, the heating can be stopped and the bath can be shut down.
During restarting, heating power is first supplied in order to reach the boiling point.
In addition, further salt withdrawals may also be required.

Aftertreatment:

After bromination:
It is necessary first to remove the adhering salt residues by thorough rinsing.
Hot water is frequently used in combination with ultrasound.
Drying/corrosion protection:
To prevent corrosion immediately after rinsing, corrosion protection is applied to the surface.

Only a layer about 1 μm thick is formed during bromination, which is why the dimensional accuracy is not impaired. In addition to the method presented here, the rapid bromination can also be used, wherein a broaching solution is applied manually directly to the component.

Why Black-oxide – Applications and Industry

Even broaching layers have only weak corrosion protection, which can be significantly improved by applying fats or oils. The layer produced is resistant to a temperature of about 300° C. and is therefore suitable, for example, for use in machine and plant engineering.

The list of suitable components for brewing is long and includes, inter alia, the following tools and machine elements:

Drilling
Miller
clamping jaws
Springs
Flanges
Fittings
Screws
spindles
washers
Control elements
Drilling chuck sleeves

In addition, the method is also very popular in the optical processing of objects. The black color of the uppermost layer makes it possible to achieve an attractive “antik effect”.

Which materials can be brewed?

In principle, brewing can be used in all oxidizable materials (e.g. steel, stainless steel, iron, brass, copper, bronze) or in casting. Rust-resistant steel or non-ferrous metals cannot form a mixed oxide layer and are therefore not suitable for the process.

Surface treatment with broaching system at KNEISSLER Brüniertechnik GmbH

Advantages of the Manufacturing Process

In comparison to other surface processes, the bromination is distinguished by a number of advantages:

Corrosion protection:
The component is protected from corrosion by Black-oxideing.
The initial protection is comparatively low, but can be improved by fats or oils.

Activity:
Since the resulting broaching layer is very thin (1 μm), the dimensional accuracy is hardly impaired.
Responsibility:
Brought layers are largely resistant to bending and abrasion and, in addition, up to 300 °C temperature resistant.
Optical upgrade:
Black-oxide is accompanied by a characteristic “black color” of the component.
This gives the workpiece an antique appearance.

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