Black chromating (colloquially also referred to as “black zinc coating”) is a process in electroplating technology where an already galvanized surface is further treated with a special dark passivation. This chemical post-treatment creates a deep black or anthracite-colored chromate layer, which enhances corrosion protection and gives the surface a distinctive dark appearance.
How Does Black Chromating Work?
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Zinc Layer as a Basis
At the beginning, the metal (e.g., steel) is galvanized (either electroplated or hot-dip galvanized), so that a zinc layer is present as a basic protective coating.
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Dark Passivation
After galvanizing, the passivation/chromating process is carried out using a special solution, which, due to its composition, results in a black coloration. Traditionally, chromate solutions containing chromium(VI) were used for this purpose, but these are now heavily regulated due to health and environmental laws. Modern chromium(III)-based or other chromium(VI)-free processes have since become standard and provide a very dark finish.
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Reaction and Layer Formation
In the passivation solution, the existing zinc layer reacts with certain chemicals, forming a conversion layer (chromate layer). This layer is black in color and provides additional protection against corrosion.
Advantages and Applications
- Distinctive Appearance: The black surface is popular for design or camouflage reasons (e.g., military applications).
- Enhanced Corrosion Protection: The chromate layer shields the underlying zinc from oxidation, thereby extending its lifespan.
- Versatile Applications: Black-chromated components are used in the automotive industry, for screws and fasteners, in electronics, for DIY applications, and for decorative purposes.
Chromium(VI)-Based vs. Chromium(VI)-Free Systems
- Chromium(VI)-Containing Processes: Provided an intensely black coloration, but are heavily restricted or banned due to environmental and health concerns (e.g., REACH regulation).
- Chromium(VI)-Free/Chromium(III)-Based Systems: Offer a dark coloration, are more environmentally and health friendly, and are continuously developed to match earlier methods in terms of color intensity and corrosion protection.
Limitations of Black Chromating
- Susceptibility to Wear: Mechanical stress (e.g., friction, scratches) can wear down the dark chromate layer.
- Precision in Process Control: To ensure a uniformly black result, baths and process parameters (temperature, time, chemicals) must be precisely controlled.
- Dependence on Zinc Quality: A well-prepared zinc layer is essential because the black tint will only adhere properly if the zinc is clean and evenly applied.
Conclusion
Black chromating, often also called “black zinc coating,” is a method of dark passivation for galvanized surfaces. The result is a deep black or dark appearance along with an additional boost in corrosion protection. In chromium(VI)-free variants, black chromating today represents a more environmentally and health-friendly way of achieving an appealing, black surface.
