If your company has parts that it wants blackened or if you create metal sculptures and want them blackened before being sold, they can be sent to another company or you can do it in your workshop. There are three blacking processes: hot, mid-temperature, and cold metal blacking. They are chemical processes that darken the metal so that parts don’t give off glare, to give sculptures or cabinet hardware an antique appearance, or to give engine parts a distinctive look.
Hot Metal Blacking
Ferrous metals are blackened during the hot metal blacking process by bathing them in hot chemical baths containing sodium hydroxide, nitrates, and nitrites. These chemicals convert metal surfaces into magnetite. To avoid creating a steam explosion while the metals are being bathed, water is periodically added to the hot bath.
After being converted to magnetite, the metals are blackened when they are dipped into tanks containing alkaline cleaner, water, and caustic soda at a temperature of 140.5°C, which is the blackening compound. Afterwards, a sealant, which is usually oil, is applied to prevent corrosion.
With mid-temperature blackening, metal surfaces are also converted to magnetite but the temperatures they are bathed at are far cooler at 104°C to 108°C. The main advantage to the mid-temperature black oxide process is that it doesn’t produce caustic fumes. This metal blackening process meets some military specifications such as MIL-DTL-13924 and AMS 2485.
Cold Metal Blacking
The cold metal blacking process is done at room temperature, which makes it ideal for factories or workshops. Instead of having a conversion coating, the blackening is done by depositing a copper selenium compound on metal surfaces. Although it produces a similar colour to the hot blacking process, it can be rubbed off and it isn’t as abrasion-resistant.
To seal the coating applied during the cold metal blacking process, then oil, wax, or lacquer is applied as it provides almost the same corrosion resistance as hot blackening. The cold metal blacking process is used for tooling and to apply a patina to steel parts used in architectural designs.
If your company decides to do their own cold metal blacking in house, there are kits that provide all the necessary chemicals. There are also kits to give metals patinas so that they have aged or antique-looking finishes. The kits can be used for industrial purposes or by hobbyists who work with metal to make art pieces or weaponry such as swords or knives.
However, if the parts that you produce need a more durable finish, you can contract with companies that specialise in hot metal blacking. By sending your metal to blacking specialists, you won’t have to worry about having caustic chemicals in your factory. Also, you won’t have to make space for the process and hire trained personnel who know how to do it.
Sending your parts, hardware, and other metal products to be blackened will help your business save money while giving your products a durable finish that meets customers’ specifications for the parts that they order from your company.