Black Oxide Coating: All You Need to Know
Black oxide coating is a conversion coating applied to a lot of metal surfaces and/or components. If you want to learn more about black oxide, how it works, and what the different types of black oxide are, this guide can provide you with valuable information.
What is Black Oxide?
Black oxide is also known as magnetite, and it forms when the ferrous metal is subjected to oxidizing chemicals. There are multiple advantages of having this thin layer of coating which is used in multiple applications.
It is a kind of surface finish that is used to enhance the aesthetics and durability of a machined component. It differs from other types of surface finishing by having a very thin layer which does not disrupt the dimensional requirements of the component.
How Black Oxide Works
Black oxide, being a chemical conversion process, is opted for ferrous metal surfaces. The steps involved in applying the black oxide coating are given below:
- The process starts by cleaning the parts which are subjected to black oxide coating. Contamination like oil, dust, grease, or rust is removed by cleaning the surface. During this cleaning, an alkaline detergent may be used.
- The cleaned components are then immersed in the blackening solution. This solution contains oxidizing salts, sodium hydroxide, nitrites, and other chemicals. The process temperature range is around 285-305°F.
- The chemicals in the solution and the iron on the surface react and create magnetite. The amount of blackening depends on the time for which the part is immersed in the solution.
- After removing the parts from the solution, they are rinsed with water.
- Oil or sealing compound is applied to the metal coated with black oxide so that its durability and resistance against corrosion are enhanced.
Benefits Of Black Oxide Coating
Black oxide coating offers multiple benefits, which are given as follows:
- It improves the corrosion resistance of ferrous metals, which protects them from oxidation and rusting.
- It provides a uniform and matte black finish to the component making it suitable for functional, as well as decorative applications.
- Black oxide coating even adds wear resistance, enhanced hardness, and low friction to the metal surface and tends to increase the lifespan of the components.
- This type of coating is applied in millionths of inches, which means that no excessive thickness is added to the components, and the parts have tight tolerance and high precision.
- It is more cost-effective when compared to other types of coating options like powder coating and electroplating.
3 Key Types of Black Oxide Coating
There are 3 types of black oxide coating, and the process of each is discussed below:
Hot Black Oxide Process
The hot black finish is a widely opted type of black oxide coating. In this process, the component is dipped in the salt bath at a high temperature. There are different salt baths used, such as nitrates, nitrites, and caustic soda.
The process consists of dipping the parts from one bath to another with the help of automated transporters. The temperature maintained during the process ranges between 275 to 295°F.
The temperature range of the black oxide coating process is very crucial and is to be maintained. In case the temperature is low, the process will take more time. If the temperature is high, the component will rust.
The hot black oxide can be applied by using iron chloride as well, in which the component is dipped in iron chloride and then in hot water, but it is suitable for steel surfaces only.
Mid-Temperature Black Oxide Process
The process of mid-temperature black oxide is the same as that of hot black oxide; however, the temperature range is different.
The temperature during this process ranges between 194 to 248°F. This temperature converts the metal into magnetite, but it doesn't produce caustic fumes, which is usually a problem with the hot black oxide process.
Cold Black Oxide Process
The cold black oxide process is also called room temperature black oxide, and it consists of depositing copper selenide on the surface of the metal component.
The temperature range in this process is around 68 to 86°F. This process does not offer as much wear resistance as the other two processes. However, if further secondary finishing is done, wear resistance improves.
Uses and Applications for Black Oxide Parts
Black oxide parts are used in different applications, such as:
Medical: Black oxide components offer light-absorbing properties, which make them suitable for medical applications. They can be used in X-ray machines if they are light-sensitive.
Automotive: Most of the automotive components are coated with black oxide finishing like spark plugs, suspension bushings, brake valve components, etc.
Electronic: Electrical components like gears for timers, wire strippers, and cutters are also coated with black oxide.
Military: Military applications, mid-temperature black oxide is used to coat shotgun shell magazines and military humvees.
Construction: Assembly tools like gauges, fasteners, and bearings are also coated with black oxide.
Conclusion
Black oxide coating adds an enhanced finish to the parts, whether you need to coat large parts or small parts.
At DEK, we offer black oxide coating services that can enhance the performance and durability of your metal parts using various techniques. Contact us now!
FAQs
Does black oxide rust?
Black oxide does not rust. It is a conversion coating applied on metal surfaces to enhance its properties without adding much thickness.
What were the early uses of black oxide coating?
The early uses of black oxide consisted of enhancing the wear resistance and appearance of the tools and machinery.
What’s the difference between black oxide and other surface finishes?
Black oxide retains the dimensions of the metal component, whereas other types of coating add a thick layer of protection, which alters the dimensions.
What materials are commonly coated with black oxide?
Black oxide is mostly applied to steel and iron as they are ferrous metals. It can also be applied on non-ferrous materials like copper and stainless steel but requires some other processes.
