Black oxide coating is one of several surface treatments used in machining. Surface treatments are crucial in precision machining because they can increase the lifespan and aesthetics of machined parts. However, a problem with many surface treatments is that they tend to increase the dimensions of the finished precision machine parts. This negatively affects the functionality of these parts, as precise dimensions are required to fit correctly.
However, in this aspect, the black oxide coating stands out compared to other metal surfaces. This is due to its ability to transfer the good properties of surface coatings to machined parts with minimal dimensional expansion. In this article, we explain the black oxide coating process, the types of black oxide coatings, their applications, and everything else you need to know about this surface coating.
What is black oxide coating?
Black oxide coating is a type of coating that belongs to the conversion coatings category. A conversion coating is a type of coating that uses a chemical process to create the coating. The name “black oxide” comes from the fact that the coating turns the color of any metal surface black/matte.
During black oxidation, the processed metal parts are immersed in an alkaline solution. The solution then converts the iron on the metal's surface into a compound called magnetite. This magnetite forms a black layer on the surface of the metal. Metals that can be oxidized black include stainless steel, metal powder, copper, silver solder, zinc, etc.
How is a black oxide coating applied?
The reverse oxide coating process generally includes five steps:
- cleaning
- To wash
- Descaling/pickling
- Black oxide finish
- After graduation
This section explains each of these steps in detail.
cleaning
This process prepares the part for the final black oxide coating. Machinists degrease and clean the part with an alkaline solution.
To wash
In this process, the alkaline solution used for cleaning is washed with distilled water.
Descaling/pickling
This step is only necessary if the parts have rust or scale. All rust and oxide stains are removed from finished parts by soaking them in acid.
Neutralizing
This process is only necessary if the finished parts have blind holes or are an assembly of parts. The piece is then washed with an alkaline solution. This prevents bleeding that could affect the surface of the part.
Demetallization
This conditional condition only occurs if the finished part is coated with another material (e.g. chromium, zinc, cadmium, etc.). The machinist must remove the part because the black oxide layer cannot adhere to the coated parts.
Black oxide finish
This is where the driver finally applies the finishing touches. The machinist immerses the finished metal parts in containers containing various alkaline solutions, such as sodium hydroxide, nitrates and/or nitrates. At this stage there are three different temperatures that determine the coating material on the surface of the part.
At high temperatures (141°C) and low temperatures (90 – 120°C), the process leaves a layer of magnetite (Fe2O3) on the part. However, at low operating temperatures, the process leaves a layer of copper selenium (Cu2Se) on the surface of the part.
post-processing
Without post-treatment, the finished part is still very susceptible to corrosion. However, this rule does not apply to black stainless steel or brass surfaces. This is because these metals have inherent anti-corrosive properties. However, for other metal parts, post-treatment is very important. The usual post-treatment agents are oil, wax and varnish.
Oil is the most commonly used finishing agent by machinists. It guarantees a shiny surface and also improves lubricating and anti-corrosion properties. Wax provides a more opaque surface, but has even better lubricating and anti-corrosive properties. Lacquer is also a good choice, especially if you need to finish the pieces quickly, as it dries very quickly.
Types of Black Oxide Finishes
As mentioned previously, there are different operating temperatures at which machinists perform the bluing process. These temperatures also distinguish between different types of bluing. That includes:
Warm black oxide finish
The warm black finish is the most popular type of black oxide finish. The piece is immersed in salt baths at high temperatures. The different salt baths used include caustic soda (NaOH), nitrate and/or nitrite salt baths.
Automated conveyors move parts from one bath to another to fully influence the process. The temperature range of this process is typically around 275°F to 295°F.
It is important that the temperature remains within this range during the black oxide coating process. This is because lower temperatures prolong the process, while higher temperatures would lead to rust on the surface of the finished piece.
The combination of the salts and the increased temperature of the fins creates magnetite (Fe2O3) instead of rust (another iron compound).
Another method used sparingly to apply a hot black oxide coating is to use ferric chloride. However, it only works on steel surfaces. The machinist repeatedly dips the steel part into a bath of ferric chloride and then into a bath of hot water.
Medium Temperature Black Oxide
This process is similar to hot black oxidation and differs only in operating temperature. The operating temperature here is between 90 °C and 120 °C. At this temperature the metal can also be converted into magnetite. However, this process has the advantage over hot black oxidation in that no corrosive fumes are produced during the process.
cold black oxide
This process is also called room temperature black oxide and does not follow the same method as the other types. During this process, copper selenide is deposited on the metal surface at 20 to 30 °C. The result of this process is a black oxide surface that is not as wear-resistant as others. However, once you add the post-treatment, it looks just as good.
Black Oxide Coating Applications for Precision Engineering Projects
Due to the minimal dimensional expansion this surface provides for precision machined parts, it is no surprise that it is popular across many industries. Other properties such as light absorption properties, wear resistance, etc. make it an excellent surface despite its low price. In this section we compile some applications of the black oxide surface:
Medical applications
Due to the light-absorbing properties of black oxide surface parts, they can be used in light-sensitive medical instruments such as X-ray machines.
Electronic parts
Wire strippers, timer gears, cutters, etc. are examples of black oxide surface electrical components.
Auto parts
Many auto parts also use black oxide surfaces. You will find them in oil filter cans, suspension bushings, spark plugs, brake valve components, etc.
Military parts
Medium temperature black oxide coating is a common coating for military applications. It is widely used in military vehicle turrets, rifle cartridge magazines, etc.
Assembly Tools
Precision machine tools used in assembly manufacturing also often have a black oxide finish. Tools such as bearings, fasteners and gauges are good examples.
Factors to Consider When Using a Black Oxide Finish
Before you decide to use black oxide coating on a precision machined part, you need to consider several parameters. These factors will help decide if black oxide coating is a good choice for the part. Examples of these factors are:
Part application
For which industry did you produce the parts? Are they intended for use in light-absorbing environments, such as the medical industry, or in abrasion-resistant environments, such as the military? The intended use of your precision parts determines whether black oxide is a suitable surface for the machined part.
Location of use of the part
The location where the piece will be used is also crucial if you choose black oxide as a finish. In general, surface finishes last longer when used indoors. The same applies to black oxide. Due to the thin layer, external conditions such as humidity and sustained high temperatures can cause faster wear of the black oxide.
The desired final look
The final appearance of a piece is also important, especially when the aesthetics of the piece play an important role. After applying the black oxide coating, you can achieve two final appearances: matte or glossy. The type of finishing treatment chosen will determine the final appearance of a black oxide piece. Therefore, limiting the final appearance of black oxide parts is an important parameter to consider.
Suggested shelf life
The length of time you plan to use precision machined parts is also critical to selecting the best surface finish. A black oxide finish can last a long time due to its rust-preventing properties. However, this factor depends on other factors such as the application and location of the part.
Common Black Oxide Coating Defects and Solutions
As with any other surface finishing process, various defects frequently occur in bluing processes. Most of these problems have a known cause and solution. Below we highlight some common defects in the bluing processes and their solutions:
Gray surfaces
This happens when the black oxide coating has a gray tint instead of the expected black color. This often occurs when there are temperature fluctuations during the black oxide coating process.
Solution
To fix this, make sure the temperature of the salt baths is at a predetermined level and kept constant throughout the process.
Brown mops
This is characterized by the precision machined part emitting a brown (rusty) color when rubbed against the surface. Large amounts of colloidal iron in the salt bath or the use of higher temperatures than necessary are common causes of this problem.
Solution
If you notice this problem, replace the salt baths in use and ensure that the salt bath temperature remains at the specified level. This should completely resolve the issue.
Discontinuities in black oxide coating
This occurs when the black oxide layer does not cover several areas of the finished part. The cause of this is usually contamination of the surface of the metal part.
Solution
To resolve this issue, you will need to subject the piece to the cleaning process again. After
After cleaning the part properly, you can repeat the black oxide coating process.
Black Oxide Coating Performance Tests
Various performance tests can serve as a benchmark for the success of a black oxide coating. However, these tests would increase the production time of precision parts and incur additional costs. Below are two of the most popular black oxide coating performance tests:
Dirt Test
This process is a physical test that determines how well the coating adheres to the metal surface. It also helps determine if there is any black residue on the metal surface. The machinist simply rubs his hand across the metal surface before applying the finish. If the coating is removed, there is a problem with the black oxide coating process. Otherwise, the process is considered successful.
Relative Humidity Test
This is a corrosion resistance test of the black oxide part. The test examines the water resistance of the black oxide part when used at 100% relative humidity.
Advantages and Disadvantages of Black Oxide Coating
Like most other surfaces, black oxide has properties that make it an excellent surface for a variety of applications. However, it may also have deficiencies in some areas. Below are some of the pros and cons of the black oxide finish.
Per
- The benefits of black oxide coating include:
- This finish is one of the most affordable surface finishes available in precision machining. Compared to other processes such as electroplating, it is much cheaper.
- It only slightly increases the dimensions of the part, which is a big advantage. This makes black oxide a suitable surface for materials used in precision applications.
- Due to the versatility of the surface, parts coated with it can be used in the most diverse sectors.
- The post-treatment guarantees lubricating properties and thus facilitates the connection of finished parts to their counterparts.
- Finally, you can oxidize the black surface and thus ensure an even more beautiful aesthetic.
Disadvantages
Disadvantages of the black oxide coating process include:
- Continuous use of black oxide parts in hot locations can cause the surface to wear easily and rust.
- Compared to other surface treatments, black oxide coating is a weak surface treatment.
Choose a black oxide finish for your precision machined parts
Using surface finishes on precision machined parts is an important method for improving the quality of these parts. Black oxide finishing is one of several options and is popular due to its affordable application cost and wide range of applications. While this finish is great for machined parts, it requires machining experience to perform correctly.
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Common questions
What type of black oxide coating is best?
The best method is black oxide coating at high temperatures. It gives the best finish, but caustic gases are released during the process.
Is black oxide harmful?
Yes, hot bluing can be dangerous due to the high temperatures and corrosive vapors released.
What materials are commonly used for black oxide treatment?
Examples of materials commonly used in black oxide treatment are caustic soda and nitrate baths, the acid bath, alkaline rinse solution, and post-treatment.
Does black oxide increase the dimensions of parts?
Yes, this is true, but only to an extremely small extent. The increase is usually between 5 and 10 millionths of an inch.
