Everything you need to know about black oxide finish
If you are looking for a sleek, modern look for your metal hardware, you may want to consider using a Black Oxide coating. This process creates a smooth, dark finish that can make your hardware stand out.
This blog post will discuss the benefits of Black Oxide , how it is applied, and what to expect from the finished product. We will also answer some common questions about Black Oxide coating so that you can decide if it is the right choice for your project!
What is Black Oxide?
Black oxidation treatment is a common means of chemical surface treatment, and the principle is to produce a layer of oxide film on the metal surface to isolate the air and achieve the purpose of rust prevention. Appearance requirements are not high when you can use black treatment, steel parts of the black surface treatment, also known as blue. Black oxidation treatment is a surface coating treatment method commonly used by military cutting tools because it can eliminate reflection.
After the Black Oxide process is complete, the metal hardware will have a smooth, dark finish. This finish is typically applied to steel or brass hardware and can be used to create a sleek, modern look.
Therefore, It base copper material, we can also call brass a copper alloy.
The zinc content of brass alloy helps increase the flexibility and strength of brass-based copper materials. The higher the zinc content of brass, the more elastic the brass alloys. In addition, the color can vary from red to even yellow depending on how much zinc is added.
How does it works?
The Black Oxide process is an immersion process where metal parts are placed into a solution of hot Black Oxide and water. An electrolytic reaction takes place that converts the surface of the metal to Black conversion coating.
The Black Oxide process works by chemically reacting with the surface of the metal to create a black oxide film. This film is very thin and does not alter the dimensions of the metal hardware. The Black Oxide is applied using a black oxide solution and can be applied to most metals, including steel, brass, and aluminum.
Black oxidation Process
The Black Oxide process is simple and does not require any special equipment. The Black Oxide solution can be applied with a brush, dipped, or sprayed onto the metal surface. After the Black Oxide solution is applied, it must be heated to a temperature of between 300-400 degrees Fahrenheit to activate the chemical reaction.
- Workpiece clamping:
Black oxide treatment is easy to operate, the workpiece needs to be fixed on the workbench or hanging in the Black oxide tank. According to the shape of the workpiece, size, and design, special fixture or spreader. The purpose is to keep enough gap between the workpiece; the workpiece can not contact each other to make each workpiece can be completely immersed in the oxidation liquid oxidation.
- Deoiling:
The purpose is to remove oil from the surface of the workpiece. After machining (blue, black is the last process), the surface of the workpiece is inevitably not left with oil, rust prevention oil for interprocess rust prevention is even more so. Any oil pollution will seriously affect the formation of ferric tetroxide, so must be removed before blue and black.
- Acid pickling:
The purpose of pickling is to remove rust on the workpiece surface. Because rust, rust spots, prevent the formation of a dense layer of ferric tetroxide. Even if the workpiece has no rust, it should also be pickling because it makes the oil further removed clean. Pickling will improve the activation energy of the molecules on the surface of the workpiece, which is conducive to the oxidation of the next process and can generate a thick layer of ferric tetroxide.
- Oxidization:
Oxidation is the main process of bluing and blackening. Whether the film is dense, smooth, and has sufficient thickness depends on the oxidation stage.
The process temperature for bluing is 550℃. The process temperature for blackening is 130 ~ 145℃. The immersion time ranges from 50 to 80 minutes. High carbon steel with high carbon content, oxidation speed is faster, and immersion time can be shorter. Low carbon steel with low carbon content, the oxidation rate is slow, and immersion time needs to be longer. Alloy steel, especially high alloy steel, the workpiece surface has certain residual austenite, has a strong resistance to alkali solution, not easy to generate Fe3O4. Hence, the immersion time needs to be longer. Table 2 lists the temperature and immersion time of the solution for the oxidation (blackening) of the steel.
- Heating:
After the Black oxide solution is applied, it must be heated to a temperature of between 300-400 degrees Fahrenheit to activate the chemical reaction.
- Saponification:
The purpose is to form a thin film of ferric stearate to improve the corrosion protection of the workpiece. Commonly used saponification solution concentration is 30 ~ 50g (soap) /L. The saponification solution was heated to 80 ~ 100℃, and the oxidized workpiece was immersed in the saponification solution for about 10min.
- Checking Quality:
After oxidation, the workpiece should be checked to see whether the Fe3O4 film is qualified. The method of inspection is: arbitrary extraction of three pieces of work, placed in the concentration of 2% copper sulfate solution to soak for the 20S, do not return the black is qualified. If more than one of the three pieces of work is not qualified, the whole groove workpiece is considered unqualified and needs to be oxidized again to deepen the thickness of the iron TEtroxide.
Why choose Black Oxide?
There are many benefits to using Black Oxide finishes. Black Oxide finishes are durable, corrosion resistant, and heat resistant. Black Oxide finishes are also less expensive than other blacking surface finishes. The following are all of the Benefits of Black Oxide Finishes.
① Black oxide has a wide range of uses, including everything from medical instruments to aerospace components.
② Black oxide is a safe and environmentally friendly process that does not produce hazardous waste products.
③ Black oxide is an economical finishing option that can be applied to a variety of metals.
④ Black oxide provides excellent corrosion resistance and abrasion resistance.
⑤ Black oxide is a highly versatile finish that can be applied to complex shapes and difficult-to-reach areas.
Defects and solutions of black oxidation
In the process of metal parts blackening, some defects may appear as follows.
Loose oxide film:
The oxide film should be very dense and firmly bound to the base. This is not a normal phenomenon if the oxide film is loose and easy to erase. This situation is mainly caused by the lack of iron ions in the newly prepared oxidation solution. The solution is to add some old oxidation solution or add some clean steel shavings and boil 30min to increase the iron concentration of the oxidation solution; this phenomenon will be eliminated.
The surface of the workpiece is reddish brown rust:
The oxide film should be very dense and firmly bound to the base. This is not a normal phenomenon if the oxide film is loose and easy to erase. This situation is mainly caused by the lack of iron ions in the newly prepared oxidation solution. The solution is to add some old oxidation solution or add some clean steel shavings and boil 30min to increase the iron concentration of the oxidation solution; this phenomenon will be eliminated.
The surface of the workpiece is light gray:
The surface of the workpiece loses black luster and is light gray, which is caused by too low a concentration of sodium hydroxide in the oxidation solution. In the solution, add the right amount of sodium hydrogen hydride.
No oxide film is generated:
After 40 ~ 50min of treatment, the surface of the workpiece is still not colored (the formation of oxide film), which is mainly due to the solution temperature being too low, and sodium nitrite concentration being insufficient. The solution is to raise the solution temperature and increase the amount of sodium nitrite appropriately.
Oxide film surface flower:
The workpiece appears sporadic white spots on the black matrix, and sometimes the white spots are denser. This phenomenon is called “hair flower” by workers. The main reason for the flower is not enough oxidation time. The solution is to extend the oxidation time (i.e., soaking time) and add a small amount of sodium hydroxide.
The workpiece surface is green:
Suppose the oxide film on the surface of the workpiece is not black but green. In that case, this is the result of the peroxidation of the workpiece, which is caused by the high temperature of the oxidation solution and the high concentration of sodium nitrite. The SOLUTION IS to add an appropriate amount of cold water (NOTE: when adding water, must wear protective gear, slowly add, so as not to splash the tank, causing burns) and add a small amount of sodium hydroxide to reduce the solution temperature and the concentration of sodium nitrite.
Types of Black Oxidizing Process
Many different surface treatment processes are currently used to achieve black oxide coatings on metal surfaces. However, there are three main categories of oxide black. They are hot, mid-temperature and cold blackening processes. Each category has a unique formulation for a proprietary coating to complete the surface treatment. Here, we will discuss these two categories in detail.
Hot Black Oxide Coating Process
The Hot Black Oxide coating is achieved by heating the metal hardware in a black oxide solution. This process creates a smooth, dark finish that is typically used on steel or brass hardware. The Hot Black Oxide coating is not recommended for aluminum hardware as it may cause the aluminum to discolor.
The stages of the hot black oxide finish process are as follows:
① Cleaning: The metal hardware is cleaned using a degreaser or solvent to remove any dirt, oil, or other contaminants.
② Water rinsing: The metal hardware is rinsed with water to remove residual degreaser or solvent.
② Heating: The metal hardware is heated in a black oxide solution until it reaches the desired temperature.
③ Quenching: The metal hardware is quenched in water to stop the Black Oxide reaction.
④ Passivating: The metal hardware is placed in a passivating solution which creates the black oxide film on the surface of the metal.
Mid temperature black oxide:
Mid-temperature black oxide is the most popular among commercial metal finishers because it can be performed at temperatures lower than hot black oxide, which reduces energy costs. The mid-temperature process also produces a more consistent and durable finish.
Cold Black Oxide Process
The Cold Black Oxide finish is achieved by immersing the metal hardware in a black oxide solution. This process creates a smooth, dark finish that is typically used on steel or brass hardware. The Cold Black Oxide coating can also be used on aluminum hardware without causing discoloration.
In addition, black oxidation is also divided into iron black oxidation, nickel black oxidation and copper black oxidation.
Other Blacking Surface Finishes
Other blacking surface finishes are available, but Black Oxidizing is the most common. Some of the other blacking surface finishes include Black Anodize, Black Nitride, Black Plating, and Black Phosphate.
Black Anodize:
Black Anodize is an electrochemical process that creates a durable oxide coating on the surface of metals. and the appearance surface is matte finish, Black Anodize is a good choice for applications where Black Oxidizing would be too expensive.
Black Nitride:
Black Nitride is a process that creates a durable nitride coating on the surface of metals. Black Nitride is a good choice for applications where Black Oxide would be too expensive.
Black Plating:
Black Plating is a process that creates a thin layer of Black Oxide on the surface of metals. Black Plating is a good choice for applications where Black Oxide would be too expensive.
Black Phosphate:
Black Phosphate is a process that creates a thin layer of Black Oxide on the surface of metals. Black Phosphate is a good choice for applications where Black Oxide would be too expensive.
What material parts can be finished in Black Oxide surface finishes？
The Black Oxide finish can be applied to most metals, including steel, brass, and aluminum. It is important to note that the Black Oxide coating is not recommended for use on zinc or lead as it may cause these metals to discolor. Additionally, the Black Oxide finish should not be used on copper or bronze as it may cause the metal to corrode.
While this metal surface finishing process may occur in a low-tech environment through trial and error, it is important to have some equipment on hand. For example, manufacturers that tend to produce parts in large quantities will need:
Blast furnaces
Temperature control systems
Salt bath
Mechanized cleaning and rinsing facilities
Automation has also gone a long way in helping manufacturers produce large quantities of black oxidized coated components. Also, it is important to remember that black oxide finishes are made with treatments that work with oils, waxes, lacquers, and acrylics. This further improves the corrosion resistance of the part.
Common Applications for Black Oxide Finish
- Automotive
- Electrical
- Aerospace
- Firearms
- General Hardware
Which Black Oxide Finish is Right for My Application?
The type of Black Oxide finish that is right for your application depends on the requirements of your application. If you need a durable finish that can withstand high temperatures and mild corrosion resistance, Black Iron Oxide is a good choice. Black Nickel Oxide is a good choice if you need a finish that is more resistant to wear and provides better corrosion resistance. If you need the best corrosion resistance available, Copper Black Oxide is a good choice.
No matter which type of Black Oxide finish you choose, Black Oxide is a good choice for applications where other blacking surface finishes would be too expensive. Black Oxide provides a durable finish that can withstand high temperatures and is resistant to corrosion. Black Oxide is also a good choice for applications where Black Nickel Oxide or Black Copper Oxide would be too expensive.
FAQs
Q: What is the difference between hot and cold black oxide finishes?
A: The main difference between hot and cold black oxide finishes is the temperature at which the finish is applied. Hot black oxide finishes are applied at temperatures around 1200 degrees Fahrenheit, while cold black oxide finishes are applied at room temperature.
Q: Is black oxide finish durable?
A: The Black Oxide finish is not as durable as other finishes, but it can last many years if properly cared for.
Q: Is the black oxide finish cheaper or expensive?
A: The Black Oxide finish is a relatively inexpensive metal finishing option.
Q: Is the black oxide finish harmful?
A: The Black Oxide finish is not considered harmful. However, following the manufacturer’s instructions when applying the finish is important to avoid inhaling fumes.
Q: What are the drawbacks of black oxidizing?
A: The drawback of black oxide finishing is that it is not as durable as other finishes and can be difficult to remove. Additionally, a black oxide finish is not recommended for use in humid environments.
Q: How long does black oxide finish last?
A: The Black Oxide finish can last many years if properly cared for. However, the finish is not as durable as other finishes and may need to be reapplied more often. Additionally, the finish can be damaged by exposure to humidity or chemicals.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.