Black oxide coating is a widely used metal finishing process that helps protect ferrous metals from corrosion while also improving their overall performance. Ferrous metals are known for their exceptional strength and hardness. However, they are equally well known for their tendency to rust, due to the oxidation of iron when exposed to oxygen and water.
Because rusty metal leads to weak and potentially dangerous structures, it is important to prevent oxidation of ferrous materials—by any means possible. One of the most effective ways to do this is by applying a black oxide layer, helping to improve corrosion resistance and extend the functional lifespan of metallic components.
For ferrous metals and other materials like copper and zinc, black oxide finishes are the most effective. Sometimes called blackening or blacking, black oxide coating is a type of chemical conversion coating, which means it actually alters the surface of the metal rather than adding an extra layer on top of it.
This article goes over the basics of black oxide coating, looking at why it is so important in manufacturing with steels. We cover its main advantages and disadvantages, and the ideal situations in which to deploy it.
What Is Black Oxide?
Black oxide coating is a type of conversion coating used to improve corrosion protection, minimize light reflection, and improve the appearance of metal. Corrosion resistance prolongs the lifespan of metal products; reflection minimization can improve the usability of items like surgical tools; and the attractive, matte black appearance of black oxide can increase the value of various products.
The process of blackening has existed for centuries. It is achieves a higher level of corrosion resistance than the older but comparable process of “bluing,” which is still commonly used in firearms production.
Black oxide coating is principally applied to ferrous metals, including:
- Steel
- Carbon steel
- Cast iron
However, slight modifications allow the process to be used on stainless steel (which behaves differently to most ferrous metals), as well as non-ferrous metals like:
- Copper and its alloys
- Zinc
As with other conversion coatings, black oxide coating is a chemical treatment that converts the surface of the metal into an adhering thin protective layer. This is achieved via a chemical reaction between the metal’s iron atoms and oxidizing salts within a chemical bath.
For ferrous metals, the black oxide coating that forms is iron(II,III) oxide (Fe₃O₄), which occurs in nature as the mineral magnetite. This oxide has other industrial uses besides corrosion-resistant coating: for example, it is widely used as a black pigment under the name “Mars Black.”
The oxide differs depending on the substrate material. For example, the black oxide for copper is copper(II) oxide or cupric oxide, which occurs in nature as tenorite.
In the metalworking industry, the copper black oxide process is known by the trade name Ebonol C. The equivalent for zinc is known as Ebonol Z. Patents for both processes were originally held by chemical company Enthone, now trading as MacDermid Enthone, a subsidiary of Florida-headquartered Element Solutions.
How the Black Oxide Process Works？
At its core, black oxide coating is a chemical conversion process. The main ingredient in the process is a blackening solution containing oxidizing salts—special salts that function as electron acceptors and that decompose to release oxygen.
Step-by-Step Process Overview
The step-by-step black oxide coating process below shows how typical blackening works. However, variants of the process are explained in subsequent sections.
- Cleaning: The metal parts are thoroughly cleaned to remove dirt, grease, and other contaminants, enabling a consistent, defect-free coating across the surface of the part.
- Rinsing: The parts are rinsed to remove any traces of cleaning agents that could interfere with the chemical reaction.
- Immersion: The clean metal parts are submerged within the blackening solution containing oxidizing salts as well as sodium hydroxide, nitrates, and nitrites.
- Temperature control: A steady temperature—very hot, fairly hot, or room temperature, depending on the process—is maintained while the parts soak, typically for a period ranging from a few minutes to an hour, depending on the degree of blackening required.
- Reaction: The oxidizing agents in the solution react with iron atoms on the surface of the metal, forming the black oxide.
- Rinsing: The oxidized metal parts are removed from the solution and rinsed thoroughly to end the chemical reaction.
- Sealing: A sealant containing oil or wax is applied to the finished parts, sinking through the gaps in the porous oxidized layer and adding an essential extra degree of corrosion resistance. This step should be repeated at regular intervals over the part’s life cycle.
Process Variants by Temperature
The most important variant in the black oxide coating process is the temperature of the blackening solution.
Hot black oxide coating or hot blackening uses a high-temperature chemical bath, resulting in a more consistent and durable finish. Cold or room-temperature blackening uses a cold black oxide solution and is easier to perform in a non-industrial environment.
Hot Black Oxide Coating
The hot black oxide process is the most common variant. It involves soaking the metal parts in a boiling solution containing the oxidizing salts and other ingredients. The temperature of the solution is maintained at around 138–143 °C (280–290 ºF), which allows for the most effective blackening without overheating (which would lead to red rust).
Because the parts must be moved quickly between stages at high temperatures, this process is usually automated in a factory setting. This allows machines to subject the parts to cleaning, rinsing, blackening, further rinsing, and sealing without them having to cool down.
Mid-Temperature Black Oxide Coating
Mid-temperature black oxide coating involves heating the blackening solution to a lower temperature of 90–120 °C (194–248 °F). Though this results in a lower degree of corrosion resistance, it has the benefit of not producing caustic fumes and therefore requires fewer safety precautions.
Cold Black Oxide Coating
Cold or room-temperature black oxide coating requires low temperatures of 20–30 °C (68–86 °F). In order for this to work, the solution is quite different to the regular black oxide mix of salts, nitrates, and nitrites, instead containing selenium compounds like selenium dioxide and other chemicals.
The main advantage of cold blackening is that it can be carried out in non-industrial environments while producing a comparable level of corrosion resistance. But it is easier to use but provides only mild corrosion resistance compared to hot and mid-temperature methods. One disadvantage is that the oxide surface is more easily rubbed or scratched away, making it unsuitable for certain mechanical parts.
Types of Black Oxide by Material
Black oxide for steel is more common than blacking for other metals and their alloys. However, black oxide can form on a number of different material substrates, including some non-ferrous metals. For some metals, the process is quite different to the primary process. For example, stainless steel black oxide requires caustic and sulfur salts.
The table below shows the different black oxide coatings that can be formed on different metals, including notes on process variations and outcomes.
| Metal | Black Oxide | Best Process | Trade Name | Notes |
| Carbon steel, alloy steel, cast iron | Iron(II,III) oxide / magnetite (Fe₃O₄) | Hot black oxide coating | Excellent finish and corrosion resistance; maintains dimensional accuracy | |
| Stainless steel (200, 300, 400 alloys; precipitation-hardened 17-4 PH) | Iron(II,III) oxide / magnetite (Fe₃O₄) | Specialized mid-temperature stainless black oxide process; caustic and sulfur salts | Reduces surface reflection; good corrosion resistance | |
| Copper, brass, bronze | Copper(II) oxide / cupric oxide / tenorite (CuO) | Cold / room-temperature black oxide coating | Ebonol C | Excellent matte black or satin finish; good corrosion resistance |
| Zinc | Zinc oxide / zincite (ZnO) | Specialized cold black oxide coating at 72–82 °C (160–180 °F) | Ebonol Z | Jet black finish; fair corrosion resistance |
| Aluminum | N/A | Black anodizing: electrolytic passivation combined with black dye | Not a black oxide process; excellent corrosion and wear resistance |
Benefits of Black Oxide Coating
Adding a black coating on steel or another metal does more than change its appearance. Black coated steel exhibits a number of important physical, chemical, and thermal properties that can improve the function of coated parts. Some black oxide benefits include:
- Corrosion resistance: The newly formed layer of black oxide acts as a protective coating, blocking air and moisture and preventing the underlying metal from rusting. Adding a protective oil or wax seal is important for maintaining corrosion resistance.
- Surface finish: Black oxide coating can produce a range of finishes, but the appearance is typically a sleek matte black.
- Reflection reduction: The matte finish reduces reflection and glare, which is beneficial for parts like stainless steel surgical instruments, where glare can affect a surgeon’s ability to operate with precision.
- Dimensional stability: The conversion coating makes only a small change to the dimensions of the part, typically just one or two micrometers. This means it can be used on parts where tight tolerances are specified.
- Abrasion resistance: Blackening can provide a certain level of abrasion resistance, extending the lifespan of parts.
- Anti-galling: Black oxide coating can reduce friction and prevent wear between sliding parts.
- Maintains electrical conductivity: The thin coating does not affect electrical conductivity in metals like copper and zinc.
- Heat dissipation: Blackened steel is better at dissipating heat than uncoated steel.
- Easy to apply: While hot black oxide coating generally requires specialist equipment, room-temperature blackening can be carried out at home.
- Cost effective: Can cost significantly less than other surface finishing processes.
Limitations and Disadvantages
Despite its many benefits, black oxide coating has some drawbacks and limitations. These concern its material versatility and the limited extent of its benefits in certain situations. The main black oxide disadvantages are:
- Extra coating required: Unlike plating and some other processes, the additional step of adding the oil or wax seal is required to maintain corrosion resistance.
- Limited material compatibility: The true black oxide coating process is limited to ferrous metals, while its variants for non-ferrous metals are not quite as effective.
- Limited scratch resistance: Some black oxide coatings, particularly those applied at cold temperatures, can be easily chipped or scratched, limiting their long-term effectiveness.
- Process demands: Thorough cleaning of the parts and use of very high temperatures can make the process fairly labor-intensive.
Black Oxide vs Other Metal Finishes
Chemical blacking is not the only protective surface finishing process for ferrous metals like steel and cast iron. Some black oxide alternatives are listed in the table below, focusing on processes that are similarly suitable for ferrous metals. For a wider range of options for ferrous metals, non-ferrous metals, and other materials, browse our surface finishing services page.
| Thickness (µm) | Corrosion Resistance | Appearance | Cost |
| Black Oxide | 1–2 | Moderate | Matte/satin black | Low |
| Phosphating | 5–15 | Moderate–good | Grey | Low–medium |
| Zinc Plating | 5–25 | Good | Bright/matte silver | Low–medium |
| Black Zinc Plating | 5–25 | Good | Matte/satin black | Medium |
| Electroless Nickel Plating | 5–25 | Very good–excellent | Bright/matte nickel | Medium–high |
| Powder Coating | 50–150 | Very good | Choice of color and texture | Medium |
Common Applications
Blackened steel parts can be found across a number of industries. The process is efficient for large batches of small parts, and its benefits—both functional and aesthetic—are desirable in many sectors. Some popular black oxide applications include:
- Hardware: In terms of sheer volume, one of the top applications of black oxide coating is for metal fasteners like screws, nuts, and bolts, as well as hardware like hinges.
- Medical: The black oxide matte finish is ideal for medical devices like surgical instruments, as the corrosion resistance and reflection reduction can make these instruments more practical and long-lasting.
- Firearms: Blackening and the simpler process of bluing have been an essential part of firearms production for centuries, helping to extend their lifespan.
- Optical: In devices like cameras, minimally reflective hardware can improve functional performance while also maintaining dimensional stability in miniaturized parts.
- Electronic: Black oxide does not reduce electrical conductivity, making it useful for copper parts and other elements of electronic systems.
- Automotive:For automotive parts, stable matte finishes and dimensional accuracy are essential.
Conclusion: Is Black Oxide Right for Your Application?
A blackened metal finish can improve the look and corrosion resistance of many parts, whether they are made from steel or certain other metals. Although material compatibility is fairly narrow, the low cost and functional benefits of metal black oxide coating are significant, prolonging the lifespan of parts while giving them a model, sleek appearance.
Of course, blacking is not the only option for metal parts. Expert guidance from a specialist like 3ERP can help you decide whether your parts are best suited to black oxide, or an alternative process like metal plating or powder coating. The answer will depend on your base material and the application of the parts themselves.
For your steel blackening needs and beyond, request a quote from 3ERP.
Frequently Asked Questions (FAQ)
What is black oxide coating?
Black oxide coating is a chemical conversion process that forms a protective layer over metal parts to prevent corrosion resistance and provide other benefits. It is mostly used on ferrous metals, but workarounds exist for other materials.
How long does black oxide last?
A black oxide coating applied professionally can last for years. However, certain factors can reduce its lifespan, namely the temperature of the blackening process, the material itself, the level of moisture in the environment, and the level of general wear and tear.
Is black oxide coating expensive?
No, compared to some other surface finishing procedures, professional blackening is an affordable process. Furthermore, at-home black oxide coating kits are available for cold black oxide processes.
Is black oxide coating bad for the environment?
Steel blackening is relatively environmentally friendly, as it does not require harsh chemicals (except for certain metals). It is fairly easy to safely dispose of the used chemical solution after the process has been carried out.
Does black oxide add thickness to parts?
Compared to other protective surface finishing treatments, black oxide makes a negligible different to part thickness, rarely more than a couple of micrometers. This is one of its main advantages.
Is black oxide safe for food or medical use?
Black oxide is not safe for food products. However, it is safe for many medical uses, including surgical instruments.
Does black oxide rust?
Although one of its main purposes is to prevent corrosion, black oxide can still rust over time, particularly if it is not sealed with an oil or wax coating. Regular reapplication of this coating is required to maintain the corrosion resistance of the black oxide.
Is black oxide a conversion coating?
Yes, black oxide is a conversion coating. For steel parts, the surface layer is turned into magnetite. For other metals, a different oxide is produced.
Can I do black oxide coating at home?
Room-temperature black oxide coating can be carried out in non-industrial environments. Premade solutions are available from many suppliers, and the waste materials can be disposed of safely. However, bear in mind that results from a professional provider of hot black oxide coating services will achieve superior results.
