Surface Finishes
Black Oxide Coating Services
Black oxide is utilized mainly for the black smooth appearance and for gently enhancing corrosion resistance. Black oxide fertilized with oil can offer the highest amount of rust resistance.
ECOREPRAP Black Oxide Coating Treatment Services
Black oxide treatment is one of the most commonly used surface finishes for carbon steel and alloy steel CNC parts. ECOREPRAP has rich production experience in carbon steels such as 1010, 1020, and 1045 CNC parts with black oxide finishing services.
As a renowned supplier of high-quality CNC OEM parts in China, we not only provide custom CNC parts but also offer over 20 types of surface treatments. Our one-stop solution for on-demand parts makes us the best choice for customers seeking both precision and versatility in their CNC components.
| Applicable Materials | Cosmetic Availability | Visual Appearance |
| Steel | NA | Smooth, matte black |
Black Oxide Parts
Learn Black Oxide Coating Definition, Process, Material Below.
Table of Contents
1. What is Black Oxide Coating?
Black oxide, also known as Black Oxide or Blackening, is a surface finishing process that forms a dense black oxide coating on the surface of metals through a chemical reaction.
This coating helps to improve the metal’s corrosion resistance, wear resistance, and enhances its overall appearance.
Essentially, black oxide is a controlled corrosion process that brings beneficial effects to the metal’s surface.
2. Which Materials Can Have Black Oxide Treatment?
Black oxide treatment is selective because it relies on the involvement of iron in the chemical reaction.
2.1. Can Steel Be Blackened?
Iron-based alloys are the most suitable materials for black oxide treatment.
Carbon steel and low alloy steel are the most commonly used and standard materials for this process.
For example, low carbon steel like 1018, 1020, Q235, medium carbon steel like 1045, and alloy steels such as , 4140, and 4340 can get the best results with a uniform, deep black color.
Cast iron can also be effectively blackened and is commonly used in engine parts, hardware, and other components.
2.2. Can Stainless Steel Be Blackened?
Stainless steel contains a high proportion of alloying elements like chromium and nickel, which naturally form a dense chromium oxide passivation layer on the surface. This passivation layer can hinder the reaction of traditional black oxide agents.
However, with special processes that break or penetrate the passivation layer, it is still possible to achieve the black oxide finish and form a black oxide layer.
The blackening of stainless steel mainly relies on two chemical methods: acidic chemical blackening and high-temperature oxidation. These methods generate black oxides or compounds by reacting with elements such as chromium and nickel on the surface of the stainless steel.
2.2.1. How Stainless Steel Black Oxide?
Acidic Chemical Blackening
Acidic chemical blackening is also known as low-temperature blackening.It is the most commonly used and mainstream method for blackening stainless steel.
The process involves immersing stainless steel parts into an acidic solution containing selenium salts (usually sulfuric acid or chromic acid) and performing a chemical reaction at room temperature or slightly elevated temperatures (but not high temperatures).
The chemicals in the solution react with elements like chromium and iron on the stainless steel surface to form black chromium selenide (Cr₂Se₃) or metal oxides. This black layer is a compound, rather than a simple oxide.
The key advantages of this method is the uniform black oxide layer it produces, which offers a visually appealing finish and strong adhesion to the base material, providing superior wear resistance compared to carbon steel blackening.
Additionally, the surface’s corrosion resistance can be significantly enhanced when sealed with oil or wax after treatment.
However, there are some drawbacks. The treatment solution often contains heavy metals like selenium and chromic acid, which raise environmental and safety concerns, requiring specialized waste disposal.
Furthermore, the cost of acidic chemical blackening tends to be higher compared to other methods.
High-Temperature Blackening
The process of high-Temperature blackening is that stainless steel parts are heated to very high temperatures (around 700°C) and placed in steam or a chemical environment for thermal oxidation.
At high temperatures, the elements on the stainless steel surface react with oxygen to form a thick black iron oxide layer.
This method creates a thick, durable black iron oxide layer that offers excellent protection and longevity.
However, the high temperature used in the process can cause undesirable side effects, such as annealing, which may lead to part deformation, softening, or reduced hardness.
2.2.2. Which Stainless Steels Can Be Blackened?
Austenitic stainless steel,ferritic stainless steel and martensitic stainless steel can be blackened.
Austenitic Stainless Steel (e.g., 304, 316)
These stainless steel grades are commonly used for acidic chemical blackening. They contain high levels of chromium and nickel, and although they form a passive layer on the surface, special blackening processes can still generate a black oxide layer.
Ferritic Stainless Steel (e.g., 430, 441, 444)
Ferritic stainless steels have lower alloying elements, making them more suitable for blackening.
Martensitic Stainless Steel (e.g., 410, 420, 440C)
Martensitic stainless steels, known for their high hardness and relatively low alloy content, can also undergo blackening treatment. The resulting black oxide layer helps improve corrosion resistance and wear resistance, making them suitable for high-strength applications.
2.2.3. Which Stainless Steels Can Not Be Blackened?
High-Alloy stainless steel can not be blackened.
High-alloy stainless steels like 904L, 2205, Super Duplex 2507 contain high proportions of nickel, molybdenum, and copper, which form a stable passive layer that prevents the formation of the black oxide layer. These alloys are generally unsuitable for standard blackening processes.
2.3. Can Copper Have Black Oxide?
Copper itself cannot undergo “blackening” in the traditional sense used for steel, as it does not contain iron.
However, copper can be oxidized in an alkaline solution to form a black or dark brown copper oxide film.
A more common method for blackening copper is the use of sulfurization (Liver of Sulfur), which is frequently applied for antiquing purposes in decorative items and crafts.
2.4. Can Zinc Have Black Oxide?
Die-cast zinc parts cannot undergo traditional blackening.
However, there is a specialized process known as “zinc blackening”, typically involving acid copper plating followed by blackening treatment.
This process results in a finish that resembles traditional blackening, giving the parts a blackened appearance.
2.5. Can Aluminum be Blackened?
Aluminum cannot undergo traditional blackening.
Instead, the black surface on aluminum is achieved through black anodizing, a completely different electrolytic process that produces a very hard and corrosion-resistant oxide layer on the surface.
For more information about anodizing, please visit our blog: Aluminum Alloy Anodizing-MIL-PRF-8625F Specifications and Standards Explained
Below is a material chart suitable for black oxide for reference.
| Material Type | Suitable for Traditional Blackening | Remarks |
| Carbon Steel, Alloy Steel | Highly Suitable | The most common and best-performing materials for traditional blackening. |
| Cast Iron | Highly Suitable | Provides good results and is widely used in blackening processes. |
| Stainless Steel | Requires Special Process | Requires acidic blackening or high-temperature oxidation, not a traditional method. |
| Copper and Copper Alloys | Requires Special Process | Uses oxidation or sulfurization, which are different from traditional blackening methods. |
| Zinc and Zinc Alloys | Requires Special Process | There is a specialized zinc blackening process, which is not traditional. |
| Aluminum and Aluminum Alloys | Not Suitable | Achieves black appearance through black anodizing, a completely different electrolytic process. |
3. What is the Process of Black Oxide Treatment?
The black oxide treatment process generally includes several key steps. The specific steps may vary based on the material, treatment requirements, and solution used.
Generally speaking, there are 3 main steps, pre-treatment, blackening and post treatment.
Below is the typical process for black oxide treatment (for carbon steel and alloy steel).
3.1. Surface Cleaning and Preparation
Degreasing and Cleaning:
Before blackening, the metal surface must be thoroughly cleaned to remove oil, rust, and other contaminants. Typically, powerful cleaners, solvents, or acidic cleaning solutions are used to remove surface impurities.
Water Rinse:
After cleaning, the metal surface is rinsed with clean water to ensure no cleaning agent residue remains.
3.2. Chemical Blackening Treatment
Immersion:
The cleaned metal parts are immersed in a chemical solution containing an oxidizing agent. The blackening solution typically contains chemicals such as sodium hydroxide (NaOH) and sulfates (e.g., copper sulfate, sodium sulfate). This solution promotes an oxidation reaction on the metal surface, forming a black oxide layer.
Reaction Process:
The metal reacts with the oxidizing agents in the solution, forming an oxide layer. For example, iron reacts with oxygen to form magnetite (Fe₃O₄), which creates a thin black film on the metal surface. For stainless steel, in addition to iron oxidation, chromium oxide (Cr₂O₃) may also form.
3.3. Cooling and Water Rinsing
Cooling:
After the blackening reaction is complete, the metal parts are removed from the solution and cooled. Cooling is generally done by air cooling or rinsing with cold water.
Water Rinse:
The metal parts are washed again with water to remove any residual chemicals, ensuring no solution residue remains.
3.4. Sealing and Drying
Sealing Treatment:
To further enhance the protective effect, the surface is often sealed using oil, wax, or synthetic solvents. The purpose of sealing is to protect the oxide layer from being damaged and improve the corrosion resistance.
Drying:
Finally, the metal parts are dried to ensure no moisture or other impurities remain on the surface.
3.5. Inspection
After blackening, the metal parts should be inspected to ensure an even black color and that the oxide layer is free of defects.
Below is a summary of the blackening process.
| Stage | Core Steps | Purpose | Key Control Points |
| Pre-treatment | Degreasing, Water Wash, Acid Wash, Water Wash | Achieve a completely clean and active surface | Cleanliness is the key to success |
| Core Treatment | High-Temperature Alkaline Blackening | Form Fe₃O₄ black oxide layer | Temperature, time, and solution concentration |
| Post-treatment | Water Wash, Sealing (Oil Immersion) | Enhance corrosion resistance and appearance | Thorough rinsing, quality and temperature of oil |
4. What are the Chemical Reactions of Black Oxide Treatment?
Below is a more detailed explanation of the chemical blackening processes for different materials.
4.1. Alkaline Oxidation Blackening for Carbon Steel / Low-Alloy Steel
The alkaline oxidation blackening of carbon steel and low-alloy steel is a classic high-temperature alkaline oxidation process.
The core of this process is the transformation of iron on the steel surface into magnetite (Fe₃O₄), a black oxide, under specific conditions.
The main components of the blackening solution are sodium hydroxide (NaOH) and sodium nitrite (NaNO₂) or sodium nitrate (NaNO₃). The former creates a strong alkaline environment, while the latter acts as an oxidizing agent.
First Stage: Alkaline Corrosion and Initial Reaction
The steel is immersed in a high-temperature (approximately 135-155°C) alkaline solution, where the surface iron reacts with sodium hydroxide to form sodium ferroate (Na₂FeO₂):
Fe + 2NaOH → Na₂FeO₂ + H₂↑
(Iron + Sodium Hydroxide → Sodium Ferroate + Hydrogen Gas)
Second Stage: Oxidizing Agent Reaction and Further Reaction
Sodium ferroate (Na₂FeO₂) reacts with sodium nitrite (NaNO₂) in the solution, further oxidizing to form sodium ferrate (Na₂Fe₂O₄):
2Na₂FeO₂ + NaNO₂ + H₂O → Na₂Fe₂O₄ + NaOH + NH₃↑
(Sodium Ferroate + Sodium Nitrite + Water → Sodium Ferrate + Sodium Hydroxide + Ammonia Gas)
Note: This reaction regenerates some NaOH to replenish the consumption.
Third Stage: Film Formation Reaction
Sodium ferroate (Na₂FeO₂) and sodium ferrate (Na₂Fe₂O₄) interact at the interface, ultimately forming the desired magnetite (Fe₃O₄) black oxide layer:
Na₂FeO₂ + Na₂Fe₂O₄ + 2H₂O → Fe₃O₄ + 4NaOH
(Sodium Ferroate + Sodium Ferrate + Water → Magnetite + Sodium Hydroxide)
4.2. Acidic Oxidation Blackening for Stainless Steel
Stainless steel (e.g., 304) contains chromium (Cr), so the blackening process involves not only the oxidation of iron but also the oxidation of chromium.
This forms a composite oxide film containing both Fe and Cr, as shown in the typical reactions below:
In an acidic solution containing nitric acid (HNO₃) or sulfuric acid (H₂SO₄), the passive chromium oxide layer (Cr₂O₃) is broken down, and oxidation occurs:
Fe + HNO₃ (dilute) → Fe (NO₃)₂ + NO↑ + H₂O
(Iron + Nitric Acid (dilute) → Iron Nitrate + Nitric Oxide + Water)
Cr + HNO₃ (dilute) → Cr (NO₃)₃ + NO↑ + H₂O
(Chromium + Nitric Acid (dilute) → Chromium Nitrate + Nitric Oxide + Water)
Subsequently, metal ions combine on the surface to form a black oxide film (e.g., FeCr₂O₄, a spinel-structured oxide):
Fe²⁺ + 2Cr³⁺ + 4O²⁻ → FeCr₂O₄↓
(Iron, Chromium Ions, and Oxygen Ions Combine to Form Black Composite Oxide Film)
4.3. Chemical Blackening for Copper and Copper Alloys
The blackening of copper is typically achieved through sulfurization or oxidation to form black copper sulfide (CuS) or copper oxide (CuO) films.
Sulfurization Blackening:
Cu + S²⁻ (from sodium sulfide, etc.) → CuS↓ (black)
(Copper + Sulfide Ions → Copper Sulfide (black))
Oxidation Blackening:
2Cu + O₂ (under the action of an oxidizing agent like potassium dichromate) → 2CuO↓ (black)
(Copper + Oxygen → Copper Oxide (black))
5. What is Thickness of the Black Oxide Coating?
The thickness of the oxide coating formed during the black oxide treatment is typically very thin, usually ranging from 0.0001 inches (approximately 0.0025 mm) to 0.0005 inches (approximately 0.0127 mm).
When discussing the thickness of the black oxide coating, the primary consideration is how to make allowances during the CNC machining.
6. What are Advantages and Disadvantages of Black Oxide Treatment?
Black oxide treatment has many advantages.
6.1. Is Black Oxide Corrosion Resistant?
Yes, black oxide treatment improves corrosion resistance, especially for metals like steel.
The chemical oxidation process forms a thin and dense oxide layer (typically iron oxide or chromium oxide) on the metal surface. This layer effectively isolates the metal from exposure to oxygen, moisture, and other corrosive substances, reducing the likelihood of oxidation and corrosion.
The main advantage of black oxide treatment is the improved corrosion resistance it offers.
6.2. Does Black Oxide Rust?
The black oxide layer, especially when sealed with oil, significantly enhances rust resistance. The oxide film, often composed of iron oxide (Fe₃O₄) or chromium oxide (Cr₂O₃), is dense and acts as a barrier to air and moisture, slowing down the oxidation process and preventing rust formation.
Other benefits of black oxide treatment include:
Uniform black or blue-black appearance, which is aesthetically pleasing.
The film is extremely thin, so it does not affect the dimensions, making it suitable for precision parts and fasteners.
Unlike electroplating, black oxide does not induce hydrogen embrittlement, making it ideal for high-strength steels.
Reduced light reflection, making it useful for parts in military or optical instruments where matte finishes are required.
However, black oxide treatment also has its drawbacks.
Corrosion resistance is much lower than electroplating (such as galvanizing or nickel plating) or anodizing.
The oxide layer is relatively soft, offering limited abrasion resistance and can be easily scratched.
The color choice is limited to black; other colors are not possible.
7. Black Oxide Coating Services from ECOREPRAP
Through this blog, readers should now have a clear understanding of the principles, process, and the materials that can undergo black oxide treatment.
For carbon steel and low-alloy steel, both black oxide treatment and phosphating are viable options. But what are the differences between these two treatments, and how should one choose between them?
Stay tuned for our next blog black oxide vs black phosphate to learn more!
FAQs
Black oxide is a conversion coating applied to ferrous metals, such as steel, to enhance corrosion resistance and create a black appearance.
Black oxide provides improved corrosion resistance, reduced light reflection, and an attractive black finish.
Black oxide forms a protective layer on the metal’s surface, reducing the risk of rust and corrosion.
No, black oxide is primarily used on ferrous metals like steel and iron.
Black oxide has a minimal impact on part dimensions, making it suitable for precision components and can be applied to parts with complex shapes and intricate features.
Black oxide enhances corrosion resistance but may not be sufficient for extended exposure to extreme outdoor or marine environments.
It is common to apply a rust inhibitor or sealant after black oxide to enhance corrosion resistance and durability, especially in harsh environments.
