Bluing of metals: Processes, materials & standards
Bluing is one of the oldest chemical surface treatment processes for metals. At the same time, it is one of the most underestimated.
If you know the differences between hot bluing, cold bluing and electrochemical bluing, you will make better purchasing decisions. This helps to avoid costly mistakes when designing parts. An overview.
What is bluing?
During bluing, the metal surface is chemically transformed in an alkaline-oxidizing bath.
This creates a thin conversion layer of magnetite (Fe₃O₄) - the black iron oxide. This layer is firmly bonded to the base material, not applied, but chemically grown.
The black oxidation layer is electrically conductive and hardly changes the component dimensions: typical layer thicknesses are between 1 and 3 µm. This makes bluing particularly attractive for precision parts where no distortion or oversize can be tolerated.
The process is a conversion coating. Unlike galvanic processes, no foreign material is applied. The base material itself is converted into a protective compound.
Steel and cast iron are particularly suitable. Aluminum, stainless steel and non-ferrous metals require special processes or are generally unsuitable.
At a glance: What is black oxidation used for?
- Corrosion protection (especially in combination with oil or wax)
- Decorative surface finishing - typical black for tools, weapons and instruments
- Reduction of light reflection on optical and precision mechanical components
- Run-in aid and friction optimization for moving parts
- Preparation before subsequent oil or lacquer impregnation
Bluing: Process explained
The quality of the black oxide layer depends crucially on the pre-treatment. Oils, rust, scale and anti-corrosion greases prevent the formation of a uniform layer. Typical pre-treatment steps are
Depending on the application, this is followed by black finishing:
- Re-oiling with corrosion protection oil, standard-compliant in accordance with DIN 50938 and MIL-DTL-13924 is mandatory, not optional. Treat components with lubrication points with defined impregnating oils
- Waxes or impregnating agents (for optical components or decorative parts)
- Painting or sealing for higher protection requirements
- Quality inspection according to DIN 50939 or customer-specific approval
Advantages and disadvantages of bluing
| Advantages | Disadvantages |
| Dimensionally neutral - no tolerance problems, no regrinding | Low corrosion protection without re-oiling |
| Electrically conductive - earthing and earth contacts unproblematic | Re-oiling or sealing mandatory in accordance with standards, not optional |
| Cost-effective, therefore ideal for large series | Particularly suitable for steel and cast iron |
| No foreign material, no heavy metals | Risk of hydrogen embrittlement in high-strength steels |
| Running-in aid for moving parts (with oil) | Mechanically and tribologically less resilient - no substitute for DLC or hard chrome |
| Matt black look without additional coating | Operational safety: hot alkaline bath, risk of nitrosamine, risk of deflagration if aluminum or non-ferrous metal comes into contact with the bath |
| Short cycle times, easy to integrate into production lines |
Bluing: A comparison of processes
Hot bluing
Hot bluing is the most commonly used industrial bluing process. It takes place at 130-145 °C in an alkaline oxidation bath (sodium hydroxide solution with sodium nitrite/sodium nitrate). This creates a black, dense magnetite layer on the surface of the steel.
The layer thickness is typically 0.5-2.5 µm, with an average value of 1-1.5 µm. Values up to 3 µm are only achieved with multiple dips (so-called „double dip“).
As part of the layer grows inwards through conversion, black oxidation is very dimensionally neutral after pure passivation. It is one of the most dimensionally neutral functional processes. According to MIL-DTL-13924, it does not affect critical dimensions.
Note for precision mechanics: An upstream acid etching process can remove 1-2 µm of material. This must be taken into account for tight tolerances. According to DIN 50938, the coating quality is tested by appearance, coating thickness and salt spray resistance.
- Very good coating quality and reproducibility
- Broad application in industry and defense technology
- Process temperature requires adapted system technology
Cold bluing
Cold bluing is carried out at room temperature and is particularly suitable for repair work, small series or workshop applications. Acid solutions based on selenium dioxide (or selenious acid), copper(II) salts (e.g. copper sulphate or copper nitrate) and an acid - usually phosphoric acid - are used.
The resulting layer does not chemically consist of magnetite (Fe₃O₄) as in hot bluing. Instead, it consists mainly of copper selenide (CuSe/Cu₂Se). The more noble copper is deposited on the surface of the steel and reacts with the selenide to form a black coating. This fundamentally different chemistry explains why cold black oxidation coatings are significantly weaker than hot alkaline coatings in terms of mechanics and corrosion chemistry.
The layer thickness is usually 0.2 to 1 µm. It is often a few hundred nanometers. This is clearly less than with hot bluing. The cold process is therefore primarily decorative and reparative, not functional.
Corrosion protection is even lower without oil post-treatment than with the hot process - sealing is therefore mandatory, not optional.
- Easy handling, no special equipment required
- Unsuitable for series production with high quality requirements
- Suitable for repairs, prototypes, individual parts
Electrochemical bluing
Electrochemical bluing is the generic term for all bluing processes in which an external current controls the layer formation process. These include the anodic process (workpiece = anode), the cathodic process (workpiece = cathode) as well as alternating current and pulse processes.
In industrial practice, anodic black oxidation dominates. The controlled current flow creates a particularly uniform magnetite layer. This also applies to complex geometries and undercuts.
It also applies in areas that are difficult to access.
There, chemical dipping processes often deliver inconsistent results.
Electrochemical bluing is used specifically where classic alkaline hot bluing reaches its limits:
- Stainless steel (passive layer prevents chemical reaction)
- Nitrided steels (ε/γ‘ compound layer blocks layer formation)
- High-alloy tool steels with increased Cr or Ni content
The process is more complex and more expensive than thermal hot bluing. It is therefore mainly used in precision mechanics, optics and defense technology. It is used when the material or component geometry leave no alternative.
Bluing of metals: All materials
Not every metal is equally suitable for bluing. Suitability depends on whether the material can form a stable magnetite layer with the bluing solution. Unalloyed and low-alloy steels, tool steels and spring steels are well suited. Here is an overview:
1. black oxidation of steel - ✅ Very suitable
Steel and structural steel are the classic applications for black oxidation. All three processes work reliably and deliver reproducible coating qualities. Hot black oxidizing is suitable for high-quality industrial parts with defined corrosion protection requirements. Cold bluing is the most economical option for small series or repairs.
For purchasers, steel from black oxidation is the most straightforward case - no process change, full process control.
2. black oxidation of cast iron - ✅ Well suited
Cast iron behaves chemically similar to steel and can be black oxidized without any problems. Hot bluing is particularly suitable for engine parts, machine frames, hydraulic components and gear parts. The somewhat rougher cast structure requires careful degreasing and pre-treatment. Residues of mold release agents from the casting process can otherwise impair the coating quality.
3. black oxidation of stainless steel - ⚠️ Conditionally suitable
Stainless steel cannot be treated using the classic hot bluing process. The natural chromium oxide passive layer, which gives stainless steel its corrosion protection, prevents magnetite formation.
Blackish coatings are possible using special processes (e.g. electrolytic black oxidation in adapted baths or thermal oxidation in high-temperature furnaces). However, these are considerably more complex and expensive. For purchasers: Stainless steel and standard black oxidation do not go together.
Industrially relevant alternatives to thermal oxidation are the INCO process (chemically in CrO₃/H₂SO₄, produces interference colors), black chrome (galvanic according to AMS 2438, but REACH problematic), anodic black oxide for stainless steel and increasingly PVD coatings (CrN, DLC, ta-C) for the highest tribological requirements.
4. black oxidation of aluminum - ❌ Not suitable
Aluminum cannot be black oxidized. The natural aluminum oxide layer prevents any magnetite formation in the black oxidation bath. As an alternative, anodizing (anodic oxidation), chromating or zirconium/silane-based conversion coatings are used.
Purchasers should pay attention to this with mixed assemblies made of steel and aluminum. Aluminum must not enter the bluing bath under any circumstances.
The reaction with the hot sodium hydroxide solution (2 Al + 6 NaOH → 2 Na₃AlO₃ + 3 H₂) is highly exothermic. It produces hydrogen explosively. There is an acute risk of deflagration and splashing in a 140 °C alkali bath.
In addition, the resulting sodium aluminate poisons the bath chemistry and usually makes a complete bath replacement necessary. The practical consequence of this is that hangers and racks are only used in steel or titanium.
Aluminium screws, anodized auxiliary parts or non-ferrous metal components must not be used. Components with soft soldered joints or zinc surfaces are also critical.
5. black oxidation of brass and copper - ⚠️ Conditionally suitable
Copper and brass can be chemically blackened using special black oxidation processes. It is technically similar to bluing, but with different bath chemistry (e.g. selenium dioxide or copper sulphate solutions).
The result is a decorative, black oxide layer. This layer is hardly suitable for industrial corrosion protection without sealing. Classic steel baths cannot be used for copper materials.
6. black oxidation of die-cast zinc - ❌ Not suitable
Zinc die casting cannot be black oxidized in any meaningful way. Without iron in the base material, no magnetite layer is formed. Alternatively, galvanic processes, chromating or painting can be used. For high decorative requirements, black galvanic nickel plating or powder coating is the better choice.
Bluing vs. other corrosion protection methods
When is black oxidation the right choice? And when is it better to use galvanizing, phosphating or organic coatings?
Bluing is ideal when dimensional tolerance, a black appearance and economical processing are the main priorities. Even if the coating is intended as a system solution with oil, wax or lacquer.
In the salt spray test according to DIN EN ISO 9227 (NSS), the coating alone only offers 2-8 hours to red rust. With corrosion protection oil, the value increases to 24-72 h, with oil-wax sealing to 100-200 h. For comparison: galvanic zinc + Cr(III) passivation achieves 96-240 h, zinc-nickel 720-1000 h. Bluing is therefore primarily a decorative-functional process - not a primary corrosion protection system.
Compared to phosphating, black oxidation offers a visually more appealing, darker appearance and an even thinner coating thickness. Compared to galvanic processes, there is no need for complex plant chemistry and heavy metal baths. This significantly reduces costs and environmental requirements.
Relevant standards and specifications
For procurers and designers, the following standards are of central importance when tendering for bluing services or providing drawing details:
- DIN 50938:2018-04 - Bluing of steel; requirements and test methods (authoritative German standard)
- MIL-DTL-13924E - US military specification for black oxide coatings on steel
- AMS 2485 - Black oxidation for steel (aerospace)
- DIN EN ISO 9227:2017-07 - Salt spray test for the evaluation of corrosion protection after bluing and oiling
- OEM-specific factory standards (e.g. Volkswagen TL standards, BMW GS standards) - binding for automotive suppliers
The tender should always specify the process (hot or cold bluing), the required coating thickness or coating quality by appearance, the post-treatment (oiling, waxing) and corrosion protection requirements.
Checklist for purchasers: Requirements for the job coater
- Certification according to DIN EN ISO 9001 or industry-specific (IATF 16949 for automotive)
- Proven bath control and documented process parameters
- In-house testing facility for coating thickness and optical quality control
- Experience with the required process variant (hot / cold bluing / electrochemical)
- Environmental certification and REACH-compliant bathroom chemicals*
- References from our own industry or for comparable part geometries
- Clear information on delivery times, batch sizes and minimum quantities
*Nitrite is hazardous to water (WGK 2) and poses a nitrosamine risk in the exhaust air. Selenium toxicity must be taken into account for cold bluing. This is because AbwV Annex 40 limits selenium in waste water to 0.3 mg/L. Black chromium as an alternative process is subject to authorization according to REACH Annex XIV (Cr(VI)).
What does bluing cost? Influencing factors for buyers
Bluing is one of the most cost-effective surface treatments. The unit costs are influenced by several factors:
- Part geometry and wall thickness: Complex cavities require greater process effort for complete wetting.
- Batch size: Bluing is particularly suitable for large series and mass-produced parts - rack or barrel goods reduce the unit costs considerably.
- Choice of process: Electrochemical bluing is more complex and more expensive than thermal hot bluing.
- Pre-treatment costs: Heavily contaminated or rusted parts increase the costs for cleaning and pickling processes.
- Post-treatment: Additional oil, waxing or varnishing have a noticeable effect on the overall costs.
- Environmental and disposal costs: bluing baths produce bath sludge that has to be disposed of properly - costs that job coaters factor in.
Bluing: Industries & Applications
Bluing is one of the few surface processes that is firmly established in series production and at the same time offers attractive decorative properties. The combination of low cost, low dimensional change, dark appearance and multifunctionality makes it the first choice. It is suitable where steel has to be coated in large quantities. Visual requirements are also important.
| Industry | Typical components | Primary goal |
| Defense technology | Barrels, bolt carriers, lock parts, magazine housings, trigger groups, system components | Corrosion protection, reflection reduction |
| Automotive | Camshafts, piston rings, springs | Running-in aid, tribology |
| Mechanical engineering | Shafts, gear wheels, hydraulic components | Inlet protection, corrosion protection |
| Toolmaking | Hand tools, cutting tools, clamping devices | Optics, corrosion protection |
| Precision mechanics & optics | Instruments, housings, brackets | Reflex reduction, dimensional neutrality |
| Aerospace | Structural components, fasteners | Corrosion protection according to AMS 2485 |
| Standard parts & screws | Screws, springs, stamped parts | Drum goods, bulk price |
Bluing with FACTUREE
As a digital procurement platform for drawing parts, FACTUREE offers access to a qualified network of specialized contract coaters. These companies take on the bluing of individual components through to series production.
Are you looking for a job coater for bluing near you? For regional inquiries, we will find the right company directly and easily. Be it bluing in Berlin, Munich, Hamburg or other locations. Alternatively, we can draw on our nationwide and international network if capacity, specialization or certification have priority.
In addition to steel and cast iron, we also offer special applications. These include bluing in accordance with industry standards (DIN 50939, MIL-DTL-13924). We also comply with aerospace standards (AMS 2485). Contact us - we will clarify feasibility and availability quickly and directly.
Your advantages with procurement via FACTUREE: Qualified network of certified electroplating companies (DIN EN ISO 9001, NADCAP, etc.) Quick quotations for standard and special coatings | Transparent specific requirements directly in the inquiry Support with the selection of standards (DIN EN ISO 4042, DIN 50961 etc.) Flexibility with quantities: Prototype to large series
