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Metal Blackening: Oxide Coating Technology for Protection and Aesthetics

Metal Blackening: Oxide Coating Technology for Protection and Aesthetics

Oxidising metals is a surface treatment process used in a variety of industries, from automotive to precision engineering. This technique involves the formation of a thin layer of iron oxide, which protects the metal from corrosion and gives it an aesthetic, matte appearance. The blackening method is particularly valued for its protective and decorative properties, as well as for its applicability to various metals such as steel or aluminium.

Meaning and applicability scope

Oxidation is the process of forming a black oxide film on the surface of metals, mainly steel, by a chemical or electrolytic reaction. As a result of this process, a thin oxide layer is formed on a metal, which protects it from corrosion and improves its aesthetic properties. In the chemical method, steel components are immersed in specially prepared chemical baths that accelerate the forming of the protective layer. In the electrolytic method, submerging components in suitable salt solutions and the flow of an electric current also lead to the creation of an oxide coating.

One of the most important aspects of the use of blackening in industry is its role in preserving carbon steel in anti-corrosion oil. After the blackening process, the metal is often covered in anti-corrosion oil, which further protects the coating from mechanical damage and further corrosion. This method of preservation is particularly recommended for machine parts, tools or structural components that require long-term protection against corrosion, as well as maintaining their aesthetic appearance.

About oxide coatings

Oxide coatings, formed during the oxide coating process, are obtained by a chemical or electrolytic reaction on the surface of metals, particularly steel. They offer protection against corrosion and give a metal its characteristic black finish. Their main task is to protect metals from the effects of moisture, oxygen, salt and other environmental factors that can provoke corrosion processes.

Protective properties and visual appeal

Oxide coatings significantly increase a metal’s resistance to rust and corrosion, allowing them to be used effectively in a wide variety of environmental conditions. Additionally, by insulating a metal from direct contact with aggressive external agents, these coatings extend the life of metal components.

Aesthetically, oxidation gives metals a deep, uniform shade of black. Such an appearance is often desired in applications such as decorative elements, photographic equipment, precision tools or optical components.

Thickness of oxide coatings

The thickness of an oxide coating has a direct impact on its protective and aesthetic properties. Thinner coatings, although less visible, can offer adequate protection for internal applications or in less aggressive environments. Thicker coatings, although more visible and aesthetically appealing, offer better protection in outdoor or industrial environments where components are more vulnerable to damage.

In the blackening process, it is possible to obtain:

  • Very thin coatings with a few nanometres of thickness that do not change the appearance of steel surface and do not protect against corrosion;
  • Thick coatings of more than 2 mm with a matte black or greyish black colour, with low mechanical resistance;
  • 0.6-0.8 mm thick coatings suitable for technical applications with good mechanical properties and some corrosion resistance.

Surface preparation for oxidation coatings

The preparation of a metal’s surface prior to oxidation is a priority to ensure the quality and durability of an oxide coating. The process requires careful and precise cleaning of the surface to remove any contaminants that could interfere with the creation of an oxide coating.

Surface preparation steps

  1. Degreasing: The first step is metal degreasing, which consists of removing oils, grease and other organic substances from the surface. Degreasing can be done with organic solvents or by using alkaline methods, where components are soaked in degreasing solutions. This process is very important, as even small amounts of organic residues can disturb the uniformity of the oxide coating.
  2. Removal of dirt and corrosion: After degreasing, the next step is the mechanical or chemical removal of any impurities such as rust, corrosion or other residues. In the case of steel, sandblasting or etching is often used to effectively remove surface contaminants and corrosion products, preparing the metal for the next stage of treatment.
  3. Rinsing: Each cleaning stage usually ends with a rinse in clean water (preferably demineralised) to remove residual chemicals and contaminants that might remain on the metal from previous processes.

Requirements for components' cleanliness prior to blackening

The cleanliness of components prior to the blackening process is absolutely critical. Any residue on the surface can result in uneven or incomplete oxide coatings, which not only detract from the aesthetics but also reduce the corrosion protection’s effectiveness.

Characteristics of blackening baths and devices

The oxidation process requires not only a properly prepared surface, but also a specially composed chemical bath and precisely designed apparatus.

Chemical composition of a blackening bath

A blackening bath is a solution containing selected chemical compounds that react with a metallic surface to form the desired oxide layer. For steel, typical bath ingredients include sodium hydroxides, sodium nitrates and sodium nitrites. These substances react with the iron when exposed to elevated temperatures. This creates a black iron(III) oxide layer, known as magnetite. The entire process requires a carefully controlled temperature and duration to produce a uniform and durable coating.

Typically, the temperature of a bath is maintained between 138°C and 145°C. The duration of the process depends on the desired thickness and quality of the coating, typically ranging from 10 to 90 minutes.

Technical description of the blackening, rinsing and maintenance baths

The baths used in oxidation are specially designed to handle the chemicals and high temperatures necessary for the process. They are usually made of stainless steel or coated with special coatings to resist the corrosive effects of the chemicals. The dimensions of the baths are varied and adapted to the variety and size of the workpieces to be treated, and can therefore accommodate small workpieces as well as large components.

Furthermore, each blackening process is usually supported by filtration and circulation systems that keep the bath in optimum condition. After blackening, the components are transferred to rinsing baths where they are thoroughly rinsed with clean water. Then, they are transferred to preservation baths, where they can be protected with anti-corrosion oil.

The dimensions (length x width x depth) of the tubs available at CPP “PREMA” S.A. are:

  • Blackening: 1500 x 700 x 950 mm,
  • Rinsing: 1000 x 700 x 950 mm,
  • Preserving: 1000 x 700 x 950 mm.

Blackening procedures

The blackening process is complex and multi-step. Oxidising requires careful execution of each step to ensure the quality and durability of the final product. Blackening steps include:

  1. Surface preparation: This includes degreasing, removal of dirt and corrosion products. The purity of a surface ensures that the blackening bath can react evenly with a metal.
  2. Blackening bath: After appropriate surface preparation, the components are submerged in the blackening bath. The bath temperature, chemical composition and duration are strictly controlled to achieve the desired oxide coating attributes.
  3. Rinsing: After blackening, the components are immediately transferred to the rinsing baths. There, they are thoroughly rinsed in clean water to remove residual chemicals and prevent further chemical reactions.
  4. Preservation in anti-corrosion oil: The final step is to immerse the components in anti-corrosion oil. The oil forms an additional protective barrier that protects the coating from mechanical damage and corrosion.

Blackening process parameters such as bath temperature, duration, chemical composition and surface preparation methods have a direct impact on the quality and durability of an oxide coating. Improper settings can lead to a non-uniform coating, increased susceptibility to corrosion or an unsuitable appearance.

Quality control of blackened coatings

Quality assurance plays a fundamental role here, as every step, from surface preparation to final maintenance, must meet rigorous standards. We will focus on two main aspects of quality control: visual inspection and evaluation of a coating’s adhesion.

Quality control methods and techniques

  1. Visual inspection: The first and most basic type of quality control that quickly identifies possible coating defects such as unevenness, blistering, chipping or discolouration. Visual inspection is carried out by trained personnel immediately after each stage of the blackening process.
  2. Coating adhesion assessment: An advanced inspection technique that determines how tightly the oxide coating adheres to the metal base. Adhesion is a critical parameter, as poor adhesion can lead to the coating falling off under usage or weather conditions.

Quality control of blackened coatings is a continuous and systematic process that also involves monitoring and analysing data from all stages of production. This allows us to react quickly to any irregularities and adjust the process parameters to ensure optimum results. The long-term durability and visual qualities of the blackened components are directly dependent on the meticulousness of the control processes.

Typical defects of blackened coatingss

During the blackening process, defective coatings may form as a consequence of not meeting the appropriate conditions. Defective coatings can be removed using a 15-20% hydrochloric acid solution. Typical defects that may occur on the surface of a workpiece are:

  • Lack of coating: Caused by bathing in too concentrated or diluted water.
  • White tarnish: Appears during storage of an object that has not been sufficiently rinsed after blackening.
  • Light or uneven colouring: Caused by too short a blackening time or too low a concentration of sodium hydroxide.
  • Light brown tarnish: Caused by an excess concentration of sodium hydroxide.
  • Yellowish green tarnish: Forms if the recommended bath temperature is exceeded.

Bath maintenance and regeneration

In order to ensure the continuity of the high-quality blackening process and the long-term effectiveness of the blackening bath, systematic control of the parameters and regeneration of the solutions is essential.

Ingredient control procedures

Inspection of the blackening bath’s ingredients is carried out regularly to ensure that all chemical components are in the correct proportions. For this purpose, we use various chemical analysis techniques such as spectrometry and chromatography. Regular chemical tests allow early detection of any irregularities that could affect the quality of the oxidation process.

Reconditioning and replacement

Over time, as a result of continuous use, the composition of the blackening bath degrades, which can affect the quality of oxide coatings. Therefore, bath regeneration procedures are recommended, which include the addition of new portions of chemical components to restore the optimal concentration and effectiveness of the solution. This regeneration is carried out based on the results of chemical analyses.

In cases where bath regeneration is not sufficient, a complete bath replacement is carried out. The new bath is prepared according to a precisely defined recipe, ensuring that all subsequent blackening processes are carried out to the highest standards.

Importance of maintaining bath quality

Proper bath maintenance and regeneration are essential not only for a good quality blackening process, but also for environmental protection. Good bath management practices allow waste to be minimised and chemicals to be disposed of more efficiently.

Occupational safety and health (OSH) aspects

Health and safety are extremely important aspects of any industrial process, and they become particularly relevant when working with chemicals and hot baths in the blackening process.

Precautions and OSH guidelines when working with chemicals and hot baths

  1. Personal protection: Workers involved in the blackening process are provided with appropriate personal protective equipment (PPE), which includes chemical-resistant suits, gloves, goggles and protective masks. Skin and eye protection is particularly important as the chemicals used in the blackening process can be corrosive or toxic.
  2. Health and safety training: Every employee involved in the blackening process receives specific health and safety training. This includes chemical management, dealing with spills or other accidents and first aid. This training is repeated regularly to maintain a high level of employee awareness and preparedness.
  3. Risk management: Monitoring and controlling the working environment (including bath temperature and chemical concentrations) is carried out to ensure safe working conditions.
  4. Emergency management: Emergency procedures that include instructions for evacuation, dealing with chemical spills, fire or other incidents. All these procedures are regularly updated based on the latest safety standards.
  5. Ventilation and extraction: As hot baths can generate fumes, adequate ventilation must be maintained in work areas. Extraction and ventilation systems are essential to maintain clean and safe air on the production floor.

Summary

Blackening, as a surface treatment method, provides both protective and aesthetic solutions for a wide range of metal components. Using this technique, it is possible not only to extend the life of products, but also to significantly enhance their appearance.

At CPP “PREMA” S.A., we focus on innovation and improvement of our processes. Thanks to technologically advanced equipment and strict adherence to procedures, we are able to control every stage of the blackening process, from surface preparation to final conservation. Our experience and technological approach allow us to adapt the blackening parameters to the individual needs of the customers, which underlines our attention to detail and individualised approach to each project.

We encourage you to familiarise yourself with the wide range of galvanic blackening products on offer at CPP “PREMA” S.A., as well as to contact us and cooperate in the creation of a wide range of high quality galvanic products.