I. Introduction
In the modern industrial system, fasteners, as key basic components for connecting various machinery, equipment, and structures, directly affect the stability and reliability of the entire system with their performance.
From daily used electronic products to large-scale building structures and aerospace equipment, fasteners are ubiquitous and play an indispensable role. Surface treatment technology, for fasteners, is like putting on a layer of “protective armor”, playing a decisive role in enhancing their corrosion resistance, extending service life, and improving functionality.
Dacromet surface treatment technology, as an advanced and mature process, is gradually emerging in the field of fasteners. With its unique technical advantages and outstanding protective performance, it has become the first choice for many industries.
It not only revolutionizes the traditional concept of surface treatment but also opens up new paths for the development of fasteners in terms of environmental protection and performance improvement.
This article will conduct an in-depth analysis of the surface Dacromet treatment technology for fasteners. Starting from the origin and development context, it will gradually delve into the process principle, detailed procedures, performance advantages, cost analysis, quality inspection and control, as well as extensive application fields and case sharing.
The aim is to present a comprehensive, systematic and in-depth knowledge system of Dacromet technology for the readers, helping industry practitioners better understand and apply this advanced technology, and also providing valuable references and inspirations for technology research and development, production management, quality control, etc. in related fields.
II. The Origin and Development of Dacromet Technology
2.1 Origin and Background
The birth of the Dacromet technology was not accidental but rather a response to specific environmental challenges. In the late 1950s, in cold regions such as North America and Northern Europe, thick ice layers on winter roads severely affected the operation of motor vehicles.
To address this issue, people used the method of spreading salt to lower the freezing point to ensure road accessibility. However, this led to the strong corrosion of the steel substrate by chloride ions in sodium chloride, causing severe damage to transportation vehicles. This serious problem prompted scientists to actively seek effective protective solutions.
After in-depth research, American scientist Mike Martin successfully developed a highly dispersed water-soluble coating mainly composed of zinc sheets, with aluminum sheets, chromic acid and deionized water as solvents. When this coating is applied to a metal substrate and subjected to a full closed-loop circulation coating and baking process, a thin layer of coating can be formed.
This coating demonstrates an astonishing ability to resist chloride ion erosion, successfully breaking through the bottleneck of short anti-corrosion life in traditional processes and bringing new hope to the field of metal protection. The Dacromet technology was thus born and quickly adopted by the US military due to its outstanding performance, becoming an important anti-corrosion military technology (US military standard MTL – C – 87115).
2.2 Development History
Since its inception, the Dacromet technology has undergone continuous development and innovation on a global scale. In the 1970s, NDS Company in Japan astutely recognized the huge potential of Dacromet technology, introduced it from MCI Company in the United States, and purchased the exclusive rights to use it in the Asia-Pacific region.
Subsequently, it also took a controlling stake in MCI Company in the United States. Due to the significant amount of steel parts corroding in Japan each year, the country placed great emphasis on anti-corrosion technology. As a result,
Dacromet technology was further improved and optimized in Japan. In Japan, over 100 coating plants and 70 pharmaceutical units rapidly developed, and the application scope of Dacromet technology continuously expanded, with its influence also growing. Some developed countries followed suit and introduced Dacromet technology, enabling it to spread and be applied more widely on a global scale.
China officially introduced the Dacromet technology from Japan in 1994. Initially, it was mainly applied in the defense industry and domestic auto parts sector, playing a significant role in enhancing the quality and reliability of products in these crucial fields.
As the technology matured and was promoted, its application scope gradually expanded to numerous industries including power, construction, marine engineering, household appliances, small hardware and standard parts, railways, bridges, tunnels, highway guardrails, petrochemicals, biotechnology, medical devices, and powder metallurgy, becoming a vital force driving product upgrades and technological advancements in various sectors.
2.3 Technological Development Trends
With the continuous advancement of technology and the increasing demands for environmental protection, the Dacromet technology is constantly innovating and developing. In terms of environmental protection, chromium-free Dacromet technology has become a research and application hotspot.
Traditional Dacromet technology contains some chromium ions that are harmful to the environment and human health, especially hexavalent chromium ions which are carcinogenic. To address this issue, researchers have successfully developed chromium-free Dacromet technology by researching new alternative materials and processes, enabling it to meet environmental protection requirements while maintaining or even enhancing the performance of the coating.
In terms of process optimization, the introduction of intelligent equipment and advanced control technologies has enabled precise control and automated operation of the production process. For instance, sensors are used to monitor various parameters during the coating process in real time, such as coating thickness, temperature, and humidity, and the computer control system automatically adjusts them.
This has significantly enhanced the stability and consistency of product quality, while reducing labor costs and production errors.
In terms of performance improvement, through in-depth research on the structure and composition of the coating, a Dacromet coating with higher comprehensive performance such as corrosion resistance, wear resistance, and high-temperature resistance has been developed.
For instance, by using nanotechnology to modify the coating and adding nanoparticles to it, the hardness and wear resistance of the coating have been effectively enhanced; by optimizing the microstructure of the coating, the coating’s ability to block corrosive media such as water vapor and oxygen has been strengthened, further improving its corrosion resistance.
III. Principles of Dacromet Process
3.1 Components and Functions of Dacromet Treatment Solution
Although there are various types of Dacromet coating solutions, their basic composition mainly includes the following types of components:
1. Metal components: Primarily composed of ultrafine flaky zinc and ultrafine flaky aluminum. Zinc and aluminum play a crucial role in the coating, as they possess excellent electrochemical activity and physical shielding properties. Zinc has a more negative standard electrode potential and can lose electrons and be oxidized preferentially in a corrosive environment, thereby providing cathodic protection to the substrate metal.
Aluminum, on the other hand, can form a dense layer of aluminum oxide on the coating surface, enhancing the chemical stability and corrosion resistance of the coating. Additionally, the flaky structure of zinc and aluminum enables them to interlock and stack in the coating, creating a multi-layer shielding structure that effectively hinders the progress of corrosive media such as water and oxygen reaching the substrate, thus serving an isolating shielding function.
2. Inorganic acid components: Such as chromic acid, etc. During the Dacromet process, chromic acid undergoes chemical reactions with zinc, aluminum powder, and the substrate metal to form a dense passivation film. This passivation film has extremely low solubility and good chemical stability, significantly improving the corrosion resistance of the coating.
Moreover, chromic acid can act as a catalyst, promoting other chemical reactions during the coating formation process to ensure the quality and performance of the coating. However, due to the environmental and health hazards posed by hexavalent chromium ions in chromic acid, with the increasing environmental protection requirements, it is gradually being replaced by chromium-free or low-chromium alternative systems.
3. Solvents: Generally inert organic solvents, such as ethylene glycol, etc. The main function of solvents is to uniformly disperse solid particles such as metal components and inorganic acid components in the coating solution, forming a stable suspension system, facilitating the coating application.
Additionally, during the baking process after coating, solvents can rapidly evaporate, allowing the solid components in the coating to firmly bond and cure on the substrate surface.
4. Special organic substances: Mainly white powders such as cellulose, serving as a viscosity-increasing and dispersion-stabilizing component in the coating solution. They can increase the viscosity of the coating solution, improve its rheological properties, enabling the coating solution to adhere better to the surface of the workpiece during the coating process and form a uniform coating.
Moreover, special organic substances can also enhance the dispersion stability of various components in the coating solution, preventing solid particles from aggregating and settling during storage and use.
3.2 Chemical Reaction Mechanism of Coating Formation
The formation of Dacromet coatings is a complex chemical reaction process, mainly including the following stages:
1. Coating stage: When the workpiece is immersed in the Dacromet treatment solution or the treatment solution is applied to the surface of the workpiece by spraying or other methods, the solvent in the treatment solution quickly wets the surface of the workpiece, allowing the metallic substances, inorganic acid components, etc. to evenly adhere to the surface of the workpiece.
At this time, the zinc and aluminum flakes in the treatment solution gradually spread out and interlace on the surface of the workpiece under the action of gravity and surface tension.
2. Initial baking stage: The coated workpiece is placed in a baking furnace for baking. As the temperature rises, the solvent begins to rapidly evaporate, and the solid components in the treatment solution gradually concentrate and start to undergo chemical reactions.
Chromium acid first undergoes redox reactions with zinc and aluminum powder, generating oxides of zinc and aluminum and low-valent chromium compounds. At the same time, iron atoms on the surface of the substrate also react with chromium acid, forming a thin passivation film on the substrate surface.
3. Curing stage: As the baking temperature further increases to around 300°C (the curing temperature of the Dacromet coating), the chemical reactions in the coating intensify. The oxides of zinc and aluminum and the low-valent chromium compounds undergo condensation polymerization reactions, forming a complex inorganic polymer network structure.
This network structure tightly binds the zinc and aluminum flakes together and firmly bonds them to the passivation film on the substrate surface through chemical bonds, thereby forming a dense and hard zinc-chromium coating on the surface of the workpiece. During this process, the moisture and unreacted volatile substances in the coating are completely evaporated, and the structure and performance of the coating gradually stabilize.
4. Cooling stage: The workpiece after baking and curing is removed from the baking furnace and rapidly cooled through a cooling system. During the cooling process, the volume of the coating will shrink to a certain extent, but due to the strong chemical bond between the coating and the substrate and the dense structure of the coating itself, it can maintain good adhesion and integrity, ultimately forming a Dacromet coating with excellent corrosion resistance.
3.3 Relationship between the Microstructure and Properties of Dacromet Coatings
The microstructure of Dacromet coatings exhibits unique characteristics, which are closely related to its excellent performance. At the microscopic level, Dacromet coatings are mainly composed of multiple layers of interlaced and stacked flaky zinc and aluminum, which are interconnected by an inorganic polymer network, forming a structure similar to “fish scales”. This special microstructure endows Dacromet coatings with the following performance advantages:
1. Excellent corrosion resistance: The multi-layer flake structure greatly increases the path length for corrosive media to penetrate to the substrate surface. Corrosive media such as water and oxygen need to pass through layers of flakes and take a tortuous path to reach the substrate. During this process, the diffusion speed of the corrosive media is significantly slowed down.
At the same time, zinc and aluminum flakes can play a cathodic protection role in the corrosive environment. Even if there is local damage to the coating, zinc and aluminum will be preferentially corroded, thereby protecting the substrate metal from erosion. In addition, the passivation film and inorganic polymer network structure in the coating can effectively prevent further invasion of corrosive media, jointly ensuring the coating has excellent corrosion resistance.
2. Good adhesion: The coating is bonded to the substrate through chemical bonds, and the inorganic polymer network is closely connected to the passivation film on the substrate surface, which makes the Dacromet coating have good adhesion. In practical applications, the coating is not easily detached from the substrate surface and can maintain its protective performance for a long time.
3. High heat resistance: The inorganic polymer network structure in the coating has high thermal stability and is not easily decomposed or structurally changed in high-temperature environments. At the same time, the melting points of zinc and aluminum flakes are relatively high, allowing them to withstand high temperatures to a certain extent.
Therefore, Dacromet coatings can be used for long periods in high-temperature environments above 300°C, and their appearance and performance remain largely unchanged, demonstrating good heat resistance.
4. No hydrogen embrittlement: The Dacromet treatment process does not involve acid washing, electroplating, or other hydrogen-releasing steps, fundamentally eliminating the possibility of hydrogen atoms invading the substrate metal.
This makes Dacromet coatings particularly suitable for high-strength fasteners and other parts sensitive to hydrogen embrittlement, avoiding part failure and safety hazards caused by hydrogen embrittlement.
5. Good penetration: Due to the good fluidity and penetration of the Dacromet treatment solution, it can penetrate into complex parts such as deep holes, narrow slits, and the inner walls of pipes. In these areas, the treatment solution can also undergo chemical reactions and form a complete coating, effectively solving the problem of uneven coating of complex structural parts in traditional electroplating processes.
IV. The entire process of Dacromet treatment for fasteners
4.1 Pretreatment Process
1. Degreasing and oil removal: During the processing, storage, and transportation of fasteners, their surfaces are usually contaminated with various oils and impurities such as engine oil, anti-rust oil, and dust. These oils and impurities can seriously affect the adhesion between the Dacromet coating and the substrate, reducing the protective performance of the coating. Therefore, degreasing and oil removal are important pretreatment steps for Dacromet.
Common degreasing methods include organic solvent degreasing, alkaline solution degreasing, and ultrasonic degreasing. Organic solvent degreasing utilizes the dissolving effect of organic solvents on oils, immersing the fasteners in solvents such as acetone or gasoline to quickly dissolve and remove the oils from the surface.
Alkaline solution degreasing involves a saponification reaction between alkaline substances and oils, converting the oils into water-soluble substances that can be removed through water washing. Ultrasonic degreasing uses the cavitation effect of ultrasonic waves to generate tiny bubbles that rapidly burst, enhancing the stripping ability of oils and improving the degreasing effect.
In actual production, the appropriate degreasing method can be selected based on the type of oil contamination, the degree of contamination, and the material of the workpiece.
2. Surface cleaning: After degreasing and oil removal, the surface of the fasteners needs to be further cleaned to remove burrs, rust, oxide scales, and other impurities. These impurities not only affect the smoothness and appearance of the coating but can also become the starting points of corrosion, reducing the corrosion resistance of the coating.
Common surface cleaning methods include mechanical cleaning and chemical cleaning. Mechanical cleaning methods include sandblasting, shot blasting, and grinding. Sandblasting involves using compressed air to spray abrasive materials (such as quartz sand, steel shot, etc.) at high speed onto the surface of the workpiece.
Through the impact of the abrasive, surface impurities are removed, and at the same time, a certain degree of roughness is formed on the surface of the workpiece, which increases the adhesion between the coating and the substrate. Shot blasting, on the other hand, uses a shot blasting machine to throw shot at high speed onto the surface of the workpiece to achieve the purpose of cleaning and strengthening.
Grinding is generally used for fasteners with high surface quality requirements. It removes minor surface defects through manual or mechanical grinding. Chemical cleaning mainly uses acid washing, where acid solutions react chemically with rust, scale, etc., to dissolve and remove them.
However, for high-strength bolts and nuts, due to the potential hydrogen embrittlement after acid washing, which can affect product quality, in the Dacromet process, mechanical methods such as sandblasting are usually preferred for rust removal to avoid the risk of hydrogen embrittlement.
3. Water washing and drying: After degreasing and surface cleaning, fasteners will have residues of degreasing agents, acid solutions, abrasives, etc. on their surfaces, which need to be thoroughly removed through water washing. Water washing is generally carried out using a multi-stage countercurrent rinsing method to save water and improve the cleaning effect.
First, the workpieces are immersed in clean water to dissolve most of the impurities, and then they are successively rinsed through multiple water washing tanks to ensure that all impurities on the surface of the workpieces are completely removed. After water washing, the surface of the fasteners contains a large amount of water.
If coating treatment is carried out directly, the water will affect the stability of the Dacromet treatment solution and the coating effect, and may also cause defects such as bubbles and pinholes in the coating. Therefore, the fasteners after water washing need to be dried.
Drying is usually carried out in an oven, with the temperature generally controlled between 80 – 120℃. The drying time is determined based on factors such as the size, quantity, and moisture content of the workpieces, and is generally 15 – 30 minutes.
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