There are many types of surface treatments for fasteners, such as electroplating, blackening, and phosphating, which are common to us. There is a surface treatment called Dacromet, which may be unfamiliar to many people. They may have seen products with this surface treatment, but they do not know that it uses the Dacromet process.
History of Dacromet
In the 1950s in the United States, some regions experienced freezing temperatures that caused road surfaces to ice over. To combat this, salt was spread on roads to lower the freezing point and melt ice.
However, the sodium chloride in the salt, when melted, caused sodium ions to splash onto passing vehicles. Over time, this corroded automotive components, causing severe damage to vehicles and, in the long term, compromising driving safety.
Upon identifying this issue, scientists developed a novel solvent. When applied to metal surfaces and baked at high temperatures, it formed a thin protective coating—the Dacromet process. This became a U.S. military standard (MTL-C-87115) for corrosion protection. After refinement in Japan during the 1970s, it gained global adoption.
Due to its exceptional corrosion resistance, Dacromet was initially deployed on military equipment requiring stringent anti-corrosion protection. Through continuous refinement, it has since gained widespread adoption across diverse industrial sectors.
What Is Dacromet Coating Technology?
Dacromet treatment technology involves using a dispersed aqueous solution (also known as water-based coating or water-soluble anticorrosive coating) composed of ultra-fine flake zinc powder and aluminum powder (with a thickness of micrometers or even nanometers) and water-soluble organic and inorganic substances (such as ethylene glycol and anhydrous chromic acid).
The surface of the treated object (metal part) is uniformly coated with this aqueous solution, and then subjected to high-temperature baking (around 300°C). During this process, Cr6+ is reduced by organic substances such as ethylene glycol, generating water-insoluble amorphous mCrO3·nCr2O3 as a binder.
This binder combines with dozens of layers of interlaced zinc and aluminum flakes on the surface, forming a tightly bonded inorganic coating—the Dacromet coating or zinc-chromium coating—on the metal substrate. Its anti-corrosion film coating can isolate the substrate from the air, thus protecting the workpiece.
Composition of Dacromet
- Metallic components: Composed of substances such as zinc and aluminum, primarily ultra-fine, flake-shaped zinc and ultra-fine, flake-shaped aluminum.
- Solvent: An inert organic solvent, such as ethylene glycol.
- Inorganic acid component: Such as chromic acid.
- Special organic compounds: These are viscosity-increasing and dispersing components of the coating solution, mainly consisting of cellulose-based white powder. (Corrosion protection mechanism)
Dacromet Process Flow
Surface cleaning
Oil and grease removal: Use organic solvents (such as acetone, ethanol) or alkaline cleaning agents (such as sodium hydroxide solution) to thoroughly remove oil, cutting fluid, and other contaminants from the workpiece surface to prevent affecting coating adhesion.
Rust removal treatment: Oxide scale and rust are removed from the workpiece surface through sandblasting (commonly using quartz sand or glass beads), shot blasting, or acid pickling (such as hydrochloric acid or sulfuric acid solution) to ensure a surface roughness of Ra 6.3~12.5 μm, providing a good mechanical anchoring base for the coating.
Washing and drying
Washing: Rinse the workpiece with deionized water to remove residual cleaning agents or rust, preventing impurities from affecting the coating performance.
Drying: Use hot air drying (temperature about 60~80℃) or natural air drying to ensure that the workpiece surface is free of moisture and to prevent bubbles or uneven coating during coating.
Coating material preparation
Composition Ratio: Dacromet coatings are mainly composed of zinc powder (60%~80%), aluminum powder (5%~15%), chromic acid (2%~5%), and deionized water. They must be prepared strictly according to the formula to ensure uniform dispersion of zinc and aluminum particles and to avoid sedimentation.
Viscosity control: The viscosity of the coating is adjusted by adding deionized water, usually controlled at 18~25 seconds (Ford Cup 4 test). Too high a viscosity will result in an overly thick coating, while too low a viscosity will result in an overly thin coating and reduced anti-corrosion performance.
Coating method
Dip coating: The workpiece is immersed in a coating tank, then removed and excess coating is allowed to drain. This method is suitable for workpieces with simple structures and for mass production, resulting in good uniformity of coating thickness.
Spray coating: Using air spraying or electrostatic spraying, this method is suitable for workpieces with complex structures and allows for precise control of coating thickness. However, attention must be paid to the spraying pressure (generally 0.3-0.5 MPa) and distance (15-25 cm) to avoid sagging or incomplete coverage.
Brush coating: Manual operation, suitable for local repairs or small batches of workpieces. It is necessary to ensure that the coating is uniform and avoid missing any spots.
Coating thickness control
Choose the number of coating layers based on application requirements:
- One coat, one bake: thickness 2-5 μm, suitable for mildly corrosive environments;
- Two coats, two bakes: thickness 6-10 μm, the standard for general corrosion protection;
- Three coats, three bakes: thickness 8-13 μm, suitable for highly corrosive environments (such as marine and chemical industries).
Curing temperature and time
Low-temperature pre-curing: After coating, dry at 80-120°C for 10-15 minutes to remove moisture from the coating and prevent bubble formation during curing.
High-temperature curing: Place the workpiece in a curing oven and keep it at 300~320℃ for 20~30 minutes to allow chromic acid to react chemically with zinc and aluminum to form a dense inorganic ceramic coating. Too low a temperature will result in incomplete curing and reduced corrosion resistance; too high a temperature may cause the coating to discolor and deteriorate in performance.
During the curing process, it is necessary to maintain air circulation in the oven to avoid the formation of a reducing atmosphere (such as carbon monoxide), preventing insufficient oxidation of zinc and aluminum particles and affecting coating performance.
Cooling and inspection
For applications requiring high corrosion resistance, an organic sealant (such as epoxy resin or silane coupling agent) can be applied to the coating surface to further improve salt spray resistance (up to 1000 hours or more) and wear resistance, while also enhancing insulation performance.
Dacromet Corrosion Protection Mechanism
Dacromet coating, which has a matte silver-gray appearance, is composed of extremely fine flake-shaped metallic zinc, aluminum, and chromate components. After degreasing and shot blasting, the workpiece is dipped in Dacromet solution.
Dacromet solution is a water-based treatment solution. Metal parts are immersed or sprayed in the water-based treatment solution, then cured in an oven and baked at about 300°C to form a film, thus forming an inorganic coating of zinc, aluminum, and chromium.
During curing, as the water and organic (cellulose) substances in the coating evaporate, the oxidizing properties of the high-valence chromium salts in the Dacromet solution cause the zinc and aluminum flakes, which have a more negative electrode potential, to react with the iron substrate, forming Fe, Zn, and Al chromate compounds.
As the film layer is obtained by direct reaction with the substrate, the anticorrosive layer is extremely dense, (the anticorrosive layer obtained by zinc plating or zinc dipping method can not be compared with) the coating in the corrosive environment, it will form countless proto-cells, i.e., first corrode away from the potential of the negative Al, Zn salts, until they are consumed before it is possible to corrode to the substrate itself.
Because Dacromet anti-corrosion mechanism is a sacrificial anode and cathodic protection for the integration of the body of the coating, so its anti-corrosion performance is proportional to the thickness of the film.
The protective effect of Dacromet layers on steel substrates can be summarized as follows:
- Barrier effect: due to the lamellar zinc, aluminum layer overlap, hindering the process of water, oxygen and other corrosive media to reach the substrate, can play a kind of isolation shielding effect.
- Passivation effect: During the Dacromet treatment process, chromic acid reacts with zinc, aluminum powder, and the base metal to form a dense passivation film. This passivation film has excellent corrosion resistance.
- Cathodic Protection: The primary protective function of the zinc-aluminum-chromium coating, similar to galvanized coatings, is to provide cathodic protection for the substrate.
Characteristics of Dacromet
Dacromet coating offers high corrosion resistance and features a matte silver-gray appearance. The Dacromet surface treatment technology combines phosphating and oxidation processes, employing a formulation similar to paint and a physical deposition method akin to dip coating. After baking and curing, it achieves an appearance resembling electroplated layers. However, compared to these surface technologies, Dacromet possesses several unique properties, as outlined below.
- Excellent Corrosion Resistance
An 8μm-thick Dacromet coating exhibits salt spray corrosion resistance exceeding 1000 hours, with some achieving over 2000 hours.
This performance stems from the layered structure of zinc and aluminum flakes within the coating, combined with the passivation effect of inorganic chromate. Zinc acts as a sacrificial anode protecting the substrate, while aluminum flakes form a physical barrier. This dual-protection mechanism delivers outstanding performance in harsh environments such as humidity, acid rain, and marine climates.
Products treated with the Dacromet process exhibit corrosion resistance dozens of times greater than traditional galvanizing. Salt spray tests reveal that standard Dacromet achieves 20 times the performance of galvanizing. For instance, transmission tower bolts in coastal regions treated with Dacromet can extend their service life by 3-5 times.
Long-life Dacromet coatings demonstrate even greater durability. Components in the nuclear industry treated with Dacromet can achieve service lives of up to 30 years.
Unlike other electroplating processes, Dacromet does not crack or fracture under tensile stress or mechanical loading.
- High-Temperature Resistance
Dacromet exhibits excellent stability in high-temperature environments, significantly outperforming traditional galvanized coatings in heat resistance.
The Dacromet corrosion-resistant coating cures at approximately 300°C, ensuring that even after prolonged exposure to high temperatures, the appearance of the workpiece remains unchanged with excellent resistance to thermal corrosion. In contrast, the chromate passivation layer on conventional zinc-plated surfaces breaks down at around 100°C, developing micro-cracks. At temperatures between 200-300°C, discoloration occurs and corrosion resistance declines sharply.
- Excellent Weather Resistance and Chemical Stability
Test results show that unpurified zinc coatings typically corrode by 1 micron within 10 hours of salt spray testing. A 3-micron-thick rainbow-colored purified coating, however, withstands 200 hours of salt spray testing before being corroded through. Dacromet coatings resist corrosion for 100 hours in salt spray tests before losing 1 micron. Compared to traditional galvanizing processes, Dacromet treatment enhances corrosion resistance by seven to ten times. For instance, headlight brackets coated with Dacromet at a Shanghai automotive lighting factory achieved over 1,000 hours of salt spray resistance.
- Hardness
Zinc coating hardness ranges from 75 to 88 HV0.05. Silver-gray Dacromet coating hardness ranges from 210 to 232 HV0.05.
- No Hydrogen Embrittlement
Dacromet treatment does not cause hydrogen permeation into steel, thereby preventing hydrogen embrittlement, as no acid treatment is performed during the process. When electroplating elastic components, thin-walled parts, and high-strength steel components, hydrogen removal is essential. Failure to do so may lead to severe consequences if hydrogen embrittlement fractures occur. Therefore, applying Dacromet treatment to such components helps prevent hydrogen embrittlement and fatigue fractures, ensuring product safety and reliability.
Hydrogen embrittlement remains an inherent limitation of traditional galvanizing processes. Due to its unique process characteristics—involving no acid treatment during application and eliminating hydrogen permeation issues present in electroplating—coupled with high-temperature curing of the coating, Dacromet inherently prevents hydrogen embrittlement. This makes it suitable for corrosion protection on high-strength components requiring high tensile strength.
- Pollution-Free
With the development of the global economy, environmental protection has increasingly become an integral part of industrial production. Various surface treatment processes—whether painting, electroplating, phosphating, or others—all pose significant pollution issues involving waste water, waste gas, and solid waste. This has long been a major challenge hindering the advancement of industrial surface treatment.
Unlike some traditional electroplating processes, Eco-friendly Dacromet coating free of hexavalent chromium (EU RoHS certified) produces less pollution during manufacturing. Consequently, certain highly polluting surface treatment processes are gradually being replaced by Dacromet.
As a “green electroplating” process, Dacromet employs a closed-loop system. During pretreatment, oils and dust are collected and treated using specialized equipment. During coating and curing, there is no wastewater containing acids, alkalis, chromium, or other heavy metals—unlike traditional electroplating processes. The only byproduct is water vapor evaporating from the coating, which has been tested and found to contain no harmful substances regulated by national standards.
- Soft Appearance
The Dacromet coating exhibits a matte silver-gray finish and can be overcoated. This feature meets the coating requirements for components demanding high aesthetic quality, such as bicycle fasteners and wire baskets. After applying the Dacromet coating, a topcoat of gloss paint is required.
- Excellent Permeability
Due to its water-soluble nature, Dacromet treatment solution can penetrate minute voids, enabling even complexly shaped parts—such as geometrically intricate blind holes—to be thoroughly impregnated with coating. After sintering, this forms a complete and uniform Dacromet coating.
Traditional electroplating techniques are often affected by factors such as uneven distribution of electric field lines due to the complex shape of the parts or insufficient dispersion of the plating solution. This often results in defects such as incomplete plating or insufficient coating thickness in the dead corners of the parts (a drawback of many organic coating electrostatic spraying processes).
Products treated with the Dacromet process exhibit excellent impact resistance, making it highly suitable for load-bearing components. This is due to Dacromet’s superior penetrability, which ensures strong bonding between the surface coating and the substrate.
- Low Surface Friction Coefficient of the Coating
The friction coefficient of standard Dacromet coatings ranges from 0.20 to 0.28, while self-lubricating coatings formed by incorporating dry lubricants exhibit a coefficient of 0.13 to 0.15—significantly lower than cadmium’s friction coefficient of 0.22.
Its lubricating surface remains dry, preventing contamination of parts during use. Moreover, its lubricating properties persist even after immersion in solvents or cleaning processes.
- Excellent Adhesion to Other Coatings
Due to its unique surface characteristics, Dacromet coating exhibits excellent adhesion to other coatings, making it highly suitable for surface recoating. The resulting composite coating system not only addresses the limitations of traditional Dacromet coatings—such as surface hardness, scratch resistance, wear resistance, and acid/alkali resistance—but also further enhances corrosion resistance, improves decorative effects, and broadens application areas (e.g., applications for black Dacromet).
- Widely Used
Dacromet coating does not affect the choice of material for parts; this differs significantly from electroplating, phosphating, and oxidation treatments, as it can be applied to various metal surfaces.
Fields of Application
Dacromet is widely used in industries with high anti-corrosion requirements, such as construction, hardware, fasteners, road engineering, and petrochemicals. Typical applications include:
| Industry | Cases | Effect |
| Wind Power | Salt spray testing of offshore wind turbine bolts reached 2000 hours (compared to the national standard of 480 hours), extending the replacement cycle from six months to five years. | Annual maintenance costs of $310,000 |
| Agricultural Machinery | Wear resistance of Northeast agricultural machinery gears increased by 3 times, with wear volume reduced by 80%. | Annual savings of $210,000 in material costs |
| Chemical Engineering | Flange bolts withstand immersion in 98% concentrated sulfuric acid for 240 hours, with zero leakage over 18 consecutive months. | Avoid environmental fines exceeding $710,000 |
| Automobile | Engine components, chassis parts (heat-resistant up to 300°C) | Solving the problem of high-temperature failure in traditional galvanizing |
Comparison with Traditional Technologies
Advantages and Limitations of Dacromet Compared to Electrogalvanizing and Hot-Dip Galvanizing:
| Characteristics | Dacromet | Traditional Galvanizing (Electrogalvanizing / Hot-Dip Galvanizing) |
| Corrosion Resistance | Salt spray testing for over 2000 hours, with a thickness of only 6–8μm | Salt spray test: 100–200 hours, thickness: 5–15μm |
| Eco-friendly | No wastewater or exhaust gases, but contains hexavalent chromium (carcinogenic); chromium-free technology is being promoted. | Wastewater containing heavy metals requires strict treatment. |
| Hydrogen Embrittlement | None | High-strength steel is prone to this |
| Evenly Coated | Excellent (uniform surface finish on complex parts) | Poor (thin plating in blind holes and crevices) |
| Heat Resistance | Stable below 300°C | Peeling and flaking at 100°C |
| Costs | High per-piece processing fee | Low cost, but requires frequent maintenance |
Development Trends of Dacromet
Chromium-Free Technology: Chromium-free Dacromet coating solutions have become an industry focus, utilizing trivalent chromium or non-chromium passivators. However, cost and technical maturity remain challenges.
Composite Coating Technology: Integrating Dacromet coatings with other finishes (such as organic or ceramic coatings) further enhances corrosion resistance, wear resistance, and functionality to meet high-end application demands.
Process Optimization and Cost Reduction: Enhancing production efficiency and lowering application costs through improved coating equipment (e.g., automated spray lines) and reduced curing times.
Intelligent and Digital Solutions: Integrating smart inspection technologies (e.g., online coating thickness monitoring, corrosion resistance prediction models) to improve process stability and product quality control.
Future Trends: Expansion into green processes (e.g., water-based coatings) and corrosion protection for new energy vehicle components.
Nano-modified Dacromet coatings (such as those incorporating graphene) are currently under development, promising further enhancements in coating toughness and self-healing capabilities. The adoption of water-based Dacromet technology, particularly with the emergence of new formulations like Jumeite, Meijiali, and Delken (Deike), will address the VOC residue issues associated with existing solvent-based coatings. As large-scale production drives down costs, its application scope may expand from high-end manufacturing into consumer markets.
Disadvantages of Dacromet
- Insufficient coating hardness, unable to resist abrasion, scratches, etc.
- Limited color options. Currently, only silver and white Dacromet coatings are commercially available, resulting in a monotonous palette. Achieving other colors requires additional post-treatment processes. Although black is under development, no optimal technical solution has been established. This monochromatic range falls far short of meeting the diverse color demands of industries like automotive and defense, which require black, military green, and other multi-color options.
- Curing temperature is excessively high. Dacromet curing requires temperatures of 300°C or higher, resulting in high energy consumption and costs. This contradicts the principles of building a resource-conserving and environmentally friendly society.
- Poor electrical conductivity makes it unsuitable for parts requiring electrical connections, such as grounding bolts in electrical appliances.
- Some Dacromet coatings contain chromium ions harmful to the environment and human health, particularly hexavalent chromium ions which are carcinogenic. (Under environmental regulations, Dacromet coatings no longer contain hexavalent chromium ions.)
- Repair difficulty. Dacromet coatings bond to substrates mechanically (not metallurgically), making them prone to peeling under intense friction. Once damaged, the entire coating must be reapplied; spot repairs are impossible, increasing maintenance costs.
- The coating exhibits a matte silver-gray appearance, which is less aesthetically pleasing than electroplated bright zinc.
- The cost is relatively high. Dacromet treatment requires specialized equipment, and the coating materials (such as high-purity zinc-aluminum powder) are expensive, making its cost 30%-50% higher than conventional galvanizing. For low-value-added fasteners, it is not economically viable.
Points to Note
Coating Repair: If the coating is locally damaged, use a specialized repair agent to restore it and prevent exposure of the base metal.
Compatibility with Other Coatings: The Dacromet coating surface is smooth. If subsequent organic coatings such as paint are required, surface roughening must be performed first to enhance adhesion.
Wastewater Treatment: Although the Dacromet process is low-chromium, wastewater generated during production must undergo strict treatment to ensure compliance with discharge standards.
Summary
Dacromet was created to provide better corrosion resistance for automobiles, and it can be said that it spread globally through the automotive industry. It improved upon many shortcomings of processes like galvanizing, leading to its widespread adoption. Its environmental concerns have spurred the development of chromium-free technologies. Users can weigh the application of traditional and new Dacromet processes based on their specific needs (such as cost and environmental standards). As technology advances, the search for even better and more perfect surface treatment processes continues.
