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How to Distinguish Blackened Bolts Screws from Dacromet Black Bolts Screws and Which Excels in Salt Spray Testing
In the fields of machinery manufacturing, automotive assembly, and construction, screws play an irreplaceable role as core fasteners. Among the various types of screws, blackened screws and Dacromet black screws are widely used due to their sleek black appearance and certain corrosion - resistant properties. However, many professionals and purchasers often confuse the two because of their similar color. Moreover, when it comes to harsh working environments, especially those with high humidity and corrosiveness, their salt spray corrosion resistance becomes a key factor affecting the service life of the entire equipment.
Before delving into the distinction and salt spray resistance, it is essential to understand the fundamental differences in the surface treatment processes of the two screws, as this forms the basis for their subsequent performance disparities.
1.1 Blackened Bolts&Screws
Blackened screws, also known as blued screws, undergo a chemical surface treatment. The common processes mainly include two types: hot blackening and cold blackening. Hot blackening is the more traditional method. It involves immersing the screw in a high - temperature alkaline oxidation solution (usually composed of sodium hydroxide and sodium nitrite) at 135 - 145°C. Through a chemical reaction, a dense black oxide film (primarily composed of iron trioxide) is formed on the surface of the screw. Cold blackening, on the other hand, is carried out at room temperature. It relies on a room - temperature blackening agent to generate an oxide film on the screw surface quickly.
The primary purpose of blackening treatment is to enhance the decorative effect of the screw and provide basic rust prevention. However, the oxide film formed by this process is relatively thin, generally only a few micrometers thick. It has poor adhesion and cannot withstand long - term exposure to harsh environments. Usually, after blackening, the screws need to be soaked in oil or coated with anti - rust oil to improve their short - term rust - prevention ability.
1.2 Dacromet Black Bolts&Screws
Dacromet black screws are processed using the Dacromet coating technology, which originated in Europe. The process involves applying a water - based coating containing zinc, aluminum flakes, and a special binder to the surface of the screw. Then, through a curing process at 280 - 300°C, the coating is firmly bonded to the screw substrate. The black color of Dacromet black screws is achieved by adding black pigments to the coating formula during the preparation process.
Different from the oxide film of blackened screws, the Dacromet coating is a composite protective layer. The zinc and aluminum flakes in the coating overlap each other to form a barrier structure, which can effectively isolate the screw substrate from the external corrosive medium. Additionally, the coating has strong adhesion, wear resistance, and heat resistance. It does not easily fall off even when subjected to external forces such as assembly and disassembly.
2. Key Methods to Distinguish Blackened Bolts&Screws from Dacromet Black Bolts&Screws
2.1 Visual and Tactile Inspection
Blackened screws typically have a matte or slightly glossy black surface. The color may appear uneven, especially for hot - blackened screws, which may have subtle color differences in local areas. When touched, the surface of blackened screws often has a slightly oily feel if they have been oiled for rust prevention. In contrast, Dacromet black screws have a more uniform and delicate black surface with a matte texture similar to powder coating. Their color is consistent across the entire surface, and the tactile sensation is dry and non - oily, without the sticky feeling of oil stains.
2.2 Adhesion Test
The oxide film of blackened screws has weak adhesion. We can perform a simple scratch test with a sharp tool such as a utility knife. Gently scratch the surface of the screw; the oxide film of blackened screws is prone to peeling or falling off, and the silver - white substrate of the screw can be seen quickly. For Dacromet black screws, even with the same scratching force, the coating is not easy to peel off. It requires greater force to damage the coating, and the scratched area does not show large - area flaking.
2.3 Solvent Test
Prepare a common organic solvent such as acetone or alcohol. Soak a cotton swab in the solvent and wipe the surface of the screw repeatedly. For blackened screws, the solvent will not cause obvious changes to the oxide film, but if there is anti - rust oil on the surface, the oil will be dissolved, and the cotton swab will be stained with oil. For Dacromet black screws, the coating is resistant to organic solvents, and the cotton swab will not be stained with black pigment after wiping. In contrast, some inferior blackened treatments may have unstable colorants that fade when wiped with solvents, which can also serve as an auxiliary distinction method.
2.4 Cross - Section Observation
If conditions permit, we can observe the cross - section of the screw through a magnifying glass or a microscope. The oxide film of blackened screws is extremely thin, and the boundary between the film and the substrate is not very distinct under low magnification. The Dacromet coating has a thickness of 8 - 12 micrometers, which is significantly thicker than the blackened oxide film. Under magnification, a clear layered structure can be seen between the coating and the substrate, and the zinc - aluminum flake layer can be faintly observed.
3. Salt Spray Test Performance Comparison
The salt spray test is a common method to evaluate the corrosion resistance of metal materials and their surface treatments. It simulates the corrosive environment in marine or high - humidity areas by spraying a saltwater mist. The longer the time a sample can withstand the salt spray without rusting, the better its corrosion resistance.
3.1 Salt Spray Resistance of Blackened Bolt&Screws
Blackened screws have very limited salt spray resistance. The thin oxide film can only provide basic protection. In the neutral salt spray test (the most commonly used salt spray test standard, with a 5% sodium chloride solution), ordinary blackened screws without oiling will start to show rust spots on the surface within 2 - 8 hours. Even if they are oiled, the rust prevention effect can only be extended to 24 - 48 hours. Once the oil film is damaged by friction or erosion, the oxide film will quickly lose its protective effect, and the screw will rust rapidly. This is because the oxide film itself does not have self - repairing ability. Once there is a gap or damage, the corrosive medium will directly erode the substrate.
3.2 Salt Spray Resistance of Dacromet Black Bolts&Screws
Dacromet black screws have excellent salt spray resistance, which is one of their core advantages. The zinc - aluminum flake structure in the Dacromet coating can play a role in cathodic protection. When the coating is slightly damaged, the zinc and aluminum with higher activity will undergo an electrochemical reaction first, sacrificing themselves to protect the iron - based substrate of the screw. In the neutral salt spray test, qualified Dacromet black screws can withstand salt spray corrosion for 500 - 1000 hours, and some high - performance Dacromet coatings can even exceed 1000 hours. This performance is far superior to that of blackened screws, making them suitable for long - term use in coastal areas, high - humidity workshops, or outdoor equipment.
3.3 Test Result Analysis
The huge gap in salt spray resistance between the two types of screws stems from the fundamental differences in their surface treatment principles. The blackening process only forms a thin oxide film on the surface, which is a passive protective layer. It can only isolate the substrate under ideal conditions. In contrast, the Dacromet coating combines barrier protection and cathodic protection. The overlapping zinc - aluminum flakes form a physical barrier, and the electrochemical reaction of the metal flakes provides active protection. This dual protection mechanism enables Dacromet black screws to maintain good corrosion resistance in harsh environments.
4. Selection Suggestions Based on Application Scenarios
When choosing between blackened screws and Dacromet black screws, we should make decisions based on the specific application scenario, service life requirements, and environmental conditions, as follows:
- For indoor equipment, furniture assembly, or temporary fixing scenarios where the environment is dry and there is no high requirement for corrosion resistance, blackened screws can be selected. They have the advantages of low cost and simple processing, which can control the overall cost.
- For scenarios such as automotive chassis parts, marine equipment, outdoor construction machinery, or electrical equipment in high - humidity areas, Dacromet black screws are the preferred choice. Their excellent salt spray resistance can ensure the stability and safety of the equipment during long - term use and reduce the maintenance cost caused by screw rust.
In conclusion, blackened screws and Dacromet black screws can be effectively distinguished through multiple methods such as appearance, adhesion, and solvent tests. In terms of salt spray resistance, Dacromet black screws are far more outstanding than blackened screws. Choosing the appropriate type of screw according to the actual needs can not only ensure the performance of the product but also avoid unnecessary losses caused by improper selection.
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