The color of a bolt is a direct manifestation of its surface treatment process, which significantly affects its anti-corrosion performance, mechanical properties, and service life.
Understanding Corrosion Resistance and Applicable Scenarios from Color Perspectives
This article systematically reviews the technical principles, core features, and typical applications of mainstream processes such as blackening, galvanizing, electrophoresis, and Dacromet, providing a selection guide based on environmental, strength, and functional requirements to help make the best decisions among functionality, reliability, and economy.
Bolts, as indispensable fasteners in our daily life and work, their surface color is not merely an aesthetic choice but a direct reflection of the surface treatment process. This “coat” determines the bolt’s anti-corrosion performance, mechanical properties, service life, and applicable scenarios.
Today, we will peel back the surface of bolt colors and sort out the various common surface treatment processes, jointly analyzing their technical principles and differences.
01 Black Series: The Fusion of Classic and Functionality
Black series bolts usually give people a visual impression of sturdiness and professionalism, but the technical paths behind them are diverse, with significant performance differences. Let’s take a look at each one respectively.
1. Oxidation Blackening/Bluing
- Principle: Through chemical or high-temperature methods, the surface of steel and iron reacts with alkaline solutions or hot air to form a dense layer of iron oxide (Fe₃O₄) film, typically only 0.6-1.5 microns thick. This process does not involve metal deposition.
- Core Features: This process has the lowest cost and is easy to operate. However, the oxide film it generates is extremely thin, with very limited anti-corrosion ability. The time to rust in a neutral salt spray test is usually only 3-5 hours. Therefore, bolts that have undergone blackening must be used in conjunction with anti-rust oil coating to prevent rust in the short term by isolating air and moisture with an oil film. The black appearance is actually the optical interference color of the oxide film, not the color of a coating.
- Typical Applications: Common tools, internal structural components of mechanical equipment, and some non-critical connection parts that emphasize a uniform black appearance where anti-corrosion requirements are not high.
2. Black Zinc Plating
- Process principle: Based on electro-galvanizing, the zinc coating undergoes a special black passivation treatment. Specific components in the passivation solution (such as silver salts, copper salts, etc.) react with zinc to form a black conversion film.
- Core features: It has significantly better anti-corrosion performance than oxidation blackening, with a salt spray test duration of approximately 12 hours. It combines the cathodic protection effect of electro-galvanizing and the barrier protection of the passivation film, presenting a uniform deep black or blackish gray color and good appearance quality.
- Typical applications: Situations requiring certain anti-corrosion capabilities and a black appearance, such as the fixation of small household appliance shells, internal structures of electronic products, photographic equipment, and the installation of some outdoor black guardrails.
3. Electrophoretic black
- Process Principle: It belongs to organic coating technology. The workpiece is immersed in water-soluble black electrophoretic paint, and under the action of an electric field, the resin and pigment in the paint solution deposit on the anode (or cathode) workpiece to form a uniform coating, which is then baked and cured.
- Core Features: The coating is uniform and dense, without the edge effect of electroplating processes, and can cover the inner and outer surfaces of complex workpieces. It has excellent anti-corrosion performance, with salt spray tests reaching over 300 hours or even longer. The coating also has good wear resistance and chemical resistance. The color is pure black.
- Typical Applications: Scenarios with high requirements for both corrosion resistance and appearance, such as high-end automotive chassis components, precision instruments, outdoor high-value equipment, and building curtain wall fastening.
02 Silver White Series: A Balance of General and Professional
Silver white is the most classic color spectrum for bolts, covering a wide range of solutions from economical and general to professional and high-end. Let’s take a look at what they are respectively.
1. White Zinc Plating (Electroplating)
- Process Principle: Utilizing the principle of electrolysis, a layer of pure zinc is deposited on the surface of the bolt. Usually, a blue-white passivation (trivalent chromium or colorless passivation) is carried out subsequently to form an extremely thin transparent conversion film for enhanced protection.
- Core Features: This is the most economical and widely applied anti-corrosion process. The zinc layer acts as a sacrificial anode, effectively protecting the underlying steel. Its anti-corrosion performance is moderate, with salt spray test duration varying depending on thickness, typically ranging from 24 to 72 hours. However, this process poses a risk of hydrogen embrittlement, especially for high-strength bolts of grade 8.8 and above, which require strict dehydrogenation treatment. The appearance is a shiny silver-white with a bluish-white tint.
- Typical Applications: It is widely used in most indoor and general outdoor environments in both industrial and civil fields, such as furniture assembly, mechanical equipment, and steel structure connections.
- Process principle: A layer of nickel is deposited on the substrate through an electrochemical method. It can be either decorative bright nickel or functional dark nickel.
- Core features: The coating has a bright and lustrous appearance, excellent wear resistance, and certain corrosion resistance (approximately 6-12 hours in salt spray tests). Its protective mechanism mainly relies on a physical barrier. The cost is higher than that of galvanizing, and it also has the risk of hydrogen embrittlement. In a humid environment, the nickel coating may induce pitting corrosion of the underlying steel.
- Typical applications: It is mainly used for decorative parts, bathroom hardware, frequently disassembled precision instrument connectors, and situations with mild requirements for electromagnetic shielding.
3. Stainless steel natural color
- Principle of Technology: It is not a surface coating but the base material of the bolt itself is austenitic (such as 304, 316) or martensitic (such as 410) stainless steel. Its rust prevention ability comes from the dense passivation film formed by chromium (Cr) in the alloy.
- Core Features: The anti-corrosion performance depends on the material. For instance, SUS304 is suitable for general outdoor use (salt spray 48-96 hours), while SUS316 has stronger resistance to acids, alkalis, and salt spray. It has a wide range of strengths and no hydrogen embrittlement issue. However, it is costly, and some lower-grade stainless steels may suffer from stress corrosion cracking in environments with chloride ions.
- Typical Applications: Food processing equipment, medical devices, chemical plants, ships, coastal buildings, and other highly corrosive environments and fields with strict hygiene requirements.
03 Color Series: Balancing Identification and Performance
Color bolts are not only used to distinguish specifications, but their colors often directly relate to specific passivation processes and anti-corrosion grades. This series is rare, let’s take a look at some of the uncommon varieties.
1. Color Zinc Plating / Yellow Zinc Plating
- Principle: After electro-galvanizing, the part is treated in a chromate passivation solution containing hexavalent chromium or trivalent chromium to form a thick passivation film. Due to the interference of the film layer, it presents a rainbow color or yellowish green.
- Core Features: The passivation film itself provides additional barrier protection and self-healing ability. When the thickness of the galvanized layer is the same, the anti-corrosion performance of the colored passivation is superior to that of the blue-white passivation. The salt spray test time can be extended to 72-200 hours or even longer, depending on the type of passivation and sealing process. Hexavalent chromium passivation is being replaced by the environmentally friendly trivalent chromium passivation.
- Typical Applications: It is widely used in industries such as automotive parts, power facilities, and general machinery, especially in scenarios where medium to high-level corrosion resistance is required and color coding for bolt grades or workstations is necessary.
2. Blue Zinc (Blue-White Zinc)
- Process principle: After electro-galvanizing, a thin passivation film with a bluish tint is formed in a specific passivation solution.
- Core features: Its anti-corrosion performance lies between that of white zinc and colored zinc, with salt spray test duration typically ranging from 48 to 120 hours. The appearance is a uniform light blue color, with a refreshing hue.
- Typical applications: Electronic products, electrical appliance casings, high-end sheet metal parts connections, etc., where there is a certain requirement for anti-corrosion and a preference for light-colored appearances.
3. Green Zinc (Military Green)
- Process Principle: It employs a special dark green passivation process, typically containing special organic or silicate sealants.
- Core Features: This is a leading performer in the passivation process in terms of anti-corrosion, with salt spray test times exceeding 200 hours. The coating is relatively thick, presenting a deep military green color, with a slightly oily feel and slightly lower wear resistance.
- Typical Applications: Military equipment, outdoor heavy machinery, and fasteners that are exposed to harsh environments for long periods but are not suitable for hot-dip galvanizing.
04 High-Performance Coatings: A Technical Benchmark Beyond Color
This type of process may not have a bright color, but it represents the pinnacle of anti-corrosion technology. It is not uncommon, and many friends have likely used it before.
1. Dacromet (Zinc-Aluminum Coating)
- Process principle: A slurry containing ultrafine zinc flakes, aluminum flakes, chromic acid and organic substances is applied by dip coating or spray coating onto the workpiece, and then cured at high temperature (around 300°C) to form a multi-layered inorganic-organic composite coating with a shingle-like structure.
- Core features: No hydrogen embrittlement, excellent corrosion resistance (can pass neutral salt spray test for over 600-1000 hours), high heat resistance (can withstand up to 300°C). The coating is matte silver-gray or can be adjusted to black as required. The drawbacks are that it is non-conductive, which may affect the precision of threads, and it is not suitable for point contact electrical connections.
- Typical applications: High-heat and high-corrosion areas such as automotive chassis and engines; high-speed rail and power transmission towers; standard anti-corrosion solution for high-strength bolts (grade 10.9 and 12.9).
2. Hot-dip galvanizing
- Process principle: After pickling, the bolts are immersed in molten zinc at approximately 450-480℃ to form a zinc-iron alloy layer and a pure zinc layer.
- Core features: The coating is thick (typically 50-150 microns), has a long service life, and is a classic method for heavy-duty anti-corrosion. However, the coating is heavy and uneven, prone to forming zinc blisters, which can affect the thread fit and require post-threading. High temperatures may have an adverse effect on the mechanical properties of high-strength bolts.
- Typical applications: Transmission towers, highway guardrails, large outdoor steel structures, bridges, and other infrastructure exposed to the atmosphere for long periods.
05 Selection Guide: Return to the Essence from Color
In our daily work or engineering projects, when choosing the appropriate surface treatment for bolts, I believe the following logic should be followed:
1. Environment determines the demand: Clearly identify the corrosion factors that the bolts will face (humidity, salt, chemicals, temperature). For indoor dry environments, economical processes can be selected; for outdoor, humid, or industrial atmospheric environments, choose galvanized zinc, hot-dip galvanizing, or Dacromet; for harsh environments (such as chemical or marine), prioritize stainless steel, Dacromet, or special coatings.
2. Strength matches the process: For bolts of grade 8.8 and above, processes without hydrogen embrittlement risk should be preferred, such as Dacromet, phosphating, zinc diffusion, or powder zinc diffusion. If electro-galvanizing is necessary, the effectiveness of dehydrogenation treatment must be specified and verified.
3. Functional additional considerations: If electrical conductivity is required (such as for grounding), non-conductive coatings like Dacromet should be avoided; for parts that need frequent disassembly, nickel plating or bolts with lubricating coatings (such as PTFE) can be considered; for applications with food contact requirements, specific grades of stainless steel or coatings with relevant certifications must be used.
4. Comprehensive assessment of cost and environmental impact: Balance the initial cost and the total life cycle cost while meeting performance requirements. At the same time, pay attention to environmental regulations (such as RoHS, REACH), and prioritize environmentally friendly processes such as chromium-free Dacromet or trivalent chromium passivation.
06 Simple Summary
The color world of bolts is essentially a condensed history of materials science and surface engineering. From the ancient bluing process to modern chromium-free Dacromet, each color corresponds to a unique technical solution.
When choosing, we must not “judge a bolt by its color” but should deeply understand the process essence, performance parameters, and application boundaries behind the color to make the best decision in terms of functionality, reliability, and economy.
In the future, with the tightening of environmental regulations and the increase in performance requirements, bolt surface treatment technologies will continue to evolve towards more environmentally friendly, intelligent, and functionally integrated directions.
If you are looking for suitable bolts and nuts, please visit us here https://hktl-fastener.com/bolts-and-nuts/.
