1. Core Purposes of Surface Finishing on Bolts
Corrosion & Rust Prevention (Primary Function) A protective layer separates the bolt substrate from oxygen, moisture, salt spray and chemical liquids. It greatly extends service life for outdoor, coastal, marine and chemical industrial applications.
Adjust Friction Coefficient & Anti-Loosening Performance Different coatings deliver stable friction values. Engineers can calculate accurate tightening torque. Special coatings like PTFE effectively stop thread galling and loosening under long-term vibration.
Decorative Appearance & Grade Identification Uniform finishes create neat surface looks. Different colors help buyers distinguish material grades, tensile strength and application scenarios at a glance.
Protect Base Material & Avoid Processing Defects Proper surface treatment eliminates risks such as hydrogen embrittlement for high-tensile bolts. It also prevents scratches during assembly and optimizes welding, painting compatibility.
2. Functions of Common Bolt Surface Treatments
Zinc Plating (Blue/Color Zinc)
Provides basic anti-corrosion protection with 48–96 hours neutral salt spray resistance.
Low cost for mass production.
Limitation: Not recommended for Grade 8.8 / 10.9 / 12.9 high-strength bolts due to hydrogen embrittlement risk.
Suitable for indoor carbon steel fasteners under dry conditions.
Hot Dip Galvanized (HDG)
Thick zinc coating offers outstanding rust resistance (over 500h salt spray).
Perfect for outdoor construction, solar brackets, bridges and anchor bolts exposed to rain and sunlight.
Resists abrasion during transportation.
Note: Larger tapped nuts are required to match thick coating threads.
Black Oxide
Forms a thin magnetite protective film for mild short-term rust resistance.
Matte black non-reflective appearance.
Nearly zero change to thread size, stable tightening torque.
Must cooperate with anti-rust oil; not fit for damp or outdoor environments.
Stainless Steel Passivation (A2 304 / A4 316)
Removes free iron impurities left from machining to avoid surface rust spots.
Reinforces the native chromium oxide film on stainless steel, boosting salt spray and acid-alkali resistance.
No extra coating, does not affect thread fitting.
Widely used for marine, food and chemical stainless fasteners.
Dacromet / Magni Coating
Zinc-aluminum composite coating with excellent anti-corrosion performance (500–1000h salt spray).
Zero hydrogen embrittlement, the top choice for high tensile 10.9 & 12.9 bolts.
Stable friction coefficient, heat-resistant and vibration-proof.
Standard finish for automotive chassis, new energy vehicle and wind power fasteners.
Mechanical Zinc Plating
Low-temperature coating process without strong acid electrolysis, completely avoiding hydrogen embrittlement.
Better anti-rust capacity than ordinary electroplating, lower cost than Dacromet.
Uniform coating for long bolts and special-shaped fasteners.
Phosphating (Manganese / Zinc Phosphating)
Acts as a base pretreatment to strengthen adhesion of paint, primer and anti-rust oil.
Forms porous film to store lubricant, improving assembly smoothness and wear resistance.
Poor anti-corrosion alone; must be coated with oil or paint afterwards.
ED Electrophoretic Coating
Even organic resin coating covers every corner including thread gaps.
Combined anti-rust and wear resistance with elegant matte black look.
No hydrogen embrittlement risk, widely adopted for home appliances and new energy parts.
Nickel / Chrome Plating
Bright decorative surface with good wear resistance and weak anti-corrosion ability.
High production cost; hydrogen embrittlement risk for high-strength bolts.
Mainly used for hardware, furniture and sanitary decorative screws.
PTFE / Molybdenum Disulfide Coating
Ultra-low friction coefficient to prevent thread galling and seizing.
Heat-resistant and resistant to acid, alkali and salt corrosion.
Enables precise torque control for high-temperature pipelines and chemical equipment bolts.
Short Product Description Snippets
Zinc plated: Basic anti-corrosion finish for indoor dry environments
Hot dip galvanized: Heavy-duty rust protection for outdoor construction & solar projects
Dacromet coating: Hydrogen embrittlement free, high salt spray resistance for high tensile bolts
Passivation: Enhance corrosion resistance of 304 & 316 stainless steel fasteners
Black oxide: Matte decorative finish for indoor mechanical components
PTFE coated: Anti-seize & low friction coating for high-temperature chemical equipment
3.We can provide you with a summary of commonly used surface treatments.
1. Electroplating Zinc (Blue/White Zinc / Color Zinc / Black Zinc)
Advantages
-Lowest cost, fast processing, high cost-effectiveness in bulk production
-The exterior is glossy and has a decent decorative appeal.
-Basic salt spray protection: Blue-white zinc 8–24h, Colored zinc 48–120h
Disadvantages
-Hydrogen embrittlement poses an extremely high risk; ordinary electroplating is strictly prohibited for high-strength bolts (grade 8.8 and above).
-The coating is thin, has poor wear resistance, and the zinc easily peels off with a slight bump.
-High temperature environments can cause peeling and failure.
2. Hot-dip galvanizing (Hot-dip zinc plating)
Advantages
-Thick coating (60–100 μm), extremely strong outdoor corrosion resistance, withstands hundreds of hours of salt spray.
-No hydrogen embrittlement, suitable for high-strength bolts of grades 8.8 and 10.9.
-Wear-resistant and scratch-resistant, suitable for steel structures, power transmission towers, and outdoor equipment.
Disadvantages
-The thick coating results in a tight thread fit, requiring the use of a special hot-dip galvanized nut.
-Rough surface, unattractive appearance
-High-temperature processing carries a risk of slight deformation of the bolts.
-Cost higher than electroplating zinc
3. Dacromet
Advantages
-No hydrogen embrittlement, suitable for all strength bolts
-Excellent corrosion resistance, neutral salt spray test 500–1000h
-The thin plating does not affect the thread fit, so there is no need to enlarge the nut.
-It can withstand temperatures up to around 250℃, as well as acids, alkalis, and moisture.
Disadvantages
-The cost is higher than electroplating zinc.
-The surface is matte gray-black, with poor decorative properties.
-The coating is not wear-resistant and is easily worn under friction conditions.
-The coating decomposes and fails at temperatures above 300°C.
4.Blackening
Advantages
-Extremely low cost and fast processing speed
-extremely thin coating does not change thread dimensions at all
-matte black appearance, smooth assembly.
Disadvantages
-It has almost no anti-corrosion ability, and the rust-preventive oil only provides short-term rust prevention.
-It rusts within days in humid, outdoor environments.
-Poor abrasion resistance, no protective layer, suitable only for use in dry indoor environments.
5. Natural Stainless Steel (201/304/316, no additional coating)
Advantages:
-Top-tier corrosion resistance;
-rust-free in seawater and acid/alkali environments for extended periods;
-No coating peeling or hydrogen embrittlement issues;
-High temperature resistance and a beautiful, bright finish.
Disadvantages:
-Extremely high material cost
-Prone to seizing and locking requiring anti-seizing grease
-Lower strength tensile strength is lower than high-strength carbon steel bolts of the same specifications
-Risk of galvanic corrosion will accelerate carbon steel rusting when used with carbon steel components.
6.PTFE Coating (Fluorocarbon Coating)
Advantages
-Extremely low coefficient of friction, effective against thread galling and seizing;
-Excellent corrosion resistance, resistant to acids, alkalis and salt spray;
-Self-lubricating, no additional oil lubrication required.
Disadvantages
-High material and processing cost;
-Soft coating surface, prone to scratches upon impact or collision;
-Coating will degrade and fail when operating temperature exceeds 260°C.
Every surface finish has its unique advantages and limitations. There is no one-size-fits-all solution for all fastening scenarios. Factors you need to consider before ordering include corrosion resistance standards, tensile grade, vibration conditions, welding processes and budget. Feel free to send us your drawing, application industry and performance requirements, we will recommend the most cost-effective surface treatment for your bolts and fasteners.
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