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5 Essential Surface Coatings For High-strength Structural Bolts

Mar 27, 2025

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Contents
  1. Introduction
  2. Why Coatings Are Critical For Structural Bolts?
  3. 5 Surface Coatings for High-Strength Bolts
    1. Phosphating
      1. Pros
      2. Cons
    2. Black oxide finish
      1. Pros
      2. Cons
    3. Zinc Plating (Z/P)
      1. Pros
      2. Cons
    4. Hot-dip galvanizing (HDG)
      1. Pros
      2. Cons
    5. Dacromet coating
      1. Pros
      2. Cons
  4. 5 Surface Coatings Detailed Comparison

 

Introduction

 

High-strength bolts play a critical role in modern steel structures and industrial engineering. By applying preload tension, they tightly connect structural components, transferring loads through friction or bearing pressure, significantly enhancing joint strength, stiffness, and fatigue resistance.

 

From long-span steel bridges and skyscrapers to industrial steel workshops, bolts are everywhere. You may notice they come in silver, black, or even colored finishes. But why do these surface coatings vary, and what purpose do they serve? This article delves into the details.

 

Why Coatings Are Critical For Structural Bolts?

 

1

Corrosion Resistance: Protects against rust in harsh environments (e.g., marine, industrial).

2

Friction Enhancement: Improves grip between bolts and connected parts, maintaining preload.

3

Durability: Extends service life by preventing wear and hydrogen embrittlement.

4

Compliance: Meets international standards (ASTM, ISO) for safety and reliability.

 

5 Surface Coatings for High-Strength Bolts

 

There are 5 common surface treatments for steel structure bolts, which are: phosphating, black, Z/P, hot-dip galvanized (HDG), and dacromet.

 

Phosphating

 

plain phosphined 109s grade structural bolts

 

The phosphating surface coating improves corrosion resistance and wear resistance by forming a phosphating layer on the high-strength bolts.

 

Pros

Basic corrosion resistance: offers fundamental rust and corrosion resistance in mild environments.

Frequent assembly/disassembly friendly: Ideal for steel structural parts requiring regular maintenance or reassembly.

Cost-effective: Low production cost and easy application compared to more complex treatments.

Enhanced paint adhesion: Phosphate coating serves as an excellent primer layer, improving the bonding of subsequent coatings.

 

Cons

Limited aesthetic appeal: Only available in a gray color, lacking decorative options.

Short-term protection: It cannot provide long-term anti-corrosion or rust resistance.

Dependent on anti-rust oil: It needs anti-rust oils for extended protection, increasing maintenance effort.

 

Black oxide finish

 

black oxide high strength bolts

 

Black oxide is a widely used surface treatment for high-strength steel structural bolts. The process involves immersing the bolt in an alkaline chemical solution containing an oxidizing agent and heating it to form a thin ferric oxide (Fe3O4) layer (0.6-0.8μm) on the surface. This black oxide coating provides basic corrosion resistance while maintaining dimensional accuracy, making it ideal for high-strength and precision fasteners.

 

Pros

The black oxide film isolates the bolt from air, slowing rust formation in mild environments.

Simple process with low production costs, ideal for mass production.

The finish shows a uniform black color, with a good appearance.

Low hydrogen embrittlement risk (safe for high-strength bolts).

The thickness of the film hardly affects the use of bolts, suitable for high-precision steel structures.

 

Cons

Limited ability to prevent rust and corrosion.

It needs to be used together with anti-rust oil.

 

Zinc Plating (Z/P)

 

electro-galvanizing high strength bolts

 

The surface treatment of electro-galvanizing is to coat the surface of the bolt with a zinc layer (3-30μm) through and electrochemical reaction. The resulting silver-white finish offers both functional and aesthetic benefits, making it a popular choice for industrial fasteners. Galvanized high-strength bolts typically withstand 72 hours in salt spray tests.

 

Pros

Superior corrosion resistance: Zinc acts as a sacrificial layer, providing cathodic protection to prevent rust even if the coating is scratched.

Z/P surface coating is cost-effective.

Bright silver-white appearance is suitable for visible applications.

The thickness of the zinc layer can be adjusted according to different application requirements.

 

Cons

Higher risk of hydrogen embrittlement (unsuitable for Grade 10.9 bolts).

The zinc layer is highly susceptible to wear and tear during the assembly process.

Sewage generated during the electroplating process needs to be treated strictly, otherwise it affects the environment.

 

Hot-dip galvanizing (HDG)

 

hot dip galvanized high strength structural bolts

 

Hot-dip galvanizing (HDG) is the industry-preferred surface treatment for structural steel bolts, providing unmatched corrosion protection through a zinc coating (>50μm thick).

 

Pros

A thick zinc layer offers long-term protection in harsh environments.

The hard zinc layer withstands friction and vibration, ideal for dynamic loads.

Hot-dip galvanizing has no risk of hydrogen embrittlement and is suitable for high strength bolts of grade 10.9 and grade 12.9.

 

Cons

The process of hot-dip galvanizing is complicated, and the cost is 30%-50% higher than Z/P.

The thicker zinc layer may affect the accuracy of the threads.

Hot-dip galvanized wastewater needs professional treatment.

 

Dacromet coating

 

darcromet structural hex head bolts

 

Dacromet is an advanced zinc-aluminum-chromium composite coating that delivers exceptional corrosion protection and thermal stability for critical fastener applications.

 

Pros

Excellent corrosion resistance and wear resistance.

Exceeds 500 hours in salt spray tests.

No risk of hydrogen embrittlement, suitable for high-strength bolts.

Dacromet coating can withstand high temperatures up to 300℃.

Uniform coating does not affect the thread accuracy.

 

Cons

Not suitable for grounding/conductive applications.

Damaged coatings are difficult to repair.

 

5 Surface Coatings Detailed Comparison

 

Coating Types

Phosphating

Black

Z/P

HDG

Dacromet
Main advantages
  • Basic rust protection.
  • Suitable for frequent assembly/ disassembly.
  • Low cost and simple process.
  • Suitable for mild environments.
  • Uniform black finish with aesthetic appeal.
  • Low hydrogen embrittlement risks
  • Good corrosion resistance.
  • Uniform zinc layer with an attractive silver-white finish.
  • Strong corrosion resistance.
  • High hardness improves wear resistance.
  • No hydrogen embrittlement risk.
  • Excellent corrosion resistance.
  • High-temperature resistance.
  • Uniform coating preserves thread precision.
Main disadvantages
  • Single color
  • Limited rust protection
  • Requires rust-oil

Weak protection
  • High hydrogen embrittlement risk.
  • Wastewater requires strict treatment.
  • Rough surface impact aesthetics.
  • 30%-50% higher cost than Z/P.
  • High processing cost.
  • Difficult to repair if damaged.
Application

Indoor, steel components requiring frequent disassembly

Indoor, mildly corrosion environments (e.g., steel workshops)

General indoor environments

Harsh environments( e.g., bridges, offshore platforms)

Extreme environments(e.g., aerospace, automotive)

 

Conclusion

 

Selecting the right coating for structural bolts hinges on environmental exposure, mechanical demands, and budget. Need tailored solutions for your project? Contact our team for expert guidance!