Hot Dip Galvanizing Process for Fasteners: Complete Guide (HDG Coating, Thickness & Standards)

Hot-Dip Galvanizing (HDG) is a widely used corrosion protection process where steel fasteners like hex bolts, hex nuts, washers, and studs are coated with molten zinc at ~450°C. This creates a durable, metallurgical bond that protects against rust for decades. The process involves surface cleaning, pickling, fluxing, galvanizing, and inspection. HDG is ideal for heavy-duty applications such as construction, automotive, and infrastructure projects. Compared to electroplating, HDG offers thicker coatings (40–200 microns) and superior longevity. At New Star Industries, we ensure precision galvanizing for high tensile fasteners to meet global standards and long-term performance requirements.

Why Hot-Dip Galvanizing is Important for Fasteners

Fasteners are critical load-bearing components. Without proper coating:

• Rust reduces strength
• Threads get damaged
• Structural integrity is compromised

Hot-dip galvanizing ensures:

• ✔ Uniform coating even in threads
• ✔ High abrasion resistance
• ✔ Long service life in outdoor conditions
• ✔ Reduced maintenance cost

At New Star Industries, HDG is widely applied to:

• Hex Head Bolts / Screws
• Heavy Hex Structural Bolts
• Hex Nuts & Heavy Hex Nuts
• Studs (B7, Double End Threaded)
• Washers (Structural, Plain, DTI)

Step-by-Step Hot-Dip Galvanizing Process

1. Surface Preparation (Degreasing)

• Removes oil, grease, dirt
• Uses alkaline or acidic cleaning solutions
• Temperature: 50–90°C

2. Pickling (Acid Cleaning)

• Removes mill scale and rust
• Uses Hydrochloric Acid (HCl) or Sulfuric Acid (H₂SO₄)
• Typical concentration: 10–15%
• Time: 5–30 minutes

3. Rinsing

• Removes acid residues
• Prevents contamination of flux solution

4. Fluxing

• Uses Zinc Ammonium Chloride (ZnNH₄Cl)
• Prevents oxidation before galvanizing
• Improves zinc adhesion

5. Drying

• Removes moisture before dipping
• Temperature: ~100–120°C

6. Hot-Dip Galvanizing (Main Process)

• Immersion in molten zinc bath
• Temperature: 445°C – 455°C
• Reaction forms zinc-iron alloy layers

Coating Layers Formed:

• Gamma Layer (Fe3Zn10)
• Delta Layer (FeZn7)
• Zeta Layer (FeZn13)
• Eta Layer (Pure Zinc Outer Layer)

7. Cooling / Quenching

• Air cooling or water quenching
• Sometimes chromate passivation applied

8. Inspection & Testing

• Coating thickness measurement
• Adhesion testing
• Visual inspection

Coating Thickness & Dimensions

Thread Considerations for Fasteners

Hot-dip galvanizing affects thread dimensions:

• Zinc coating adds thickness
• Internal threads (like Hex Nuts) are tapped oversize
• External threads (like Hex Bolts) may be undercut

Standard Adjustments:

• Nut oversizing: +0.3 mm to +0.5 mm
• Thread tolerance: As per ISO 965 / IS 1367

Standards for Hot-Dip Galvanizing

• ASTM A153 / A153M – Fasteners
• ASTM A123 – Structural steel
• ISO 1461 – General galvanizing
• IS 2629 / IS 4759 – Indian standards

New Star Industries follows strict quality compliance aligned with:

• ISO/TS 16949:2016
• ISO 9001:2015

Advantages of Hot-Dip Galvanizing

1. Long-Term Protection
   20–50 years lifespan depending on environment
2. Complete Coverage
   Covers threads, edges, cavities
3. Strong Bond
   Metallurgical bonding, not just surface coating
4. Cost Effective
   Low maintenance over lifecycle
5. Eco-Friendly
   Zinc is recyclable

Limitations of Hot-Dip Galvanizing

• Not suitable for very tight tolerance parts
• Thread fitting requires adjustments
• Surface finish is rougher than electroplating

Applications of Hot-Dip Galvanized Fasteners

Hot-dip galvanized fasteners are used in:

• Infrastructure (bridges, highways)
• Power transmission towers
• Automotive sector
• Construction & steel structures
• Railways & heavy engineering

New Star Industries: Expertise in Galvanized Fasteners

With 60+ years of manufacturing experience, New Star Industries specializes in:

• ✔ High tensile fasteners
• ✔ Automotive & industrial components
• ✔ Advanced galvanizing support
• ✔ Precision-engineered Hex Bolts, Hex Nuts, Studs, Washers

The company operates a state-of-the-art facility with CNC machines, forging presses, heat treatment furnaces, and quality testing labs

Internal Linking Opportunities (SEO Boost)

To strengthen SEO and product visibility, link this blog to:

• Hex Head Bolts / Screws (HDG coating importance)
• Hex Nuts & Heavy Hex Nuts (thread adjustment after galvanizing)
• Structural Bolts (EN 14399)
• Washers (DTI, Structural, Plain)
• Studs (B7 Studs, Double End Studs)

Hot-Dip Galvanizing is not just a coating—it’s a long-term investment in durability and performance. For industries relying on strength, safety, and longevity, HDG-coated fasteners like hex bolts and hex nuts are essential.

At New Star Industries, we combine precision manufacturing with advanced surface protection techniques to deliver products that perform reliably in the toughest environments.