Galvanizing

A Complete Guide to Hot-Dip Galvanized Fasteners

Ultimate Corrosion Protection for Demanding Applications

When you need the highest level of corrosion protection for steel fasteners, few finishes can compare to the rugged durability of hot-dip galvanizing. It provides a thick, tough coating that can protect steel from the elements for decades, making it the premier choice for construction, infrastructure, and heavy industrial applications.

This guide explains the galvanizing process in detail, its benefits, the key technical standards, and the critical considerations required for threaded fasteners.

What is Hot-Dip Galvanizing?

Hot-dip galvanizing is a process where a steel fastener is completely immersed in a bath of molten zinc heated to around 450°C.

This is not just a simple surface coating; a metallurgical reaction occurs between the zinc and the steel. This reaction forms a series of tough zinc-iron alloy layers, which are bonded directly to the steel. A final layer of pure zinc sits on the outside. The result is a multi-layered, armoured shell that is exceptionally resistant to damage and corrosion.

The Galvanizing Process in Detail

To achieve a perfect, long-lasting coating, a controlled multi-stage process is essential.

  1. Stage 1: Surface Preparation
    • This is the most critical stage. The steel surface must be perfectly clean for the molten zinc to react with it.
    • > Degreasing - First, dirt, oil, and organic residues are removed using a caustic solution.
    • > Pickling - The fastener is then dipped in an acid bath (typically hydrochloric acid) to remove mill scale and iron oxides (rust).
    • > Fluxing - Finally, the fastener is treated with a flux solution (usually zinc ammonium chloride) to remove any remaining oxides and provide a final protective layer before entering the zinc bath.
  2. Stage 2: Galvanizing
    • The clean fastener is fully immersed in the bath of molten zinc. During this time, the zinc-iron alloy layers form on the surface. The component remains in the bath until its temperature rises to match the zinc, at which point the reaction is complete.
  3. Stage 3: Inspection
    • After being removed from the bath and cooled, every component is inspected. The inspection checks for a smooth, continuous coating and verifies that the coating thickness meets the requirements of the specified standard.

Key Benefits of Galvanizing

  • Exceptional Durability - The underlying zinc-iron alloy layers are harder than the steel itself, providing outstanding resistance to abrasion and impact damage.
  • Complete and Total Coverage - Because the process involves full immersion, every part of the fastener is coated, including sharp corners and edges.
  • Long-Term Service Life - A hot-dip galvanized coating can protect steel from corrosion for 50-100 years in most environments.
  • Sacrificial Protection - If the coating is scratched down to the steel, the surrounding zinc will corrode first, cathodically protecting the damaged area.

Galvanizing vs. Zinc Plating: What's the Difference?

  • The Process - Galvanizing involves dipping in molten zinc, creating thick alloy layers. Zinc plating uses an electric current in a chemical bath to deposit a thin layer of pure zinc.
  • Thickness & Durability - Galvanizing is much thicker and more durable, suitable for harsh outdoor use. Plating is thin and cosmetic, best for mild or indoor environments.
  • Appearance - Galvanizing is typically dull grey and matte. Plating is bright, shiny, and smooth.


Galvanizing and Fasteners: The Crucial Oversize Tapping Issue


The galvanizing process creates a very thick coating. This means a standard-sized nut will not fit a galvanized bolt.

The correct engineering solution is *oversize tapping*.

  1. The nut is hot-dip galvanized first.
  2. After galvanizing, the internal thread is re-cut (tapped) to a larger dimension.
    1. This removes the zinc from the internal threads, creating clearance for the coated bolt thread.

Once assembled, the large zinc coating on the bolt's thread provides sacrificial protection to the nut's bare steel threads, protecting them from corrosion.

Important Considerations & Limitations

  • High-Strength Fasteners - For very high-strength fasteners (Grade 10.9 and above), there can be a risk of hydrogen embrittlement or a change in the steel's mechanical properties due to the heat of the galvanizing bath. Special procedures are required.
  • Heat Distortion - The high temperature of the process can cause thin or long, slender parts to warp.
  • Size Constraints - The size of a component that can be galvanized is limited by the physical size of the galvanizer's zinc bath.

Trojan's Galvanized Fastener Solutions


At Trojan Special Fasteners Limited, we are experts in supplying and modifying galvanized fasteners to meet your exact project requirements. We can offer galvanized nuts in two configurations:

  • Standard Fit - For use with non-galvanized or zinc-plated bolts.
  • Oversize Fit - Correctly tapped oversize after galvanizing for use with hot-dip galvanized bolts.

Furthermore, Trojan Special Fasteners Limited can retap your existing galvanized nuts to the correct oversize dimension, ensuring a perfect fit.

Relevant Standards and Specifications

  • BS EN ISO 1461:2022 - The primary UK, European, and International standard for hot-dip galvanized coatings.
  • ASTM A153 / A153M-16a - The main US standard for zinc coating on smaller hardware and fasteners.
  • ASTM F2329 / F2329M-15 - A highly detailed US standard focused exclusively on hot-dip galvanizing fasteners.