How do you specify hot-dip galvanizing?

Author: Cheryl

Oct. 21, 2024

Galvanizing Specifications

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Hot-dip galvanizing is done in accordance with long established ASTM specifications.  There are three main specifications (ASTM A123, A153, and A767) governing the coating thickness, adherence, and finish for hot-dip galvanized steel and a handful of supporting specifications referenced in these specifications; cover design practices, repair and touch-up, and painting/powder coating over galvanizing.

ASTM Specifications

Specifying hot-dip galvanizing is simply done by using ASTM specifications developed to produce high-quality galvanized coatings. The main specifications that pertain to hot-dip galvanizing are:

  • ASTM A123 - general iron and steel products
  • ASTM A153 - small parts
  • ASTM A767 - reinforcing steel (rebar)

ASTM A123

ASTM A123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products covers the requirements for galvanizing by the hot-dip process on iron and steel products made from rolled, pressed, and forged shapes, castings, plates, bars, and strips. It covers both un-fabricated products and fabricated products, for example, assembled steel products, structural steel fabrications, large tubes already bent or welded before galvanizing, and wire work fabricated from uncoated steel wire. This specification also covers steel forgings and iron castings incorporated into pieces fabricated before galvanizing or which are too large to be centrifuged (or otherwise handled to remove excess galvanizing bath metal). ASTM A123 does not apply to wire, pipe, tube, or steel sheet which is galvanized on specialized or continuous lines. The specification includes minimum coating thicknesses according to material category as well as finish and adherence requirements.

ASTM A153

ASTM A153 Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, covers zinc coatings applied by the hot-dip process on iron and steel hardware. It is intended to be applicable to hardware items that are centrifuged or otherwise handled to remove excess zinc. ASTM A153 also contains information on minimum coating thickness as well as finish and adherence requirements.

ASTM A767

ASTM A767 Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement, covers zinc coatings applied by the hot-dip process on individual steel reinforcing bars before fabrication used in concrete. The specification says bars shall have NO bare spots and shall be free from tears or sharp spikes which make the bar hazardous to handle, and bars that stick together after galvanizing shall be rejected. Minimum bend diameters for finished bars are included in the specification to prevent the coating from flaking on bend radii, as are requirements for coating thickness according to bar size. All rebar fabrications shall be galvanized to the requirements of ASTM A123.

Other Specifications 

All members of the AGA must follow the ASTM standards. However, sometimes a galvanizer may receive requests from designers/specifiers who use other galvanizing standards such as international ones or those produced by other organizations (SSPC, AASHTO, etc.). Here are some other specifications for hot-dip galvanizing you occasionally see used in North America. These specifications have slightly different requirements for coating thickness, but most refer back to the ASTM specifications, and by following ASTM specifications the galvanizer will meet the requirements outlined in them.

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  • ISO : Hot-Dip Galvanized Coatings on Fabricated Iron and Steel Articles Spec and Test Methods
  • CSA G164: Galvanizing of Irregularly Shaped Articles
  • AASHTO M111: Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products (ASTM A123)
  • AASHTO M232: Zinc Coating (Hot-Dip) on Iron and Steel Hardware (ASTM A153)
  • M180: Corrugated Sheet Steel Beams for Highway Guardrail

Specifying Hot-Dip Galvanizing

Specifying hot-dip galvanizing is simply done by using ASTM specifications developed to produce high-quality galvanized coatings. The main specifications that pertain to hot-dip galvanizing are: 

  • ASTM A123 Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products
  • ASTM A153 Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
  • ASTM A767 Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement 

All three of these specifications govern coating thickness, adherence, and finish for hot-dip galvanized coatings. A123 is the main specification and covers all types of galvanized products except fasteners and small parts which are covered by A153, and reinforcing steel bars, covered by A767.

All AGA members follow the ASTM standards. However, sometimes a galvanizer receives requests from designers/specifiers who use other galvanizing standards such as international ones or those produced by other organizations (SSPC, AASHTO, etc.) Here are a few common specifications from other entities occasionally used in North America:

  • ISO  Hot-Dip Galvanized Coatings on Fabricated Iron and Steel Articles - Specification and Test Methods
  • CSA G164 Galvanizing of Irregularly Shaped Articles
  • AASHTO M111 Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products (similar to ASTM A123)
  • AASHTO M180 Corrugated Sheet Steel Beams for Highway Guardrail
  • AASHTO M232 Zinc Coating (Hot-Dip) on Iron and Steel Hardware (similar to ASTM A153)

These specifications have slightly different requirements for coating thickness, but most refer back to the ASTM specifications, and by following ASTM specifications the galvanizer will meet the requirements outlined in them.

Supporting Specifications

In addition to these three main specifications, there are a collection of supporting specifications related to hot-dip galvanized steel.  There are four specifications the architect, engineer, and fabricator should be aware of before galvanizing, as they give best practices on steel selection and proper design:

  • ASTM A143 Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement
    • Practices to avoid strain-age embrittlement from cold working; absorption of hydrogen
  • ASTM A384 Practice for Safeguarding Against Warpage and Distortion During Hot-Dip Galvanizing of Steel Assemblies
    • Design practices to minimize issues due to the galvanizing temperature/stress relief
  • ASTM A385 Practice for Providing High-Quality Zinc Coatings (Hot-Dip)
    • Details on steel selection, design practices such as venting/draining, welding, etc.
  • ASTM A Practice for Life-Cycle Cost Analysis of Corrosion Protection Systems on Iron and Steel Products
    • Calculation to analyze different steel corrosion protection systems (lccc.galvanizeit.org automates this calculation)

There are also three specifications specifiers may need for post-galvanizing considerations including:

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  • ASTM A780 Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings
    • Accepted forms of touch-up and proper repair for both in-plant and in-field
  • ASTM D Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting
    • Details proper surface preparation for galvanized steel before painting
  • ASTM D Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating
    • Details proper galvanized surface preparation for powder coatings 

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