ASTM A123/A123M Hot-Dip Galvanizing Standard: Coating Thickness, Inspection & Acceptance

Hot-dip galvanizing is one of the most widely used corrosion protection methods for steel structures, fabricated assemblies, and industrial steelwork. The ASTM A123/A123M specification defines the minimum requirements for zinc (hot-dip galvanized) coatings on iron and steel products—covering scope, coating thickness grades, finish/appearance, sampling, test methods, repair limits, and acceptance criteria.

This guide breaks down ASTM A123 in a practical, inspector-friendly way—so you can apply it in QA/QC, coating inspection, and site acceptance.

Note: ASTM A123 has newer editions beyond the 2002 version you shared (A123/A123M-24 is listed on ASTM’s site). Always confirm the project’s required revision in the contract.

Coating Inspection Procedure Guide

1) Scope: What ASTM A123 Covers (and Excludes)

Covered steel products (batch galvanizing)

ASTM A123 applies to hot-dip galvanizing on iron and steel products made from:

• Rolled, pressed, and forged shapes
• Castings
• Plates, bars, and strips
• Unfabricated and fabricated items (structural fabrications, assembled steel products, bent/welded tubes before galvanizing, fabricated wire work)

Not covered by ASTM A123

This standard does not apply to:

• Wire, pipe, tube, or steel sheet galvanized on specialized/continuous lines
• Steel thinner than 22 gauge (0.76 mm)

Related ASTM standards you must reference

• ASTM A153/A153M: hot-dip galvanizing for hardware/fasteners that are centrifuged
• ASTM A767/A767M: zinc-coated reinforcing steel bars
• ASTM A780: repair (renovation) of damaged/uncoated areas
• ASTM E376 / A90 / B487: coating thickness measurement methods

2) Key Definitions Inspectors Must Understand

Single-specimen vs multi-specimen articles

• Single-specimen article: surface area ≤ 160 in² (or centrifuged items). Entire surface = one specimen.
• Multi-specimen article: surface area > 160 in². Divide into three continuous sections, each section = one specimen (for thickness testing).

Specimen coating thickness

For each specimen, take five or more thickness readings, widely dispersed, and average them. That average = specimen coating thickness.

3) Minimum Coating Thickness Grades (ASTM A123 Table 1)

Coating thickness requirements depend on:

• Material category (structural shapes, strip/bar, pipe/tubing, wire)
• Measured steel thickness range

Common practical keywords: ASTM A123 coating thickness, galvanized coating thickness requirements, hot dip galvanizing thickness microns, zinc coating inspection.

Minimum average coating thickness grade (summary)

How acceptance works (critical)

• All specimens tested must meet the required average coating thickness grade for their category/thickness.
• Any individual specimen is allowed to be one grade lower than the Table 1 requirement (minimum specimen allowance).
• A few thin readings do not automatically fail the job—only the specimen average and article/sample averages control acceptance (as per the standard’s method).

For a clear industry explanation of ASTM A123 thickness requirements and inspection logic, see the American Galvanizers Association overview.

4) Finish & Appearance Requirements (Visual Acceptance)

ASTM A123 requires the coating to be:

• Continuous
• Reasonably smooth and uniform (as practical for shape/handling)
• Free from:
  • Uncoated areas
  • Blisters
  • Flux deposits
  • Gross dross inclusions
• No heavy zinc lumps/projections that interfere with intended use
• Holes ≥ 1/2 in should be reasonably clean and usable

Tip for inspectors: marks from handling (tongs/straps) are acceptable unless they expose base metal and exceed allowable repair limits.

5) Repair (Renovation) Limits Using ASTM A780

Uncoated or damaged areas can be repaired only within limits:

• Each repair area must be ≤ 1 in (25 mm) in its narrowest dimension
• Total repair area per article must be the lesser of:
  • 0.5% of accessible surface area, or
  • 36 in² per short ton of piece weight
• Repair thickness must meet the required grade; if using zinc paint, thickness should be 50% higher than Table 1 requirement, but not over 4.0 mils
• If repair exceeds limits (or is inaccessible) → reject the coating

6) Sampling: How Many Pieces to Inspect Per Lot

If not otherwise agreed, ASTM A123 uses the following minimum sample sizes:

7) Coating Thickness Test Methods (How Inspectors Measure)

ASTM A123 allows three main approaches (plus microscopy):

1) Magnetic thickness gauge (most common)

• Use ASTM E376
• Take 5+ readings per specimen, widely dispersed
• Best for most structural steel and fabrications

2) Stripping method (destructive)

• Use ASTM A90/A90M
• Converts coating mass to thickness grade

3) Weigh before and after galvanizing

• Weigh after pickling/drying and after cooling
• Calculate coating weight per area → convert to thickness grade

4) Microscopy cross-section (destructive)

• Use ASTM B487
• Take 5+ dispersed cross-section measurements and average

8) Inspection, Rejection & Retest (Real-World QA/QC Flow)

• The galvanizer is responsible for compliance via in-plant inspection.
• The purchaser/third-party may inspect at the plant and has access during contract work.
• If rejected (not embrittlement), the lot may be:
  • Sorted (remove nonconforming pieces)
  • Repaired within limits
  • Stripped and regalvanized, then resubmitted