ASTM A653 Galvanized Coating Designations

ASTM A653 covers steel sheet, zinc-coated (galvanized) by the hot-dip process. Coating designations range from G01 to G360 (imperial) and Z001 to Z1100 (metric). The designation represents the minimum triple-spot total both sides coating weight.

Common Coating Designations & Thickness Equivalents

Imperial (G)Metric (Z)Total oz/ft²Total g/m²Per Side oz/ft²Per Side g/m²Thickness per Side (µm)Thickness per Side (mils)
G30Z0900.30900.1545~6.3~0.25
G40Z1200.401200.2060~8.4~0.33
G60Z1800.601800.3090~12.6~0.50
G90Z2750.902750.45137.5~19.3~0.76
G115Z3501.153500.575175~24.5~0.96
G140Z4501.404500.70225~31.5~1.24
G185Z5501.855500.925275~38.5~1.52
G210Z6002.106001.05300~42.0~1.65
G235Z7002.357001.175350~49.0~1.93

Note: 1 oz/ft² = 305 g/m². Zinc density = 7.14 g/cm³. Thickness calculated as: coating weight per side ÷ density. Actual thickness may vary ±10-15% due to surface roughness and steel chemistry.

Unit Conversion Quick Reference

Standard conversion factors for zinc coating measurements:

305
1 oz/ft² = 305 g/m²
7.14
Zinc Density = 7.14 g/cm³
1.7
1 oz/ft² ≈ 1.7 mils (43 µm) zinc thickness

Conversion Formulas

Weight to Thickness

Thickness (µm) = Coating Weight (g/m²) ÷ 7.14

For single-side coating: divide total g/m² by 2 first.

Imperial to Metric

g/m² = oz/ft² × 305

Example: G90 = 0.90 oz/ft² = 0.90 × 305 ≈ 275 g/m²

ASTM A123 Structural Galvanizing Requirements

ASTM A123 covers zinc coating (hot-dip galvanizing) on iron and steel structural shapes, plate, pipe, tubing, and wire. Minimum average coating thickness depends on material category and measured steel thickness.

Minimum Average Coating Thickness (µm) by Material Category

Material Category<1.6mm1.6–3.2mm3.2–4.8mm4.8–6.4mm6.4–16.0mm>16.0mm
Structural Shapes45657575100100
Strip and Bar4565757575100
Plate4565757575100
Pipe & Tubing454575757575
Wire355060658080
Reinforcing Bar————100100
Forgings & Castings———100100100

Note: ASTM A123 requires individual specimen coating thickness to be at least one coating grade below the minimum average. All coating thickness requirements are minimums; no maximum exists in the specification.

ISO 1461 Hot Dip Galvanizing Standard

EN ISO 1461 is the European and international standard for hot-dip galvanized coatings on fabricated iron and steel articles (non-centrifuged).

Minimum Coating Thickness Requirements (Non-Centrifuged)

Steel Thickness (mm)Local Thickness Minimum (µm)Average Thickness Minimum (µm)Coating Mass (g/m²)
≥ 6 mm7085505
≥ 3 mm to < 6 mm5570395
≥ 1.5 mm to < 3 mm4555325
< 1.5 mm3545250

For grit-blasted surfaces prior to galvanizing, a nominal coating thickness of 1000 g/m² (140 µm) can be specified for steel ≥6 mm.

GB/T 13912 Chinese National Standard

GB/T 13912-2020 specifies technical requirements and test methods for hot-dip galvanized coatings on fabricated iron and steel articles (excluding continuous sheet and tube).

Coating Thickness Requirements (Non-Centrifuged)

Steel Thickness (mm)Average Thickness (µm)Local Thickness Minimum (µm)Coating Mass (g/m²)
≥ 6 mm> 8570505
> 3 mm to < 6 mm> 7055395
> 1.5 mm to ≤ 3 mm> 5545325

Centrifuged Articles (e.g., Fasteners)

Diameter (mm)Local Thickness Minimum (µm)Coating Mass (g/m²)
> 6 mm (threaded)40285
≤ 6 mm (threaded)20140

GB/T 13912-2020 updated from 2002 version with additional terminology definitions and expanded repair requirements.

International Standards Comparison

Comparison of common coating designations across major international standards (total both sides).

GB/T 2518 (CN)JIS G3302 (JP)EN 10327 (EU)ASTM A653 (US)Total g/m²
Z060Z06Z060—60
Z080Z08Z080—80
Z090Z09Z090G3090
Z100Z10Z100—100
Z120Z12Z120G40120
Z180Z18Z180G60180
Z250Z25Z250—250
Z275Z27Z275G90275
Z350Z35Z350G115350

Weight Increase Due to Zinc Coating

Galvanizing adds weight to the base steel. The actual increase depends on steel thickness, shape, surface area-to-weight ratio, and coating thickness.

Typical Weight Increase by Steel Type

Coating ThicknessSteel TypeSurface Area (ft²/ton)Zinc Added (lbs)Weight Increase %
3.0 mils (1.7 oz/ft²)Light Truss500990~2.6%
4.0 mils (2.2 oz/ft²)Typical Mix Shapes250660~1.7%
5.0 mils (2.8 oz/ft²)Heavy Structural100330~0.9%
7.0 mils (3.9 oz/ft²)Reactive Steel (High Si/P)5002,309~6.1%

On average, the galvanized coating increases the weight of black steel by up to 15%, depending on factors including steel chemistry (silicon/phosphorus content significantly affects coating thickness), shape, size, and surface conditions. Typical increase is 3-6% for most structural fabrications.

Coating Weight Estimation

Quick Estimation for Pipes

Galvanized Weight ≈ Black Steel Weight × (1 + 3% to 6%)

For precise calculations, use the steel coil weight calculator with galvanized density (~8.05 g/cm³).

Coating Thickness Measurement Methods

Four primary methods are used to measure hot-dip galvanized coating thickness, per ASTM E376 and ISO 2178.

Measurement Techniques

  • Magnetic Thickness Gauge (Most Common) — Non-destructive, measures non-magnetic coating on magnetic steel substrate. Calibration per ASTM E376. Accuracy: ±5% of reading.
  • Weigh-Strip-Weigh (Referee Method) — Sample weighed, coating stripped in inhibited acid, then re-weighed. Most accurate laboratory method.
  • Weighing Before and After Galvanizing — Practical for large articles; measures total zinc pickup.
  • Optical Microscopy — Cross-section examination; used for research and failure analysis.

ASTM E376 Key Points

  • Surface roughness can cause gauges to read high (Base Metal Reading effect).
  • Gauges should be calibrated on uncoated substrate with similar roughness and metallurgy.
  • Minimum 5 readings per measurement area, average reported.
  • Electronic/digital gauges offer best repeatability.

Frequently Asked Questions

What does G90 coating designation mean?
G90 is an ASTM A653 designation meaning 0.90 oz/ft² total zinc coating on both sides. If evenly applied, each side has 0.45 oz/ft² (approx. 137.5 g/m²). Equivalent to Z275 in metric designation. The "G" stands for "galvanized".
How do I convert oz/ft² to g/m²?
Multiply oz/ft² by 305. Example: G90 = 0.90 × 305 = 274.5 g/m² (rounded to Z275). For single-side conversion, divide the total by 2 first. 1 oz/ft² ≈ 305.15 g/m² precisely.
What is the difference between ASTM A653 and ASTM A123?
ASTM A653 covers continuously hot-dip galvanized steel sheet and coil (pre-galvanized). ASTM A123 covers hot-dip galvanizing of fabricated structural steel articles (post-fabrication galvanizing). A123 coating thicknesses are generally thicker than A653 sheet coatings.
How thick is G90 galvanizing in microns?
G90 provides approximately 19-21 µm of zinc thickness per side. Calculation: 275 g/m² total ÷ 2 = 137.5 g/m² per side; 137.5 ÷ 7.14 (zinc density) ≈ 19.3 µm per side.
What is the minimum galvanizing thickness per ISO 1461?
ISO 1461 minimum average thickness ranges from 45 µm (for steel <1.5 mm) to 85 µm (for steel ≥6 mm). Local thickness minimums are approximately 10-15 µm lower than average requirements.
How much weight does galvanizing add to steel?
Typical weight increase is 3-6% for most structural fabrications. Light-gauge trusses with high surface area-to-weight ratio may see 2-3% increase, while reactive steels can add 6-10% or more. Maximum can reach 15% in extreme cases.
What is the most common galvanized coating for sheet steel?
G90 (Z275) is the most widely specified coating for construction and general industrial applications. G60 (Z180) is common for automotive and appliances. G40 (Z120) is used for light-duty indoor applications.
Does GB/T 13912 match ISO 1461?
GB/T 13912-2020 is technically aligned with ISO 1461, with identical coating thickness requirements for most categories. The 2020 revision harmonized terminology and measurement methods with international practice.