I. Chinese Standard (GB) Hot-Dip Galvanizing Specification

The thickness of Chinese-standard hot-dip galvanized coatings is mainly governed by GB/T 13912, 'Metallic Coatings - Hot Dip Galvanized Coatings on Fabricated Iron and Steel Articles - Requirements and Test Methods.' The coating thickness requirement is tied to the thickness of the base material. The standard applies to hot-dip galvanized coatings on iron and steel articles (including some castings) with a total alloying element content not exceeding 2%.

1. Core Thickness Standards (by Workpiece Thickness)

Note: The values in the table apply to non-centrifuged (non-spun) articles; the local thickness is the lowest value at any measuring point, preventing premature corrosion at the thinnest location.

2. Weight Conversion Standards

Municipal and steel-structure projects often use zinc mass per unit area as an indicator. At a zinc density of 7.14 g/cm3, 1 μm ≈ 7.14 g/m2:

The phrase 'average zinc coating ≥ 275 g/m2' commonly seen in engineering mostly refers to continuously hot-dip galvanized sheet (see the ASTM A653 section). It belongs to a different system from the thickness grading by base-material thickness in GB/T 13912 and should not be directly equated or converted.

3. Measurement Methods and Scope

Measurement method: the magnetic method (GB/T 4956) is primary, with the gravimetric (weight) method used for arbitration; the coating thickness gauge measures from the steel substrate surface to the outer surface of the coating, including the total thickness of the outermost pure-zinc layer and the zinc-iron alloy layer.

Applicability and exclusions: this standard targets hot-dip galvanized coatings on iron and steel articles; coatings that have been centrifuged and galvanized coatings on cast iron are classified or agreed upon separately under the standard.

II. U.S. Standard (ASTM) Hot-Dip Galvanizing Specification

U.S. standards assign different specifications by product type; there is no single value applicable to all products.

1. Structural Steel and Fabricated Products: ASTM A123 / A123M

'Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products' applies to rolled, pressed, and forged sections, plates, bars, steel pipes, and welded assemblies; it does not apply to continuously galvanized sheet. It gives the minimum average coating grade by material category and steel thickness:

2. Fasteners and Hardware

Small parts require centrifuging (spin-off) after galvanizing, yielding a thinner coating that cannot be mixed with the structural-member standard. The main current standards are as follows:

Tip: fasteners require matching nuts with enlarged holes after galvanizing to ensure fit; high-strength bolts (roughly above 33 HRC) carry hydrogen-embrittlement risk and the process should be discussed with the galvanizer in advance.

3. Continuous Hot-Dip Galvanized Sheet: ASTM A653

ASTM A653 covers continuously hot-dip galvanized steel strip (coil) and uses 'coating designations' ordered by zinc mass per unit area rather than specifying thickness directly. G denotes a pure-zinc coating; the number is the total double-side zinc mass (oz/ft2, triple-spot test method).

Note: the 275 g/m2 of G90/Z275 is a double-side total; if split evenly, each side is about 19-21 μm. This cannot be directly compared with the single-side 85 μm (≈ 606 g/m2) for structural members with base ≥ 6 mm under GB/T 13912.

III. European Standard (EN ISO 1461) Hot-Dip Galvanizing Specification

The European standard (EN ISO 1461) also grades by base-material thickness and is divided into two requirements: 'non-centrifuged' and 'centrifuged' parts. It targets finished iron and steel articles and does not include continuously hot-dip galvanized sheet (which belongs to the EN 10346 system).

1. Non-Centrifuged Parts (Conventional Structural Members)

2. Centrifuged Parts (Bolts, Nuts, Washers, and Other Small Parts)

3. Observations on the Pattern

Coating thickness varies in the same direction as steel thickness: the thicker the steel, the longer the heating and dipping time, the more fully the zinc-iron alloy layer grows, and the thicker the minimum coating the standard allows; the thinner the steel, the lower the requirement (e.g., for non-centrifuged parts under 1.5 mm, the local minimum is 35 μm and the average is 45 μm). Therefore, the claim that 'the thicker the steel, the lower the coating requirement' is the opposite of what the standards stipulate.

IV. Core Differences Comparison

V. Selection and Acceptance Tips

Identify the product type clearly: structural steel follows A123, fasteners follow F2329/A153, continuously galvanized sheet follows A653 - the three cannot substitute for one another.

Distinguish single-side from double-side when converting: coil designations (e.g., G90/Z275) are mostly double-side totals, and their thickness cannot be directly compared horizontally with single-side hot-dip galvanized parts.

State the standard, thickness, test method, and edge distance in the contract to avoid inconsistent acceptance criteria.

Thickness is not the only indicator: reactive steel with high silicon content accelerates coating growth; high-strength parts require hydrogen-embrittlement evaluation; enclosed cavities must reserve vent/drain holes.