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How to measure coating thickness? Fastener plating measurement methods and acceptance essentials

2026-10-30

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why coating thickness disputes happen - the wrong method, not just thin plating

The supplier measures 8 um on a batch of zinc-plated bolts with a magnetic gauge while the buyer's metallographic cross-section reads 6 um. Who is right? Possibly both, possibly neither. The first rule of coating thickness measurement is matching the method to substrate and coating: the magnetic method (ISO 2178 / GB/T 4956) works only for non-magnetic coatings on ferrous substrates, such as electro- or hot-dip galvanizing on steel; the eddy-current method (ISO 2360 / GB/T 4957) applies to non-conductive coatings on non-magnetic metals, like anodizing on aluminium; X-ray fluorescence (ISO 3497 / GB/T 16921) is nearly substrate-independent and can simultaneously report thickness and alloy composition for zinc-nickel or zinc-flake systems; the metallographic method (ISO 1463 / GB/T 6462) is the destructive referee method with precision down to about 0.8 um. Reading an eddy-current gauge on steel, or pressing a magnetic probe onto a head-marking impression, can introduce errors of up to 30 percent - which is exactly where disputes begin.

Choosing among four measurement methods: accuracy, speed and cost

For quick shop-floor checks the magnetic method is first choice: a portable gauge returns a reading in about 2 seconds, typically 0-2000 um range with roughly 10 percent accuracy. Keep probe points at least 5 mm from edges, away from head markings and oil films, take 3-5 points per part and average them, and note the minimum substrate thickness (about 0.5 mm for steel, probe-dependent) - thin parts need calibration on certified foils. Hot-dip galvanizing is judged against ISO 1461 / GB/T 13912 minimum mean thickness: at least 85 um for fasteners above M20, 70 um for M8-M20, with local values never below the minimum local thickness (70 / 55 um). Electroplated zinc follows GB/T 5267.1: 5 um or 8 um average depending on the Fe/Zn class, measured magnetically or by X-ray. For zinc-nickel and zinc-flake coatings, X-ray fluorescence gives thickness plus alloy ratio in one pass and is the lab standard. In disputes, the metallographic cross-section is the referee: cut along the bolt centreline, mount, polish and measure 3-5 positions with a calibrated eyepiece.

Sampling and acceptance: write the criteria into the contract

Acceptance hinges on minimum local thickness, not the batch average: GB/T 5267.1 requires 5 points per part judged in groups of three parts, and any single point below the limit rejects the batch - much stricter than an average-based check. Sample per ISO 2859-1 / GB/T 2828.1 general inspection level II with AQL 1.0-2.5, and AQL 1.0 for safety-critical fasteners. Three red lines: calibrate the gauge with certified foils before every use; distribute measurement points across head, shank and thread - on threaded sections the plating must respect the 6g tolerance class, so a GO/NO-GO thread gauge after plating says more than thickness alone; and the batch report should record instrument model, calibration data, a point-layout sketch, tied to an EN 10204 3.1 certificate. Specifying the coating system, minimum local thickness, measurement method and judgement standard once in the contract removes the argument about whose gauge counts. For zinc, zinc-flake or alloy-plated fasteners with measurement reports and 3.1 certificates, contact Chiali Industrial (HK) for samples and a proposal.

How to measure coating thickness? Fastener plating measurement methods and acceptance essentials

Why coating thickness disputes happen - the wrong method, not just thin plating

Choosing among four measurement methods: accuracy, speed and cost

Sampling and acceptance: write the criteria into the contract

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