🔩 Thickness of non-conductive coatings (paints, varnishes, plastics, enamels, powder coatings, anodic oxide layers) on non-magnetic conductive metals — non-destructive measurement by the amplitude-sensitive eddy-current method; PN-EN ISO 2360

Materials and NDT PN-EN ISO 2360

Overview

WHAT THE STANDARD COVERS. ISO 2360:2017 (clause 1) specifies the non-destructive measurement of the thickness of non-conductive coatings on non-magnetic, electrically conductive base metals with amplitude-sensitive eddy-current instruments. The word "coating" covers here e.g. paints and varnishes, electroplated coatings, enamels, plastic coatings, claddings and powder coatings. The method is particularly suitable for most oxide layers produced by anodizing, but not for all conversion coatings — some are too thin (clause 6). It can also be used to measure non-magnetic metallic coatings on non-conductive substrates, although the phase-sensitive method of ISO 21968 is then more accurate; it is not used for non-magnetic metallic coatings on conductive metals (here ISO 21968; the exception being very thin coatings of very low conductivity). On magnetic substrates its use is not recommended — the magnetic method of ISO 2178 is used there (separate entry of this database); only for very thick coatings, above approx. 1 mm, can the amplitude method also be used. Normative references: ISO 2064, ISO 4618 and ISO/IEC Guide 98-3 (GUM). The "Scope" field of the PKN card of PN-EN ISO 2360:2017-10 (English version, scope description in Polish), fragment: "In this document the term »coating« is used for materials such as, for example, paints and varnishes, electroplated coatings, enamel coatings, plastic coatings, powder coatings and claddings. This method is particularly applicable to measurements of the thickness of most oxide coatings produced by anodizing, but is not applicable to all conversion coatings, because some are too thin to be measured by this method" (our translation).

PRINCIPLE (clause 4). A high-frequency electromagnetic field from the probe induces eddy currents in the base metal under the coating; they change the field around the coil, and thus the amplitude of the coil impedance. The density of the eddy currents depends on the distance of the coil from the metal surface, so after calibration with standards the change in impedance is a measure of the coating thickness. The coil is usually part of a resonant circuit — the change in impedance shifts the resonant frequency, which becomes a measure of the thickness. The probe and the instrument may form one device; basic requirements for instruments are given in Annex C (informative).

FACTORS OF UNCERTAINTY (clause 5). The sensitivity of the probe decreases with thickness: in the lower part of the range the absolute uncertainty is constant, in the upper part — approximately a constant fraction of the thickness. The conductivity of the base metal determines the measuring effect, which is why calibration and measurement are carried out on the same material (there are instruments compensating for conductivity). A base metal that is too thin disturbs the generation of eddy currents — it should be thicker than the critical minimum thickness, which is determined experimentally or estimated with the formula tmin = 3·δ0 (δ0 — standard penetration depth). Measurements too close to edges, holes and corners may be invalid (the amplitude method is more sensitive to this than the phase method), the curvature of the base metal has a stronger effect the smaller the radius and the thinner the coating (adjustment on a base metal of the same geometry, microprobes), and roughness causes systematic and random errors (calibration on an uncoated base metal of similar roughness, many readings at different places). A gap between the probe and the coating — e.g. fine contamination — acts like an additional coating (lift-off), so the probe tip is checked frequently; the pressure shall be the same during adjustment and measurement and as small as possible for soft coatings (spring-loaded probes; a protective foil of known thickness, which is subtracted from the result); the probe is applied perpendicularly. The temperature of the probe and the base metal (the conductivity of metals depends on temperature) should be close to the calibration conditions; intermediate coatings and strong external electromagnetic fields also have an effect.

CALIBRATION AND MEASUREMENT (clauses 6–7). Before use, each instrument is calibrated or adjusted according to the manufacturer's instructions with thickness standards and a base metal of material, geometry and surface similar to the test specimens, at the same temperature. The standards are coated base metals or foils laid on an uncoated base metal; foils and coatings shall be non-conductive and non-magnetic (usually plastic — unlike the magnetic method, conductive foils, e.g. of copper alloys, must not be used), with a documented thickness, uncertainty at a confidence level of e.g. 95 % and metrological traceability, visually checked before use; a foil can deform under the probe and must not leave a gap (especially on concave surfaces). Adjustment: on the base metal alone ("zeroing"), on the base metal and one standard, on the base metal and several standards or on several standards; the uncertainty decreases as the number of points increases and the better they cover the expected thickness range. Calibration at the inspection site is repeated when the instrument is put into operation, when the material or geometry of the specimens changes and when the conditions change (e.g. temperature), and periodically during use. The thickness is given as the arithmetic mean of several readings from a specified area, with the standard deviation; unless otherwise agreed, at least five readings are recommended.

STATUS (PKN card as of 27.10.2026). The card of PN-EN ISO 2360:2017-10 (English version) has no withdrawal label: publication 26-10-2017, 46 pages, Metallurgy Sector, KT 106 Corrosion and Corrosion Protection of Metallic Materials, "Adopts: EN ISO 2360:2017 [IDT], ISO 2360:2017 [IDT]", "Replaces: PN-EN ISO 2360:2006 - Polish version", ICS 25.220.20. PKN search, search term "ISO 2360" (27.10.2026): alongside it there is the German version PN-EN ISO 2360:2017-10; the editions PN-EN ISO 2360:1998, 2004 and 2006 are withdrawn, and there is no Polish version of the 2017 edition in the results. The fourth edition ISO 2360:2017 (ISO/TC 107) replaced ISO 2360:2003. In the iTeh catalogue (read on 27.10.2026) ISO 2360:2017 has the status "Published" and the stage "9093 – International Standard confirmed", and EN ISO 2360:2017 — "Published".

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database refreshed from the production server on 27.10.2026 and current PCA documents downloaded on that day). Number 2360 appears in 9 records at 6 Polish laboratories: AB 023 (four entries in two laboratories), AB 240, AB 369, AB 394, AB 904, AB 910. In all 6 current PCA documents the standard is in the same 9 rows; no entry is suspended. Forms: "PN-EN ISO 2360:2017-10" (AB 240, AB 394, AB 910 — with an asterisk), "PN-EN ISO 2360" without the year (AB 023 partly with the addition "N", AB 369, AB 904). The magnetic standard PN-EN ISO 2178 is held in the same entries by AB 023 and AB 240, and PN-EN ISO 2808 (thickness methods for paint coatings) — by AB 023 and AB 369 (method 7C). The "Laboratories" tab ("PN-EN ISO 2360") shows 9 laboratories: these 6 Polish ones and 3 Czech ones (tab query on labcoda.pl, 27.10.2026).

WHAT THE LABORATORIES TEST (database copy and PCA documents). Eddy-current method: coating products for the protection of metal elements (AB 023: 4–30 µm and 4–2000 µm, also in the field), elements of skeletal metal structures and coatings (AB 023, also the range 0–500 µm "by the magnetic method" with a Minitest instrument in the entry with three standards), non-conductive coatings — oxide coatings on aluminium and its alloys up to 1000 µm alongside non-magnetic metallic coatings on a magnetic substrate (AB 240, the latter part according to PN-EN ISO 2178), paint systems and anti-corrosion coatings 11.4–1018 µm (AB 369), non-conductive coatings on a non-magnetic conductive substrate 0–800 µm (AB 394; AB 904 — "N-Fe substrate"), protective coating of metal products 0–960 µm (AB 910).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — substrate: the method concerns non-magnetic conductive substrates; on steel (magnetic substrate) the magnetic method ISO 2178 is appropriate, and the amplitude method does not measure a metallic coating on a conductive metal — an entry with three standards and "the magnetic method" must state in the report by which method a given result was measured. Second — calibration on a different material: the conductivity of the base metal strongly changes the reading, so adjustment on a different aluminium alloy gives a systematic error, and the reference foils must be non-conductive (a copper-alloy foil, good for the magnetic method, falsifies the result here). Third — "lift-off": a particle under the probe tip adds to the thickness. Fourth — edges, curvature and thin base metal: amplitude instruments are more sensitive to edges than phase instruments, and a base metal thinner than the critical minimum thickness (without other data tmin = 3·δ0) and changes in its thickness give a reading error (point 5.3). Fifth — temperature: hot coatings and temperature differences between the probe, instrument and specimen can give large errors. Sixth — thin conversion coatings are sometimes beyond the reach of the method.

WHAT WE DO NOT GIVE. The fragment of ISO 2360:2017 we read ends at point 7.2. We do not give the method of determining uncertainty (clause 8, Annexes B, E, F), precision data (clause 9, Annex G), requirements for instruments (Annex C) or the examples of experiments from Annex D.

Method principle

A probe with a coil is applied perpendicularly to the coating; the high-frequency electromagnetic field induces eddy currents in the non-magnetic base metal, whose density depends on the distance of the coil from the metal, and the change in the amplitude of the coil impedance (usually a shift in the resonant frequency of the circuit), after calibration with thickness standards on the same base metal, is a measure of the thickness of the non-conductive coating (ISO 2360:2017, clauses 4 and 6).

Applications

• Non-conductive coatings — paints, varnishes, plastics, enamels, powder coatings, claddings — on non-magnetic conductive metals (ISO 2360:2017, clause 1)
• Anodic oxide layers on aluminium and its alloys (method particularly useful); not all conversion coatings
• Non-destructive measurements in the laboratory and in the field (AB 023 — also in situ)
• In accreditation scopes: 6 Polish laboratories, ranges from 4–30 µm to 0–2000 µm

Key parameters

Standard

Standard number

PN-EN ISO 2360:2017-10 (wersja angielska i wersja niemiecka; wprowadza EN ISO 2360:2017 i ISO 2360:2017 [IDT])

Title (PL)

Powłoki nieprzewodzące na podłożu niemagnetycznym przewodzącym elektryczność — Pomiar grubości powłok — Metoda amplitudowa prądów wirowych

Title (EN)

Non-conductive coatings on non-magnetic electrically conductive base metals — Measurement of coating thickness — Amplitude-sensitive eddy-current method

Step-by-step procedure

1. Calibration and adjustment

   According to the manufacturer's instructions, with thickness standards on a base metal similar to the specimens, at the same temperature; adjustment method chosen for the required accuracy (clause 6).

2. On-site check

   When putting the instrument into operation, when the material or geometry of the specimens changes and when the conditions change; periodically during work (point 7.1).

3. Measurement

   Probe clean, applied perpendicularly, with the same pressure as during adjustment; away from edges and holes; taking curvature and roughness into account (clause 5).

4. Result

   Arithmetic mean of at least 5 readings from a specified area and the standard deviation (point 7.2). PN-EN ISO 2360:2017-10 — current (PKN card as of 27.10.2026; English and German versions).

Required equipment and apparatus

Health and safety (OHS)

• TECHNICAL NOTE: on a magnetic substrate (steel) the magnetic method ISO 2178 is appropriate, not ISO 2360
• TECHNICAL NOTE: calibration on a base material different from the specimen gives a systematic error (point 5.2)

Frequently asked questions