🔩 Thickness of non-magnetic coatings on a magnetic substrate (paints, varnishes, electroplated and zinc coatings, enamels, plastics) — magnetic method, tactile and non-destructive measurement; PN-EN ISO 2178:2016-06 (ISO 2178:2016)

Materials and NDT PN-EN ISO 2178

Overview

WHAT THE STANDARD COVERS. The scope on the PKN card of PN-EN ISO 2178:2016-06 is, in full, the Polish wording of the scope of ISO 2178:2016, including the closing note with the reference to ISO 2361; the evs.ee catalogue (read on 08.10.2026) gives it in English, sentence by sentence the same: "ISO 2178:2016 specifies a method for non-destructive measurements of the thickness of non-magnetizable coatings on magnetizable base metals. The measurements are tactile and non-destructive on typical coatings. The probe or an instrument with integrated probe is placed directly on the coating to be measured. The coating thickness is displayed on the instrument. In ISO 2178:2016 the term "coating" is used for material such as, for example, paints and varnishes, electroplated coatings, enamel coatings, plastic coatings, powder coatings, claddings. NOTE This method can also be applied to the measurement of magnetizable coatings on non-magnetizable base metals or other materials (see ISO 2361)." We did not have the text of the published edition. We read the principles of the method in the FINAL DRAFT ISO/FDIS 2178 (voting from 26.11.2015 to 26.01.2016), from the cover to clause 5.2, with the table of contents; the scope in the draft reads the same as in the description of the 2016 edition, but the rest of the text may have changed after the vote.

ACCORDING TO THE FINAL DRAFT ISO/FDIS 2178 (clauses 1–5.2). Committee: ISO/TC 107 "Metallic and other inorganic coatings"; the third edition was to replace the second (ISO 2178:1982). References (2): ISO 2064 (definitions and conventions concerning the measurement of the thickness of metallic and inorganic coatings), ISO 4618 (terms for paints and varnishes), ISO 5725-1:1994 (accuracy of measurement methods) and ISO/IEC Guide 98-3 (GUM). Terms (3): adjustment of a measuring system is the set of operations after which the system provides indications corresponding to given values (e.g. zero adjustment, offset adjustment, span adjustment) and after which it is usually recalibrated; calibration is a prerequisite for adjustment and the two must not be confused — colloquially, adjustment is wrongly called "calibration", and calibration — "verification". Principle (4.1): the magnetic flux density near the field source (permanent magnet or electromagnet) depends on the distance from the magnetizable substrate, which makes it possible to determine the thickness of a non-magnetic coating; all methods of the standard evaluate the flux density, converted into current, voltage or force; the methods of 4.3 and 4.4 may be combined in one probe with the eddy current method (ISO 2360, ISO 21968); the requirements for thickness gauges are given in Annex B. Pull-off method (4.2): the attractive force between a permanent magnet and the substrate decreases with distance, so it is a measure of thickness; the instrument has a permanent magnet, a pull-off device with a steadily increasing force (springs or an electromagnetic system) and a scale; some instruments compensate for gravity, the others may be used only in the position indicated by the manufacturer; the measuring point must be clean, free from liquid or tacky coatings, the magnet — free from particles, and electrostatic charges must be removed. Induction principle (4.3): the inductance of a coil changes in the vicinity of iron; the probes usually have two coils — a primary one producing an alternating low-frequency field and a secondary one in which the induced voltage is measured; the dependence of the voltage on thickness is non-linear and depends on the permeability μr of the substrate, so it is determined by calibration; a core of high permeability increases sensitivity and reduces the influence of geometry, a bipolar probe averages over the surface and a unipolar one measures locally; the frequency, usually below one kilohertz, does not induce eddy currents, so conductive and non-conductive coatings can be measured. Magnetic flux thickness gauge (4.4): the detector — a Hall sensor or a magnetoresistive sensor — near the magnet measures the flux density, which decreases as the proportion of non-magnetizable material increases; this dependence is also non-linear, depends on μr and is determined by calibration. Influencing factors (5, according to the table of contents): the coating thickness itself, the magnetic properties of the substrate, the electrical properties of the coating, the substrate thickness, the edge effect, the surface curvature, roughness, cleanliness (lift-off of the probe), probe pressure, its tilt, temperature and external electromagnetic fields. According to 5.1 the sensitivity of the probe decreases with increasing thickness: in the lower part of the range the uncertainty is constant in absolute terms, in the upper part — approximately a constant fraction of the thickness. According to 5.2 the dependence of the result on thickness depends strongly on the permeability of the substrate, so calibration and measurements must be made on the same material.

STATUS (as of 08.10.2026). The card of PN-EN ISO 2178:2016-06 (English version) has no withdrawal label: title "Non-magnetic coatings on magnetic substrates -- Measurement of coating thickness -- Magnetic method" (our translation), publication 20-06-2016, 50 pages, price group T, Metallurgy Sector, KT 106 Corrosion and Corrosion Protection of Metallic Materials, "Introduces: EN ISO 2178:2016 [IDT], ISO 2178:2016 [IDT]", "Replaces: PN-EN ISO 2178:1998 – Polish version", ICS 25.220.40 and 25.220.50. Card of PN-EN ISO 2178:1998 (Polish version): "Withdrawn and replaced by PN-EN ISO 2178:2016-06 – English version", publication 26-02-1998, withdrawal 20-06-2016, 8 pages, it introduced EN ISO 2178:1995 and ISO 2178:1982 and itself replaced PN-H-04623:1986; its scope named two principles — measuring the magnetic attractive force between a magnet and the base metal, or the change of the magnetic flux. PKN search, term "ISO 2178" (08.10.2026): 7 items, only these two editions with the number 2178. The current edition thus has only an English version. evs.ee catalogue: ISO 2178:2016 in force since 17.03.2016, in the history ISO 2178:1982.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database and current PCA documents, read on 08.10.2026). In the database copy the number appears in 12 records at 8 laboratories, in the current PCA documents (issues from 16.07.2025 to 02.09.2026) — in 12 entries at the same laboratories; none is suspended. Notation: "PN-EN ISO 2178:2016-06" — AB 067 (three entries: zinc coating test of insulators, alongside the standards PN-EN 61109, PN-EN 62231, PN-EN 61952, PN-EN 60383-1 and PN-EN 60168), AB 240 (alongside PN-EN ISO 2360:2017-10) and AB 910 (with an asterisk — opinions and interpretations); "PN-EN ISO 2178:2016-05" — AB 089 (the PKN catalogue does not know an edition with this month — there is 2016-06, publication 20-06-2016); without a year — AB 023 (three entries, alongside PN-EN ISO 2360 or PN-EN ISO 2808, one with the mark "N"), AB 128 (alongside PN-EN ISO 2361 and PN-EN ISO 2808 method 7B2), AB 369 (alongside PN-EN ISO 2808 method 7B.2) and AB 904. The "Laboratories" tab (term "PN-EN ISO 2178") shows all 8 laboratories (11 of the 12 records — one record of AB 023 begins in the database with PN-EN ISO 2808); the term does not pick up standards with other numbers.

WHAT THE LABORATORIES TEST (PCA documents). Quantity: coating thickness, by the magnetic or magnetic induction method. Ranges: 0–500 µm (AB 089 and one entry of AB 023), 4–2000 µm (AB 023, in one cell with the eddy current method), 0–800 µm (AB 904 — non-magnetic coating on an Fe substrate), up to 900 µm (AB 067 — three entries of the zinc coating test of insulators), 0–960 µm (AB 910 — protective coating on metal products), 0–1000 µm (AB 240 — non-magnetic metallic coatings on a magnetic substrate, excluding nickel coatings), 11.4–1018 µm (AB 369). Subject: composite, polymer, ceramic and glass insulators — zinc coating test (AB 067); elements of a metal frame structure, coatings and coating products for the protection of metal elements (AB 023); metals and alloys, plastics, composites, polymers and paint coatings (AB 089); organic and inorganic coatings on metal products (AB 128); metals and alloys with metallic and inorganic coatings (AB 240); paint systems and anticorrosive coatings and coverings (AB 369); coatings (AB 904); products made of metallic materials (AB 910).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — the coating–substrate system: the standard applies to non-magnetizable coatings on a magnetizable substrate; non-conductive coatings on a non-magnetic substrate, e.g. oxide coatings on aluminium, are measured by the laboratories by the eddy current method according to ISO 2360 (this is how AB 240 divides it, which additionally excludes nickel coatings), and for magnetizable coatings on a non-magnetizable substrate the note in the scope of the standard refers to ISO 2361 (it stands alongside at AB 128). Second — the substrate: the dependence of the reading on thickness depends strongly on the permeability of the substrate, so calibration on a material other than the product tested gives an error. Third — adjustment versus calibration: the draft standard distinguishes these operations and notes that the colloquial "calibration" of an instrument is usually an adjustment. Fourth — geometry and surface condition: the result depends, among other things, on the substrate thickness, the proximity of an edge, curvature, roughness, cleanliness, the pressure and tilt of the probe, temperature and external fields. Fifth — the range: the sensitivity of the probe decreases with increasing thickness, and the uncertainty in the upper part of the range increases in proportion to the thickness. Sixth — notation: four laboratories write the number without a year, AB 089 gives a month of edition that is not in the PKN catalogue ("2016-05" instead of "2016-06"), and in several entries the magnetic method stands in one cell with the eddy current method and with ISO 2808, so the report should show by which method the measurement was made.

WHAT WE DO NOT GIVE. We did not have the text of the published edition ISO 2178:2016 or of PN-EN ISO 2178:2016-06. From the final draft we do not have clauses 5.3–10 or the annexes: the details of the influence of the individual factors, the methods of calibration and adjustment, the course of the measurement and the number of measurements, the estimation of uncertainty, the precision (repeatability r and reproducibility R), the content of the report and the requirements for thickness gauges.

Method principle

The magnetic flux density near the field source (a permanent magnet or an electromagnet) depends on the distance from the magnetizable substrate, so it allows the thickness of a non-magnetic coating lying on it to be determined. According to the final draft ISO/FDIS 2178 three principles are used: pull-off (the attractive force of a permanent magnet decreases with thickness), induction (change of the voltage in the secondary coil of the probe in an alternating low-frequency field) and magnetic flux (Hall sensor or magnetoresistive sensor); the dependence of the reading on thickness is non-linear and depends on the permeability of the substrate, which is why the instrument is calibrated on the same material on which the measurement is made. The measurement is tactile and non-destructive — the probe is placed on the coating, and the thickness is displayed by the instrument.

Applications

• Paints, varnishes and powder coatings on steel — thickness of protective coatings and paint systems; in the current accreditation scopes 8 laboratories
• Zinc coatings — zinc coating test in the testing of insulators (AB 067, alongside standards for insulators)
• Electroplated coatings, enamels and plastic coatings on a magnetizable substrate; claddings
• Measurements on structures and finished products — tactile, non-destructive measurement

Key parameters

Standard

Standard number

PN-EN ISO 2178:2016-06 (wersja angielska; wprowadza EN ISO 2178:2016 [IDT] i ISO 2178:2016 [IDT])

Title (PL)

Powłoki niemagnetyczne na podłożu magnetycznym — Pomiar grubości powłok — Metoda magnetyczna

Title (EN)

Non-magnetic coatings on magnetic substrates — Measurement of coating thickness — Magnetic method

Step-by-step procedure

1. Choice of method for the system

   Check whether the coating is non-magnetizable and the substrate magnetizable; non-conductive coatings on a non-magnetic substrate — by the eddy current method (ISO 2360); magnetizable coatings on a non-magnetizable substrate — see ISO 2361 (note in the scope).

2. Calibration and adjustment

   Calibrate on a substrate of the same material as the product tested (draft, 5.2); WE DO NOT GIVE the methods of calibration and adjustment of clause 6.

3. Preparing the measuring point

   Surface clean, free from liquid or tacky coatings; with the pull-off method the magnet free from particles, electrostatic charges removed (draft, 4.2).

4. Measurement

   Place the probe directly on the coating and read the thickness; take into account the factors of clause 5 (edge, curvature, roughness, probe pressure and tilt, temperature, external fields). WE DO NOT GIVE the number of measurements and the evaluation of the results (clause 7).

5. Uncertainty and report

   WE DO NOT GIVE the estimation of uncertainty (clause 8), the precision (clause 9) and the content of the report (clause 10); in the report state the edition of the standard and the method, when several stand in the scope.

Required equipment and apparatus

Health and safety (OHS)

• Non-destructive measurement; when working on structures and products outside the laboratory the safety rules of the given site apply
• TECHNICAL NOTE: only non-magnetizable coatings on a magnetizable substrate; calibration on the same substrate material; the principles of the method are described according to the final draft ISO/FDIS 2178 — we did not have the published text

Frequently asked questions