ISO 21968:2019

Overview

ISO 21968:2019 specifies the phase-sensitive eddy-current method for non-destructive measurement of the thickness of non-magnetic metallic coatings on metallic and non‑metallic substrates. It covers common coating/base pairs such as zinc, cadmium, copper, tin or chromium on steel and copper or silver on composite materials. The standard explains the measurement principle, factors affecting accuracy, calibration and adjustment, procedures for measurement and evaluation, and guidance on estimating measurement uncertainty in line with ISO/IEC Guide 98‑3 (GUM).

Key benefits of the phase-sensitive method highlighted in the standard:

• Suited to smaller measurement areas and stronger surface curvatures than amplitude methods (ISO 2360).
• Less sensitive to magnetic properties of the base material.
• More sensitive to electrical properties of the coating material.

Key Topics and Requirements

• Principle of measurement: detects phase-shift of probe coil impedance caused by eddy currents induced in coating and base material; phase angle is used to infer coating thickness after calibration.
• Applicability condition: reliable measurement on metallic/metallic systems requires the product of conductivity and permeability (σ·μ) of one material to be at least twice that of the other.
• Factors affecting accuracy (Clause 5): coating electrical properties, substrate geometry and thickness, edges, surface curvature, surface roughness, lift-off, probe pressure and tilt, temperature, intermediate layers, and external electromagnetic fields.
• Calibration & adjustment: use of traceable thickness reference standards and recommended adjustment methods; Annex C gives basic performance requirements for instruments.
• Uncertainty & precision: procedures for uncertainty estimation according to GUM, examples and repeatability/reproducibility guidance are provided (Annexes B, F, G).
• Measurement procedure: recommended number of measurements, evaluation methods and reporting requirements (Clause 7 and Clause 10).

Practical Applications

ISO 21968:2019 is intended for organizations and professionals who need accurate, non-destructive coating-thickness data:

• Quality control in metal finishing, electroplating, and surface treatment operations.
• Inspection of plated components in automotive, aerospace, electronics and industrial equipment.
• Composite part verification with metallic top-coats (e.g., copper or silver on composites).
• NDT laboratories and metrology labs performing routine or contract inspections.
• Instrument manufacturers designing phase-sensitive eddy-current coating thickness gauges.

Practical uses include in-process QC, acceptance testing, conformity assessment and materials research where curvature, small areas or magnetic substrates make amplitude-sensitive methods unsuitable.

Related Standards

• ISO 2360 - amplitude-sensitive eddy-current method (complementary comparison)
• ISO 2064 - definitions and conventions for coating thickness measurement
• ISO 4618 - paints and varnishes terms and definitions
• ISO/IEC Guide 98‑3 (GUM) - uncertainty of measurement guidance

This standard is essential when selecting or validating phase-sensitive eddy-current instruments and establishing robust, traceable coating-thickness measurement programs. Keywords: ISO 21968:2019, phase-sensitive eddy-current method, coating thickness measurement, non-destructive testing, non-magnetic metallic coatings, measurement uncertainty.