ISO/DIS 21227-4:2026(en)

ISO/TC 35/SC 9/WG 31

Secretariat: BSI

Date: 2026‑01-02

Paints and varnishes — Evaluation of defects on coated surfaces using digital image processing — Part 4: Evaluation of filiform corrosion on coated corrosion test samples

© ISO 2026

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Contents

Foreword iv

Introduction v

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 Calibration 1

5 Image enhancement 2

5.1 General 2

5.2 Examples of image enhancement: 2

5.2.1 Gamma correction 2

5.2.2 Sigmoid function 2

5.2.3 Histogram-based contrast increase 2

5.2.4 Median filter 2

5.2.5 Gaussian filter 2

5.2.6 Binarisation 2

6 Evaluation 2

7 Test report 5

Bibliography 7

Foreword

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This document was prepared by Technical Committee ISO/TC 35, Paints and varnishes, Subcommittee SC 9, General test methods for paints and varnishes in collaboration with the European Committee for Standardization (CEN) Technical Committee CEN/TC 139, Paints and varnishes, in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).

This second edition cancels and replaces the first edition (ISO 21227‑4:2008), which has been technically revised.

The main changes are as follows:

— the title of the document has been changed to “Part 4: Evaluation of filiform corrosion on coated corrosion test samples”;

— the text has been revised considering the new methods for digital image processing added in ISO 21227-1:—;

— the calibration (Clause 4) has been changed;

— the normative references have been updated.

A list of all parts in the ISO 21227 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

Filiform corrosion is a type of corrosion proceeding under a coat of paint, varnish or related product, in the form of filaments, generally starting from uncoated edges or from local damage to the coating. The visualization of the filiform corrosion is performed using one of the methods described in ISO 21227‑1. The result is a digital image of the sample. Measurement and evaluation of filiform corrosion are performed using digital image processing techniques. Here, the area and spread of the delamination in the form of filaments are of interest.

Paints and varnishes — Evaluation of defects on coated surfaces using digital image processing — Part 4: Evaluation of filiform corrosion on coated corrosion test samples

This document specifies methods and measured values for objective and quantitative evaluation of filiform attack on coated corrosion test samples. It takes into account the analogous evaluation of filiform corrosion in accordance with ISO 4628‑10 in digital form.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 129‑1, Technical product documentation (TPD) — Presentation of dimensions and tolerances — Part 1: General principles

ISO 4618, Paints and varnishes — Vocabulary

For the purposes of this document, the terms and definitions given in ISO 4618 and the following apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at https://www.electropedia.org/

3.1

filiform corrosion

type of corrosion proceeding under a coat of paint, varnish, or related product, in the form of threads (filaments), generally starting from bare edges or from local damage to the coating

Note 1 to entry: Usually the threads are irregular in length and direction of growth, but they can also be nearly parallel and of approximately equal length. They usually follow the extrusion direction and do not cross over one another. They need to be initiated by aggressive ions.

[SOURCE: ISO 4623‑1:2018, modified: "threads" have been changed to "threads (filaments)" in the definition]

Metric calibration is performed separately in the X and Y directions of the image sensor, as different calibration factors are typically required for horizontal and vertical measurements depending on the pixel geometry and imaging. The calibration factors are stored in the system and are applied to each measurement. The measured values are always specified in geometric length and area dimensions.

The image capture system is used to capture a calibration plate with a defined length scale or pattern size as a digital image. Digital tools are used to measure defined distances in the image of the calibration plate, both horizontally and vertically. The algorithm uses this information to calculate the number of pixels per unit of length, which is typically specified as resolution in dpi (dots per inch), as well as the calibration factors for converting pixels into a geometric measurement. The calibration process shall be documented.

For preparation of the analytical image evaluation, digital image processing algorithms can be applied to the sample image. Methods for increasing contrast and dynamic range and for removing image defects and noise can be used.

The application of image processing methods shall be documented so that the measurement results can be reproduced subsequently. The individual steps shall be saved.

In a gamma correction, the greyscale values of the image are converted into a new image based on a non-linear characteristic curve using the function Y = X^γ. The exponent γ determines the profile of the characteristic curve and therefore the effect on the image.

EXAMPLES  

γ = 2,2: Convex, brightens the image and increases the dynamic range in dark regions.

γ = 0,5: Concave, generates increased contrast in the image.

The Sigmoid function combines the two characteristic curve profiles of the gamma correction and has an S‑shape. In the lower greyscale range the sigmoid characteristic is concave, in the upper range it is convex. Application of the sigmoid function increases the contrast a great deal.

A histogram is a representation of the brightness distribution across the image. On the x‑axis the brightness runs from left (dark) to right (light). If the difference between light and dark is only slight, this means that there is generally little contrast in the image. Spreading the brightness values across the full bandwidth generates more contrast in the image.

The median filter removes noise and point-like interference from the image.

A Gaussian filter is used to smooth the image, including edges.

A binary black and white image is generated via threshold processing.

The filament length is determined via the distance between the mechanical damage (scribe) and the filament head. The measurement is performed at a 90° angle to the scribe. The curvature of the filament is neglected during the measurement.

If the test panel has multiple scribes that are to be assessed (e.g. X‑shaped or T‑shaped scribe), then each scribe section is evaluated separately. At the ends of each of the scribe an adjustable length of the scribe is excluded from the measurement (typically 5 mm to 10 mm). In cases where the scribes cross over (X‑shaped scribe) the inner cut surface is not considered in the measurement. The area to be excluded has the shape of a diamond that extends 5 mm along each scribe from the intersection of the cuts.

The evaluation is performed separately on both sides of the scribe. In the following, we speak in general terms of the left and right-hand sides. In the case of horizontal scribes, in accordance with ISO 129‑1 the dimensions are to be viewed from the right, i.e. “left” refers to the lower side, “right” to the upper side of the scribe.

The following are measured and calculated for every scribe or scribe section:

— corrosion area

— A, in square millimetres (mm²), total, left-hand side A_L, right-hand side A_R

— mean corrosion area A_m per cm of scribe length, in square millimetres per centimetre (mm²/cm)

— longest filaments

— longest filament L_max, in millimetres (mm), left-hand side L_{max L}, right-hand side L_{max R}

— arithmetic mean  and median  calculated from the 6 longest filaments per scribe (see Figure 1).

— most common filament length

— measurement of the distance M_L and M_R, in millimetres (mm), until the majority of filaments have developed from the scribe, both on the left and right-hand sides.

— calculation of the most common filament length M.

— filament frequency normalized for the length of the scribe (based on GSB Regulations Part VII – Measuring and Testing Methods, 05/2013, 14.2 Evaluation^[4])

— For the scribe section displaying the strongest corrosion damage, all filaments with a defined minimum length are measured.

— N = number of filaments measured

—  = mean filament length, in millimetres (mm)

— l = length of the scribe, in millimetres (mm)

— H = filament frequency: H = N/l

— index for the filiform attack (see Figure 2):

— F =  ⋅ H

Key

Red corrosion area

Green 6 longest filaments per scribe section

Blue marked areas are excluded from the measurement

Figure 1 — Results image showing the corrosion area

Key

Red corrosion area

Green 6 longest filaments per scribe section

Blue marked areas are excluded from the measurement

Figure 2 — Results image showing filament lengths on the scribe section displaying the worst damage

The test report shall contain at least the following information for every measurement:

a) report for every measurement:

1. original image of the sample

2. binary results image with marking of the corrosion areas and filament lengths

3. original image with superimposed results image, scale grid and ruler (examples see Figure 3 and Figure 4)

4. table with data and numerical results

5. parameters with applied image processing steps

b) process data:

1. reference to this document (i.e. ISO 21227-4:—)

2. date and time of the test

3. name of the examiner

4. stress duration of the sample

5. sample identification

c) measured values table, for each scribe or scribe section:

Key

Red corrosion area

Green 6 longest filaments per scribe section

Blue marked areas are excluded from the measurement

Figure 3 — Original image and results image showing the 6 longest filaments per scribe section, scale grid and ruler

Key

Red corrosion area

Green 6 longest filaments per scribe section

Blue marked areas are excluded from the measurement

Figure 4 — Original image and results image showing filament lengths on the scribe section displaying the worst damage, scale grid and ruler

Bibliography

[1] ISO 4623‑1:2018, Paints and varnishes — Determination of resistance to filiform corrosion — Part 1: Steel substrates

[2] ISO 4618‑10, Paints and varnishes — Evaluation of quantity and size of defects, and of intensity of uniform changes in appearance — Part 10: Assessment of degree of filiform corrosion

[3] ISO 21227‑1, Paints and varnishes — Evaluation of defects on coated surfaces using optical imaging — Part 1: General guidance

[4] GSB Regulations Part VII – Measuring and Testing Methods, 05/2013, 14.2 Evaluation^[1]

1. GSB International e.V., Fritz-Vomfelde-Straße 30, D-40547 Düsseldorf, Phone: +49 (0) 211-4796-450/-451, e-Mail: info@gsb-international.de ↑