ISO 21746:2019

Composites and metal assemblies - Galvanic corrosion tests of carbon fibre reinforced plastics (CFRPs) related bonded or fastened structures in artificial atmospheres - Salt spray tests

This document specifies the apparatus, the reagents and the procedure to be used in conducting the neutral salt spray (NSS), acetic acid salt spray (AASS) and copper-accelerated acetic acid salt spray (CASS) tests for assessment of the galvanic corrosion resistance of joints and bonded structures between carbon fibre reinforced plastics (CFRPs) and metallic materials, with or without permanent or temporary insulation for the galvanic current.

Assemblages composites et métal — Essais de corrosion galvanique des structures en plastiques renforcés de fibres de carbone (CFRP) jointes ou fixées en atmosphères artificielles — Essais au brouillard salin

General Information

• Status: Published
• Publication Date: 25-Apr-2019
• ICS: 83.120 - Reinforced plastics
• Technical Committee: ISO/TC 61/SC 13 - Composites and reinforcement fibres
• Drafting Committee: ISO/TC 61/SC 13/WG 7 - Composites and metal assemblies
• Current Stage: 9093 - International Standard confirmed
• Start Date: 03-Sep-2024
• Completion Date: 13-Dec-2025

Overview

ISO 21746:2019 - "Composites and metal assemblies - Galvanic corrosion tests of carbon fibre reinforced plastics (CFRPs) related bonded or fastened structures in artificial atmospheres - Salt spray tests" - defines standardized salt spray methods to assess galvanic corrosion between CFRP and metallic components. The standard covers test apparatus, reagents and procedures for neutral salt spray (NSS), acetic acid salt spray (AASS) and copper‑accelerated acetic acid salt spray (CASS) to evaluate galvanic corrosion resistance of bonded or fastened CFRP/metal joints, with or without insulation.

Key topics and technical requirements

• Test methods covered: NSS, AASS and CASS tailored to CFRP‑metal assemblies.
• Test solutions: Preparation of a 50 g/L ± 5 g/L sodium chloride solution; requirements for reagent purity (e.g., Cu and Ni < 0.001 % mass fraction) and impurity limits for salt.
• pH control: Specified pH ranges for sprayed solutions - NSS: pH 6.5–7.2; AASS: pH ~3.1–3.3; CASS: uses added copper chloride plus acetic acid to simulate more aggressive, copper‑accelerated acid environments.
• Apparatus and cabinet performance: Details on spray cabinet design, reference specimens, cabinet corrosivity checks, and recommended operating conditions and durations.
• Specimen preparation and evaluation: Guidance on test specimen geometry, arrangement, scribing of organic coatings, post‑test treatment, mass loss measurements, and result evaluation.
• Supporting annexes: Example cabinet schematic, alternative zinc reference method, and normative guidance for preparing coated test panels.

Practical applications and users

ISO 21746:2019 is intended for organizations and professionals concerned with durability and corrosion performance of CFRP assemblies, including:

• Aerospace and automotive OEMs integrating CFRP with metallic fasteners or fittings.
• Materials and corrosion engineers evaluating galvanic coupling risks and insulation integrity (sealants, coatings, injected caulking).
• Test laboratories performing standardized salt spray tests for type certification, qualification, or maintenance procedures.
• MRO (maintenance, repair and overhaul) planners who need to assess long‑term corrosion behavior and repair strategies for CFRP structures.

Key practical benefits include detection of galvanic current leak paths, evaluation of lightning strike protection layers (Cu or Al mesh/foil) in acidic conditions, and support for qualification and maintenance decision‑making.

Related standards

Relevant complementary standards referenced in ISO 21746:2019 include ISO 9227 (general salt spray testing guidance) and test/measurement standards such as ISO 1514, ISO 2808, ISO 8044 and ISO 8407 for panels, film thickness, corrosion terminology and removal of corrosion products.

Keywords: ISO 21746:2019, galvanic corrosion, CFRP, carbon fibre reinforced plastics, salt spray tests, NSS, AASS, CASS, galvanic corrosion resistance, salt spray cabinet.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 21746
First edition
2019-04
Composites and metal assemblies —
Galvanic corrosion tests of carbon
fibre reinforced plastics (CFRPs)
related bonded or fastened
structures in artificial atmospheres
— Salt spray tests
Assemblages composites et métal — Essais de corrosion galvanique des
structures en plastiques renforcés de fibres de carbone (CFRP) jointes
ou fixées en atmosphères artificielles — Essais au brouillard salin
Reference number
©
ISO 2019
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test solutions . 2
4.1 Preparation of the sodium chloride solution . 2
4.2 pH adjustment . 3
4.2.1 pH of the salt solution . 3
4.2.2 Neutral salt spray test (NSS) . 3
4.2.3 Acetic acid salt spray test (AASS) . 3
4.2.4 Copper-accelerated acetic acid salt spray test (CASS) . 3
4.3 Filtration . 3
5 Apparatus . 4
6 Method for evaluating cabinet corrosivity . 5
6.1 General . 5
6.2 Reference specimens . 5
6.3 Arrangement of the reference specimens. 6
6.4 Duration of tests . 6
6.5 Determination of mass loss (mass per area) . 6
6.6 Satisfactory performance of cabinet . 6
7 Test specimens. 7
8 Arrangement of the test specimens . 7
9 Operating conditions . 8
10 Duration of tests . 8
11 Treatment of specimens after test . 9
11.1 General . 9
11.2 Non-organic coated specimens: metallic and/or inorganic coated . 9
11.3 Organic coated specimens . 9
11.3.1 Scribed organic coated specimens . 9
11.3.2 Organic coated but not scribed specimens . 9
12 Evaluation of results . 9
13 Test report .10
Annex A (informative) Example schematic diagram of one possible design of spray cabinet
for galvanic corrosion test with means for optional treating fog exhaust and drain .11
Annex B (informative) Complementary method for evaluating galvanic corrosion test
cabinet corrosivity using zinc reference specimens .13
Annex C (normative) Preparation of galvanic corrosion test panels with organic coatings .15
Annex D (normative) Required supplementary information for testing galvanic corrosion
test panels with organic coatings .16
Annex E (informative) Recommended specimen geometries — Salt spray tests to metal and
CFRP assemblies .17
Bibliography .19
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 13,
Composites and reinforcement fibres.
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.
iv © ISO 2019 – All rights reserved

Introduction
This document specifies the testing method for galvanic corrosion of composites and metal assemblies
subject to salt spray environment using a bonded or fastened specimen.
The potential benefits to CFRP-metal users of implementing a galvanic corrosion test based on this
document are:
a) addressing corrosion risks relating CFRPs of drastically nobler galvanic potential than metals to
form a global cell between CFRP and metal – new risks drastically exceeding the scope of ISO 9227
for a local cell of isolated metal – utilizing the resources of ISO 9227;
b) expanding CFRP applications to the fields of corrosive environments that still require the
combinations with metallic components;
c) the detection or the prevention of galvanic current insulation loss, such as ion migration and
time-related degradation in sealant film, injected calking layer and glass fibre reinforced plastics
(GFRPs) layer;
d) demonstrating the conformity to specified conditions for type certification requirements in the
engineering such as aircraft developments;
e) evaluating the corrosion related procedures for maintenance, repair and overhaul (MRO) in the
engineering operations such of CFRP aircrafts.
It is not the intent of this document to imply the need for:
— omitting relevant field tests for CFRP related engineering;
— generally specifying the dimensions of test specimen to represent CFRPs related bonded or fastened
structures;
— superimposing test results for specific applications of the parameters that exceed the range of this
document;
— comparative testing as a means of ranking different protections with respect to corrosion resistance.
INTERNATIONAL STANDARD ISO 21746:2019(E)
Composites and metal assemblies — Galvanic corrosion
tests of carbon fibre reinforced plastics (CFRPs) related
bonded or fastened structures in artificial atmospheres —
Salt spray tests
1 Scope
This document specifies the apparatus, the reagents and the procedure to be used in conducting the
neutral salt spray (NSS), acetic acid salt spray (AASS) and copper-accelerated acetic acid salt spray
(CASS) tests for assessment of the galvanic corrosion resistance of joints and bonded structures
between carbon fibre reinforced plastics (CFRPs) and metallic materials, with or without permanent or
temporary insulation for the galvanic current.
2 Normative references
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 1514, Paints and varnishes — Standard panels for testing
ISO 2808, Paints and varnishes — Determination of film thickness
ISO 3574, Cold-reduced carbon steel sheet of commercial and drawing qualities
ISO 6361-2, Wrought aluminium and aluminium alloys — Sheets, strips and plates — Part 2: Mechanical
properties
ISO 8044, Corrosion of metals and alloys — Basic terms and definitions
ISO 8407, Corrosion of metals and alloys — Removal of corrosion products from corrosion test specimens
ISO 17872, Paints and varnishes — Guidelines for the introduction of scribe marks through coatings on
metallic panels for corrosion testing
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8044 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
reference material
material with known test performance
3.2
reference specimen
portion of the reference material that is to be exposed with the intention to check the reproducibility
and repeatability of the test results for the test cabinet in use
3.3
test specimen
specific portion of the samples upon which the testing is to be performed
3.4
substitute specimen
specimen made of inert materials (such as ceramic or glass) used for the substitute of a test specimen
Note 1 to entry: Examples of inert materials are ceramic or glass.
3.5
control specimen
specimen made of CFRP, which is identical to the CFRPs of test specimen, used to check the
reproducibility, repeatability and deviation or drift by solution absorption of the test results
3.6
neutral salt spray test
NSS
test method in which a 5 % sodium chloride solution is atomized under a controlled environment
Note 1 to entry: It particularly applies to:
— CFRPs and metals or their alloys in fastened or bonded form;
— sacrificial protections (anodic and cathodic);
— organic coatings on pinned or riveted joints of CFRPs and metals.
3.7
acetic acid salt spray test
AASS
test method in which a 5 % sodium chloride solution with the addition of glacial acetic acid is atomized
under a controlled environment
Note 1 to entry: It is especially useful for testing CFRPs with lightning strike protection layer (LSP) of Cu or Al
mesh or foil in acid-rain or exhaust gas environment.
3.8
copper-accelerated acetic acid salt spray test
CASS
test method in which a 5 % sodium chloride solution with the addition of copper chloride and glacial
acetic acid is atomized under a controlled environment
Note 1 to entry: It is useful for modelling an aged structure of CFRPs with LSPs and Al alloys in fastened or
bonded form in acid-rain or exhaust gas environment.
3.9
scribed specimen
specimen with organic or inorganic coating, which is intentionally line-damaged with scriber needle
3.10
purity of salt
mass fraction of sodium chloride excluding contaminant
4 Test solutions
4.1 Preparation of the sodium chloride solution
Dissolve a sufficient mass of sodium chloride in distilled or deionized water with a conductivity not
higher than 20 μS/cm at 25 °C ± 2 °C to produce a concentration of 50 g/ l ± 5 g/l. The conductivity shall
be measured just before the preparation, as dissolution of carbon dioxide in atmospheric environment
2 © ISO 2019 – All rights reserved

can drift the value. The sodium chloride concentration of the sprayed solution collected shall be
50 g/l ± 5 g/l. The specific gravity range of a 50 g/l ± 5 g/l solution is from 1,029 to 1,036 at 25 °C.
The sodium chloride shall contain less than 0,001 % mass fraction of copper and less than 0,001 %
mass fraction of nickel, as determined by atomic absorption spectrophotometry or another analytical
method of similar sensitivity. It shall not contain more than 0,1 % of a mass fraction of sodium iodide,
or more than 0,5 % of a mass fraction of total impurities calculated for dry salt. Industrial salt is not
recommended due to possible deviation of the impurities.
4.2 pH adjustment
4.2.1 pH of the salt solution
If necessary, control the pH of distilled or deionized water to 7,0 by the aeration of nitrogen gas
- dissolution of carbon dioxide can drift the pH – in the preparation of the sodium chloride solution.
Adjust the pH of the salt solution to the desired value on the basis of the pH of the sprayed solution
collected.
4.2.2 Neutral salt spray test (NSS)
Adjust the pH of the salt solution (see 4.1) so that the pH of the sprayed solution collected within the
test cabinet (5.2) is 6,5 to 7,2 at 25 °C ± 2 °C. Check the pH using electrometric measurement or in
routine checks, with a short-range pH paper, which can be read in increments or 0,2 pH units or less.
If pH electrodes are used, they shall be suitable for measuring pH in weakly buffered sodium chloride
solutions in de-ionized water. Make any necessary corrections by adding hydrochloric acid, sodium
hydroxide or sodium bicarbonate solution of analytical grade.
4.2.3 Acetic acid salt spray test (AASS)
Add a sufficient amount of glacial acetic acid to the salt solution (see 4.1) to ensure that the pH of
samples of sprayed solution collected in the test cabinet (see 5.2) is between 3,1 and 3,3. If the pH of the
solution initially prepared is 3,0 to 3,1, the pH of the sprayed solution is likely to be within the specified
limits. Check the pH using electrometric measurement at 25 °C ± 2 °C. If pH electrodes are used, they
shall be suitable for measuring pH in weakly buffered sodium chloride solutions in de-ionized water.
Make any necessary corrections by adding glacial acetic acid or sodium hydroxide of analytical grade.
4.2.4 Copper-accelerated acetic acid salt spray test (CASS)
Dissolve a sufficient mass of copper (II) chloride dihydrate (CuCl ·2H O) in the salt solution (see 4.1) to
2 2
produce a concentration of 0,26 g/l ± 0,02 g/l [equivalent to (0,205 ± 0,015) g of CuCl per litre].
Adjust the pH using the procedures described in 4.2.3.
4.3 Filtration
Test solutions prepared using the procedures described in 4.1 and 4.2.1 in laboratory environment are
suitable for the spraying applications. However, if necessary, filter the solution before placing it in the
reservoir of the apparatus, to remove any solid matter or contaminant which might affect the apertures
of the spraying device. Any incident relating to the solid matter shall be reported as described in 13.2 r).
5 Apparatus
5.1 Component protection. All components in contact with the spray or the test solution shall
be made of, or lined with, materials resistant to corrosion by the sprayed solution and which do not
influence the corrosivity of the sprayed test solution.
If necessary, electric insulation shall be devised for the apparatus to prevent the influence from inside
or outside of the apparatus.
The supports for the test specimen shall be constructed such that different substrate types do not
influence each other. It shall also be constructed so that the supports themselves do not influence the
test specimens.
5.2 Spray cabinet. The cabinet for galvanic corrosion test shall be such that the conditions of
homogeneity and distribution of the spray are met. Due to the limited capacity of cabinets smaller than
0,4 m3, the effect of the loading of the cabinet on the distribution of the spray and temperature should
be carefully considered. The sprayed solution shall be fell down naturally on the test specimens, and the
upper parts of the cabinet shall be designed so that drops of spayed solution formed on its surface do not
fall on the test specimens being tested (see Annex A).
If a cabinet for galvanic corrosion test has been used once for an AASS or CASS test, or has been used for
any other purpose with a solution differing from that specified for the NSS test, it shall not be used for
the NSS test.
The size and shape of the cabinet shall be such that the collection rate of solution in the cabinet is within
the limits specified in Table 3, measured as specified in 9.3.
Preference shall be given to apparatus that has a means for properly dealing with fog after the test,
prior to releasing it from the building for environmental conservation, and for drawing water prior to
discharging it to the drainage system.
5.3 Heater and temperature control. An appropriate system for galvanic corrosion test maintains the
cabinet and its contents at the specified temperature (see Table 3). The temperature shall be measured
at least 100 mm from the walls.
5.4 Spraying device. The device for spraying the test solution comprises a supply of clean air, of
controlled pressure and humidity, a reservoir to contain the solution to be sprayed, and one or more
atomizers.
The compressed air supplied to the atomizers shall be filtered to remove all traces of contaminants
such as oil or solid matter, and the atomizing pressure shall be at an overpressure of 70 kPa to 170 kPa.
The pressure should be 98 kPa ± 10 kPa.
In order to prevent the evaporation of water from the sprayed droplets (aerosol), the air shall be
humidified before entering the atomizer by passing through a suitable humidifier. The humidified air
shall be saturated such that the concentration of the fallout solution falls within the specifications of
4.1. The humidified air shall also be heated such that when mixed with the salt solution, there is no
significant disturbance of the temperature in the cabinet. The appropriate temperature depends on
the pressure used and on the type of atomizer nozzle. Temperature, pressure or humidification, or
a combination thereof, shall be adjusted so that the rate of collection of the spray in the cabinet and
the concentration of the collected spray are kept within the specified limits (see 9.3). A common used
humidifier is the saturation tower, where temperature and pressure are controllable. Table 1 gives
guidance values on temperature and pressure combinations for the saturation tower.
4 © ISO 2019 – All rights reserved

Table 1 — Guiding values of galvanic corrosion test for the solution temperature in the
saturation tower
Guiding values for the temperature, in °C, of the hot water in the sat-
Atomizing overpressure
uration tower when performing the different salt spray test
Neutral salt spray (NSS) and ace- Copper-accelerated acetic acid
kPa
tic acid salt spray (AASS) salt spray (CASS)
70 45 61
84 46 63
98 48 64
112 49 66
126 50 67
140 52 69
The atomizers shall be made of inert material such as ceramic and glass. Baffles may be used to prevent
direct impact of the spray on the test specimens, and the use of adjustable baffles is helpful in obtaining
uniform distribution of the spray within the cabinet. For this purpose, a dispersion tower equipped
with an atomizer may also be helpful.
The supply salt solution to the nozzle shall be kept stable to ensure a continuous and uniform fall out as
described in 9.3. A stable level of spraying can be achieved by either controlling the level of salt solution
in the reservoir or restricting the flow of salt solution to the nozzle such that a continuous spray is
achieved.
Distilled or deionized water with a conductivity no
...