ISO 21746:2019

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.