Photovoltaic (PV) backsheet salt spray corrosion testing is a standardized accelerated aging procedure used to verify the resistance of solar module backsheets to the saline atmospheres encountered in coastal, offshore, floating, and high-salinity deployment environments. Performed in accordance with recognized standards such as IEC 61730-2, IEC 60068-2-52, ISO 9227, and ASTM B117, the test exposes backsheet materials and complete modules to controlled salt fog to reveal corrosion, delamination, adhesion loss, and insulation degradation before products reach the field. This article explains the applicable standards, sample types, test conditions, evaluation criteria, and practical applications of salt spray corrosion testing for photovoltaic backsheets.
Technical Overview
The photovoltaic backsheet is the multilayer polymeric sheet laminated to the rear surface of a crystalline silicon solar module, where it provides electrical insulation, moisture barrier protection, and long-term weathering resistance for the cells and encapsulant. A typical construction consists of an outer weathering-resistant fluoropolymer or coated layer, a polyethylene terephthalate (PET) core that delivers dielectric strength and water vapor barrier performance, and an inner adhesion layer compatible with ethylene vinyl acetate (EVA) or polyolefin elastomer (POE) encapsulants.
In coastal, island, offshore, floating, and saline-alkali desert environments, airborne chloride particles deposit on the module rear surface and, combined with humidity and condensation cycles, accelerate hydrolysis of the PET core, degrade interlayer adhesives, and corrode exposed metallic components such as lead-out ribbons, junction box terminals, and aluminum barrier foils. Salt spray corrosion testing reproduces these degradation drivers in an accelerated, reproducible laboratory environment, enabling material screening, design verification, and safety certification before field deployment.
The relevant framework of standards includes:
- IEC 61730-2 – Photovoltaic (PV) module safety qualification, which includes a salt mist corrosion test applicable when a module is declared suitable for installation in corrosive atmospheres.
- IEC 60068-2-52 – Environmental testing, Test Kb: salt mist, cyclic, the method referenced by IEC 61730-2 for module-level evaluation.
- IEC 60068-2-11 – Environmental testing, Test Ka: salt mist, for basic neutral salt fog exposure of components and materials.
- ISO 9227 – Corrosion tests in artificial atmospheres, defining the neutral (NSS), acetic acid (ASS), and copper-accelerated acetic acid (CASS) salt spray methods.
- ASTM B117 – Standard practice for operating salt spray (fog) apparatus.
- GB/T 2423.17 and GB/T 10125 – Chinese national standards technically aligned with the IEC and ISO salt spray methods.
Testing Samples
Backsheet Film Specimens
Flat coupons cut from production backsheet rolls, covering fluoropolymer-based structures (PVF/PET/PVF, PVDF/PET/PVDF), coated PET structures, coextruded multilayer backsheets, and foil-containing barrier backsheets. Specimens are prepared at standardized dimensions (for example 150 mm × 75 mm or 100 mm × 100 mm), conditioned at 23 °C and 50% relative humidity before exposure, and placed in the chamber at an inclination of 15° to 30° from vertical so that condensate drains freely.
Laminated Composite Coupons
Mini-laminates reproducing the real module lay-up of glass, EVA or POE encapsulant, and backsheet are tested to evaluate how salt-laden moisture attacks the encapsulant-to-backsheet interface, which is often the weakest path for moisture ingress and delamination in the field.
Backsheet Assemblies with Auxiliary Components
Specimens integrating junction box mounting adhesives, lead-out ribbon pass-throughs, cut edges, and sealing tapes are used to assess edge corrosion and adhesive degradation at realistic mechanical interfaces.
Complete Photovoltaic Modules
Full modules with frame, junction box, cables, and connectors are tested when certification under IEC 61730-2 requires evidence of performance in corrosive environments; the module-level test evaluates the entire weather protection system rather than the backsheet material in isolation.
Testing Items
Depending on the product level and the specification invoked, the following characteristics are typically measured before and after salt spray exposure:
- Visual appearance: blistering, cracking, delamination, warping, salt residue staining, and surface corrosion of metallic parts.
- Interlayer and encapsulant adhesion: 90° or 180° peel strength and its retention relative to unexposed controls.
- Moisture barrier performance: water vapor transmission rate (WVTR) after exposure.
- Mechanical properties: tensile strength and elongation at break retention.
- Optical properties: color difference (delta E) and yellowing index (YI) of the exposed surface.
- Electrical safety: insulation resistance and wet leakage current, particularly for complete modules.
- Corrosion of metallic elements: busbars, lead-out ribbons, aluminum foil layers, frame edges, and junction box terminals.
- Dimensional and mass stability: shrinkage, swelling, and weight change after exposure and reconditioning.
Testing Methods
Neutral Salt Spray (NSS)
The most widely applied method for backsheet evaluation, performed per ISO 9227, ASTM B117, GB/T 2423.17, or IEC 60068-2-11, with the following key conditions:
- Solution: sodium chloride (NaCl) at 50 ± 5 g/L (approximately 5%) in distilled or deionized water.
- pH: 6.5 to 7.2 measured at 25 °C.
- Chamber temperature: 35 ± 2 °C.
- Fog collection rate: 1.0 to 2.0 mL per 80 cm² per hour, averaged over the collection period.
- Common exposure durations: 24, 48, 96, 168, 240, 500, 720, or 1000 hours, selected according to the target service environment and the invoked specification.
Cyclic Salt Mist Test (IEC 60068-2-52, Test Kb)
The cyclic method mandated for module-level assessment under IEC 61730-2, because it reproduces the wet-dry cycling that drives chloride transport into laminates more realistically than continuous fog:
- Each 24-hour cycle combines a 2-hour neutral salt spray phase with an extended humid storage phase of approximately 22 hours at 40 °C and at least 93% relative humidity.
- Severity level (1), comprising four cycles, is the configuration referenced for PV module safety testing.
- Higher severity levels with additional cycles are available for offshore and marine applications with more demanding service conditions.
Acidified Salt Spray (ASS and CASS)
For research purposes or highly aggressive atmospheres, ISO 9227 also defines the acetic acid salt spray test (ASS: pH 3.1 to 3.3 at 35 °C) and the copper-accelerated acetic acid salt spray test (CASS: copper(II) chloride addition, pH 3.1 to 3.3 at 50 °C). These variants are occasionally used to benchmark coated backsheets and metal-containing structures under accelerated acidic conditions typical of industrial or agricultural environments.
Post-Exposure Evaluation Procedure
After exposure, specimens are gently rinsed with running water to remove salt deposits, dried, and conditioned at standard laboratory atmosphere before assessment. Evaluation normally combines visual rating against defined defect criteria with quantitative comparisons of peel strength, tensile properties, WVTR, and, for modules, a wet leakage current test and insulation verification in accordance with IEC 61730-2. A specimen is generally considered nonconforming when corrosion products penetrate functional layers, delamination exceeds specified limits, adhesion retention falls below the agreed acceptance threshold, or electrical safety requirements are no longer met.
Applications
- Certification support for PV modules and backsheets intended for coastal, island, and offshore solar farms, where tender documents frequently require documented salt mist endurance.
- Floating photovoltaic projects on reservoirs, lakes, and nearshore waters, where the water surface raises ambient humidity and chloride concentration around module rear sides.
- Desert and saline-alkali land PV plants, where salt-laden dust and dew cycles create strong corrosion and hydrolysis stress.
- Material research and development: comparing fluoropolymer, coated PET, coextruded, and foil-based backsheet platforms under identical corrosive stress.
- Incoming quality control and supplier qualification for module manufacturers verifying batch-to-batch consistency of backsheet corrosion resistance.
- Failure analysis and warranty investigation, correlating laboratory salt spray response with field degradation such as backsheet cracking, yellowing, or delamination reported from marine installations.
FAQ
Is salt spray corrosion testing mandatory for every PV backsheet?
No. It is a conditional test under IEC 61730-2, applied when the module manufacturer declares that the product will be installed in corrosive atmospheres such as coastal or offshore sites. Many project developers nevertheless request it proactively as a durability indicator.
Which duration or severity should be selected?
For module-level assessment under IEC 61730-2, the cyclic method of IEC 60068-2-52 Test Kb, severity (1), is the standard choice. For backsheet material screening, durations between 96 and 1000 hours of neutral salt spray are common, with longer durations reserved for marine and floating PV applications.
What is the difference between ASTM B117 and IEC 60068-2-52?
ASTM B117 and the NSS method of ISO 9227 describe continuous exposure to a neutral salt fog, making them suitable for material comparison. IEC 60068-2-52 Test Kb alternates salt spraying with humid storage, reproducing the wet-dry cycling of real coastal environments, and is the method invoked for PV module safety testing.
What results indicate a failure?
Typical failure signatures include interlayer blistering or delamination, hydrolytic whitening and embrittlement of the PET core, corrosion of metallic foil or ribbon elements, adhesion retention below the specified threshold, and, at module level, failure of the wet leakage or insulation tests.
Can salt spray results predict service life?
Salt spray testing is an accelerated comparative test, not a direct service-life predictor. Correlation with field performance requires combining salt spray data with damp heat (85 °C / 85% RH), humidity-freeze, UV, and thermal cycling results, together with field reference data for similar material systems.
Where should the test be performed?
Salt spray corrosion testing should be carried out by accredited third-party laboratories or qualified in-house facilities whose chambers, instrumentation, and procedures are validated against ISO 9227, IEC 60068-2-52, or ASTM B117, with calibrated fog collection rates and documented temperature and pH control.