Salt spray corrosion testing of thermal conductive gel for photovoltaic modules is an accelerated environmental test that evaluates how thermally conductive silicone gels withstand chloride-rich, corrosive atmospheres typical of coastal and offshore solar installations. Conducted in accordance with internationally recognized standards such as ISO 9227, ASTM B117, and IEC 61701, this testing verifies that the gel maintains its thermal transfer, adhesion, and electrical insulation performance after prolonged salt fog exposure. This article explains the sample types, test items, methodologies, evaluation criteria, and typical applications of the test for module manufacturers, material suppliers, and project developers.
Technical Overview
Thermal conductive gel is a low-hardness, highly conformable silicone elastomer loaded with thermally conductive ceramic fillers such as alumina or aluminum nitride. In photovoltaic modules, it is applied at critical heat-transfer interfaces, including junction box interiors, bypass diode assemblies, cell-to-backsheet gaps, and module-integrated power electronics, where it replaces air gaps and lowers thermal resistance. Because photovoltaic power plants are increasingly deployed in coastal zones, islands, offshore platforms, and floating installations, the gel and its adjacent metallic interfaces are continuously exposed to salt-laden moisture.
Salt spray corrosion testing simulates this aggressive environment in an accelerated laboratory chamber by atomizing a controlled sodium chloride solution onto test specimens. Although the gel itself is a non-metallic polymer, the test is essential for several reasons: chloride ions can migrate through or along the gel and attack protected metal surfaces, the gel can lose adhesion or swell under humid saline conditions, filler interfaces can degrade, and ionic contamination can compromise electrical insulation. The test therefore confirms that the thermal interface material will not become a corrosion initiation point or fail to perform thermally over the module service life. Accredited third-party laboratories routinely perform this test as part of material qualification and module-level certification programs.
Testing Samples
Bulk Cured Gel Specimens
Free-standing plaques and sheets of fully cured gel are prepared according to the manufacturer's specified mixing ratio, curing time, and curing temperature. These specimens, typically 2 mm thick, are used to measure changes in hardness, tensile properties, thermal conductivity, volume resistivity, mass, and extractable ion content before and after salt fog exposure.
Gel-on-Substrate Assemblies
The gel is dispensed and cured onto substrates representative of actual module hardware, including tinned copper busbars, copper foil, aluminum alloy heat spreaders, and junction box terminal metals. These assemblies evaluate adhesion retention, interface corrosion, and whether the gel acts as an effective barrier against chloride penetration to the metal surface.
Component-Level Assemblies
Complete functional subassemblies, such as junction boxes filled or lined with thermal gel, bypass diode packages, and small power-electronic units bonded with the gel, are tested to observe real-world interaction between the gel, solder joints, encapsulants, plastic housings, and metallic contacts under salt fog conditions.
Module-Level Specimens
For full verification against IEC 61701, complete photovoltaic modules or representative module sections containing the thermal gel are exposed in the salt spray chamber. Pre- and post-exposure electrical performance measurements, including maximum power determination per IEC 61215 series requirements, quantify whether the internal materials have degraded.
Testing Items
The evaluation program for thermal conductive gel combines visual, physical, thermal, electrical, and chemical test items conducted before and after salt fog exposure:
- Visual appearance inspection: blistering, cracking, chalking, discoloration, surface tack change, oil bleeding, and delamination, rated with reference to ISO 4628 degradation assessment methods.
- Corrosion assessment of adjacent metals: white rust on zinc or zinc-aluminum coated surfaces, red rust on steel, and tarnishing or pitting of copper, rated in accordance with ISO 10289 or equivalent evaluation practices.
- Adhesion performance: 90-degree or 180-degree peel strength and lap shear strength retention on standard metal substrates.
- Thermal performance: thermal conductivity and thermal impedance measured by methods such as ASTM D5470 or ISO 22007-2, confirming retention of heat-transfer capability.
- Mechanical properties: Shore 00 hardness per ASTM D2240, tensile strength and elongation per ISO 37, and compression deformation behavior.
- Physical stability: percentage change in mass and volume, checking for saline water absorption or swelling.
- Electrical properties: volume resistivity per IEC 62631 series and dielectric strength per IEC 60243-1, confirming insulation integrity in wet, ionic environments.
- Ionic contamination analysis: extractable chloride, sodium, and potassium ions quantified by ion chromatography to confirm the gel does not leach corrosive species.
Testing Methods
Applicable Standards
- ISO 9227: Corrosion tests in artificial atmospheres, covering neutral salt spray (NSS), acetic acid salt spray (AASS), and copper-accelerated acetic acid salt spray (CASS) methods.
- ASTM B117: Standard practice for operating salt spray (fog) apparatus, widely referenced for material-level qualification in North America.
- IEC 61701: Salt mist corrosion testing of photovoltaic modules, defining severity levels and post-test evaluation requirements at module level.
- IEC 60068-2-52: Environmental testing, Test Kb, cyclic salt mist exposure combining salt fog phases with humidity storage phases.
- GB/T 10125: The Chinese national standard for corrosion tests in artificial atmospheres, technically aligned with ISO 9227.
Test Conditions
For the most common neutral salt spray (NSS) method, the chamber maintains a 5 percent by mass sodium chloride solution atomized at 35 degrees Celsius, with fog collection rates of 1 to 2 mL per hour per 80 square centimeters and a collected solution pH between 6.5 and 7.2. Acetic acid salt spray lowers the pH to 3.1 to 3.3 for more aggressive evaluation, while CASS adds copper(II) chloride and raises the temperature to 50 degrees Celsius for severely corrosive simulation. Specimens are positioned 15 to 30 degrees from vertical on inert supports, ensuring uniform fog contact and avoiding contact between samples that could create crevice effects.
Exposure Duration
Exposure durations are selected according to the target severity level and the corrosivity category of the intended installation environment, classified under ISO 9223. Typical material-level durations include 48, 96, 168, 336, 500, and 1000 hours. For module-level verification under IEC 61701, severity levels define cumulative salt mist exposure ranging from short qualification durations up to extended exposures of 1000 hours or more for the harshest marine environments.
Procedure and Evaluation
The standard workflow proceeds as follows: baseline characterization of all specimens, chamber conditioning and calibration verification, loading of samples with defined orientation, continuous or cyclic exposure for the specified duration, followed by a controlled recovery period in which specimens are gently rinsed and stabilized in standard laboratory atmosphere before evaluation. Post-test measurements repeat the baseline test items, and results are compared against acceptance criteria. Typical pass criteria, defined by the product specification or the client's technical requirements, include no red rust on protected metal surfaces, no gel delamination or cracking, thermal conductivity retention of at least 90 percent of the initial value, retention of minimum adhesion and dielectric strength values, and visual degradation ratings within specified limits.
Applications
Salt spray corrosion testing of thermal conductive gel supports the photovoltaic industry across multiple scenarios:
- Qualification of new thermal interface materials during module design and development, confirming suitability for coastal and offshore deployment.
- Supplier change management and incoming material control, verifying that alternate gel formulations deliver equivalent corrosion resistance.
- Module certification support, providing documented evidence for IEC 61701 salt mist compliance required in coastal tenders and bankability reviews.
- Site-specific engineering decisions, matching material selection to the atmospheric corrosivity category of the project location.
- Floating solar and near-shore installations, where high humidity and direct salt deposition impose the most demanding conditions on internal module materials.
- Root cause failure analysis, comparing degraded field returns against accelerated test references to identify chloride-driven degradation mechanisms.
FAQ
Why is salt spray testing necessary for a non-metallic gel?
Although the gel does not rust, it operates in direct contact with metals it must protect. Salt fog testing verifies that the gel resists moisture absorption and chloride transmission, maintains adhesion, does not leach corrosive ions, and continues to provide thermal and electrical performance after corrosive exposure. A gel that fails these checks can accelerate corrosion of busbars and diode terminals rather than prevent it.
Which test method should be selected: NSS, AASS, or CASS?
Neutral salt spray is the default method for photovoltaic applications and aligns directly with IEC 61701 requirements. Acetic acid and copper-accelerated variants are reserved for materials intended for severely corrosive micro-environments or for accelerated comparative screening, because their lower pH and added copper ions produce substantially faster attack.
How long should the exposure last?
Duration depends on the installation environment and specification requirements. Inland or mildly corrosive sites may accept 96 to 168 hours, while coastal, island, and floating installations typically require 500 to 1000 hours or more. The selected duration should be documented together with the corresponding severity level to ensure consistent acceptance criteria.
What results indicate a passing sample?
A sample generally passes when post-exposure inspection shows no cracking, delamination, or significant surface degradation of the gel; no unacceptable corrosion of adjacent metals beyond agreed rating limits; thermal conductivity retention within the specified tolerance, commonly 90 percent or higher; maintained peel or shear adhesion above the minimum value; and preserved dielectric strength and insulation resistance.
Does salt spray testing predict actual service life?
No single accelerated corrosion test translates directly into years of field service, because real degradation depends on wet-dry cycling, UV radiation, temperature cycling, and pollutant chemistry. Salt spray results should be interpreted as a comparative and qualification tool, ideally combined with damp heat, thermal cycling, and humidity-freeze testing per the IEC 61215 series for a complete reliability picture.
How many specimens are required for a valid test?
Best practice requires a minimum of three to five replicate specimens per condition for each test item, plus retained control specimens, so that measured changes can be reported with statistical confidence and compared against baseline values from identical, unexposed material.