Foam Sulfur Dioxide Corrosion Testing: Standards, Methods, and Evaluation Criteria

Foam sulfur dioxide corrosion testing is an accelerated environmental test that evaluates whether foam materials such as EVA, PE, PU, silicone, and EPDM foams cause corrosion of adjacent metal components or degrade when exposed to humid sulfur dioxide atmospheres. This testing is critical for electronics, automotive, and industrial manufacturers who need to verify that foam gaskets, cushioning pads, and sealing components will not tarnish silver-plated connectors or other sensitive metal parts. This article explains the applicable standards, sample types, test methods, evaluation criteria, and practical applications of SO2 corrosion testing for foam materials.

Technical Overview

Sulfur dioxide (SO2) corrosion testing is a laboratory environmental test in which materials are exposed to a controlled atmosphere containing a defined concentration of sulfur dioxide gas at elevated temperature and relative humidity. In the presence of moisture, SO2 dissolves on metal surfaces to form sulfurous and sulfuric acids, which aggressively attack silver, copper, nickel, tin, and steel. The test therefore simulates aggressive industrial atmospheres and accelerates corrosion processes that would otherwise take years to develop in the field.

Foam materials, including EVA, PE, XPE, PU, silicone, and EPDM foams, are widely used in electronic assemblies as cushioning pads, gap fillers, shock absorbers, dust seals, and gaskets. Although foams themselves are non-metallic, they may contain sulfur-bearing additives such as vulcanizing agents, peroxide curing residues, flame retardants, antioxidants, or release agents. These compounds can outgas slowly and, together with ambient moisture, promote sulfidation of nearby silver-plated contacts, copper traces, and connector terminals. Sulfur dioxide corrosion testing verifies two aspects of foam quality: whether the foam is chemically inert toward adjacent metals, and whether the foam retains its physical properties after SO2 exposure.

Testing is typically performed in a gas-tight environmental chamber equipped with SO2 gas injection, temperature control, humidity regulation, and continuous gas concentration monitoring. Foam specimens are exposed either alone, to assess material degradation, or together with standardized metal coupons such as silver, copper, or brass strips, to assess corrosivity toward surrounding parts. After exposure, the specimens and coupons are evaluated for tarnish, corrosion products, mass change, and contact resistance shift. Testing is commonly carried out by accredited third-party laboratories or in-house reliability laboratories using internationally recognized standards.

Testing Samples

Polyolefin Foams (EVA / PE / XPE)

EVA and polyethylene foams are common in consumer electronics, packaging, and display modules. Specimens are usually cut into sheets or plaques of a defined size, for example 50 mm × 50 mm with the nominal product thickness, and are conditioned at standard laboratory atmosphere before testing. Both plain foam and laminated or skinned versions can be evaluated.

Polyurethane Foams (PU / Poron-Type Urethane)

Urethane foams used for shock absorption and sealing in smartphones, laptops, and battery packs are frequently screened for corrosive volatiles. Specimens may be tested as die-cut parts with or without pressure-sensitive adhesive, since adhesive systems can also contribute sulfur-containing residues.

Silicone and EPDM Foams

Silicone foam is cured using peroxide or platinum systems, and EPDM foam is sulfur- or peroxide-vulcanized; both processes can leave sulfur-bearing residues. These foams are tested when used near silver-plated contacts, antennas, busbars, or optical components. Tube, sheet, and molded profiles are all acceptable specimen forms.

Foam Tapes and Composite Parts

Double-sided foam tapes, thermoformed foam components, and foam-metal or foam-film laminates are tested in their end-use configuration. Testing composite parts in the assembled state is recommended because interfacial layers can trap moisture and concentrate corrosive species.

Testing Items

Depending on the product specification and referenced standard, the following items are typically evaluated:

• Visual corrosion rating of paired metal coupons (silver, copper, brass) including tarnish, discoloration, pitting, and sulfide growth
• Mass change and mass gain of metal coupons caused by corrosion product formation
• Contact resistance of silver- or tin-plated contact pairs before and after exposure
• Tensile strength and elongation retention of the foam after SO2 exposure
• Compression set and compression deflection retention
• Peel adhesion strength retention for adhesive-backed foams
• Hardness stability (Asker C or Shore OOO scale for soft foams)
• Dimensional stability and warpage after exposure
• Surface and chemical analysis of corrosion deposits using SEM/EDS or ion chromatography to confirm sulfur-bearing species
• Screening for total sulfur and volatile sulfur outgassing as a complementary chemical characterization

Testing Methods

Reference Standards

• IEC 60068-2-42 — Environmental testing, Test Kc: Sulfur dioxide test for contacts and connections
• GB/T 2423.19 — Chinese national equivalent of IEC 60068-2-42
• JIS C 0027 — Japanese national adoption of the SO2 test for electronic components
• DIN 50018 — Corrosion testing in a saturated condensing SO2 atmosphere (Kesternich-type test)
• ASTM G87 — Standard practice for conducting moist SO2 exposure tests
• ISO 3231 — Determination of resistance to humid atmospheres containing sulfur dioxide, commonly applied to coatings on foam-adjacent surfaces

Low-Concentration Flowing Gas Method (IEC 60068-2-42 / GB/T 2423.19)

This method is the most widely used approach for foam and electronic assemblies. Typical test conditions are a temperature of 25 °C ± 2 °C, a relative humidity of 75% ± 5%, and an SO2 concentration of approximately 25 ppm by volume. Standard exposure durations are 4 days, 10 days, or 21 days, selected according to the intended severity level. Foam specimens are placed in the chamber together with cleaned metal coupons, and the gas atmosphere is maintained continuously for the specified duration.

Kesternich Condensing Method (DIN 50018 / ASTM G87)

For heavier-duty industrial or outdoor-service simulation, a condensing cabinet test may be applied. A defined quantity of SO2, for example 0.2 L, 1 L, or 2 L introduced into an approximately 300 L chamber, is used at 40 °C with cyclic condensation, typically 8 hours of condensation followed by 16 hours of dwell in each 24-hour cycle. Test length is expressed in the number of completed cycles, commonly 1 to 12 cycles. This method is more aggressive and is preferred when foams are used near painted or coated metal surfaces.

Test Procedure

• Specimen preparation: cut foam samples to specified dimensions, mark and weigh metal coupons, and record initial contact resistance values where applicable
• Pre-conditioning: stabilize specimens at 23 °C ± 2 °C and 50% ± 5% RH for a defined period, typically 24 hours
• Exposure: load specimens and coupons into the chamber without contact between samples, then establish the specified SO2 concentration, temperature, and humidity
• Recovery: after the exposure period, remove specimens and allow them to stabilize in a clean atmosphere before evaluation
• Evaluation: perform visual rating, mass measurement, electrical measurement, and mechanical property testing, and document coupon appearance photographically

Evaluation Criteria

Acceptance criteria are defined in the product specification. Typical requirements include no visible tarnish or corrosion products on silver or copper coupons, a contact resistance shift below a specified limit such as 10% or a fixed milliohm threshold, and retention of at least 80% to 90% of the original tensile strength and adhesive performance. Coupon corrosion is commonly graded on a numerical rating scale from 0 (no change) to 4 (severe corrosion), with grade 0 or 1 usually required for foams used near precious-metal-plated contacts.

Applications

Sulfur dioxide corrosion testing of foam is applied across industries where foam components operate in proximity to metals or sensitive electronics:

• Consumer electronics: cushioning pads, antenna supports, and display sealing foams in smartphones, tablets, laptops, and wearables
• Automotive electronics: battery-pack foam pads, connector seals, and control-module gaskets exposed to sulfur-containing environments
• Telecommunications: sealing foams in base-station equipment, servers, and network enclosures with silver-plated contacts
• Display and optical modules: LCD and OLED assemblies where foam gaskets contact reflective films, metal frames, and driver circuitry
• Energy storage: foam spacers and thermal pads in lithium-ion battery modules requiring long-term chemical inertness
• Medical devices and aerospace: foam sealing and damping components in equipment that must remain reliable over long service lives
• Supplier quality control: incoming-material screening of foam lots and comparative qualification of alternative foam suppliers

FAQ

Why is SO2 corrosion testing performed on foam if foam itself does not corrode?

The test determines whether the foam is chemically aggressive toward adjacent materials. Sulfur-bearing residues inside the foam can migrate to nearby metal surfaces in humid conditions and cause tarnish or sulfide corrosion on silver-plated contacts and copper conductors, leading to increased contact resistance and eventual electrical failure.

What is the difference between the SO2 test and the H2S test?

The SO2 test (IEC 60068-2-42 / GB/T 2423.19) uses approximately 25 ppm sulfur dioxide at 25 °C and 75% RH, while the hydrogen sulfide test (IEC 60068-2-43 / GB/T 2423.20) uses 10–15 ppm H2S under similar temperature and humidity. H2S is particularly aggressive toward silver, so the two tests are often run together for connector-intensive assemblies.

How long does the test take?

Under IEC 60068-2-42, standard durations are 4, 10, or 21 days of continuous exposure. Kesternich-style tests run in 24-hour cycles, with total test length of 1 to 12 cycles. Adding specimen preparation, conditioning, and post-test evaluation, a complete project typically requires one to four weeks.

What samples should be submitted for testing?

Representative foam sheets or die-cut parts in the end-use configuration are recommended, ideally with the production adhesive if one is used. A typical submission includes several pieces of at least 50 mm × 50 mm in product thickness, along with any specified metal coupons or contact samples that need to be evaluated.

What results are reported?

A standard report includes the test standard and conditions, exposure duration, photographic documentation of coupons before and after exposure, corrosion rating, mass and contact-resistance data, mechanical property retention, and a clear pass/fail conclusion against the specified acceptance criteria.

Can a foam be qualified as non-corrosive or sulfur-free?

Yes. When foam samples pass the coupon-based SO2 test with the highest rating grade, and complementary chemical analysis confirms low total sulfur content and minimal volatile sulfur outgassing, the material can be qualified as low-corrosivity or sulfur-safe for use near precious-metal finishes. Many electronics manufacturers maintain internal approved-material lists based on exactly this type of qualification.