The sulfur dioxide (SO2) aging test is an accelerated laboratory corrosion test that evaluates how metallic materials, protective coatings, electronic components, and polymeric products withstand humid atmospheres containing sulfur dioxide, one of the most common industrial air pollutants. By reproducing aggressive industrial environments inside a controlled test chamber, this method enables manufacturers to predict long-term service durability and demonstrate compliance with internationally recognized standards such as ISO 6988, ISO 3231, and IEC 60068-2-42. This article explains the technical principles, applicable sample types, test items, methodologies, and typical applications of the SO2 aging test.
Technical Overview
Sulfur dioxide is released into the atmosphere primarily through the combustion of sulfur-bearing fossil fuels, metal smelting, and chemical processing. When SO2 dissolves in the thin moisture film that condenses on material surfaces, it forms sulfurous and sulfuric acids, which aggressively attack metals, metallic coatings, paints, and elastomers. The sulfur dioxide aging test, also known as the SO2 corrosion test or Kesternich test, reproduces these conditions in a sealed environmental chamber so that many years of exposure to a polluted industrial atmosphere can be simulated within days or weeks.
In a typical test, specimens are placed in a temperature- and humidity-controlled chamber into which a measured quantity of sulfur dioxide gas is introduced. Depending on the selected standard, the test may operate under continuous condensation conditions with relatively high SO2 dosing, or under constant climate conditions with a low, precisely regulated SO2 concentration. After exposure, specimens are assessed for corrosion, coating degradation, changes in contact resistance, and other performance criteria.
The most widely applied standards for SO2 aging and corrosion testing include:
- ISO 6988 – Metallic and other non-organic coatings: sulfur dioxide test with general condensation of moisture (Kesternich test)
- ISO 3231 – Paints and varnishes: determination of resistance to humid atmospheres containing sulfur dioxide
- IEC 60068-2-42 – Environmental testing, Test Kc: sulfur dioxide test for contacts and connections
- GB/T 2423.19 – Environmental testing for electric and electronic products: sulfur dioxide test
- ASTM G87 – Standard practice for conducting moist SO2 exposure tests
Testing Samples
Metallic Materials and Inorganic Coatings
Electroplated components such as zinc-plated, zinc-nickel plated, nickel-chromium plated, and tin-plated parts, fasteners, stampings, die castings, anodized aluminum, and passivated surfaces are frequently tested. The SO2 test is particularly effective at revealing porosity in coatings and weaknesses in conversion layers such as chromate or trivalent passivates.
Organic Coatings and Paint Systems
Coated test panels, painted components, powder-coated parts, and e-coat samples are evaluated for blistering, loss of adhesion, undercutting at scribe marks, and color or gloss changes caused by acidic humid atmospheres.
Electronic Components and Electrical Contacts
Connectors, relay contacts, switch contacts, printed circuit boards, and terminated wire assemblies are exposed to low-concentration SO2 atmospheres to detect surface tarnishing and increases in contact resistance that could compromise signal or power integrity.
Rubber, Plastics, and Elastomeric Seals
Polymeric materials used in gaskets, seals, cables, and hoses may be evaluated for changes in hardness, tensile strength, elongation, and surface appearance after exposure to sulfur dioxide, reflecting aging behavior in chemically polluted environments.
Testing Items
Depending on the material and the referencing specification, the following characteristics are typically evaluated before and after SO2 exposure:
- Visual appearance: corrosion products, white rust, red rust, tarnishing, discoloration, blistering, or cracking
- Corrosion extent and rating in accordance with ISO 10289 or the agreed rating system
- Mass change and calculated corrosion rate
- Coating adhesion, including cross-cut or pull-off testing performed after exposure
- Gloss retention and color difference for painted and organic finishes
- Contact resistance measurement for electrical contacts and connections
- Retention of mechanical properties such as hardness and tensile strength for polymeric materials
- Functional verification of components after exposure, where required by the product standard
Testing Methods
Kesternich Condensing SO2 Test (ISO 6988 / ISO 3231)
The classic Kesternich test uses a chamber of approximately 300 liters in volume. A defined quantity of sulfur dioxide gas, typically 0.2 L, 1 L, or 2 L, is introduced at the start of each 24-hour cycle. Each cycle consists of approximately 8 hours with the chamber heated to 40 °C ± 3 °C so that moisture condenses on the specimens, followed by 16 hours with heating switched off and the chamber kept closed. The number of cycles, commonly ranging from a single cycle up to 24 or more cycles, is defined by the product specification. The condensing acidic atmosphere rapidly generates realistic corrosion morphologies that correlate well with industrial field exposure.
Low-Concentration SO2 Test (IEC 60068-2-42 Test Kc / GB/T 2423.19)
This method is designed for contacts and connections used in environments containing traces of sulfur dioxide. Specimens are exposed to a continuously regulated atmosphere maintained at 25 °C ± 2 °C, a relative humidity of approximately 75%, and an SO2 concentration of about 25 ppm. The gas atmosphere is renewed continuously through a controlled flow system. Preferred test durations are 4, 10, or 21 days, after which contact resistance and surface appearance are evaluated. Because this test uses a low gas concentration without condensation, it discriminates sensitively between contact plating systems such as gold, palladium, and silver finishes.
Moist SO2 Test (ASTM G87)
This practice exposes specimens to a warm, humid atmosphere dosed with sulfur dioxide, with condensation maintained on sample surfaces throughout the exposure period. It is used to compare the relative corrosion resistance of metals, coatings, and finishes under acidic moist conditions and can be adapted to specific dosing and duration requirements agreed between parties.
Post-Test Evaluation
After the specified exposure, specimens are carefully removed, gently rinsed or cleaned only as permitted by the governing standard, and examined under defined lighting conditions. Corrosion and coating defects are rated, quantitative measurements are compared with baseline values, and a test report documenting the test conditions, standard applied, exposure duration, and results is issued by the accredited third-party laboratory or in-house facility performing the test.
Applications
The sulfur dioxide aging test supports material qualification and quality control across many industries, including:
- Automotive: validation of plated fasteners, brackets, body hardware, and electronic modules intended for use in polluted urban and industrial atmospheres
- Electrical and electronics: qualification of connectors, relays, switches, and contact platings against tarnishing and contact-resistance drift
- Coatings and paints: development and comparison of protective paint systems, pretreatments, and passivations
- Infrastructure and construction: assessment of metallic building components, fastening systems, and hardware exposed to industrial or coastal-industrial environments
- Petrochemical and power generation: screening of materials and coatings intended for plants where sulfur-containing process atmospheres are present
- Research and development: accelerated comparison of new alloys, surface treatments, and corrosion inhibitors
FAQ
What is the difference between the SO2 aging test and the neutral salt spray test?
The neutral salt spray test exposes samples to a continuous mist of sodium chloride solution, simulating marine chloride attack. The SO2 test instead creates a humid, acidic atmosphere containing sulfur dioxide, simulating industrial pollution and acid condensation. The two methods attack materials through different mechanisms and are often specified together for a more complete corrosion resistance assessment.
How many test cycles are typically required?
The required number of cycles depends entirely on the product specification. For Kesternich-type testing, common specifications call for anywhere from 1 to 24 or more 24-hour cycles, with the SO2 dosage level (0.2 L, 1 L, or 2 L per 300 L chamber) defined alongside the cycle count. For constant-climate testing per IEC 60068-2-42, durations of 4, 10, or 21 days are typical.
What sample preparation is required?
Samples should be representative of production parts or finished coatings, cleaned only as permitted by the standard to avoid removing protective films. Specimens are usually positioned at an angle of approximately 15 degrees from vertical, must not touch each other, and should allow condensate to run off the significant surfaces. Specific quantity, dimensions, and conditioning requirements are defined by the applicable standard or agreed with the testing laboratory.
Which standard should be selected for electronic contacts?
For contacts, connectors, and similar electronic components, IEC 60068-2-42 Test Kc or its national equivalent GB/T 2423.19 is the appropriate choice, because it evaluates the subtle contact-resistance changes that low-level sulfur dioxide contamination causes on precious-metal and base-metal platings.
Can the SO2 test results be correlated to real service life?
The SO2 aging test is an accelerated comparative test, not a direct service-life predictor. Its results allow reliable ranking of materials and coating systems and qualification against specification requirements. Translating results into an exact service lifetime requires additional information about the actual service environment and is generally established through field correlation data held by the manufacturer.