Sulfur Dioxide (SO2) Gas Corrosion Testing: Technical Overview, Standards, and Applications

Sulfur dioxide (SO2) gas corrosion testing is an accelerated environmental test that evaluates the resistance of metallic materials, protective coatings, and electrical contacts to corrosive industrial atmospheres. This technical article explains the corrosion mechanism behind the method, the applicable international standards such as IEC 60068-2-42, DIN 50018, ISO 6988, and ASTM G87, typical sample types, evaluation items, and industry applications. It is a practical reference for engineers seeking to qualify components or coatings for service in sulfur-polluted environments through accredited third-party laboratories.

Technical Overview

Sulfur dioxide (SO2) gas corrosion testing is an accelerated laboratory method used to assess the resistance of metals, metallic and organic coatings, and electronic components to corrosive atmospheres containing sulfur dioxide. SO2 is one of the most widespread atmospheric pollutants, released mainly by fossil fuel combustion, smelting, chemical processing, and other industrial activities. In real service environments, SO2 dissolves into thin condensed moisture films on material surfaces, forming sulfurous acid that is subsequently oxidized to sulfuric acid, producing aggressive acidic conditions that drive both general and localized corrosion.

The test is particularly significant for electrical and electronic products. Sulfur-bearing species react readily with silver, copper, and other contact metals to form poorly conductive sulfide films, such as silver sulfide, which increase contact resistance and can cause intermittent or permanent failure of connectors, relays, and switches. By exposing specimens to a controlled concentration of SO2 at elevated humidity and defined temperature, the test reproduces within days or weeks the corrosive damage that would develop over months or years of exposure in a polluted industrial atmosphere.

Typical evaluation outcomes include surface discoloration, formation of corrosion products, mass change, degradation of plating or paint systems, and changes in electrical contact resistance. The method is widely specified in the electronics, automotive, telecommunications, electrical equipment, and surface treatment industries and is routinely performed by accredited third-party laboratories.

Testing Samples

Electrical Contacts and Connection Components

Connectors, relay contacts, switch contacts, terminals, busbars, solder joints, lead wires, and crimped or welded connections. These components are tested because sulfide tarnish films directly impair conductivity, switching reliability, and long-term signal integrity.

Electronic Assemblies and Components

Printed circuit boards and assemblies with surface finishes such as ENIG, immersion silver, immersion tin, and HASL, as well as passive and active components, sensors, and modules intended for installation in industrial or urban environments with elevated pollution levels.

Metallic Materials and Electroplated Parts

Substrate metals such as steel, copper alloys, aluminum, and zinc alloys, together with electroplated and chemically deposited coatings including zinc, nickel, chromium, tin, silver, gold, and their multilayer combinations, as well as anodic oxide films and conversion coatings such as chromate and phosphate layers.

Organic Coating Systems

Paints, powder coatings, lacquers, and multilayer coating systems with organic topcoats, tested either as coated panels or as finished parts, and evaluated for blistering, corrosion creepage from scribe marks, loss of adhesion, and changes in gloss or color.

Other Products

Fasteners, springs, decorative hardware, watch components, jewelry, optical parts, enclosures, and packaging materials intended for service in sulfur-containing atmospheres.

Testing Items

Depending on the product specification and the referenced standard, a sulfur dioxide corrosion test program typically covers the following examination items:

• Visual appearance assessment: discoloration, tarnishing, staining, blistering, cracking, and corrosion product formation.
• Corrosion extent and rating of metallic coatings in accordance with ISO 10289 or ASTM D1654, including scribe-line corrosion creepage for coated panels.
• Coating degradation evaluation of paints and varnishes per the ISO 4628 series, covering blistering, rusting, cracking, flaking, and chalking.
• Contact resistance measurement before and after exposure for contacts, terminals, and connectors.
• Mass change determination by gravimetric weighing of specimens before and after testing.
• Adhesion testing after exposure, for example cross-cut or tape tests per ISO 2409 or ASTM D3359.
• Microscopic examination and, where required, SEM/EDS analysis of corrosion products to confirm sulfide formation.
• Functional and electrical performance verification of complete assemblies after exposure.

Testing Methods

Several internationally recognized methods exist, and the choice depends on the product type, industry sector, and governing specification. The principal standards are summarized below.

IEC 60068-2-42 (Test Kc) – SO2 Test for Contacts and Connections

This method, aligned with EN 60068-2-42 and GB/T 2423.19, is the established procedure for contacts and connections used in electrotechnical products. Specimens are exposed to an atmosphere of 25 ±2 °C and approximately 75% relative humidity containing an SO2 concentration of 25 cm³/m³ (ppm) by volume. Standard test severities are 4, 10, or 21 days. The main acceptance parameters are contact resistance drift and the visual condition of contact surfaces after exposure.

DIN 50018 – SO2 Corrosion Test with Condensation of Moisture

This Kesternich-type condensation test uses a chamber of approximately 300 liters into which a defined volume of SO2 gas, commonly 0.2, 0.33, 0.67, 1.0, or 2.0 liters, is introduced. Each 24-hour cycle consists of an 8-hour condensation phase at 40 ±3 °C under saturated humidity, followed by a 16-hour phase at 18–28 °C with the chamber opened for ventilation. Test severity is defined by the number of cycles, typically ranging from one to fifteen or more as specified by the product standard. This method is widely applied to evaluate coating systems on steel and general corrosion protection performance.

ISO 6988 / GB/T 9789 – SO2 Test with General Condensation of Moisture

Applicable to metallic and other inorganic coatings, this standard exposes specimens in a closed chamber where moisture condenses uniformly on the specimen surfaces in the presence of a defined quantity of SO2 gas. Testing proceeds in repeating 24-hour cycles, and the corrosion appearance of the coating is assessed after the agreed number of cycles.

ISO 3231 – Resistance of Paints and Varnishes to Humid Atmospheres Containing SO2

Painted panels are exposed to a condensing humid atmosphere in which 0.2 liters of SO2 is admitted into a 300-liter cabinet maintained at approximately 40 °C. Exposure proceeds in repeating 24-hour cycles, after which the panels are evaluated for blistering, corrosion creepage from a scribe mark, and adhesion loss.

ASTM G87 – Conducting Moist SO2 Tests

This practice describes the exposure of metals, alloys, and coatings to a warm, condensing, SO2-laden atmosphere. Specimens are positioned above water in a closed cabinet to which a metered volume of SO2 is added, creating repeating condensation and drying cycles. The method is used to compare the relative corrosion resistance of materials and protective systems under acidic sulfur-containing conditions.

Related Methods

For applications requiring pollutant concentrations closer to real electronic operating environments, ISO 10062 and IEC 60068-2-60 (Test Ke) describe mixed flowing gas tests in which SO2 may be combined with H2S, NO2, or Cl2 at ppm or sub-ppm levels under precisely controlled temperature and humidity. These methods are frequently used for long-term reliability assessment of high-value electronic hardware.

Post-Test Evaluation

After exposure, specimens are typically documented photographically, cleaned according to the requirements of the governing standard, visually rated, weighed, and electrically tested where applicable. Acceptance criteria are defined by the product specification and may include a maximum permitted contact resistance increase, a minimum corrosion rating number, absence of substrate corrosion such as red rust on steel, or a maximum corrosion creepage distance from a scribe line.

Applications

Sulfur dioxide gas corrosion testing supports product qualification, quality control, and materials research across a broad range of industries:

• Electronics and telecommunications: qualification of connectors, relays, contact finishes, and circuit board surface finishes for equipment installed in industrial or urban environments.
• Automotive: validation of electronic control units, sensors, terminals, and plated fasteners exposed to polluted service atmospheres.
• Electrical power equipment: assessment of switchgear, terminal blocks, busbar systems, and control components.
• Surface treatment and plating industry: quality control of electroplated coatings, passivation layers, and duplex protection systems.
• Coil coating, paint, and construction products: verification of long-term corrosion protection for painted building components and hardware.
• Watch, jewelry, and consumer goods industries: evaluation of tarnish resistance for decorative silver-bearing and copper-bearing finishes.
• Research and development: comparative material studies, corrosion mechanism investigations, and validation of new protective coating systems.

FAQ

What is the difference between SO2 gas corrosion testing and neutral salt spray testing?

Neutral salt spray testing, such as that described in ISO 9227 or ASTM B117, evaluates resistance to chloride-containing, marine-type corrosion, whereas SO2 testing simulates acidic industrial atmospheres rich in sulfur pollutants. The corrosion chemistry and the materials at risk differ significantly: SO2 primarily produces sulfide tarnish films and acid-driven attack, which is why electrical contact metals are normally assessed with SO2 methods rather than salt spray.

Which standard should be selected for electrical contacts and connectors?

IEC 60068-2-42 (Test Kc), together with its EN and GB/T 2423.19 equivalents, is the established method for contacts and connections. It combines a 25 ppm SO2 atmosphere with 25 °C and approximately 75% relative humidity, with standard durations of 4, 10, or 21 days, and evaluates contact resistance change and surface appearance after exposure.

How long does an SO2 corrosion test take?

Under IEC 60068-2-42, the standard severities are 4, 10, and 21 days. Condensation-based methods such as DIN 50018, ISO 6988, GB/T 9789, and ISO 3231 are performed in 24-hour cycles, with total test durations ranging from a single cycle to several weeks depending on the number of cycles defined in the applicable product specification.

What sample sizes and quantities are required?

For coating and paint evaluations, standard flat panels such as 150 mm × 100 mm specimens are commonly used. For components and assemblies, the sample configuration and quantity are defined by the product specification; as a general practice, a minimum of three specimens per test condition, plus retained unexposed reference samples, is recommended to ensure statistically meaningful results.

How are test results judged?

Acceptance criteria are set by the customer or product standard and typically include appearance ratings per ISO 10289, ASTM D1654, or ISO 4628, a maximum corrosion creepage distance from a scribe line, retention of coating adhesion, and a permitted maximum increase in contact resistance. Accredited third-party laboratories issue formal test reports documenting the test conditions, duration, specimen details, and measured results.

Can SO2 corrosion testing be combined with other environmental tests?

Yes. In comprehensive qualification programs, SO2 testing is frequently sequenced with damp heat testing per IEC 60068-2-78, thermal cycling, vibration, or salt spray exposure per ISO 9227 to build a complete picture of a product's durability under combined environmental stresses.