Rubber Gas Corrosion Hardness Testing: Determination of Hardness After Exposure to Corrosive Gas Atm

Rubber gas corrosion hardness testing measures the change in hardness of elastomeric materials after exposure to corrosive gas atmospheres such as sulfur dioxide, hydrogen sulfide, nitrogen dioxide, chlorine, and ozone. By comparing Shore or IRHD hardness values before and after controlled gas exposure, this test quantifies the degradation resistance of seals, gaskets, and rubber components intended for chemically aggressive service environments. Accredited third-party laboratories perform this testing in accordance with international standards such as ISO 48, ASTM D2240, and the IEC 60068-2 series.

Technical Overview

Elastomeric materials used in industrial service frequently encounter airborne corrosive agents, including sulfur dioxide (SO2), hydrogen sulfide (H2S), nitrogen dioxide (NO2), chlorine (Cl2), and ozone (O3). These gases react chemically with the polymer network, filler systems, and additives in rubber compounds. The resulting mechanisms, which include chain scission, additional crosslinking, surface oxidation, and filler interaction, alter the mechanical behavior of the material. Depending on which mechanism dominates, the rubber may either soften or harden, and in both cases its sealing capability, compression set behavior, and tensile properties may be compromised.

Rubber gas corrosion hardness testing provides a quantitative measure of this degradation. Test specimens are first conditioned and their initial hardness is recorded. The specimens are then exposed to a defined corrosive gas atmosphere at controlled temperature and relative humidity for a specified duration, after which the hardness is measured again. The hardness change (delta hardness) serves as the primary acceptance criterion in many material specifications and is often supplemented by a visual assessment of surface cracking, blistering, tackiness, or discoloration.

• Quantifies gas aging resistance using an objective, reproducible hardness parameter
• Simulates harsh service environments such as petrochemical plants, wastewater treatment facilities, and polluted industrial atmospheres
• Supports material qualification, incoming material inspection, quality control, and failure analysis
• Applicable to vulcanized rubber, thermoplastic elastomers, and finished rubber articles

Testing Samples

A wide range of rubber materials and product forms can be evaluated. Sample preparation requirements differ depending on the geometry and hardness method selected, and accredited third-party laboratories can advise on the most suitable specimen configuration.

Standard Vulcanized Test Sheets

Flat test plaques with a thickness of approximately 2 mm to 6 mm, cured directly from the compound of interest, are the preferred specimen form. Flat, parallel surfaces are required for reliable Shore hardness and IRHD measurements, and standard sheets minimize the influence of part geometry on the test result.

Finished Seals, O-Rings, and Gaskets

Actual production parts such as O-rings, flat gaskets, diaphragms, and stampings can be tested directly. For curved or small cross-section articles, micro IRHD methods per ISO 48 or Shore A measurements with appropriate support fixtures are used. Testing real parts provides the most direct evidence of field performance.

Rubber Components and Molded Articles

Bushings, vibration mounts, bellows, hoses, and other molded elastomeric components can be evaluated by measuring hardness on a prepared flat area of the part before and after gas exposure.

Cable Sheathing and Insulation Compounds

Elastomeric cable jacket and insulation materials, which are frequently exposed to sulfur-bearing industrial atmospheres, are commonly evaluated for hardness retention after corrosive gas aging in accordance with the IEC 60068-2 test methods.

Testing Items

The core deliverable of the test is the hardness change after corrosive gas exposure, typically reported together with supporting observations. Common reporting items include:

• Initial hardness before exposure (Shore A or IRHD)
• Hardness after corrosive gas exposure
• Hardness change in hardness points (delta hardness)
• Visual assessment of surface condition: cracking, crazing, blistering, tackiness, discoloration, or visible corrosion deposits
• Optional complementary measurements: tensile strength and elongation change, mass and volume change, and compression set after gas aging

Testing Methods

The test sequence consists of baseline hardness measurement, controlled corrosive gas exposure, and post-exposure hardness determination. The complete procedure is summarized below.

Specimen Conditioning and Baseline Hardness Measurement

Specimens are conditioned in accordance with ISO 23529, typically at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for at least 16 hours before measurement. Baseline hardness is determined using a Shore A durometer in accordance with ISO 7619-1 or ASTM D2240, or by the international rubber hardness degrees (IRHD) method in accordance with ISO 48 or ASTM D1415. A minimum of five measurement points per specimen is normally recorded and averaged.

Corrosive Gas Exposure

Exposure is carried out in programmable gas corrosion test chambers using methods aligned with IEC 60068-2-42 (sulfur dioxide), IEC 60068-2-43 (hydrogen sulfide), IEC 60068-2-60 (mixed flowing gas), and the corresponding GB/T 2423 series methods such as GB/T 2423.19, GB/T 2423.20, and GB/T 2423.51. For ozone-sensitive compounds, ozone exposure may be performed per ISO 1431-1 or ASTM D1149. Typical exposure conditions include:

• Temperature: 25 °C to 40 °C depending on the method
• Relative humidity: typically 75 % to 95 %, held constant during exposure
• Gas concentration: commonly in the range of 10 ppm to 50 ppm for SO2 and H2S, with mixed gas methods using lower, precisely controlled concentrations
• Duration: commonly 4, 7, 10, 14, or 21 days, or as specified by the product standard

Post-Exposure Hardness Determination

After exposure, specimens are removed from the chamber, gently cleaned of surface deposits, and reconditioned under standard laboratory atmosphere before measurement. Hardness is then remeasured using the same instrument, scale, and measurement locations as the baseline determination wherever possible, so that the comparison reflects the effect of gas exposure rather than measurement variability.

Calculation and Acceptance Criteria

The hardness change is calculated as the difference between the mean post-exposure hardness and the mean initial hardness, expressed in hardness points. A positive value indicates hardening, typically associated with continued crosslinking or oxidation, while a negative value indicates softening, typically associated with chain scission or plasticizer loss. Acceptance limits are product-specific; a change within ±5 to ±10 Shore A points and the absence of visible cracking are common specification requirements for many sealing applications.

Applications

Rubber gas corrosion hardness testing is widely used across industries in which elastomeric components must survive chemically aggressive atmospheres:

• Automotive and off-highway vehicles: underbody seals, engine bay components, and gaskets exposed to exhaust gases and road environments
• Oil and gas and petrochemical processing: seal rings, diaphragms, and hose linings exposed to sour gas and process atmospheres containing H2S and SO2
• Electrical and electronics: elastomeric cable accessories, connector seals, and enclosure gaskets evaluated per IEC 60068-2 environmental test methods
• Water and wastewater treatment: seals and membranes exposed to chlorine and chlorine-containing air streams
• Power generation and heavy industry: flue gas desulfurization equipment seals, expansion joints, and anti-vibration mountings
• Material development: comparative evaluation of compound formulations, antioxidant and antiozonant packages, and filler systems for improved gas aging resistance

FAQ

Which hardness scale should be used for rubber gas corrosion testing?

Shore A per ISO 7619-1 or ASTM D2240 is the most common scale for medium-hardness engineering rubbers, while IRHD per ISO 48 or ASTM D1415 is preferred for precise comparative work and for small or curved specimens using micro IRHD methods. The same scale and instrument must be used for both the baseline and post-exposure measurements.

What gases are typically used in the exposure stage?

Sulfur dioxide and hydrogen sulfide are the most frequently applied single gases, while mixed flowing gas methods expose specimens to combinations of SO2, H2S, NO2, and Cl2 at low concentrations and high humidity. Ozone exposure is used separately when ozone cracking resistance is the property of interest, following ISO 1431-1 or ASTM D1149.

How long does the test take?

Exposure durations of 4 to 21 days are typical, depending on the specification and severity level. Total turnaround also includes specimen conditioning before and after exposure, baseline and final hardness measurements, and reporting, so a standard program usually requires two to four weeks.

What does a hardness increase or decrease after gas exposure indicate?

An increase in hardness generally indicates additional crosslinking or oxidative hardening of the rubber network, while a decrease indicates chain scission, reversion, or loss of plasticizer. Both directions of change signal degradation, which is why most specifications place a limit on the absolute magnitude of the hardness change.

Can finished parts be tested instead of standard test sheets?

Yes. Finished seals, gaskets, and molded components can be tested directly, provided a suitable flat measurement area exists or an appropriate micro-hardness method is applied. Testing real parts gives the most representative assessment of gas aging performance in the intended application.