API 6A defines material classes AA to HH based on H2S and CO2 exposure. Elastomers for DD/EE and FF/HH undergo immersion in specified sour gas mixtures at temperature class levels. Testing verifies resistance to swelling, degradation, and loss of properties. Premium compounds like peroxide-cured HNBR excel in these aggressive environments https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/