It is customary to select inner ring material to be the same as the metal winding
ASME B16.20 recommends the use of inner rings if the users experience has shown inward buckling of the gasket. Flexitallic recommends the use of inner rings for the following applications:
Suitable for use with flat face and raised face flanges and specified for high pressure/temperature service or where corrosive or toxic media are present. Note on use of inner rings: ASME B16.20 which covers spiral wound gaskets requires the use of solid metal inner rings in: