An E.I. Dupont manufactured elastomer widely used in the sealing industry.
The generic name is fluorocarbon.
Although chemically compatible with a variety of fluids, many of these Viton® compounds can be attacked by water or steam.
The general rule is that what ever attacks the “cure” attacks the compound. Many of these flurocarbons are cured in a sulphur atmoshere, hence their sensitivity to water based compounds