New mild steel guide , clamped to +9m brace. Retrofit superduplex caisson. Probably have a 25mm anulus. Is a rubber liner to the guide barrel adequate for isolation? PTFE? 20year design life.
Is there any rubber liner guaranteed for 20 years? If yes. then rubber is the cheapest. I would however, opt for some spacers and some good paint on the most nobil one (the duplex). You'll get contamination only if there is direct contact.
For example Epoxy based elastomer might be able to do the job. Little more information is required on mechanics and chemical exposure conditions. For detailed information on integrated open source material advice (corrosion + permeation + mechanical retention) you may also try: