Best material for a metal willy

I want to replace my old willy with a metal willy that won´t corrode
in the asshole of my boyfriend.

What is the best metal for a willy that won´t get damaged in my
boyfriends colon ?

Iron Willy ? May rust
Titanium Willy ?
Golden Willy ?
Silver Willy ?
Palladium Willy ?
Aluminium Willy ?