If you need a power head shaft or a grounding probe, any metal in the saltwater aquarium. Stainless Steel "SS316" is safe and I am using it for many years without any problems. Titanium is the best but very expensive.
You can get parts at the link bellow.
Shaftinggroundprecision - Small Mechanical Parts And Components
You can get parts at the link bellow.
Shaftinggroundprecision - Small Mechanical Parts And Components