Marathon elements are threaded through brass ports. Historically brass has been used in salt water applications because of its resistance to corrosion. And when it does corrode the oxide layer (which is green) bonds tightly to the underlying metal so as to prevent further corrosion. Ship finders bring up brass cannons which are covered in barnacles but the metal is all there.
Normal red rust Fe2O3 does not bond to the underlying metal but flakes off exposing more metal which rusts and flakes off etc. Fe3O4 (black rust) forms a more protective coating but this type of rust is only produced at high temperatures,
Marathon elements have a stainless steel sheath but I don’t know what alloy.
Watch a Marathon being made:
Under Consumers click “features and benefits”
At the bottom click on “Marathon Tour”