I have the protank 2 also, i haven't come across this issue and have skimmed most of these posts in this thread, if i had a chance to take mine apart for cleaning I am sure i could confirm the centre tube is probably ferrous - iron containing. all the other parts for the protanks need to be cast into a mold and brass suits itself perfectly for small ornate parts. the centre tube loans itself to being made out of steel or stainless tube as it accepts threading well and has better strength. i guess the plating on the threads of the centre tube have worn a bit, the rust can be from anything like galvanic corrosion (dissimilar metals in contact) or possibly the water or juice reaction. in either case several people mentioned that rust inst intrinsically harmful. if you do ingest some you might feel slightly less anaemic. i will probably tear down my PT-II in a week and confirm. the worst case scenario for rusting in your protank is from poor machining practices whereby contamination residue from use on ferrous parts carries over to machining on protank brass parts.