i would suggest Aqua regia, leave it for a while and then wash with a lot of water

Aqua regia (nitric acid: hydrochloric acid=1:3 in volume) would make it white again, but make sure you handle it with care with personal protection.

Have a look 

ACIDIC DISSOLUTION OF IRON OXIDES AND REGENERATION OF A CERAMIC  FILTER MEDIUM

https://www.doria.fi/bitstream/handle/10024/86745/isbn%209789522653321.pdf?sequence=3

https://www.researchgate.net/post/Can_any_one_suggest_any_proper_solvent_for_dissolving_iron_oxide_nanoparticles

http://www.ehow.co.uk/how_8158123_dissolve-iron-oxide-citric-acid.html

http://www.atmos-chem-phys.net/12/11125/2012/acp-12-11125-2012.pdf

http://www.sciencedirect.com/science/article/pii/S0304386X07000618

Hydrochloric acid+water could be quite enought. If no, try EDTA+HCl+H2O2.

Rust stains may be difficult to completely eliminate from polytetrafluoroethylene (PTFE), i.e. 'Teflon'. Concentrated sulphuric acid, hot glacial acetic acid, citric acid (aprox. 10 wt %), oxalic acid (3-5 wt %), and hydrofluoric acid (3-5 wt %), may all be effective. A patented “rust stain removal formula” was proposed by Robert F. Crist (Iron Out, Inc.; Fort Wayne, IN), Patent US 6,297,208 B1, Oct. 2, 2001. Uses hot aqueous solution of ammonium bifluoride and boric acid (which react to form fluoboric acid) complemented with added oxalic acid. 'Teflon' also resists well to this solution. 

I have tried with aqua regia and it works very well. Don't put this solution under sonication! An accident may occur by effervescence of the solution. Thank you very much for your assistance!