I thought of cleaning my headphones, which has an outer aluminium casing, with Dettol liquid which I believe has strong oxidising agents. This was a bad idea. It left a tarnish on the aluminium and I'd like to know if I can undo this.
I would try any sort of weak acid that wouldn't cause damage to other parts of the headphones.
Citric Acid (found in lemon juice), Acetic Acid (found in vinegar), and Phosphoric Acid (found in cola) are probably all safe bets for sticking on a cloth and wiping off the casing. The primary difficulty you have here is the issue that comes from the fact submerging your headphones in anything would likely damage the electronics inside. You can rig up a shallow flat pan full of acid and resting the headphones on it's side in there, though I'd take caution.
My friend once asked me to clean their silver chain. I used H2O2 as a school chemistry book suggested and it all turned black. They brought it to a jewelry store and the chain was cleaned ... with H2O2/NH3. A friend of mine brought a mixture of H2O2 and NH3 from lab to home to clean some silverware. Premixed, in one bottle. These two chemicals started to react when they were going home by bus. Having a bag that makes a whistling sound and stinks like ammonia while going by bus is not fun.
DIY stain cleaning using lab chemicals often go wrong and rarely worth the trouble.