I recently suffered the embarrassment of breaking a tap in one of the backhead bushes for the firehole door supports on my Compbyne boiler.
I tried dissolving it out with concentrated sulphuric acid, which is available as a drain cleaner from hardware shops, but this time without success.
(I live in Ham-fisted City).
I had heard that a hot alum solution could do the job but did not know anybody who had tried it.
The technique is to dissolve the alum in very hot water till no further crystals will dissolve, (i.e. make a saturated solution), reheat to boiling, add the engineering mishap and “boil for about an hour”.
During this time the steel will be etched away giving you the opportunity to start again. THE PAN SHOULD BE ALUMINIUM for obvious reasons. I do not know about non-stick surfaces.
There are different forms of alum with different chemical names. I used Potassium Aluminium Sulphate obtained by Googling one of the numerous
suppliers on the web.