Tyneside Society of Model & Experimental Engineers
Dealing with a broken tap in a bronze bush.
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.
I made a replica problem seen in picture 1, a phospher-bronze bush, silver soldered into copper sheet. This was drilled tapping size for an old 1/8'' Whitworth tap which obligingly broke in use.
The part was then immersed in the boiling alum solution.
You can see the broken tap protruding and the bubbles of the boiling solution. The white haze above the tap is a stream of fine bubbles forming around the tap and emerging from the tapped hole. The coil of brass wire was to see if the alum had any adverse effect on brass. I could not detect any, it remained as clean and springy as when it went in.
I am trying to convince you that the tap is dissolving away!
I am trying to convince you that the tap is dissolving away!
I am trying to convince you that the tap is dissolving away!
I am trying to convince you that the tap is dissolving away!
When I felt it was safe to do so I added the boiler to the cook pot. Safe, because the technique seemed to work OK, (and my lady wife was out for the day!).
The process took a good bit longer than an hour, I would suggest making sure you cannot dissolve any more alum from the start. I added more as I went along, the solution gradually coming to look opalescent and tending to form a skin on the surface and at the edges, as the water boiled away. Do not let this happen.
Keep checking the work and do not give up once started. The pan was old and a bit pitted but the process did not do it any favours, the pitting was worse. In the picture below you can see this and the crystals of alum which were left in the pan as the solution cooled down.
