Broken tap in expensive unit!

Dilute hydrochloric will only attack the zinc in the brass alloy, not the copper. The resultant effect on the brass component is minimal. HCl will be much faster than alum. All such agents are effective by corroding the iron in the steel i.e. accelerated"rusting". I've used both and dilute HCl is my preference. The problem though is any HCl vapour causing corrosion of nearby components which wouldn't occur with alum solution. So, cover the area with Clingfilm if possible.

So, bit of a contradiction. I said 'Don't use Hydrochloric Acid because it attacks brass' and Fowlers Fury says 'Dilute hydrochloric will only attack the zinc in the brass alloy, not the copper.'

He's right but dezincification of the brass might matter; it would weaken the thread for example. Slightly more worrying to me is the possibility that Hydrochloric Acid will react with Iron in the tap to make Iron (III) Chloride. Now that chemical is well known for dissolving copper to make printed circuits.

Unfortunately, I'm pontificating from my armchair, whereas Fury has actually done it. As Andrew has two spoiled capacitors to experiment on, would he please try both methods and put me out of my misery?

Dave

He could try something like the link below, cheap and no chemicals, if the tap isn't too tight it will get it moving enough to be able to tap it round with a small punch.

**LINK**

I think it was Clive Foster further back who suggested making some clamps and deal with the broken taps later. I echo his opinion, things like car battery post clamps will avoid the risk of chemical attack, the problems of spark erosion and dubious drilling methods where run off into the brass is all too possible.

For that matter, if the clamps are satisfactory electrically, why bother doing any more?

Brian

"........the possibility that Hydrochloric Acid will react with Iron in the tap to make Iron (III) Chloride. Now that chemical is well known for dissolving copper to make printed circuits. "

Not quite, the reaction between HCl and Fe will produce ferrous (III) chloride irrespective of the acid concentration.

It is the ferric (II) chloride of course which is the etchant. However ferrous chloride does attack Cu to a lesser extent though with the problem described any effect would surely be insignificant.

If Andrew T does decide to use either alum or dilute HCl, it's important to keep agitating the solution with a piece of thin wire in order to displace the hydrogen bubbles which will form on the tap's surface and prevent the reaction. Also keep the reaction warm for a faster dissolution of the steel.

Forgot to add > if a thread lubricant was used before breakage then this will inhibit acid contact with the tap, so degrease as much as possible first with some solvent.

Edited By Fowlers Fury on 14/09/2017 16:21:24

Ah yes, thanks for the clarification. I'd like to blame my aged brain for the mistake. Unfortunately I remember all too well being chastised at school for not paying attention. Many times. Many, many times...

Dave

Isn't the iron (III) chloride the ferric and iron (II) chloride the ferrous?

Thor

Thor ,speaking as an ex chemist ,you are correct.

Apologies, yes of course, ferrous is divalent form.

Age is taking its toll; a moments thought about ferrous chloride being FeCl2 would have avoided the silly error.

[Message to self: do not rely on failing memory, check first]