Steel screw removal using alum - update

Hi all,

Today I picked up a couple of packs labelled "Alum (Potash)" from the pharmacy. Tonight I assembled a test piece using an aluminium end plate from a long dead magneto and a 4 BA steel screw. I put it in an old aluminium saucepan and covered it with the best part of a litre of boiling water, then added a tablespoon of alum. It dissolved completely so I added more. And more. And more. I was hoping to achieve a saturated solution, but I was also hoping not to spend a fortune on alum, so I stopped when I had added 200 g.

I left it simmering on the cooker. A few tiny bubble were rising around the steel screw. Many more bubbles were rising from the white metal bearing in the centre of the aluminium end plate. After half an hour I turned off the heat to see what had happened. Both the aluminium and steel have turned a dark grey, almost black. The steel screw shows a couple of minor pits. There aren't any noticeable pits in the aluminium. There are pits in the white metal bearing, but they might have been there already - it was in really poor condition.

I took out the aluminium end plate and just put in the steel screw on its own. It didn't bubble more than before. Despite saying that I didn't care how long this took, I'm ever the impatient chemist! And the house smells of alum. I opened the door but several hornets came in to watch the chemistry, so I had to shut it again. I'll enjoy a bottle of my favourite cider before I check on it again. But not much is happening so far...

Best wishes,

Chris

Chris:

They may have sold you the right stuff..... "Alum" is a term for a double salt containing aluminum ion, sulfate ion, and another ion. "Chrome alum" is chromium aluminum sulfate, so "Alum (potash)" may indeed be potassium aluminum sulfate. From your description of the solubility, it certainly could be. KAl(SO4)2 is very soluble in water (it actually has twelve water molecules in the structure).

"Potash" usually means K2CO3 but when it appears along with another name, it can simply mean "potassium". "Caustic potash" is KOH, "chloride of potash" is KCl. (Common names are such a pain.....the Stock method of naming compounds is a lot less ambiguous.)

Potassium alum is a common ingredient in making pickles; it may be available in grocery stores, in the 'canning' section.

Best -- Terry

Chris, why not simply try a screw extractor. They work quite well.

Harry C.

Christ> > Christ> >

So there is a difference between potash and alum potash?

Thanks. :-) Now Lloyd has to jump in. But maybe the white-metal you mentioned ruined your setup.

I got terrible results with the vinegar-salt solution when different steel alloy were in one pot at the same time. So all that might be very sensitive.

Only genuine French ...

I understood that and remember. Don't need a siren nor a broken tap. :-)

Nick

I went through this recently. See thread

GWE

Christ> Hi all,

Must be that way.

Umm ... maybe (remember: me, chemistry dummy) it needs the voltage difference and conduction?

But that is the same material. To be sure, try it with some plastic. But not on the oven. ;-)

No, but very interested. If I would, I would tell you what I found out.

Thanks for reporting.

Nick

S*it. Looks like the pharmacy b**stards sold me the wrong stuff :-(. I asked them for Potassium Alum. But this stuff says "Alum (Potash)" on it. Now I'm annoyed. It wasn't that cheap either...

Chris

Pardon my previous post. It seems that it's actually the right stuff, KAl(SO4)2:

Bash :-D.

This cider is g...o...o...d. If you like cider you should try it sometime: Sheppy's "Taylor's Gold".

It's not a tap. It's a rusty screw on my air raid siren that I want to extract.

I found a place selling 0.5 l of 70% nitric acid for £12. I might try this next.

Chris

The only set of screw extractors I had sucked. Maybe it was because they were very cheap and came with no instructions as to the right size of drill to use, though. These screws seem a little small for extractors -

Chris

Thanks Terry. It looks like they did sell me the right stuff. I am rather surprised at how many people (in the Google Groups archives) suggest alum. From my initial test it doesn't seem to work at all.

After another hour's heating the solution has become saturated. Bubbling doesn't seem to be any faster. If I just insert the steel screw on its own, bubbling doesn't seem any faster either. The white metal bearing seems to be disintegrating. The steel and aluminium have both turned a dark grey. The steel appears to be unchanged. The aluminium might be slightly etched.

Best wishes,

Chris

It seems so. I'm not a chemist either, but Terry seems to think that the word potash is used quite widely.

This is possible, although I'm doubtful because when I put in the steel screw alone, the reaction rate didn't seem to increase. I guess there's the aluminium saucepan to consider, too. What effect that might have I don't know.

Did you try this yourself and get it to work?

Best wishes,

Chris

About 15 minutes ago my solution was saturated and boiling. But there wasn't much visible chemistry taking place. A few really tiny bubbles rising from the screw, but that was it. After 90 minutes in the solution the screw is essentially unchanged apart from a dark grey coating.

What problem did you have with the nitric acid which meant that you couldn't remove the screw? Suppose you had to :-D...

Best wishes,

Chris

That's what I was thinking. I was wondering about hooking my big DC power supply up to the system to see what happens...

Chris

Thanks for the information Grant. Maybe the reaction is just occurring slowly. I'll leave it overnight and see.

Anyone know if sulphuric acid will dissolve steel without damaging aluminium? I found a source of 95% sulphuric acid.

Chris

I meant the voltage difference of the elements. Al and Fe

Nick

Don't be. "Potash" is the old - long obsolete - term for potassium. The reason it says "Alum (Potash)" is that there are other alums not normally used for medicinal purposes.

LLoyd

I am a retired chemist. You probably have the correct compound.

Frankly I was surprised that potassium alum would dissolve steel. In order to dissolve a metal you need an oxidizing agent. I don't see much oxidizing power in an alum solution.

As a chemist in a metallurgy research lab I dreaded those requests to dissolve something that was buried in a crevice or hole. In order to dissolve the beast you need to get fresh solution to the metal surface. Getting fresh solution to the bottom of a small diameter hole is difficult.

I doubt if you will get all of the screw by a dissolution method.

FYI the chem handbook says infinitely soluble in hot water. You probably found a mass of crystals when you let the solution cool.

He was wrong.

Add a drop or two of Dawn to reduce surface tension?