broken steel screws in aluminum casting - the experiment

Grant

There is a commercial product for just such thinks called "Tap Out" It is used to dissolve hss taps that have broken off in aluminum parts. Try you friendly industial supplier. It may be hard to ship becuase of the nature of the acitd involved.

That's a tough name to google on! I found a dead link:

Wish I knew what the chemical reaction is that's taking place. Alum is actually potassium aluminum sulfate, a double salt of the SO4 ion, which is of course the same ion in sulfuric acid. Since the screw is bubbling, and it doesn't stink, it must be bubbling off oxygen, which means what's going into solution is some iron sodium compound, but what? and why not the aluminum? (anyone?)

Grant

What is hot? Boiling or just enough that you don't want to put your finger it it for some seconds?

Nick

The reaction, whatever it is, goes faster when the solution is hot.

One person knowledgeable on chemistry wrote me that the salt of a strong acid is a weak base. Another wrote me that AlKSO4 dissolves into K+ and AlS04- ions, not as I'd suspected into K+, Al+ and SO4-- ions.

The plot thickens.

GWE

You can always heat the part.

Can't be me. :-)

That's quite interesting. Because I remember that someone asked the same problem in a chemistry-NG and he got no (helpful/right) answer. Furthermore, I do have some alum (after I found out the translation). Sadly enough, I don't intend to break a tap in aluminium in the near future. OTOH, I hope near enough to be able to remember the recipe.

Hope you are making progress.

Nick

Mr Erwin Would you be so kind to share this process start to finish? I have a similar problem as yours Thanks Paul

Start: I bought an old cutoff saw with aluminum base castings. There are sheet metal guards which were badly rusted and which rattled terribly. When I removed them I discovered two 10-32 screws broken off below flush. As it turned out, these were self-tapping screws, which have a very hard tip. I tried punching a hole in a piece of sheet metal about the size of a postage stamp, then I dimpled the hole out using a punch. Using a small tip on my O/A torch, I tried to weld the sheet metal to the screw. This simply resulted in welding the hole shut, with no fusion to the screw at all. Next I tried to heat the screw in situ, to add weld bead to the top. The aluminum all around it wicked the heat away too fast. Next I took a 7/64" drill, centerpunched the screws, and drilled them lengthwise which took a few drill bits because the material was so hard. Then I tried easyouts, both the left-screw type and the 4-corner type. I was able to get one screw out, but the other resisted. So I started looking at dissolving it out. People on this newsgroup were kind enough to share with me that alum, a common white chemical used in turning cucumbers into pickles, and available at any grocery store in the spice isle, will dissolve steel much faster than it dissolves aluminum. Being of an experimental bent, I then took the one screw I got out, figuring it's essentially identical to the one left in, put a couple of teaspoons of alum in some water, weighed the screw (0.7 grams at the start) and dropped it in. In 2½ days very little was left, just a little smudge on the bottom of the solution. I heated it several times, but it cooled almost immediately.

My next step is going to be trying to find some nitric acid, and repeating the experiment with 0.7 grams of the same kind of screw. Whichever works better, I will then try on my part.

Stay tuned.

GWE

ph17314 wrote:

I just broke a 4-40 tap in brass do you happen to know of something that would dissolve the tap without affecting the brass. I'm building Phil Duclos' model makers dividing head and it's much too pretty to do anything other than dissolve the tap. I'm no chemist and I don't know if nitric acid will affect brass.

I'm no chemist and I don't know

Boy, wil it ever. Very fast probably faster than the steel. I think your not going to do it chemically. ...lew...

"Paul G. Shultz" wrote in news:439BA825.80406 @alaska.net:

If I remember correctly, I've used the hot-water/alum soak to dissolve taps out of copper, so it should be safe for brass. But, try a small piece of brass first.