The ability of the solution to wet the surface isn't his issue. As it works, the solution is depleted in the active region, and there's inadequate open area to cause it to circulate. So you're left with a blind hole full of dead solution and no way to get fresh stuff back down to do the work.
LLoyd
Small pump and a fine tube stuffed into the hole? Or dump the whole mess in an ultrasonic cleaner?
This correlates with what I found. If you boil a small screw in a solution of alum, it dissolves nicely, but if you put a solution of alum above a small hole with a broken off screw in it you can sit there for weeks and nothing will happen. If the entire aluminum part could be boiled, then the agitation of the solution would probably make it work.
Broken steel screws in large aluminum castings can be nearly impossible to remove. Certainly very very difficult.
GWE
According to Lloyd E. Sponenburgh :
Hmm ... a non-metallic pump to circulate the fluid, and a medicine dropper nozzle to squirt the output of the pump directly at the problem area, which should cause reasonably fresh solution to be present full time. You don't need much flow from the pump, since you are restricted by the aperture of the medicine dropper anyway.
And I think that part of your problem is that bearing, which is probably being attacked before the broken off screw. Once that is gone (or if you can press it out), that should improve the rate of attack on the steel -- as long as it is in direct electrical contact with the aluminum, which may be why the screw lying in the bottom of the container as a test is not being attacked very fast.
Good Luck, DoN.
The alum success stories I recall were about removing broken taps, where the flutes give access to much more of the tap's surface than the solution has with a plain screw. Add to that the fact that the screw threads are almost certainly sealed by that white powdery expanding stuff that aluminum glues a corroding fastener with, and you're stuck attacking only the end of the screw.
Ned Simmons
Thanks for this field report. I can see how this is a technique for taps rather than stuck screws.
This has been a most interesting thread.
Wes S
Maybe drilling a hole in the broken screw would help, especially if it's a thru-hole.
It doesn't seem to work with a loose screw lying in a pan of boiling saturated alum solution, so I can't see it working with a screw in a piece of aluminium, even if I drill the centre of the screw. I'm going to try phosphoric or nitric acid next.
I've got visitors this weekend so I'll probably be slow to reply to messages. Thanks for the thoughts on dissolving screws and also on electric die grinders. I'll let you know how things turn out.
Best wishes,
Chris
I have to agree that spinning a 20ft pipe at 1000 rpm sounds really scary. Especially if it is only supported by shopping cart casters. Why spin it so fast? slow it down to about 100 rpm and let the abrasives do the work. Maybe spin it up briefly to 200 rpm when you get to the really fine compounds. NEVER wrap a rag all the way around it. Don't wear gloves, Use eye protection and have some way to kill that tractor NOW should anything go wrong. Have 911 memorized. Tell a friend what you are about to do. Let them watch and videotape from a distance.
Please make sure you work in a well-vented space when you apply the nitric acid.
S.
Whyfor? We're talking five molar here, not red fuming.
Mark Rand RTFM
My concern is not so much for the acid fumes themselves as for the gaseous NO2 released by the oxidation of the iron.
S.
That will happen with concentrated nitric acid. It would also lead to the iron being passivated and the reaction slowing down. With a more dilute acid, you get the formation of nitrates and the evolution of hydrogen. Clever stuff :-)
Mark Rand RTFM
I see. Well, if I erred, at least I erred on the side of caution.
S.
De nada.
Mark Rand RTFM
I tried boiling a loose screw in alum for about two hours. Although there were a few really tiny bubbles rising, it didn't seem to dissolve the screw.
No luck here either. I guess it's possible that the white metal bearing is protecting the screw, so I might try removing it, but the initial results don't look hopeful.
Best wishes,
Chris
Well, it does take a day or two.
GWE
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