Removing broken steel screws in aluminium (this time it's a siren)

Hi all,

On Friday I had some fun. I was passing a old factory building and noticed that it was being demolished. In the past I had noticed a large siren on the building. Whether it was a factory siren or a true air raid siren, I'm not sure, but I had always fancied owning a big siren. I got chatting to the demolition crew and persuaded them to unbolt it and lower it to the ground for me. A little cash changed hands and I had a siren.

It turns out that the siren is seized and has suffered from galvanic corrosion, so it's going to be a challenging restoration project, but I'm keen. I think everything can be salvaged apart from the rotor. Making a new rotor will probably be a CNC mill job, so I'll have to look out for someone with a CNC mill and some time to spare! Anyway, to get to the rotor I have to remove quite a few bolts and screws. Some have already come out, some are seized solid, and others are going to break off (one has already). They're BA screws or small Whitworth (approximately 3/16" to 1/4" diameter). Reading through the archives of this group I've found three suggestions for chemicals which will dissolve steel but leave aluminium untouched: nitric acid, sodium hydroxide and aluminium ammonium sulphate (alum). Which is best? I don't care how long it takes, and I can even tolerate a minor amount of etching on the surface of the aluminium.

It also made me wonder if there was an electrolytic process for removing corrosion from aluminium. Something similar to electrolytic de-rusting perhaps? And if there is, does it work safely when you have a component made from aluminium and steel bolted together?

I also need to undo some bolts which have badly corroded heads. I can't even grind the heads off because of their location. They're inside one cylinder, but outside another, so the only way to get to them is with a socket and extension. It might be possible to get to them with a die grinder, but for the moment I don't have a die grinder. I've seen these Irwin "Bolt Grip" sockets for sale though, and I have various uses for them if they work. Does anyone know if they're any good? Here's a link showing what I mean:

And here are pictures of my siren, for anyone who's interested:

Best wishes,

Chris

Fascinating.

Its gonna suck to be your neighbors

Good luck on the project. Way cool.

Gunner

"I think this is because of your belief in biological Marxism. As a genetic communist you feel that noticing behavioural patterns relating to race would cause a conflict with your belief in biological Marxism." Big Pete, famous Usenet Racist

Careful! Sodium Hydroxide dissolves aluminum rapidly... don't know why it's on that list.

LLoyd

The recent thread mentioned potassium aluminum sulfate, or plain old pickle making alum, for this. I asked if ammonium aluminium sulfate, or double alum, would do the same and never received an answer. (Probably asked too late in the thread as it was already wandering off topic...) My guess is that it will, but please don't rely on my question as a definitive source of which type works best.

Thanks Gunner! I'm looking forward to this one. Making a new rotor is going to be a challenge though.

Sadly our nearest neighbour died just yesterday, so there there won't be many people at close range.

Best wishes,

Chris

It was something briefly mentioned in a post archived at Google Groups. It went back a way, and I seem to recall it was an "I think..." sort of post. I'll strike that one off the list. Thanks Lloyd.

Best wishes,

Chris

Thanks. I stopped at the pharmacy to get some potassium alum today. They said they could order it but that it might not come due to "anti terror precautions" or something like that. It had better come. What is the world coming to?

Best wishes,

Chris

He probably realized what you dragged home..and took the easy way out. (sorry...twisted sense of humor)

You may consider making a pattern and having the rotor cast rather than machined. You may well be able to get by with casting it out of aluminum rather than cast iron or steel. Make the pattern out of wax or foam, cast it, then balance afterwards. "I think this is because of your belief in biological Marxism. As a genetic communist you feel that noticing behavioural patterns relating to race would cause a conflict with your belief in biological Marxism." Big Pete, famous Usenet Racist

David Billington wrote in news: snipped-for-privacy@djbillington.freeserve.co.uk:

(Probably

topic...)

I'm one of the posters from the past that used aluminium ammonium sulfate to remove small (2-56 and 4-40 taps and tap drills) from aluminum parts my students were machining. These were small and took a week (weekdays only) in a heated bath of water and alum on a stirred hot plate. I use lab grade aluminium ammonium sulfate from our chem stock room. It worked. I tried the same thing with alum from the grocery store's spice rack recently and it didn't seem too effective (I was also in a hurry too).

Ken

The original is cast aluminium, probably balanced after casting as you say. I was thinking that there are probably more people with CNC mills that aluminium casting facilities, but I might be wrong.

Best wishes,

Chris

Actually..thats a pretty complex job for a CNC mill. And will be costly unless you can find someone willing do do it for the fun of it.

There are quite a number of folks here on RCM that do aluminum casting and could likely give you a hand.

You could start out by casting a mold from what you have, making it in plaster of paris or whatever..then adding the missing areas, and cleaning it up, then recasting in foam/wax or whatever, then casting in aluminum. This has the advantage of being able to simply cast another pattern if the first one didnt turn out right, rather than having to redo the foam pattern from scratch, the second and third times

Gunner

"I think this is because of your belief in biological Marxism. As a genetic communist you feel that noticing behavioural patterns relating to race would cause a conflict with your belief in biological Marxism." Big Pete, famous Usenet Racist

Chipping in.. I'm one of the other posters from the past that sheared off a brake caliper bleed nipple. I removed the remains with 5 molar nitric acid in about an hour IIRC. The aluminium was in as good or better condition at the end as it was in the beginning since the nitric acid encourages oxide film formation.

I got the nitric acid from the chem lab at work, but nowadays I buy it from:-

in the UK

Mark Rand (usual disclaimers) RTFM

Indeed. I'm quite doubtful that I can afford to pay to have it done professionally. It's especially complex because it has lobes on both sides: five on the top and six on the bottom to give the two-tone sound.

I think this may well be possible. I think I may be able to build it up using "chemical metal" body filler, which would be plenty robust enough for a pattern. A casting is also easier because I don't have to make a CAD model of the rotor. My only concern is that a casting might not be as strong as a machined rotor, and even if the original casting was strong enough, it doesn't mean that my replacement will be. My gut feeling is that a rotor machined from a solid chunk of aluminium will be less likely to fail in a nasty way. This thing does about 6000 rpm so I'm keen to avoid a face full of shrapnel :-D.

Chris

I'd have to go out to the workshop and try it on some ally foil to find out and it's late! I would suspect that it would be nowhere near as good as the nitric because it dissolves steel quite slowly and iron phosphate isn't noticeably soluble _and_ takes up more space than the original iron.

Whereabouts are you? I'm in Rugby with a litre of Nitric on hand if that helps.

Mark Rand RTFM

Looks like a useful supplier. Pity about their £50 minimum online order value. I wonder if this applies to telephone orders? I only need £5 worth of acid!

Chris

Actually, I have some concentrated phosphoric acid. Would that have the same effect?

Chris