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hornluv
10-02-2006, 08:58 PM
I repair brass musical instruments and there are a lot of Trumpet makers out there who insist on using aluminum for their valve stems, which are screwed into raw brass barrels. After a few years, the aluminum starts to seriously corrode into this fine white powder and the stems become impossible to remove, then the threads eventually just start crumbling after a decade or so. Is there something I can do to slow down the reaction between the brass (or the copper in the brass) and the aluminum? Would greasing the threads help?

Thanks,
Stuart

JCHannum
10-02-2006, 09:12 PM
Are you sure the reaction is between the aluminum and brass, not the aluminum and spit?

I have seen that same corrosion occur between two pieces of aluminun that were exposed to wetting and drying cycles.

Evan
10-02-2006, 09:17 PM
You might try a wrap of teflon pipe tape. However, if there is any metal to metal contact it is going to corrode in the presence of saliva. Saliva is a pretty good electrolyte. I don't know what material is used for the valves but 1100 aluminum is the most corrosion resistant although a pig to machine. 3000 series is next best. Stay away from 2000, 6000 or 7000 series.

hornluv
10-02-2006, 09:52 PM
The aluminum parts are not part of the sounding tube, so they're never in contact with saliva or condensation (probably valve oil though, which is basically paraffin oil). They're the short stem that has the finger buttons screwed into them. These are then screwed into a hollow brass barrel at the top of the valve and hold in the spring and valve guide. I'll see if I can post a photo a little later. The valve itself is usually made of either Monel (on the higher end horns) or nickel-plated brass.

Stuart

Shaidorsai
10-03-2006, 12:20 AM
I suspect that the best long-term solution is to remake the stems out of another material. German Silver (Nickle Silver - but really a kind of Bronze) comes to mind, since it is fairly easy to machine and will polish up to a nice silver finish. If the stems are short I doubt the additional weight will affect the finger action, except for the most discriminating Musician. It is a material that is closer in the Galvanic series to the basic metals of the instrument, so corrosian should be much reduced, if not eliminated

ptjw7uk
10-03-2006, 05:35 AM
The white powder is aluminium oxide (aluminium rust) and can be started by lots of things and is very difficult to stop. One way is anodising which creates a thin layer on the surface which is quite hard wearing and should resist corrosion far longer. Any dissimilar matals in contact can produce corrosion over time and are best avoided if possible.
Peter

Millman
10-03-2006, 06:16 AM
My daughter used some kind of grease for clarinet. Can't remember the name.

Scishopguy
10-03-2006, 02:06 PM
Brass and aluminum are a prime example of active metals forming a galvanic cell. Even if kept dry they will react with each other. Add something like sea water (or sweaty fingers) and they start working. Most any time I saw brass screwed into aluminum or vice versa, I knew it was going to be a long day in the repair shop. The aluminum forms aluminum oxide crystals. These are the same crystals as grinding wheels are made out of. Imagine dipping a bolt in grinder dust and then screwing it into a nut.

Grease, like silicon grease, or teflon tape will help but like others have said, a different material woould be better. Titanium rod might be the answer. It is a little harder to machine but is lighter than aluminum and is pretty inert to corrosion.

Good luck with it.

Evan
10-03-2006, 02:33 PM
Aluminum is a really bad choice for those conditions, especially any of the harder alloys. They can corrode by themselves because of intergranular corrosion with the alloying metals like copper and zinc. Throw in a major source of copper and zinc nearby and you set up several different galvanic couples that eat both materials.

The suggestion to use nickel silver (aka German silver) is a good one. It is very corrosion resistant and easy to work with. I have used it to make jewelry items and stuff like belt buckles. It will last forever.

If you must stay with aluminum then use the 1100 aluminum as it has virtually no alloying ingredients. It will have the least tendency to oxidise. It also will accept a clear anodise coat extremely well without significant change in appearance.

jkeyser14
10-03-2006, 04:53 PM
Anodize the aluminum first, even if it's just a clear anodizing. It will greatly help with preventing corrossion.

See my website for a how to:
http://engineeringhobbyist.com/projects/completed/anodizing/

J Tiers
10-03-2006, 05:19 PM
Nickel silver is fine, UNLESS you have sulphur-bearing pollutants or ambient gases.

We had a LOT of problems with nickel-silver contacts, particularly rarely-used ones that were supposed to keep a contact "made". The polluted environment in bars etc would corrode the N-S enough to open the connections. Looked ugly too.

I was able to replicate that in a kludged-up test chamber.

A particular plating (a common one, done a certain way) fixed that entirely.

However, it would likely be far better for the valves, and in fact I think it IS used for clarinet parts.

Evan
10-03-2006, 05:33 PM
I don't recall sulphur being a problem with nickel silver. It doesn't have any silver in it. It's an alloy of nickel, copper and zinc.

[added]

I used to use Liver of Sulphur to patina certain metals including silver. It doesn't work on nickel silver.

topct
10-03-2006, 05:41 PM
If they are totalled what do you do?

If you can get them before they completly rot I might try some de-ox(sp) on the threads. The grease they use on electrical connections when aluminum is used for a conductor.

J Tiers
10-03-2006, 09:33 PM
It isn't the (non-existent) silver, but the copper, that is evidently susceptible to sulphur, which is one part. The contacts also tend to turn a "lemony-green" nickel corrosion color over time.

The "made" contact would open and rise to several hundred thousand ohms resistance. Dunno about LoS, but we definitely had a serious problem with corrosion.

The references to N-S cutlery mention a rapid corrosion to a dull surface color.

Exposure to sulphur, and sulphur combustion products in a hot moist environment corroded open the contacts in a matter of 2 or 3 weeks. The contacts were not then the nickel corrosion color, but blackened.

Use of the plated contacts shut that problem off like a light.

Shaidorsai
10-04-2006, 12:20 AM
Nickel Silver is a type of Bronze (Copper Tin Alloy) rather than a Brass (Copper Zinc Alloy) with a lot on Nickel in the Alloying constituants. Unless you're totin' your Trumpet to Hell, Sulphur (Brimstone) is not likely to be a long term serious problem. People don't stay long (or live long for that matter) in the presence of even minute amounts of gaseous Sulphur compounds. That would be a truly tough gig. Any copper bearing alloy will turn green (form Verdigris) in the presence of sulphur bearing compounds. Best example I know of is pocket knife bolsters which will turn green if left too long in an oiled leather sheath. Got one to prove the point. It will still clean up and polish pretty well though. If a better finish is desired (chrome-like shiny, Gold, whatever) the part could always be plated with something that shines better. Try an electroless Nickel plating kit from Brownell's. These are used to touch-up wear spots on Nickle plated guns. Since it covers up the surface copper atoms on the part, any worries of sulphur corrosion would be eliminated. Most Bronzes and Brasses plate-up really well.

Nickle silver is way easier to machine than Titanium. Ti is tough stuff. I have yet to figure out a really satisfactory way to machine small parts from it in my 3" Sherline shop. Nowhere near enough rigidity in that small a tool to keep the bit cutting well in Ti. If anybody knows of a sure-fire magic cutting fluid that will solve that problem as well as curing Rheumatism, Shingles, the common cold and Cancer, please let me know. Nickle silver machines like butter in the Sherline.

J Tiers
10-04-2006, 08:31 AM
The reason I think the corrosion is relevant is that this WAS on-the-gig- corrosion!

The products in question were guitar amplifiers. The usual problem connector was a 1/4" phone plug connector on teh back which was for connecting an exernal preamp to the power amp. If you didn't plug anything in, it "normaled-through" the connection.

It took a few months for the corrosion problem in most places. Northern climates had the least trouble, overseas exports to tropical areas the most. I was never sure if the huge proportion of match-striking smokers, or the moist climate was at fault for the tropical areas. Probably both.

Plating fixed it, but isn't as practical for a moving part like a valve stem on a trumpet.

jburstein
10-04-2006, 02:25 PM
Sounds like a business opportunity. Make replacement stems. If you have a lathe it should be a pretty easy thing to do. Make 'em out of stainless or something similar.

-Justin