doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Dissimilar Metal Reaction

I saw a post on here recently referring to using a shim cut from an aluminum beer can to take up the slack in hinge pin/hook junction. Seemed like a good idea so I cut one, put it on the hinge pin/hook junction on my NID and it works great. Then I remembered a post from a while ago which mentioned a nightmare scenario where aluminum snap caps were chemically "welded" in place! I think we've got some chemists/engineers and other technical expertise on this site, whats everyones opinion, should I pull the aluminum shim ASAP or is it a non-issue. Thanks in advance. John C.

If it works and you want it in there, you should be so lucky! Might be soft and fast-wearing. Too bad steel won't weld itself to steel overnite; save all this soldering and gluing.

jack

I'm not a chemist or metalurgist, but I did have an aluminmum boat that I used for several years in salt water. The process your talking about is called electrolosis and occurs when two disimilar metals with greatly different electrovalences are pressed together. What happens is a battery is formed and one metal looses electrons to the other causing corrosion. With aluminum and steel the steel looses electrons to the aluminum causing corrosion between the metals basically locking them together with rust from the steel. If this were to occur in a salt water environment, with the salt water facilitating the chemical reaction it occurs very quickly as I soon discovered with my boat.
The grease and oil used in the hook should act as at least a partial insulator greatly slowing down in chemical reaction between the two metal, though I have no doudt that if an aluminum shim were left in place for say 20 or 30 years some corrosion would occur.
Steve

JC - I have no concerns for using an aluminum shim for "proof of concept" when tightening an action. However, as mentioned, wear will be an issue with an aluminum shim, as may battering. As mentioned, the oil and grease used for lube should make a pretty good insulator. Since you are satisfied with the results of aluminum shims, why not change over to a steel shim? Shim stock is available from many machining sources in a wide variety of thicknesses. Match your aluminum shim in steel, super/gorilla glue into place and live happily everafter - OK, maybe not literally everafter, but for a goodly number of rounds through the gun.

Steel shim stock is the way to go. The salts that are byproducts of electrolysis are acidic and further add to the corrosive electrolytic environment. As stated it takes an "electrolyte" to make this happen. Water being the most common. Oil will provide a barrier to oxidation. But I'm not too sure how effective it is in providing a barrier to electrolysis.

Worst that will happen will be that the aluminum corrodes to a pinch of white powder. But aluminum from a can is very soft, good for shimming under a clamp where it'll deform under pressure, but not for a moving joint where it'll wear. Use steel.

For many of the above reasons, I think it outta be changed out to a steel shim also.

A combination to be avoided like the plague is Aluminum and Stainless Steel. This cobination has sufficient annodic/cathodic difference to quickly corrode if in direct contact.

Chuck

Interesting coments about aluminum and SS. They must use a different alloy in aircraft aluminum. When I had my boat I was told that the aluminum alloy used in it (a mid-1950's vintage Loan Star cabin cruiser) had an electro valence similar to SS. I promptly changed out all my fasteners and other mild steel parts to SS (I also tried as best as possible to insulate the metals from each other) anyhow it pretty much solved my corrosion problems (like having mild steel parts completely ruined by rust in a matter of months).
One interesting item, the previous owner had installed a brass self bailer in the hull, when I took the bailer out I had to put a 1 foot square aluminum patch over the area because corrosion had thinned the aluminum hull so badly.
Steve

Just use steel. It is cheap enough.

http://www.smallparts.com/products/descriptions/shss.cfm

For a proper job, send it a good gunsmith. I use Dave Norin:
DavidNorin@aol.com
(847) 662-4034
c/o Schranks Smoke n Gun
2010 Washington Street, IL 60085-5132
Waukegan

He has one my guns now that was loose. I have used him in the past. He is very good. His prices are reasonable for the work done. He will not solder in a shim...

Pete

I am.

Yes, there is a potential for corrosion, but if you keep water (and especially salt water) out of the picture, and use a good lubricatant, then I see no real problem. I'd expect mechanical failure before corroson failure.

Eug