Which Shim Stock

[Mac1]

Galvanic corrosion on stainless is probably nill, but between aluminum and brass it is real. I've seen several cases in-hand.

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I think his lathe is a 1957 South Bend Heavy 10L. I doubt it has much aluminum on it.

 

[george wilson]

Andre referred to mixing aluminum and brass shim stock. But,I fail to see how corrosion could take place if it is dry. Bathed in salt water,yes!

 

[Andre]

Andre referred to mixing aluminum and brass shim stock. But,I fail to see how corrosion could take place if it is dry. Bathed in salt water,yes!

I made some rifle bits out of aluminum and used brass screws, they were front sight clamp blocks for a sporter class competition air rifle. I made 5 or 6, and in the screw threads they have started to corrode. Same with a vacuum pickup stylus tool I made for somebody, I used a brass screw in the aluminum stylus and the threads corroded as well. I have since lubricated the threads on the stylus and it seems to be behaving itself.

It doesn't seem like either base metal is the one being corroded, but a greenish white powder forms. Almost like zinc corrosion.

 

[george wilson]

Do not eat the greenish white powder!

I might theorize that since air guns make a bit of moisture when their compressed air is let off suddenly, thus you see brass barrels used for many of them(or USED to be!)

Could it be that a bit of the water vapor is getting into your shimmed area,Andre? Or at least raising the humidity near the gun momentarily? I think moisture HAS to be playing a part in the corrosion.

 

[Ulma Doctor]

aluminum will corrode when placed in proximity of stainless steel, it gets even worse when humidity is high.

 

[John Hasler]

aluminum will corrode when placed in proximity of stainless steel, it gets even worse when humidity is high.

Our 20 year old aluminum horse trailer shows no signs of corrosion around the stainless steel hardware.

 

[Ulma Doctor]

just about every piece of equipment i work on is constructed from stainless steel.
in my 25 year career, i have had to extract hundreds of stainless steel screws in corroded aluminum at their interface-
you may wish to do a closer trailer inspection. the corrosion may happen where you can't always see it.
i guarantee if you were to try to remove an untreated SS bolt from your trailer, it would soon break

 

[Ripthorn]

Galvanic corrosion has more to do with electrical contact than it does with humidity. In a dry environment galvanic corrosion will still occur, but the rate may be different. Most dissimilar metals have the potential to corrode due to galvanic corrosion, but the time scale may be on the decades or centuries time scale.

 

[george wilson]

I doubt that Andre's airgun shows decades of corrosion. I think he is about 17.

 

[Ulma Doctor]

Here is what happens at the interface of aluminum and stainless steel in a damp environment
the white/gray blob in the middle is Aluminum oxide that has bonded to the stainless steel because of electrolysis.


the picture below is the aluminum frame where the interface electrolysis is heavily occurring

the equipment shown is a Hobart 4812 meat grinder. it's assembled with #10/32x3/8" pan head 316 stainless screws from the factory.
the screws are always locked into the aluminum plate through electrolysis.
most times i just knock the heads of the screws and drill & re-tap a hole in close proximity.
the addition of antisieze or oil will aid in future removal of fasteners, especially under adverse conditions.