Heavy corrosion inside the pump tube (crosman1377)

[^^artin]

About 6 month ago I've builded this flat top piston from a M6 countersunk head screw and some brass parts.
The screw was from zinc plated steel and I also soldered the piston head onto the screw to get it fixed and air-tight by using tin/lead solder.
The 1377 was pretty new and has a aluminum valve. I mentione all this details because I think the following corrosion inside the pump tube is due to electrolytic processes caused by the presence of multiple different metalls on the gun.
The corrosion happened at a small spot from about .25 " in diameter at the top/inside of the pump tube, right in front of the flattened aluminum valve, where the brass piston head sits in its resting position.
Sorry for the bad quality Image. I took the "corrosion"-photo right thru the screw-hole where usually the screw sits that holds the valve in place.

I'm not sure how this corrosion could have happened within this short time.
Electrolytical corrosion would be one presumption.
Another guess could be water condensating at this location. But then the corrosion may had happened in a ring like shape all around the inside of the pump tube.
Would water tend to condensate at this location where compression is at the maximum ?
Otherwise perhaps it's not a good idea to build with to many different metals.
Could also be both and this point is where the brass piston head contacts the pump tube in addition when resting.

What do you guys think ?

 
     

[Wayne52]

Zinc is highly corrosive and I'm sure that's probably what is cause the corrosion.  Zinc is a bad choice when it comes to an airgun that is pneumatic or even not.  I'm use to digging up coins with my metal detector and the copper flashed zinc pennies that they're making these days might last 2 years in the ground but eventually they'll totally disolve into the ground.  A copper penny will still be there in a hundred years.

[^^artin]

Could easily be caused by the zinc.
Unfortunately almost every steel screw you can get in german hardware stores this days is zinc plated - as a corrosion protection. 
But zinc is also one of the main components of the brass.


After some research:
On the other hand zinc (and also magnesium and aluminum) is also used as a "sacrificial anode" to protect steel, what makes it rather unlikely that it causes corrosion on the steel tube.
https://en.wikipedia.org/wiki/Galvanic_anode

After that theory it's the metalls that are of higher electrochemical order (like brass/copper) that causes corrosion of the metalls of lower electrochemical order (like steel).
But after that theory also the metalls of even lower electrochemical order than the steel (like aluminum and zinc) must be corroded before the steel does.
Unfortunately, my pumptube don't seems to know that.
Of course it wouldn't be much better if the aluminum valve starts corroding first.