I am about to assemble a new barrel and new upper receiver when after inspecting the parts I noticed some bare aluminum inside the upper receiver where the barrel extension slides in. There are two very small spots, they are at the five o’ clock and seven o’ clock positions, as can be seen in the photo below. The flash from the camera makes them appear larger than they are.
Now I know I can’t see them when assembled, and that is not my concern. My concern is about possible galvanic corrosion between the bare aluminum and the steel barrel/barrel extension.
I do use AeroShell 33MS grease, which meets mil-spec requirements.
Should I coat the bare aluminum prior to assembly with something like Birchwood Casey’s Aluminum Black ? I do not belive that product has any sealing qualities, or am I wrong ?
Do I even have to be concerned about galvanic corrosion between the receiver and barrel ?
There is some concern of galvanic corrosion between steel & aluminum, but in this case it’s very minor. The steel part should have it’s own coating to protect against dissimilar metals.
If it will help you sleep better at night, the best way to protect the area would be to thoroughly degrease and prep the spots and properly apply Alodine, then cover the spots with a coat of two part epoxy paint. The upper will then be protected even from the harshest elements.
Prepping the surface and applying two part epoxy paint by itself works nearly as well and is a bit simpler.
Personally, I think you’ll be fine as is
I think you will be fine. The two metals will corrode if together and then exposed to salt, water, or other corrosive solutions. As long as you put a film of grease between the two during assembly, they will not corrode. I am a bicycle mechanic and work with dissimilar metals all of the time and have never had corrosion between bare steel and bare aluminum as long as they were assembled with a good grease. Areoshell MS33 is an excellent grease for corrosion prevention.
I once (very) briefly owned a Les Baer upper receiver, which was ‘in the white’ on the entire internal surface. Well, there was some overspray at the rear and around the ejection port. I sold it and never looked back.