What do you put on barrel extensions?

[lysander]

Any change in resistance of galvanic corrosion?

Most of the galvanic corrosion protection is handled by the anodized layer.

Anodizing converts the outermost layer of the aluminum into aluminum oxide, which is a great insulator. No electrical connection, no galvanic corrosion. Unless you have a very large break in the anodized finish where it is in contact with steel, you are fine.

EDIT:
Anodizing is why you do not worry about steel hammer-trigger pins in aluminum holes, or all those other steel parts attached to the aluminum uppers and lowers . . .