Electrolysis in copper to copper piping

[jmulhern (John Mulhern)]

Can anyone explain what would cause the electrolysis between two similar copper pipes, at couplings and elbows.

Thank you

[rcooke (Roy D. Cooke, Sr)]

Looks like the solder flux was not cleaned of properly when the job was finished .

Added

A Google search found these
https://www.nachi.org/forum/f22/copper-pipe-flux-corrosion-89835/

http://www.inspectionnews.net/home_inspection/plumbing-system-home-inspection-and-commercial-inspection/31957-corrosion-soldered-joints.html