[CQ-Contest] oxidized inner conductor when coax carries DC power

[Franki ON5ZO]

Probably not the appropriate forum but there ought to be a technically skilled ham here? Sorry that this post isn’t about cheating and what defines ‘assistance’. Some contesters 
have real issues though.

A few weeks ago I was doing some relocating and rerouting of the coaxes outside. One RG-213 is used for my active RX loop. It needed a different plug on the antenna switching side 
so I cut the existing one off and prepared the cable for a new one. Much to my surprise I found the inner conductor black from corrosion. I cut off a few centimeters at a time, but 
after having cut off two meters, it still was black. I have been working with these things for ages and I can tell you: it is NOT water ingress. Everything is sealed properly. I 
was a pain to solder the new plug to the center conductor. I had to sand the black film off and even then the tin wouldn’t flow.

Last week I took the loop down and cut away the layers of tape that kept the feed point coax dry. And dry it was. However I noticed the N male-female junction had a green mush 
developed around the mating pins of the inner conductors.

I didn’t pay attention in chemistry class, but I’d label this as corrosion. Right?
Since I have never seen this before, and this is the only coax that ever carried DC around here, I assume the DC voltage is the culprit here?

* Can I avoid this?
* Does it hurt? I seem to remember something about DC and polarity that can eat your copper away?
* My coax shields are tied to a dedicated earth system. Does this relate to the corrosion in a good or bad way?

I’m thinking of ways to improve my 80/160 RX situation, and several scenarios involve DC over the coax. So I better know what I’m up against.
Thanks and 73
Franki ON5ZO / OQ5M