Black inky coating in copper flex pipe on hot side?

[PaulOregon]

Greetings. Last weekend, I removed a 15 year old propane-fired water heater from my house and replaced it with a nice new Bradford-White 50 gallon tank. Pretty straightforward job, but I noticed something that was strange.

The connections to the top of the tank are the common 3/4" copper flex lines, soldered to 3/4" hard copper for a bit then conversion to PEX. When I was getting ready to connect the hot output flex line to the new tank, i stuck my finger up into the threads to clean out any crap and my finger was coated with the blackest inky coating. It was thin, not oily but permeated my skin like black ink. I cleaned as much as I could with a rag and reassembled the line. System is running fine now, but I wonder what that inky stuff was.

Water heater was 15 years old and was handling moderately hard water (5 gpg) for all but the last year so it's likely that the tank was full of sediment. GE water softener was installed a year ago. I wonder about dielectric reactions between the copper and the stuff in the water and maybe even the threaded nipples at the top of the tank (if they were iron and/or reactive)

Any ideas? Is this common? Thanks!

 

[Terry]

There used to a rubber seal at the end of the flex connector. That is pretty much gone now.
I only use flex connectors once. If I ever remove them, I toss em.
You should have used a new connector there.

 

[Sponsor]

 

[Master Plumber Mark]

what did it taste like??

 

[Gary Swart]

What Terry said. Never reuse flex tubing.