The water heater is located in a unconditioned utility room, but it is insulated. The corrosion seems a bit excessive.
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Looks to be/have been leaking at the connection where the copper riser and PEX line meet.
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My thought as well.
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She’s a leaker, mama! The plastic <> copper adapter had been leaking for a long time. The “corrosion” is mostly mineral deposits left behind by the evaporated water.
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what they said …
Thanks. After I posted, I noticed the streak down the copper pipe. Appreciate the input
This is common on the hot side, small leaks happen when no or not enough sealant is used on treads. The calcium in the line usually seals the leak over time.
If the coupler is a different metal, could it be a galvanic reaction between two metals making contact?
Copper and brass don’t react with each other. Brass is made with a combination of copper and zinc.
Noted. I could not tell what metals other than copper were present from the photo.
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Hey Stephen -
Just for future reference - that stuff is called “efflorescence” (although with plumbing, “corrosion” is more understandable on your report). Same stuff you’ll find on the inside of block foundation walls. Remnants of evaporated water. It’s made of calcium and sodium…and from my experience - licking it or snorting it does very little to improve your mood. 
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Ah, the lick test!
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tastes like salt…
Galvanic corrosion.
LOL!..Mark! 