I've found a few threads with similar photos, but the answer seems to be different in cases that look the same to me. I'm a complete plumbing novice.

Can anyone tell me what would cause this corrosion? The installer insists that it was properly installed (though he won't send someone out to check) and claims that it's a leak. (that mysteriously appeared after the install because the pipes were fine with the old water heater)

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