At the top of a water heater, there are two pipe nipples (short lengths of pipe) for attaching
both the cold water supply line and the outgoing hot water line.
In California (and maybe in other states), the water lines are typically flexible copper pipe to meet earthquakes requirements.
Not to get too technical, but the steel pipe nipples and the flexible copper pipe are dissimilar metals... If allowed to touch each other without an insulating material (neutral) between them... electrolysis occurs (corrosion caused by dissimilar metals touching each other).
In the photo, flexible copper pipes were threaded onto steel pipe nipples.
It is unknown whether Teflon tape or plumbers pipe thread compound was used between the copper pipe nipple and steel fitting... Or if a plastic insulator in the threading nut was compromised.
The result of not having a insulator was electrolysis, or corrosion between the two dissimilar metals: steel and copper. As the corrosion progressed, water seepage occurred and the result is shown above.
This was a 3 year old water heater, that was replaced. Had the plumbing been installed correctly, the water heater may have lasted 12-15 years on the average.
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