Chemical reaction?

[Frank Bombardiere]

Home is only 5 yrs old and neither pipe has been replaced. Why would one be so tarnished and the other not at all?

[Dom D'Agostino]

Poorly made flex connector, or no dielectric union/coupler on that piece.

[Frank Bombardiere]

There was a DE on that pipe. May not be working, but it was there. Still wondering what else it could be.

[Dom D'Agostino]

Water heater is back-drafting, and sulfur is corroding the copper?

[Billy Stephens]

The Guy who left his Foot Print on the Wall did it. :D
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[Bill Anglin]

I'm with Dom. It definitely looks like sulfur corrosion. I would say Chinese drywall but it would likely be on both lines. Check for back drafting in the direction of the line.

[Corn Walker]

Flue gasses condensing on cold water line?

[Corn Walker]

I heard that sodium reacts and catches fire in water, can you tell me the reaction and chemical formula.

Na2 + H2O -> SpAm

[Kristi Silber]

Seems like an extreme example, but heat can speed up corrosion rates. Is the bad one the hot water?

[Corn Walker]

Seems like an extreme example, but heat can speed up corrosion rates. Is the bad one the hot water?

Heat can speed up corrosion, but the type of corrosion present suggests something else. That's why I suspect it's the cold water pipe - if sulfur dioxide condenses on the cold water line with water it it will form sulfurous acid and sulfuric acid which will corrode copper as in the image.