Red Rot
Often what is called Red Rot - isn't.
Actual red rot comes from inside the metal and only shows up on the outside after it has destroyed the inside. Surface blemishes - are exactly that.
Try using a little Brass polish - or even a piece of steel wool and polish. If it comes off, it isn't red rot. If it is red rot, the tube will probably collapse. The only fix for true red rot is replacing the tubing.
"Red Rot" is commonly used to refer to any reddish patch of corrosion on a copper-based alloy, such as brass. However, this loose usage covers two distinct but related conditions, surface corrosion and deterioration of the alloy due to de-zincification.
Red rot is found in brasses, which are alloys of copper and other non-ferrous metals, such as zinc. As the alloyed metal, most often zinc, is corroded out of the brass, a latticework of copper is left which is reddish in appearance.
If this happens on the surface of the part, it is relatively harmless. Only a very thin layer of zinc has corroded away, and the part will likely continue to perform well.
However, sometimes the de-zincification is extensive. True Red Rot is the complete breakdown of the alloy, and it occurs from the inside of a tube and proceeds outward. In this case, the loss of zinc from the brass is substantial throughout the thickness of the part. It is identifiable as a reddish patch on the outside of the tube, roughly circular, with a pin-point dark spot in the very center. At this spot the alloy has failed completely, leaving a weak structure of copper behind. You can put a probe on the spot and push all the way through without effort. The metal has failed.
The German word for this zinkfrass, and sounds more accurate than red rot.