I noticed an interesting corrosion pattern on one of the stainless-steel electrodes used in the applied electronics for bioengineers course:
I think that the dark ring on the stainless-steel rod comes from welding stainless-steel wire together in the manufacturing process of making the rod, probably before the wire was drawn down to the 1/8″ diameter. Because the rod is sold for TIG welding, in which it will be melted in the normal use case, having an occasional weld in the rod where material properties change slightly should not matter. Our using the stainless steel rods as electrodes revealed a structure in the rod that is not normally visible.
I think I was lucky to see even one instance of this pattern, since I would expect that the wire-drawing process only adds welds every few hundred meters of rod (when new feedstock has to be joined), and I only sampled about 65cm of rod (26 electrodes about 2.5cm long). But maybe I’m wrong about the process that is used for making stainless-steel welding rods, and they actually have frequent joins.