It looks like what the old-timers used to call "steel worms". I would categorize it as corrosion/erosion damage from abrasives in the product or velocity-dependant corrosion mechanisms. The damage tends to concentrate in high velocity areas and once damage appears, it progresses rapidly. It can attack any discontinuity in the parts. It tends to be worse in the stationary parts rather than the rotating. But a set-screw hole where an impeller wear ring is attached can be the starting point of similar damage on the impeller. We started seeing this in many pumps a few years ago when our crude slate changed. According to our metalurgist, our new crudes produce naphthenic acid corrosion at certain temperatures. But this corrosion mechanisms depends on velocity to remove the stable corrosion by-products so it shows up as localized grooves, swirl patterns and "worm tracks" in carbon steel, CA6NM (chrome steel) and other materials. What is the case material? What is the product being pumped?
