It does resemble a stress fracture along a seam, perhaps with chemical compromising (accelerated damage) (pitting) caused by the product being pumped. If it were cavitation alone, I don't think there would be chemical residue.
Is the setting properly supported? Does this section need a flexible joint? Is the pump overated for the job at hand? If the pump is overated, then a recirc line would be needed to accomodate the flow requirements at low load. Just my initial thoughts.
