... hotspots caused by the corrosion?
I didn't think the hotspots were a result of corrosion.
My take on it is that the hotspots are an independent factor in relation to the corrosion. I expect they are a variation in residual welding stress occurring around the weld. A number of cycles of variation in residual stress might occur around the path of the weld. This will include areas that are free of residual stress and areas that might have several times the average residual stress and which might be termed 'hotspots'.