What type of water treatment chemicals are you using? Overly aggressive application of scale remover can start dissolving boiler tubes causing iron levels to increase.
High iron levels in drum could be the reason of FAC in your riser tubes. LP tubes are more susceptible, however, it can be seen on IP as well.
You can expect soon a tube failure and heavy steam leak. That's what happened to us on our LP section. It started with high iron levels in drum and followed by damage on riser elbows, tees etc. Solution was easy. We just upgraded the riser tube material from CS to low alloy steel (P12.