where is the corrosion problem in all of them or near to the heater?how many steam or BFW  Do you used? which is the inside diameter for those tubes?
what type of material are you used in those elbows?  9Cr-1Mo (steel) internally hardface with 309L/stellite overlay or similar (this is the typical material required for steam/air decoking) this have to be the material for all the elbow from the outlet heater until you reach the swing elbow
With this specification we never found any corrosion problem in our coker units
 
I’m sorry for my english

I’m far from expert, but I have heard of people using a tungsten carbide spray in the return bends to prevent return bend erosion

Do you currently rotate on spalling, pigging or steam air? Depending on how often you spall these other methods could help.
Roman Korpus
D.D.Technology Inc
713-206-6112

Have you looked into having your return bends boronized? Could maybe get away with using boronized plain carbon steel.