I have seen photos of internal corrosion but not external corrosion of coke drums. You may want to try asking members of the Linked In group: Coke Drum Mechanical Integrity. Someone else may have seen it.
Yes, it is possible to have external corrosion under insulation for coke drums. Half of the cycle the drum temperature is cold enough to allow this mechanism. During this portion of the cycle, if liquid water is in contact with the drum it is possible for conventional CUI to occur. Damage to the insulation system that allows liquid water to contact the metal is required for this to occur. Similarly, liquid water has to be in contact with the metal.
I am aware of a case that had a leaking decoking water isolation valve so that water came out of the cutting tool even when the valve was closed and that soaked the top head of the coke drum and its insulation with water. This continued for an extended time period which resulted in metal loss due to CUI that resulted in a leak during the coking phase.
I think all the piping associated with a coke drum that experiences the same thermal cycle as the drum are at risk for CUI as for half the cycle the temperature of the piping is low enough to allow liquid water to contact the metal.