See the links in this post
http://www.pprune.org/tech-log/37634...ml#post5027449
Ice crystals will normally bounce-off both cold and heated surfaces; however, on some heated surfaces if the crystals are slowed down by airflow disturbance, closed curves, etc, then depending on several variables (particularly heat flow) then some of the ice may melt and the water traps other particles resulting in ice accumulation.
This is not normally a problem for engines except in exceptional circumstances, i.e. near large Cbs.