Many thanks for your reply!!! It works. Now I can see the boundary line. :)
One remaining problem is that in the "Replace value in Array (Conditional)", I have to select a conditional array related to the Boundary Cells. However, I cannot choose the cells obtained directly from "Find Boundary Cells". I have to create a threshold mask for the boundry cells (greater than 0). You mentioned that this value would range from 0-6. I am not sure if this is the right way to do (alougth it works for now). :)
Again, many thanks for your help.