Grease is not kind to cast iron pipe.
The flash illustration to the right
shows how after a "grease mantle"
forms the grease begins to chemically
react with the cast iron pipe and over
years of time will eat the pipe away.

Eventually the side wall of the pipe
becomes so thin that the pipe
collapses on itself yielding an oval
shaped pipe of reduced diameter
with no remaining structural
integrity. Pipe in this condition is
prone to blockages and backups,
and is not  maintainable.

The typical repair for such pipe is to
incur the cost of digging it up and
replacing it with PVC piping.