After several years my pellet stove vent pipe doesn't let me get it as clean as it did when new. I use the typical pellet stove vent pipe brush and run it through until no more ash falls out. However there has been what looks like rust or corrosion building up on the inside of the pipe more and more each year. Is this buildup typical? As I mentioned I have all of the ash cleaned out nicely. It's just that "rough" surface on the inside of the pipe that's making me nervous. The pipe is solid as a rock and no issues, I am just nervous that "rough" surface could start a fire. Opinions appreciated.