How to stop rats from walking in plastic sewage pipe and damaging it?
Step one: eliminate the rats. 

Step two: find their means of ingress and block it off. 

Pictures of the pipe where it exits the house, the cover on the access port of the tank, and the area of the ground that's above the tank would help suggestions flow in to accomplish these steps. Failing that, those are the places that you'd need to inspect, to figure it out.