Centrotherm flue pipe

[NC1]

Hi. I have a centrotherm inoflue pipe hooked to a viessma 100 boiler. Heating room is inside under my stairs. The metal clamp around the pipe is rusting. Does that mean gas is leaking? My monoxide detectors arent going off. We have 8. Is there any fix? Or if there is a leak any liquid seal product available to fix? It is on a 45 joint.
Thanks in advance. Nav