Abstract: In fluid flow systems corrosion of the fluid flow passages occur as a result of the action of the fluid on the passage walls as it flows through the system. The present invention provides a corrosion indicator for use in such systems and which includes among other things a novel switch having a member which is responsive to a corrosive condition within the system to self-destruct thereby to cause actuation of a signal to indicate the condition at a time to alert the user that remedial attention to the system is necessary.