Abstract: A device for detecting and indicating the presence of corrosive fluids in a specified volume, as in the insulating blanket surrounding the pipes in a heavy water plant, is disclosed. The device includes a housing containing a primary indicator such as a bobbin with a long, brightly colored ribbon wrapped therearound, and a secondary indicator. A compression spring abuts the secondary indicator and is held in compression by a corrodable link anchored at one end to the housing and at the other end to the secondary indicator. The link is exposed to the volume being monitored and if there is a corrosive fluid present the fluid will corrode the link until it breaks. The spring force is released to act on the secondary indicator to project the primary indicator from the housing. The ribbon unwinds from the bobbin to provide a visual indication of the presence of the corrosive fluid.