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Wastage or thinning of a metal hull on a boat is problematic. Often the thinning is due to a phenomenon known as electrochemical or galvanic corrosion. One area prone to metal wastage on an aluminum boat hull is at the waterline. Many aluminum hulled boats have sacrificial anodes attached to their hulls beneath the water to control metal wastage from galvanic corrosion. These anodes are composed of a metal that is less noble than the aluminum hull. The anodes waste or give up their electrons to protect the aluminum hull. The effectiveness of these anodes is based on their composition, surface area, and condition of their surface (e.g. free of calcareous scale). A sacrificial anode that has been accidentally painted with antifouling bottom paint is no longer effective. Such were the events that led up to this inspection. When the anodes are no longer effective, the cathode (in this case the aluminum hull) gives up its electrons and the metal wastes (corrodes).

Applying Infrared Imaging Techniques to Marine Surveying … Combining IR with Audio Gauging (.pdf) - Video Cameras and Accessories