Marine heat exchangers are the common way to cool a boat's engine, using the lake, river or ocean water in which the boat floats. The water-antifreeze mixture runs through the heat exchanger dumping heat, but remaining separate from corrosive salts and chemicals found in the water the boat is floating in. Heat from the water-antifreeze mixture is then transferred to the ocean (or lake or river)
water which flows into a heat exchanger. Since this water may be corrosive the engine may be cooled by a sealed mixture of distilled water and antifreeze. If the ocean water eventually corrodes and ruins the heat exchanger it can be replaced at a fraction of the cost of replacing the engine.
To protect the marine heat exchanger from corrosive salts, a sacrificial zinc anode is screwed into the heat exchanger. This anode must be periodically replaced as part of regular maintenance. This filter must be periodically cleaned or else the flow of water to the heat exchanger will become obstructed and the engine will overheat.