Discovery of the cathodic protection principle

About 1820 the Navy Board was anxious to find the reason why copper sometimes got fouled, whilst iron was dissolving, and at other times the copper was dissolving rather too quickly. A Committee was formed with the Royal Society, of which Sir Humphry Davy was President. Davy had already in 1806 advanced the hypothesis that chemical and electrical charges may be identical, and later convinced Berzelius of this idea. Now, assisted by Michael Faraday, he began to experiment with copper and other metals, such as iron and zinc in various saline solutions, and found the electrochemical reactions he had expected.

In January 1824 Sir Humphry Davy put forward his findings that iron or copper, and for that matter any other metal in saltwater where oxygen is present, forms a galvanic battery and one metal or the other is gradually dissolved, depending on the scale of nobility in the galvanic series. He advocated a small quantity of zinc, or of even cheaper malleable iron, should be placed in contact with copper, and thereby prevent its corrosion. The following year he was able to move away from the laboratory tests and continue his research aboard a naval vessel. Davy found that small "protectors" of malleable iron preserved the copper by the iron gradually dissolving in a galvanic process.

(Source: "On the corrosion of copper sheeting by seawater, and on methods of preventing this effect, and on their application to ships of war and other ships". Proceedings of the Royal Society, 114 (1824), pp 151-246 and 115 (1825), PP 328-346.)