CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



THE PROPERTIES OF AN ALUMINIUM ANODE. 



By H. W. Morse and C. L. B. Shuddemagen. 

 Presented by John Trowbridge, December 9, 1908. Received January 6, 1909. 



I. Introduction. 



Many of the metals exhibit peculiar properties when used as anode 

 with certain electrolytes in an electrolytic cell. Iron and chromium 

 and, in less degree, uickel and several other elements, assume the so- 

 called "passive state" under these conditions. Some other metals, 

 among them aluminium, magnesium, tantalum, and niobium, show a 

 still more striking change from their usual properties when the same 

 conditions are imposed upon them. If the surface of metallic alu- 

 minium is kept free from the protecting film which usually covers it, 

 it is rapidly attacked by the oxygen of the air. It is a familiar lecture 

 experiment to carefully amalgamate a piece of clean aluminium by 

 rubbing it with pure mercury. At the places where the mercury pre- 

 vents the protecting oxide film from forming, the action of the air is so 

 rapid that a white fibrous mass of oxide, several millimeters in thick- 

 ness, grows up in a few minutes. While pure aluminium is very sensi- 

 tive to an attack of reagents, it can under some circumstances act like 

 a noble metal. As long as the film which forms on the surface retains 

 its coherence aluminium is stable in the air, and even when it is used 

 as anode in an electrolytic cell it may, under some circumstances, 

 resist corrosion and solution to a surprising extent. Metals like 

 copper and silver, which lie far down toward the negative end of the 

 electromotive force series of metals, readily go into solution when used 

 as anode in an electrolytic cell, but a plate of aluminium in contact with 

 many electrolytes merely covers itself with a protecting layer and 

 remains otherwise unattacked. / 



The protecting layer so formed offers a hindrance to the passage of 

 a current through the cell as long as the aluminium plate remains the 

 anode. If the current is reversed, the film no longer opposes the same 

 resistance to its passage. These facts determine the use of aluminium 



