I
deas
E
xchange
C olumn
l/EC's features bring in lots of fan mail. Readers want information, and they pass along ideas. Ideas Exchange Column is a careful screening of this correspondence. This is not a place for praise or criticism; it is a source of technical questions and their factual answers.
Corrosion Research DEAR M R . KRUGER:
The members of the Corrosion Research Council gazed at their crystal ball made of copper, to be sure, and raised their heads with some knowledge about the basic principles of corrosion [ I N D . E N G . CHEM.
50, 55 A (March 1958)]. I n reading the very interesting summary I started to meditate about the interpretation of the facts described. I am sure that the researchers have much more in mind than they have stated in this article. Still, they might use some ideas originated by looking at their problem from a different angle. First, the experiments contain the implicit statement that the atoms of some crystals are not equivalent from the standpoint of chemical reactivity. I think that this interesting phenomenon should be studied closer—e.g., by etching (perhaps by galvanic) experiments. Can some theory of field distribution sufficiently account for this experience? In the presence of C 0 2 ( C 0 2 + H 2 0 -> H 2 C 0 3 -> H + + H C O 3 - ) , first protons or rather hydroxonium ions ( H + + H 2 0 - » H 3 0 + ) may become absorbed on the copper surface, in a distribution oriented according to the arrangement of the Cu atoms (let Cu denote the copper in the crystal lattice). Can it be proved that absorption on (some) single crystals is oriented? T h e C u . . . H3O + adsorption com+ plex may split out C u O H 2 , leaving a C u . . . H neutral surface. T h e reaction CuOHj -* CuOH + H + regenerates the proton, while oxygen naturally cleans the C u . . . H surface from the active hydrogen. Thus the process m a y start again from the beginning, with the net result of the formation of an oriented C u O H film on the surface. In the absence of C 0 2 we might
assume the absorption of water, which being neutral is not subjected to oriented adsorption. When acted upon by oxygen which m a y come in contact with Cu surface by dissolution in the adsorbed water film, Cu.. .OHj. . .1/2 0 2 -* CuO + H 2 0 the copper atom in question m a y change its valency, a n d thus break loose from the crystal lattice. C u O m a y be reduced in the presence of C 0 2 by the C u . . . H surface formed as outlined above. How can the influence of light be explained? I a m sure that these basic studies on single metal crystals will be of great importance not only from the aspect of corrosion but also in other relations—e.g., catalysis. FRANCIS K A L L A Y
Budapest X I I Zolyomi-lepcso 19 Hungary Dr. Kruger's
Answer:
T h a n k you for your very interesting letter. Unfortunately our copper crystal ball is not sufficiently magical to allow us to look as deeply into this corrosion process as your letter suggests. We don't really know in detail the mechanism of the reactions occurring at the surface of the copper crystal. I feel that adsorption of oxygen at the copper surface is probably more important than adsorption of hydrogen ions, although some hydrogen ion adsorption probably occurs also. T h e adsorption of oxygen is considered to be the necessary first step in most theories of oxide film formation. Also, as some further work of ours indicates, after this oxygen adsorption a thin C u 2 0 film whose thickness is 30 to 50 A. or more forms extremely fast. This happens both in the presence and absence of C 0 2 . T h e role of the C 0 2 is to lower the p H of the solution by the introduction of hydrogen ions as you indicated. At the lower p H , thermodynamically, C u 2 0 is the
more stable oxide. Not enough is known to propose a definite mechanism for this fact. At p H 7, when C 0 2 is absent, C u O is thermodynamically the stable oxide. It, however, does not form next to the copper surface, but always on top of a thin film of C u 2 0 , for copper reduces the C u O to C u 2 0 . No evidence for the formation of the C u O H has been found, and it is listed in the handbooks as being of questionable existence. I cannot go into any more detail than this brief sketch without going beyond the facts which our experiments and those of others have revealed. As to the influence of the different atomic arrangements on corrosion, no one knows the exact reason for this. I can mention in an oversimplified way two possibilities. 1. It depends on the way the crystals of oxide can arrange themselves on the metal crystal surface. For some metal crystallographic planes the geometry of the oxide crystals is such that they fit well on the metal plane and thus form protective films. For poorer fits, less protective films are formed and more corrosion occurs. 2. It depends on the tendency of a given atomic arrangement to let go of its metal atoms. For closely packed arrangements this is more difficult than for loosely packed arrangements. Your suggestion for etching experiments, galvanic experiments, etc., for studying the influence of atomic arrangement in a surface is a good one and some experiments of this sort have been done by others. These show that the second reason above has some validity. However, both reasons are probably important. As for the influence of light, it probably depends on the fact that C u 2 0 is a good semiconductor whose electronic properties also influence the growth of films of this oxide. Finally, you are right about the importance of single crystals in the study of catalysis. Some very valuable work along this line has a p peared recently in the literature. VOL. 5 1 , N O . 2
•
FEBRUARY 1959
87 A
