An Electron Microscopic Study of Nucleation and Growth in Electrochemical Displacement Reactions: A Comparison of the Cu
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tributing factors in the overall cementation mechanism, identified a critical step in the process as the reduction and incorporation of the precipitating metal ions from the dilute aqueous solution into the crystal lattice of the deposit. In order to study such mechanisms, it is necessary to observe and compare these phenomena on a microscopic level in the submicron regime. The overall cementation reaction is the combination of the respective half- cell reactions: M m " + me ~M
[1] where, with reference to Fig. 1, ions from solution (M m+) having a valence m, are reduced to elemental metal (M) on the more electropositive, solid metal substrate surface (N). The arrows indicate the directions of the reaction. In the present study of the initial reactions according to Eq. [1] for the CufFe and cuiAl systems we can write Cu2+ + Fe
= Fe 2 + + Cu
[2]
and Cu2+ +"32 Al ="32 A I3+ + Cu.
[3]
In addition, there are numerous side reactions which have been discussed previously. 5,9-11 The nucleation and initial deposit growth, have so far been neither observed nor studied in any detail. In the present investigation a novel technique was devised to study the incipient structural features of copper cementation on iron and aluminum. Both transmission and scanning electron microscopy were employed in direct observations and comparison of the initial stage of copper deposition in the CufFe and cuiAl cementation systems, including "nucleation" and growth of the deposit. In the cuiAl system, the surface oxide, which is always present, provides for a certain passivation not associated with iron. Chloride ion (cr) must be added
ISSN 0360-2141178/1211-0515$00.75/0 © 1978 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 9B,DECEMBER 1978-515
to the solution in order to break down the oxide layer and expose the metal sites. Edaleanu and Evans 1 2 explained the pitting and localized attack on aluminum in chloride solutions as autocatalytic in character and the anodic corrosion having once started along a path, continued there rather than elsewhere. This can happen, however, only if the anodic and cathodic regions are well separated. Godard, et aZ1 3 similarly attributed the pitting observed in aluminum in chloride solutions to electrochemical cell action which produced local anodes where the corrosion occurred, with the surrounding area becoming a local cathode. EXPERIMENTAL METHODS Thin films of iron and aluminum estimated to be 0.20 J.lffi thick'" were vapor deposited in high vacuum onto (001) NaCI cleaved crystal blanks (1.9 cm on a side and 0.3 ern thick) as previously outlined by Murr and Inman" and Murr and Bitler. 16 Iron powder (99.997 pet) and aluminum filings (99.9999 pet) were vaporized using a resistively heated tungsten boat at a nominal pressure of 5 x 10-6 Torr. The NaCI crystal blanks were attached to a copper substrate heater which could heat them to temperatures in excess of 500°C. By heating the NaCI substrate, the grain size of the iron or aluminum films which were vapor deposited could b
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