Mass transport, directed metal oxidation

In the very early stages of oxidation the oxide layer is discontinuous both kinetic and electron microscope" studies have shown that oxidation commences by the lateral extension of discrete oxide nuclei. It is only once these interlace that the direction of mass transport becomes of importance. In the majority of cases the metal then diffuses across the oxide layer in the form of cations and electrons (cationic diffusion), or as with the heavy metal oxides, oxygen may diffuse as ions with a flow of electrons in the reverse direction (anionic diffusion). The number of metals oxidising by both cationic and anionic diffusion is believed to be small, since a favourable energy of activation for one ion generally means an unfavourable value for the other... 

Summary. An STM study has been initiated to investigate the various processes associated with electrodeposition of Cu-Ni multilayers on Cu(100). The substrates were prepared by electropolishing in phosphoric acid followed by immersion in 10 mmol/1 HCl. A (V2 x V2)R45° adlattice of oxidatively adsorbed chlorine is formed under these conditions. The adlayer stabilizes the surface steps in the <100> direction which corresponds to the close packed direction of the chloride adlattice. In dilute (millimolar) solutions of cuprous ion, reduction occurs under mass transport control with the electrocrystallization reaction proceeding by step flow in the <100> direction. At more negative potentials chloride is partially desorbed. Coincidentally, the highly kinked metal steps become Mzzy and move towards adopting the close-packed <110> orientation of the metal lattice. Preliminary experiments on heteroepitaxial nickel deposition reveal regions where electrocrystallization on Cu(100) occurs via step flow in the <110> direction. 

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