Surface formation foreign materials deposition

The deposition begins at potentials more positive than values where deposition of R occurs on bulk R. Consider, for example, the deposition of Ag on a 1-cm Pt electrode from a 0.01-L solution containing 10 M Ag". Let A = 1.6 X 10 cm and yo = yR. The potential for deposition of one-half of the silver (which forms about 0.05 monolayer) is = 0.35 V, compared to E = 0.43 V required for the same amount of deposition on a silver electrode. Deposition at potentials before that predicted by the Nernst equation with R = 1 is called underpotential deposition. The situation is much more complicated than the above treatment suggests, since the deposition potential depends on the nature of the substrate (material and pretreatment) and on adsorption of O. Also, the treatment assumes that formation of a second layer does not start until the first is complete. However, this is frequently not the case atoms of metal will often aggregate, rather than deposit on a foreign surface, and dendrites will form. Reviews on the nature of underpotential deposition and the deposition of solids in general are available (6-10). 

---