Complex surface dynamics during anodic dissolution of Ni

The evolution of the surface roughness during the anodic dissolution of polycrystalline Ni was investigated by means of ex situ AFM in acid phosphate solutions. To characterize the time and spatial scaling behavior of surface roughness, the interface width and the power spectral density of the surface at different dissolution stages were analyzed in terms of dynamic scaling theories. The time dependence of global surface roughness, W(L,t), shows an unstable behavior characterized by a continuous increase without saturation following the relation W approximately t(beta), where beta > 0.5. The unstable behavior results from the development of wide grooves that originates a surface consisting of mounds. Two scaling regimes at scales shorter and larger than the mound dimensions (l(c)) were observed. For l < l(c), we found alpha approximately 1 consistent with mounds exhibiting smooth (faceted) walls, whereas an anomalous scaling behavior with a proper local roughness exponents (alpha(loc) < 1) dominates at l > l(c). The introduction of nitrite in the solution, a common additive used in phosphating baths, leads to some changes in the scaling behavior as a consequence of different generated chemical surface conditions during dissolution. The different dissolution rates of the exposed crystal orientations and surface diffusion of adatoms were identified as the physical processes that govern the interface dynamic for this system.