Pit Growth in NiFe Thin Films

Pit growth was studied in 80Ni-20Fe sputtered thin films by analysis of images of the growing pits. The pit current density was found to increase with pit growth potential until reaching a limiting value. The limiting current density increased with decreasing film thickness. The mass-transfer resistance to the active pit wall exceeds by an order of magnitude that predicted from a simple radial-diffusion model. It is suggested that the undercut, remnant passive film collapses over the pit wall causing a constriction. A voltage component calculation matches the data rather well and indicates that pit growth below the limiting current density is limited by a combination of ohmic, concentration, and surface activation considerations. The study of pitting in thin films is of interest because of differences in behavior compared to pitting in bulk alloys. Thin films often have different properties and certainly have more stringent requirements in terms of allowable material loss. Furthermore, thin films provide a unique opportunity for studying pit growth since the whole pit is visible during the growth process. As a result, no assumptions need be made regarding the active pit area during growth. Pits were previously shown to penetrate thin metallic films quickly and reach the substrate 1 . They grow subsequently in a two-dimensional fashion with walls perpendicular to the substrate. This initial study was performed on approximately 1500 A thick Al films, and the average pit current density was calculated from images of the growing pits. The pit current densities were found to be large (tens of A/cm 2 ) and independent of time during pit growth. There was, however, an influence of pit growth potential. At the highest growth potentials, the pit current density was rather independent of potential, and the pits were very round in shape. At lower potentials, the pit current density varied approximately linearly with potential, and the pits were more irregular in shape. At the lowest growth potentials, the pit perimeters were extremely convoluted, and the calculated pit current density was again independent of potential, although the latter was determined to be an artifact of the calculation as discussed below. Pit growth was described to be under mass-transport control in the highest potential region and under mixed ohmic/charge-transfer control at lower potentials. The alloy studied in this work is Permalloy, a NiFe alloy. The early investigations of this alloy focused on its oxidation and atmospheric corrosion behavior 2-6 . Recently, studies of the