In Situ STM Study of the Effect of Chloride on Passive Film on Nickel in Alkaline Solution

In situ electrochemical scanning tunneling microscopy (STM) measurements on the growth and structure of the passive film on Ni(111) in 0.1 M NaOH + xM NaCl (x = ,0.0,0.1, and 1 M) aqueous solutions are reported. At the onset of the active-passive transition, the formation of the (2 X 2) islands, assigned to the locally ordered coadsorption of hydroxide species and water molecules on the Ni(111) unreconstructed surface, is retarded ([Cl-]/[OH-] < or = 1) or blocked ([Cl-]/[OH-] = 10) by a competitive adsorption mechanism between OH- and Cl-. A competitive mechanism, at step edges, of metal dissolution and nucleation of a 2D passive film is also observed. For ([Cl-]/[OH-] S:1), the nucleation of the passive film at the step edges is blocked at E = -500 mV/SHE (standard hydrogen electrode). The rate of retraction of the step edges is not significantly enhanced by the chlorides, but the dissolution is sustained. Increasing the potential unblocks nucleation but leads to the formation of 3D nuclei. At E > or = -175 mV/SHE, a crystalline passive film develops for [Cl-]/[OH-] < or = 0.5. At higher chloride concentration, the crystallization is blocked, and clusters of nanograms (10 plus/minus 5 nm) are formed. Further growth of the grains by coalescence is blocked. Localized attacks of the substrate are developed by preferential dissolution at the boundaries between the grain clusters.