Initiation of localized corrosion at the nanoscale by competitive dissolution and passivation of nickel surfaces

The structural modifications of Ni(1 1 1) single-crystal surfaces induced by prolonged polarization at the onset of the active/passive transition have been studied at the (sub-)nanometer scale by in situ electrochemical STM (EC-STM) in 0.1 M NaOH(aq). The results evidence the effect of the structural order in the passivating monolayer on the corrosion resistance properties and the initiation of localized corrosion. 2D pits are initiated by dissolution on the terraces covered by disordered islands of nickel hydroxide particles and at the boundaries of the crystalline Ni(OH)2(0 0 0 1) islands. The pits propagate preferentially in the disordered areas. Dissolution is blocked in the well-ordered islands of the passivating monolayer and by the 3D nuclei of the passive film formed at the pre-existing step edges of the nickel substrate. This competitive mechanism of dissolution and passivation, repeated on the terraces newly formed by local dissolution, initiates the growth of 3D pits of nanometer dimensions, i.e. localized corrosion at the nanoscale. A tip effect, promoting the dissolution reaction by a mechanical and/or chemical interaction with the passivating particles, contributed to reveal these local properties. This fundamental understanding of the depassivation/repassivation at the nanoscale opens the way to the search for functional surfaces with self-repair properties.