On Electric Field Induced Breakdown of Passive Films and the Mechanism of Pitting Corrosion

Abstract

Scanning tunneling microscopy reveals that microscopic roughness inherently exists at the smoothest of metal surfaces. Mathematical modeling indicates that the passive film on the concave region of the surface is subject to higher‐than‐average electrostatic pressure and is, therefore, a preferred site for passive film rupture and micro pit formation. The model includes the role of Cl− ion in the breakdown; Cl− increases the electrostatic pressure and/or reduces the compressive strength of the film. It follows that pit initiation could be a repeated breakdown‐repair process that deepens the micro pit until it reaches a critical depth determined by the IR voltage. Thereafter, the pit bottom remains active due to its electrode potential being in the active region of the polarization curve. The pit mostly widens but also can grow upward, eventually penetrating the outer surface and causing a short circuit of the IR voltage, shift of the electrode potential into the passive region, and cessation of pit growth at the penetration or over the entire pit surface. Repetition of the above process leads to further penetrations to produce the lace‐like pattern or, if complete cessation occurred, the reinitiation of the pit at a sharp concave site. Acidification and chloride ions promote the IR condition for pit growth.