Revisiting passivation II: electronic conduction of transition metal oxides, Mott-Schottky plots, and root cause of stainless steel grades and nickel based alloys

Abstract

The novel views on passivation and passivity are briefly reviewed, with further explanation of some key details. First, the most common polymerisation mechanism directly transforms an insoluble hydroxide into a cross-linked oxide gel. The Al type passivation corresponds to insulating oxides, and this directly leads to a nearly sealed off gel with just a residual access of the electrolyte to the metal surface. In Fe type passivation, the electronic 3d shells are incomplete, which induces a coexistence of bi- and trivalent monomers, and from elsewhere well-known quantum (or magnetic) effects, a complete ordering can suddenly induce an electronic conduction at the interatomic scale. The different grades of stainless steels or Ni based alloys, and a few other paradoxical observations, are thus explained by the required conjunction of magnetic, chemical and electrochemical features. In parallel, any applied polarisation induces a junction potential at the electrolyte boundary, with electric field, ionic space charges, and local electromigration. All are cancelled in the Faraday cage of a conductive gel, but its adaptation to each potential takes some time. Therefore, the alleged “Mott-Schottky” plots do not detect the electronic space charges of a supposedly semiconductor, but just the local ionic space charges induced by artificial polarisations.