Vacancy condensation as the precursor to passivity breakdown

Conference · Sat May 01 04:00:00 EDT 1993

OSTI ID:10158863

Macdonald, D D

This paper outlines the events that are envisaged to occur, according to the Point Defect Model (PDM), during ``chemically-induced`` breakdown of the barrier layer on passivated metals and alloys in aqueous environments. The essential hypothesis of the model is that the local generation of cation vacancies at the barrier layer/environment interface, due to the autocatalytic adsorption of a damaging species such as chloride ion into the oxygen vacancy structure, leads to an enhanced flux of cation vacancies across the film. If the vacancies that arrive at the metal/film interface cannot be annihilated by cation ejection from the metal, the vacancies will condense leading to the local decohesion of the barrier layer from the metal. A combination of film dissolution at the barrier layer/environment interface and residual stresses in the oxide film leads to the physical rupture of the film, marking a passivity breakdown event. The PDM accounts for a variety of empirically-established relationships for the nucleation of pits, including the fact that the breakdown voltage (V{sub c}) and induction time (t{sub ind}) are distributed quantities; that V{sub c} = V{sub c}{sup o} - blog (a{sub Cl{minus}}), where b > 2.303RT/F; and that log (t{sub ind}){alpha} 1/{Delta}V, where {Delta}V = V (applied voltage) - V{sub c}. Also, the model has been extended to account for alloying effects and, indeed, new alloys are now being designed on the basis of rules that have been derived from the PDM.

• Research Organization: Pennsylvania State Univ., University Park, PA (United States). Center for Advanced Materials
• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: FG02-91ER45461
• OSTI ID: 10158863
• Report Number(s): DOE/ER/45461--10; CONF-930998--1; ON: DE93012925
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360105
ALLOYS
CATIONS
CORROSION AND EROSION
DESIGN
FILMS
NUCLEATION
PASSIVATION
PITTING CORROSION
VACANCIES