Some personal adventures in passivity—A review of the point defect model for film growth

Abstract

A review is presented of the history of the development of the Point Defect Model (PDM) for the growth and breakdown of passive films that form on the surfaces of reactive metals in contact with corrosive, condensed phase environments. The PDM has passed through three generations, with each successive gen eration addressing issues that have arisen from experiment. Thus, the first Generation model (PDM I), which was developed in the late 1970s/early 1980s, assumed that the passive film was a single defective oxide layer that contained cation vacancies and oxygen vacancies that were generated and annihilated at the metal/film and film/solution interfaces. This model was inspired by the work by Wagner on high temperature oxidation. As with gas phase systems, the film was assumed not to dissolve. However, it soon became evident that this model could not account for the properties of the passive state on metals in contact with aqueous environments and, accordingly a Generation II model (PDM II) was developed to address these issues. PDM II incorporated the bi layer structure of the film comprising a defective oxide (or hydride) barrier layer that grows into the metal and an outer layer that forms by precipitation of material from the reaction of cations transmitted through the barrier layer with species in the environment (including water, HS -, etc.), introduced metal interstitials to the suite of defects, recognized barrier layer dissolution, and recognized the need to classify reactions as to whether they are lattice conservative or non conservative. PDM II has enjoyed considerable success and the author knows of no instance where it has been demonstrated to be at odds with experiment when confluence between experiment and theory has been demonstrated. A Generation III model (PDM III) has been recently developed to extend the theory to those cases (e.g., the valve metals) where the outer layer is so resistive that it controls the impedance of the interface and hence the corrosion rate. A fourth generation model that will describe passivity on alloys is now under development. The experi mental evidence upon which each generation is based is reviewed.

Studies on Pitting Corrosion of Al–Cu–Li Alloys Part III: Passivation Kinetics of AA2098–T851 Based on the Point Defect Model

“…According to the point defect model, [38,49,50,51] the thickness of the barrier oxide layer through which tunneling must occur can be written as:Lss=1−αεV+g where ε is the electric field strength, and g is a function of pH and the standard rate constants for film formation at the metal/barrier layer (m/bl) interface and the dissolution rate at the bl/s interface, among other parameters (see Equation (3)). Substitution of Equation (10) into Equation (8) yields the tunneling current as:i=truei0^e−trueβ^(1−αε)Ve−βg.…”

Electrochemical study on the corrosion behavior of Ti3SiC2 in 3.5% NaCl solution

“…2b and d. As point defect model (PDM) interpreted, [27][28][29] the passivation lms were indicated to possess a bi-layer structure. An inner defect oxide barrier layer grows into the matrix and an outer porous layer forms by precipitations produced by reactions between cations transmitted through the barrier layer and species (water, particularly OH À in current situation) in the environment.…”

Factors Influencing Passivity Breakdown on UNS N08800 in Neutral Chloride and Thiosulfate Solutions

“…Overall, the Nyquist plots present incomplete capacitance arcs, which is consistent with the EIS characteristics of the passive systems. 20,42 Fig. 5a shows Nyquist plots of UNS N08800 obtained at various passive potentials in 0.6 mol · L −1 Cl − solution, showing the dependence of real and imaginary part of impedance on frequency.…”