What Does Point Defect Model (PDM) Mean?
A point defect model (PDM) refers to a model supported the movement of point defects in associate electric field. It's planned to interpret the expansion behavior of a passive film on a metal surface.
The PDM could be a settled model describing the expansion and breakdown of passive films in terms of mass and charge flux via purpose defects across the semiconductive, defective barrier layer of the passive film, as a result of the predictions of the model are affected by the relevant natural laws.
Corrosionpedia Explains Point Defect Model (PDM)
The point defect model (PDM) states that the dissolution of metal results from the flux cations to film/solution interface, deed metal vacancies or metal holes at the metal/film edge. These metal holes tend to immerse into the bulk of the metal and therefore to disappear. If the assembly rates of this hole are larger than their submersion rate, the holes compile at the metal surface through forming voids. Since the void grows to an essential size, the passive film collapses down into the void, leading to domestically accelerated corrosion, and therefore the formation of pit.
The PDM has undergone 3 generations, the primary generation model (PDM-I), developed within the early Eighties, assumed that the passive film was one defective chemical compound layer that contained ion vacancies and chemical element vacancies that were generated and exterminated at the metal/film and film/solution interfaces.
The generation II model (PDM-II) incorporated the bi-layer structure of the film, comprising a defective an or binary compound barrier layer adjacent to the metal and an outer layer that forms by precipitation of fabric from reaction of transmitted cations with species within the setting (consisting water, carbonate, bisulfide, etc.), introduced metal interstitials to the suite of defects, recognized barrier layer dissolution, and recognized the requirement to classify reactions on whether or not they are lattice conservative or non-conservative, however assumed that management of the passive current resided within the barrier layer alone.
The generation III model (PDM-III) extends the speculation to those cases (e.g., valve metals) wherever the outer layer is thus resistive that it controls the electric resistance of the interface and therefore the corrosion rate.