Corrosion Science
- PDF / 4,565,151 Bytes
- 4 Pages / 604.8 x 806.4 pts Page_size
- 29 Downloads / 193 Views
occur on crystal surfaces in low-pressure gases (and also differences). Anions such as chloride or sulfate affect corrosion not only by changing the reaction product but also by specific adsorption and by altering the surface mobility of metal adatoms. Corrosion inhibitors such as benzotriazole work differently on Steps and terraces, and differently in chloride and sulfate. All this is done at atomic reso lution on well-defined crystal surfaces. Most corrosion-resistant materials are solid-solution alloys such as cupronickel or stainless steel. Since no two elements have the same electrochemical behavior, it is natural for less reactive elements (or those with very stable oxides) to accumulate at the surface. In the early 1980s, we began to think about how this accumulationoccursand,inparticular, howitcan produce continuous, protective layers, which often happens abruptly at particular alloy compositions. Percolation theory provides a natural framework within which to think about binary-alloy behav ior, and it forms the basis of an atomistic view involving selective dissolution (per colation), surface diffusion (annealing or blocking), and gelation (in the case of oxides). These processes are amenable to atomistic Simulation as well as experimentation, and in the article by Newman et al., the authors explain how such an approach can clarify a ränge of puzzling phenomena such as critical electrode potentials and parting limits (critical alloy Contents) for selective dissolution from alloys. Alloy corrosion can be used to produce useful nanostructures as well as being a fascinating physical and Chemi cal process. Pnssivity of pure metals is also a surprising p h e n o m e n o n that interested Michael Faraday, who termed it a "peculiar condition." A reactive metal like iron, which corrodes at half a millimeter per year in salt water, corrodes a thousand or even a million times slower in environments that allow the formation of a passivating film (a thin oxide layer). Luckily, iron forms such a film in the alkaline water within the pores of the cement used to make reinforced concrete.
Passivating oxide films are remarkably thin—less than 4 nm, and sometimes as thin as 1.5 nm—and until recently, their structures and compositions were th
Data Loading...
