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19 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 This distinction between the two pro- cesses frees the new modeling approach from making assumptions about the migration of ions through a material. Rather than presuppose the speed and direction of movement, Bobaru and Chen were able to simply input basic parame- ters and watch the process unfold. "I think that's a very good thing, because what happens at the interface is a very complicated electrochemical pro- cess that goes all the way down to the atomic scale," Bobaru says. "Outside of modeling each and every atom—which you cannot do except in very tiny regions—you cannot solve this problem computationally. That's why we wanted to simplif y this model as much as we could and let it tell us how this corrosion will happen by itself." According to the two researchers, this model can predict the pit shape and damage prof ile in materials with micro- structural heterogeneities, such as defects, interfaces, inclusions, and grain boundaries. "People have been trying to model this for many years, but they've struggled because it's diff icult to track how the interface between a corroding solution and the solid material evolves over time," Bobaru says. These struggles, he says, stemmed in part from misconceptions about the interface, which previous research had usually conceived as a mem- brane separating the pristine metal sur- face from the corrosive solution. Inspired by more recent atomic-scale investigations, Bobaru and Chen instead treated the interface as an initial layer of corrosion that indicates the presence of slight but advancing degradation just beneath the surface. "Our contribution was the modeling of this sub-surface layer as one that eventually weakens a material and could inf luence how potential cracks grow from there," Bobaru says. While formulating their model, Bobaru and Chen examined existing experimental data on corrosion in one- dimensional components that included wire. They calibrated the parameters of their model accordingly, f inding that its simulations of corrosion-related damage closely resembled the experimental out- comes. After modif ying their math and associated code, Bobaru and Chen then expanded the model to replicate corro- sion of two- and three-dimensional struc- tures. The team's model can be applied to both metallic and non-metallic materials, and is able to capture the grow th rate of corrosion on a steel pillar or the evolution of a pothole in a concrete street. "It tells us what concentration of ions we have to lose in order for a mechanical bond to break," Bobaru says. "So the ion diffusion problem is coupled with the mechanical damage to the material." Though the new corrosion model can- not yet simulate the fractures that result from broken mechanical bonds, Bobaru and Chen are now attempting to incorpo- rate them. "This is, in a sense, just a f irst step," Bobaru says. "Once you have corro- sion pits forming, a crack can emerge. This crack grow th is inf luenced by corro- sion, and the corrosion is inf luenced by the crack grow th. The solution can get in there, speeding up the material degrada- tion and potential catastrophic failure of your system. So we're now exploring that." A study describing the new corrosion model is discussed in the Journal of the Mechanics and Physics of Solids. 1 Source: Scott Schrage, University Communications, University of Nebraska— Lincoln, unl.edu. Contact Florin Bobaru— e-mail: fbobaru2@unl.edu. Bibliography Basic Corrosion Course. Houston , TX: NACE International, 2004. Reference 1 Z. Chen, F. Bobaru "Peridynamic modeling of pitting corrosion damage," J. of the Mechanics and Physics of Solids 78, 5 (2015): pp. 352-381. Information on corrosion control and prevention

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