Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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17 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 Information on corrosion control and prevention Snapshots from a peridynamic simulation of corrosion damage. The initial size of the defect in the top layer has a length of ~60 ┬Ám. The colors of the damage maps represent the mechanical degradation or damage caused by the corrosion process. Red corresponds to completely corroded material, while blue shows pristine metal without any corrosion damage. The colors between blue and red describe degrees of partially damaged material. Images courtesy of Florin Bobaru and Ziguang Chen. Corrosion model captures dynamics of pitting damage A model for the evolution of pitting corrosion damage that is capable of capturing subsurface damage has been developed by University of Nebraska- Lincoln researchers Florin Bobaru, a professor of mechanical and materials engineering, and Ziguang Chen, a postdoctoral researcher. By simulating the rate and prof ile of pitting corrosion, the model could allow engineers to more precisely forecast catastrophic structural failures as well as design materials less susceptible to corrosion. "If you can model a process, that's a great f irst step in trying to improve the design of a material or structure," says Bobaru. "Once you have that understand- ing, you can say, 'I know this piece is cor- roded, but it's going to last two more years,' or, 'I better replace this next month because there's a high potential that a crack will run through and damage the whole structure.'" Pitting corrosion is a localized form of corrosion that rapidly penetrates the sub- strate thickness and produces deep, nar- row cavities or holes in the material. Pit- ting ty pically develops at weak spots in the surface f ilm. A small area becomes anodic while the larger surrounding area remains cathodic, and corrosion is driven by the potential difference between the anodic area inside the pit and the sur- rounding cathodic area. Several factors can lead to pitting cor- rosion, including localized chemical or mechanical damage to the protective oxide f ilm; water chemistry factors that can cause breakdown of a passive f ilm; low dissolved oxygen concentrations, which tend to render a protective oxide f ilm less stable; and high concentrations of chlorides, as in seawater. Pitting can also occur in the presence of nonunifor- mities in the metal structure of the com- ponent, such as nonmetallic inclusions, or when a protective coating is poorly applied or has localized damage. Considered to be more hazardous than uniform corrosion damage, pitting is more diff icult to detect, predict, and design against. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system. Pitting corrosion is a common denomina- tor in almost all ty pes of localized corro- sion attack, and may assume different shapes, such as open, uncovered pits or pits covered with a semi-permeable mem- brane of corrosion products. Pits can be either hemispherical or cup-shaped. Continued on page 18

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