Materials Performance

DEC 2018

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 MATERIALS PERFORMANCE: VOL. 57, NO. 12 DECEMBER 2018 MP welcomes news submissions and leads for the "Material Matters" department. Contact Ben DuBose at ben.dubose@ Information on corrosion control and prevention As part of the study, the team con- ducted a series of crack injection experi- ments on two silver-gold alloys under perchloric acid (HClO 4 ) immersion condi- tions. This acid does not corrode silver- gold alloys without adding an applied voltage, thus allowing researchers to con- trol the thicknesses of the dealloyed lay- ers. These experiments were designed to separate a mechanical component of cracking from any stress-corrosion com- ponent, identif ied as simultaneous deal- loying and mechanical loading. In these experiments, each polycrys- talline sample was dealloyed under a zero-stress condition to form nanoporous gold layers of 500 nm and 2.2 µm. Then, while the dealloying potential was main- tained, each sample was rapidly loaded to cause intergranular fracture. By rapid loading, this minimizes the time that stress and dissolution can simultaneously operate. The team varied the delay time between 3 and 180 s between each sample being removed from the corrosive HClO 4 and subjected to stress, which the researchers performed by bending each sample by hand to a radius curvature of 500 µm. For shorter time frames of 10 s or less, all the samples fractured very quick ly upon being subjected to stress. To examine what happens at the lon- ger delay times, the researchers measured the penetration depth of the dealloyed portion beneath the surface layer. In those cases, TEM examination revealed the formation of elliptical voids near and parallel to the interface between the cor- roded and uncorroded layers. These voids severely degraded cohesion within the corroded layer, which ultimately pre- vented cracks from being transmitted beyond the grain boundary at that time. While this dynamic served to eventu- ally prevent crack transmission, the experiments found a time gap in which SCC leading to fracture could still occur, even when the application of stress was not simultaneous with the corrosion. "Our f indings demonstrate that grain boundary nanoporous morphologies that form as a result of corrosion can, under suitable conditions, mechanically inject cracks into a surrounding phase unaf- fected by corrosion," the researchers write in a paper on the subject. 2 "Our results have revealed that the roles of stress and dealloying corrosion can be decoupled and demonstrate that there is a separate mechanical component." In all, the researchers believe this pre- viously unidentif ied mechanical compo- nent to SCC could result in corrosion measurements that may underestimate the rate of SCC by multiplicative factors of 10 or more. Their hope is that end users will take their f indings into account when designing and searching for appropriate alloys to use on important infrastructure. "In nuclear plants, SCC maintenance and plant shutdowns are based on previ- ous experience with similarly designed reactors," Sieradzki says. "While we are not building new nuclear plants in the United States, these f indings should trig- ger the search for new, corrosion-resis- tant alloys that can be used for replace- ment parts in existing plants and in other important structural applications." Source: Arizona State University, Contact Karl Sieradzki, ASU—email: References 1 "ASU Research Finds Better Predictor of Metal Structure Failures," ASU Now, Sept. 10, 2018, https://asunow.asu .edu/20180910- asu-research-finds-better-predictors-metal- structure-failures (Oct. 25, 2018). 2 N. Badwe, et al., "Decoupling the Role of Stress and Corrosion in the Intergranular Cracking of Noble-Metal Alloys," Nature Mat eri al s, Se pt. 10, 2018, https://www. n a tu re . c o m / a r t i c l e s / s 4 1 5 6 3 - 0 1 8 - 0 1 6 2 -x (Oct. 25, 2018). —Ben DuBose

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