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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16 DECEMBER 2018 W W W.MATERIALSPERFORMANCE.COM New Research Finds Better Predictors of Stress Corrosion Cracking A n Arizona State University (ASU) (Tempe, Arizona, USA) research team has released new work regarding intergran- ular stress corrosion cracking (SCC), with insight into how new alloys can be designed to better avoid SCC failures. •The research addresses the assump- tion that intergranular SCC is the result of the simultaneous presence of a tensile stress and corrosion and demonstrates that the roles of stress and corrosion can be decoupled, or can act independently. "The f inding is the culmination of about 30 years' work on this k ind of stress corrosion problem," says lead researcher Karl Sieradzk i, a professor of materials SCC is an environmental cause of premature failure in engineered structures, including bridges, aircraft, and nuclear power plants. Image courtesy of ASU. science and engineering at ASU. 1 "We now have a v iew into how new alloys can be designed to avoid this form of stress corrosion-inducted failure." The ASU team was assisted by researchers w ith the U.S. Pacif ic Northwest National Labo- rator y (Richland, Washington, USA) for the project, which was supported by the U.S. Department of Energ y (Washington, DC, USA). When metals are exposed to water con- taining salts, the strength of the metal can be severely compromised, leading to unex- pected failure. According to the research team, the conventional paradigm for understanding such SCC conditions has been the simultaneous presence of a suffi- cient level of tensile stress, a corrosive environment, and a susceptible material. However, the ASU research challenges that perspective by illustrating that the simultaneous presence of stress and a corrosive environment is not a require- ment for SCC. Instead, SCC can occur if the corrosion happens f irst and the mate- rial is subsequently subjected to stress. In their study, the researchers exam- ined the behavior of a laboratory model silver-gold alloy, which mimics the corro- sion behavior of engineering alloys. These alloys include carbon steel pipelines along with stainless steels and nickel- base alloys, used frequently in nuclear and conventional power generating sta- tions. Corrosion in these engineering alloys, as in the model silver-gold alloy, results in the formation of nanometer- sized holes within the corroded layer. In intergranular SCC, ty pical failure mechanisms include hydrogen embrittle- ment (HE), which can occur when envi- ronmental hydrogen diffuses into the alloy, and selective metal dissolution. To isolate the dealloying process and con- sider factors outside of HE, the research- ers used the silver-gold alloy, since nei- ther of the components absorbs hydrogen. According to Sieradzki, the key parameter determining the occurrence of rapid SCC is the adhesion between the corroded layer and uncorroded alloy. Using the atomic-scale techniques of high-resolution transmission electron microscopy (TEM) and atom probe tomography, researchers determined that the assumed requirement for the simulta- neous presence of stress and corrosion exists because of time-dependent mor- pholog y changes that affect adhesion. MATERIAL MATTERS

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