Materials Performance

AUG 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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Page 19 of 92

17 MATERIALS PERFORMANCE: VOL. 57, NO. 8 AUGUST 2018 Information on corrosion control and prevention Study Explores Water Vapor Corrosion of Metals at Atomic Level S cientists with several U.S. and Chinese government agencies recently experi- mented with oxide grow th at the atomic level on a Ni-Cr alloy. 1 They say this allowed them to model the process through computer simulations to provide insights into how water vapor could change other materials, particularly at elevated temperatures, and the pathways to corrosion. According to the researchers, while engineers have long known water vapor can accelerate the corrosion of metals and alloys, the exact mechanisms behind it have not been clear. In turn, this makes the phenomenon diff icult to prevent. However, by probing the atomic-level reactions, researchers say they found that the involvement of protons speeds up the corrosion process. "Understanding how water vapor such as mist or steam corrodes metals and alloys can help engineers keep industrial systems working at peak performance longer," says Chongmin Wang, a senior research specialist at U.S. Department of Energ y's Environmental Molecular Sci- ences Laboratory (EMSL) (Richland, Washington, USA) who helped lead the study. "Armed with that knowledge, engi- neers can also improve cataly tic conver- sion processes and enhance ionic conduc- tion in materials." The researchers identif y steam gener- ators, turbine engines, fuel cells, and cat- alysts as examples of material applica- tions where water vapor is present, be it intentional or unavoidable. In their study, the researchers used in situ environmental transmission electron microscopy (TEM) to examine a single crystalline Ni-Cr alloy f ilm exposed to both pure oxygen (O 2 ) and water vapor (H 2 O) at 350 °C. By contrasting the results, they determined unique features from H 2 O exposure. For both environments, cuboid nickel(II) oxide (NiO) crystals formed on the alloy during oxidation, in which the Ni and O atoms diffused to form an NiO lattice, layer by layer. Compared with O 2 , a unique feature of H 2 O oxidation was the formation of vacancy clusters. These clus- ters of vacancies, which originate when an atom is missing from a lattice site, are described as sub-nanometer cavities formed by incorporating both Ni and O vacancies. These cavities can merge with other vacancy clusters and eventually migrate to the surface. In their study, with continued oxida- tion in H 2 O, the vacancy clusters ty pically migrated to the surface and created a surface pit after ~174 s. The surface pit then subsequently f illed up via the diffu- sion and grow th of atoms and molecules on the surface, leading to a f lat surface after ~301 s. The process, which is not observed during the grow th of NiO in pure O 2 , then repeats as oxidation pro- gresses. Hence, the vacancy formation and migration in growing NiO in H 2 O indicates a modif ied oxidation mecha- nism, according to the researchers. During the modif ied ox idation pro- cess, H 2 O molecules were adsorbed and chemically dissociated into negatively charged hydrox ide (OH − ) ions (anions) and positively charged hydron (H + ) ions (cations) on the NiO surface. From there, the O-H bonds were f urther broken to form free ox ygen ions that ser ved as the ox idizing species. According to the researchers, H + could penetrate the NiO lattice by overcoming a small diff usion barrier. This led to the formation of interstitial protons, H i , w ithin the NiO lattice. According to the study and subse- quent modeling, the presence of H i enhanced vacancy generation, further lowered the diffusion barrier, and thus promoted the clustering of those vacan- cies—which could lead to widespread surface pitting. Scanning TEM analysis also revealed the morpholog y of the oxidizing Ni-Cr surface in H 2 O and O 2 , respectively. A thick ness contrast showed the oxide layer formed in H 2 O was highly porous, w ith an average pore size of ~5 nm after 30 min of oxidation. By comparison, oxi- dation in pure O 2 did not lead to the for- mation of pores. "This indicates vacancy formation and condensation are both enhanced in The researchers identify turbine engines as one example of a material application where water vapor is present and can possibly lead to corrosion. Continued on page 18

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