Scientists with several U.S. and Chinese government agencies recently experimented with oxide growth at the atomic level on a Ni-Cr alloy.1They 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 difficult 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 Energy’s Environmental Molecular Sciences Laboratory (EMSL) (Richland, Washington, USA) who helped lead the study. “Armed with that knowledge, engineers can also improve catalytic conversion processes and enhance ionic conduction in materials.”
The researchers identify steam generators, turbine engines, fuel cells, and catalysts as examples of material applications 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 film exposed to both pure oxygen (O2) and water vapor (H2O) at 350 °C. By contrasting the results, they determined unique features from H2O 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 O2, a unique feature of H2O oxidation was the formation of vacancy clusters. These clusters 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 oxidation in H2O, the vacancy clusters typically migrated to the surface and created a surface pit after ~174 s. The surface pit then subsequently filled up via the diffusion and growth of atoms and molecules on the surface, leading to a flat surface after ~301 s. The process, which is not observed during the growth of NiO in pure O2, then repeats as oxidation progresses. Hence, the vacancy formation and migration in growing NiO in H2O indicates a modified oxidation mechanism, according to the researchers.
During the modified oxidation process, H2O molecules were adsorbed and chemically dissociated into negatively charged hydroxide (OH−) ions (anions) and positively charged hydron (H+) ions (cations) on the NiO surface. From there, the O–H bonds were further broken to form free oxygen ions that served as the oxidizing species. According to the researchers, H+ could penetrate the NiO lattice by overcoming a small diffusion barrier. This led to the formation of interstitial protons, Hi, within the NiO lattice.
According to the study and subsequent modeling, the presence of Hi enhanced vacancy generation, further lowered the diffusion barrier, and thus promoted the clustering of those vacancies—which could lead to widespread surface pitting.
Scanning TEM analysis also revealed the morphology of the oxidizing Ni-Cr surface in H2O and O2, respectively. A thickness contrast showed the oxide layer formed in H2O was highly porous, with an average pore size of ~5 nm after 30 min of oxidation. By comparison, oxidation in pure O2 did not lead to the formation of pores.
“This indicates vacancy formation and condensation are both enhanced in H2O,” the researchers write. “Therefore, water vapor directly influences the alloy oxidation processes in the early stages, which could affect the formation of a protective oxide layer.”
The researchers also performed and analyzed in situ TEM experimental measurements of oxidation kinetics after the initial oxide layer had formed. The general process of the oxide growth on Ni–Cr featured the sparse nucleation and growth of oxide islands on the initial layer, they say. With subsequent oxidation, these oxide islands reached a few microns. By tracking these islands through two series of time-resolved TEM images, the researchers found that the measured oxidation rate in H2O showed wider scattering than in O2.
“The more porous structure of the initial oxide formed in H2O leads to a relatively large variation in oxide growth rates,” the researchers write. “However, the general trend is that the growth rate of NiO islands in H2O is higher than that in pure O2.”
Separately, the researchers also found that the oxidation of pure Ni in H2O also led to the formation of vacancy clusters, which did not happen in the case of oxidation in O2.
In closing, the researchers explain that the Ni-Cr alloy was routinely shown to have an enhanced oxidation rate during the early oxidation stages in moist environments at elevated temperatures, adding that the water vapor-promoted oxidation largely stemmed from the incorporation of interstitial protons (Hi) derived from water disassociation.
“The dissolved proton in the oxide lowers vacancy formation energy, promotes vacancy clustering, and enhances cation and anion diffusion, all of which leads to an increased oxidation rate in water vapor and consequently a porous structure in the developed oxides at an early stage,” they say.
The project was worked on by scientists from the EMSL, the Pacific Northwest National Laboratory (Richland, Washington, USA), the Chinese Academy of Sciences (Beijing, China), and State University of New York at Binghamton (Binghamton, New York, USA). The work was supported by the materials sciences and engineering division of the U.S. Department of Energy (Washington, DC, USA). Research was conducted at EMSL’s facilities.
Source: EMSL, www.emsl.pnl.gov.
Reference
1 “Peering into the Mist: How Water Vapor Changes Metal at the Atomic Level,” EMSL News, May 25, 2018, https://www.emsl.pnl.gov/emslweb/news/peering-mist-how-water-vapor-changes-metal-atomic-level (June 22, 2018).