Rusting iron can be its own worst enemy
Simulations show iron catalyzes corrosion in 'inert' carbon dioxide

Date:
January 21, 2022
Source:
Rice University
Summary:
Atom-level simulations reveal the reason iron rusts in supposedly
'inert' supercritical carbon dioxide fluid. Trace amounts of water
can cause a reaction at the interface between iron and the fluid,
prompting the formation of corrosive chemicals.



FULL STORY ==========================================================================
Iron that rusts in water theoretically shouldn't corrode in contact with
an "inert" supercritical fluid of carbon dioxide. But it does.


==========================================================================
The reason has eluded materials scientists to now, but a team at Rice University has a theory that could contribute to new strategies to
protect iron from the environment.

Materials theorist Boris Yakobson and his colleagues at Rice's George
R. Brown School of Engineering found through atom-level simulations
that iron itself plays a role in its own corrosion when exposed to supercritical CO2 (sCO2) and trace amounts of water by promoting the
formation of reactive species in the fluid that come back to attack it.

In their research, published in the Cell Press journalMatter, they
conclude that thin hydrophobic layers of 2D materials like graphene
or hexagonal boron nitride could be employed as a barrier between iron
atoms and the reactive elements of sCO2.

Rice graduate student Qin-Kun Li and research scientist Alex Kutana are
co-lead authors of the paper. Rice assistant research professor Evgeni
Penev is a co- author.

Supercritical fluids are materials at a temperature and pressure that
keeps them roughly between phases -- say, not all liquid, but not yet
all gas. The properties of sCO2 make it an ideal working fluid because, according to the researchers, it is "essentially inert," noncorrosive
and low-cost.

"Eliminating corrosion is a constant challenge, and it's on a lot of
people's minds right now as the government prepares to invest heavily
in infrastructure," said Yakobson, the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry. "Iron
is a pillar of infrastructure from ancient times, but only now are we
able to get an atomistic understanding of how it corrodes." The Rice
lab's simulations reveal the devil's in the details. Previous studies have attributed corrosion to the presence of bulk water and other contaminants
in the superfluid, but that isn't necessarily the case, Yakobson said.

"Water, as the primary impurity in sCO2, provides a hydrogen bond network
to trigger interfacial reactions with CO2 and other impurities like
nitrous oxide and to form corrosive acid detrimental to iron," Li said.

The simulations also showed that the iron itself acts as a catalyst,
lowering the reaction energy barriers at the interface between iron and
sCO2, ultimately leading to the formation of a host of corrosive species: oxygen, hydroxide, carboxylic acid and nitrous acid.

To the researchers, the study illustrates the power of theoretical
modeling to solve complicated chemistry problems, in this case predicting thermodynamic reactions and estimates of corrosion rates at the interface between iron and sCO2. They also showed all bets are off if there's more
than a trace of water in the superfluid, accelerating corrosion.

The research was supported by the U.S. Department of Energy's Fossil
Energy Program, Division of Crosscutting R&D and Systems Integration
(DE-AC05- 00OR22725), through UT-Battelle LLC (4000174979).

========================================================================== Story Source: Materials provided by Rice_University. Original written
by Mike Williams. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Qin-Kun Li, Alex Kutana, Evgeni S. Penev, Boris I. Yakobson. Iron
corrosion in the "inert" supercritical CO2, ab initio
dynamics insights: How impurities matter. Matter, 2022; DOI:
10.1016/j.matt.2021.12.019 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220121145420.htm

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