Interactions between iron, water, oxygen and ions quickly become complex. MTU scientists
developed a more precise method to observe how iron minerals like rust form.

One can easily see with the naked eye that leaving an old nail out in the rain causes
rust. What does require the keen eyes and sensitive nose of microscopy and spectroscopy
is observing how iron corrodes and forms new minerals, especially in water with a
pinch of sodium and calcium.

Thanks to a new technique developed by chemists at Michigan Technological University,
the initial stages of this process can be studied in greater detail with surface analysis.
The team, led by Kathryn Perrine, assistant professor of chemistry, recently published their latest paper in The Journal of Physical Chemistry A.

The group’s main finding is that the cation in solution — positively charged sodium
or calcium ions — influences the type of carbonate films grown when exposed to air,
which is composed of atmospheric oxygen and carbon dioxide. The gradual exposure of
oxygen and carbon dioxide produces carbonate films specific to the cation. The iron
hydroxides of different shapes and morphologies are without gradual air exposure,
not specific to the cation.

A better understanding of this process and how fast the minerals form opens up possibilities
for monitoring carbon dioxide capture, water quality byproducts and improving infrastructure
management for old bridges and pipes.

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Chemists Watch Rust Form

Interactions between iron, water, oxygen, and ions quickly become complex. Studying
the air-solution-solid interface is tricky, which is why chemist Kathryn Perrine led
a team to develop a more precise, three-step method to observe how iron minerals like
rust form. Republished with permission from The Journal of Physical Chemistry A. Copyright
2021 American Chemical Society.

Methodologies Go Interdisciplinary

Even though rust and related iron minerals are a well-known part of life on Earth’s
surface, the environments they form in are quite complex and varied. Rust is usually
composed of iron oxides and iron hydroxides, but corrosion can also lead to iron carbonate
and other mineral formation. For each form, it is difficult to understand the best
conditions to prevent or grow it. Perrine points to major environmental issues like
the Flint water crisis as an example of how something as simple as rust can so easily
slip into more complicated, unwanted subsequent reactions.

“We want to measure and uncover chemical reactions in real environments,” Perrine
said, adding that her team focuses specifically on surface chemistry, the thin layers
and films where water, metal and air all interact. “We have to use a high level of
[surface] sensitivity in our analysis tools to get the right information back so we
can really say what is the surface mechanism and how [iron] transforms.”

Studying the surface science of materials is inherently interdisciplinary; from materials
science to geochemistry, from civil engineering to chemistry, Perrine sees her work
as a bridge that helps other disciplines better inform their processes, models, interventions
and innovations. To do so requires high precision and sensitivity in her group’s research.

While other methods of monitoring surface corrosion and film growth do exist, Perrine’s
lab uses a surface chemistry approach that could be adapted to analyze other reduction
and oxidation processes in complex environments. In a series of papers, they vetted
their three-stage process —assessing changes to the electrolyte composition and using oxygen and carbon dioxide
from air as a reactant, to observe real-time formation of the different minerals observed at the air-liquid-solid interface.

Precise Measurements are the Molecular Lens to Seeing Chemistry

The analysis techniques the team uses are surface-sensitive techniques: polarized
modulated-infrared reflection-absorption spectroscopy (PM-IRRAS), attenuated total
reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron
spectroscopy (XPS) and atomic force microscopy (AFM).

Bright colors from microscopy show the shape of minerals.

Polished iron exposed to electrolyte solutions will degrade and form iron carbonate
and calcium carbonate films when exposed to oxygen and a heterogeneous mixture of
platelets. Image Credit: Mikhail Trought, Perrine group. Reprinted with permission from The Journal of Physical Chemistry A. Copyright 2021
American Chemical Society.

“The spectroscopy tells us the chemistry; the microscopy tells us the physical changes,”
Perrine said. “It’s really difficult to [image] these corrosion experiments [in real-time
with AFM] because the surface is constantly changing, and the solution is changing
during corrosion.”

What the images do reveal is a sequence of pitting, chewing and degrading the surface,
known as corrosion, which produces nucleation sites for the growth of minerals. The
key part is watching the initial stages as a function of time.

“We can watch the corrosion and film growth as a function of time. The calcium chloride
[solution] tends to corrode the surface faster, because we have more chloride ions,
but also has a faster rate of carbonate formation,” Perrine said, adding that in a video her lab recorded, it’s possible to see how sodium chloride solution corrodes the surface of iron gradually
and continues forming rust as the solution dries.

She adds that since iron is ubiquitous in environmental systems, slowing down and
closely observing mineral formation comes down to adjusting the variables in how it
transforms in different solutions and exposure to air.

The team’s surface catalysis approach helps researchers better understand fundamental
environmental science and other types of surface processes. The hope is that their
method could help uncover mechanisms contributing to polluted water, find ways to
mitigate carbon dioxide, prevent bridge collapses and inspire smarter designs and
cleaner fuels, as well as provide deeper insight into Earth’s geochemical processes.

Michigan Technological University is a public research university founded in 1885 in Houghton, Michigan, and is home to more than 7,000 students from 55 countries around the world. Consistently ranked among the best universities in the country for return on investment, the University offers more than 125 undergraduate and graduate degree programs in science and technology, engineering, computing, forestry, business and economics, health professions, humanities, mathematics, social sciences, and the arts. The rural campus is situated just miles from Lake Superior in Michigan’s Upper Peninsula, offering year-round opportunities for outdoor adventure.