Surface chemistry reveals corrosive secrets

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The interactions between iron, water, oxygen and ions quickly become complex. Scientists at MTU have developed a more precise method to observe the formation of iron minerals such as rust.

It can easily be seen with the naked eye that leaving an old nail in the rain causes rust. What requires the keen eyes and sensitive noses of microscopy and spectroscopy is to observe 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 first steps in this process can be studied in more detail with surface analysis. The team, led by Kathryn Perrine, Assistant Professor of Chemistry, recently published their last article 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 that develop when exposed to air, which is made up of atmospheric oxygen and dioxide. of carbon. Gradual exposure to oxygen and carbon dioxide produces cation-specific carbonate films. Iron hydroxides of different shapes and morphologies are without gradual exposure to air, non-specific to cation.

A better understanding of this process and the speed with which minerals form opens up possibilities for monitoring carbon dioxide capture, water quality by-products and improving infrastructure management for them. old bridges and pipes.

Play Chemists Video Watch Rust Shape Video

Chemists watch the form of rust

The 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 Journal of Physical Chemistry A. Copyright 2021 American Chemical Society.

Methodologies become interdisciplinary

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

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

The study of the science of materials surfaces is inherently interdisciplinary; from materials science to geochemistry, from civil engineering to chemistry, Perrine sees her work as a bridge that helps other disciplines to better inform their processes, models, interventions and innovations. To do this, you need great precision and sensitivity in your group’s research.

Although other methods of monitoring surface corrosion and film growth exist, Perrine’s lab uses a surface chemistry approach that could be adapted to analyze other reduction and oxidation processes in plants. complex environments. In a series of articles, they examined their three step process —Assess changes in the composition of the electrolyte and use oxygen and carbon dioxide from the air as reagents, to observe the formation of the electrolyte in real time. different minerals observed at the air-liquid-solid interface.

Accurate measurements are the molecular lens to see chemistry

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

The bright colors of the microscopy show the shape of the minerals.

Polished iron exposed to electrolytic solutions degrades and forms films of iron carbonate and calcium carbonate when exposed to oxygen and a heterogeneous mixture of platelets. Credit: Catherine Perrine. Reprinted with permission from Journal of Physical Chemistry A. Copyright 2021 American Chemical Society.

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

What the images reveal is a sequence of pitting, chewing, and surface degradation, known as corrosion, that produces nucleation sites for mineral growth. The main thing is to look at the initial stages as a function of time.

“We can observe the corrosion and the growth of the film over time. Calcium chloride
[solution] tends to corrode the surface faster because we have more chloride ions, but we also have a faster rate of carbonate formation, ”Perrine said, adding that in a video recorded by his laboratory, it is possible to see how the sodium chloride solution gradually corrodes the surface of the iron and continues to form rust as the solution dries.

She adds that as iron is ubiquitous in environmental systems, slowing down and observing closely the mineral formation comes down to adjusting the variables of its transformation into different solutions and exposure to air.

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

Michigan Technological University is a public research university founded in 1885 in Houghton, Michigan, and welcomes 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 over 125 undergraduate and graduate programs in science and technology, engineering, computer science, forestry, business and economics, health professions, sciences. humanities, mathematics, social sciences, and the arts. The rural campus is located a few miles from Lake Superior in Michigan’s Upper Peninsula, providing year-round outdoor adventure opportunities.

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