It is easy to see with the naked eye that leaving old nails in the rain can cause rust. Microscopes and spectroscopy keen eyes and keen noses are needed to observe how iron corrodes and forms new minerals, especially in water containing small amounts of sodium and calcium.

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

The team’s main finding is that the cations in the solution-positively charged sodium or calcium ions-affect the type of carbonate film that grows when exposed to air, which is made up of atmospheric oxygen and carbon dioxide. The gradual exposure of oxygen and carbon dioxide produces a cation-specific carbonate film. Iron hydroxides of different shapes and forms are not gradually exposed to the air and are not specific to cations.

A better understanding of this process and the rate of mineral formation opens up possibilities for monitoring carbon dioxide capture, water quality by-products, and improving infrastructure management of old bridges and pipelines.

Methodology goes interdisciplinary

Although rust and related iron minerals are a well-known part of life on the earth's surface, the environment in which they are formed is very complex and diverse. Rust is usually composed of iron oxide and iron hydroxide, 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 grow it. Perrine uses major environmental issues such as the Flint Water Crisis as an example to illustrate how simple things like rust can easily fall into more complex and unwelcome follow-up reactions.

We want to measure and reveal chemical reactions in real environments," Perrine said, adding that her team pays special attention to surface chemistry, thin layers and films that interact with water, metal, and air. "We have to use them in our analysis tools A high level of [surface] sensitivity to get the correct information, so that we can really tell what the surface mechanism is and how [iron] changes.

The surface science of research materials is interdisciplinary in nature. From materials science to geochemistry, from civil engineering to chemistry, Perrine sees her work as a bridge to help other disciplines better inform their processes, models, interventions and innovations. For this reason, her team's research requires high precision and high sensitivity.

Although other methods of monitoring surface corrosion and film growth do exist, Perrine's laboratory uses a surface chemistry method that can be used to analyze other reduction and oxidation processes in complex environments. In a series of papers, they reviewed their three-stage process—evaluating changes in electrolyte composition and using oxygen and carbon dioxide in the air as reactants to observe the real-time observation of different minerals in air-liquid-solid Form the interface.

Accurate measurement is the molecular lens of observing chemistry

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

"Spectrum tells us chemical properties; microscope tells us physical changes," Perrine said. "[Real-time with AFM] [imaging] These corrosion experiments are really difficult because the surface is constantly changing and the solution is changing during the corrosion process."

What the image reveals is a series of pitting, chewing and surface degradation, called corrosion, which creates nucleation sites for the growth of minerals. The key part is to observe the initial phase as a function of time.

"We can observe corrosion and film growth over time. Calcium chloride [solution] tends to corrode the surface faster because we have more chloride ions, but carbonates are also formed faster," Perrine said , And added that in the video recorded in her laboratory, you can see how the sodium chloride solution gradually corrodes the iron surface and continues to rust as the solution dries.

She added that since iron is ubiquitous in the environmental system, slowing down and closely observing the formation of minerals comes down to adjusting the variables of how it transforms in different solutions and is exposed to the air.

The team’s surface catalysis method helps researchers better understand basic environmental science and other types of surface processes. It is hoped that their methods can help discover the mechanisms that cause water pollution, find ways to reduce carbon dioxide emissions, prevent bridges from collapsing, inspire smarter designs and cleaner fuels, and provide a deeper understanding of the geochemical processes.

Link to this article： How does metal corrosion occur?

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