Chemists at Michigan Technological University (MTU) (Houghton, Michigan, USA) have developed a new technique to observe and study the initial stages of iron corrosion. This corrosion process leads to the formation of new minerals such as rust, especially in water with a pinch of sodium and calcium, which can be studied in greater detail with surface analysis.
Led by Kathryn Perrine, assistant professor chemistry at MTU, the team recently published its findings in a paper for The Journal of Physical Chemistry A.
The team’s main finding is that the cation in solution—i.e., positively charged sodium or calcium ions—influences the type of carbonate films grown when exposed to air, which is composed of atmospheric oxygen (O2) and carbon dioxide (CO2). The gradual exposure of O2 and CO2 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 how this process works and how fast minerals form creates opportunities for monitoring CO2 capture, water quality byproducts, and improving infrastructure management for old bridges and pipes.
Rust and related iron minerals form in a variety of complex environments. Typically, rust is composed of iron oxides and iron hydroxides, yet corrosion can also lead to iron carbonate and other mineral formation. By studying each form, researchers hope to understand the best conditions for preventing or growing corrosion, as well as how to stop them into developing more complicated and unwanted subsequent reactions.
To that end, Perrine’s team focused specifically on surface chemistry, the thin layers and films where water, metal and air all interact.
“We want to measure and uncover chemical reactions in real environments,” says Perrine. “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.”
Perrine sees the study of the surface science of materials as inherently interdisciplinary, encompassing materials science, geochemistry, civil engineering, and chemistry. Her group conducted its highly precise research with the aim to build bridges between disciplines. For instance, Perrine’s lab used a surface chemistry approach that could be adapted to analyze other reduction and oxidation processes in complex environments, as well as developed a three-stage process that observed real-time formation of different minerals observed at the air-liquid-solid interface.
The team used surface-sensitive techniques that include 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).
“The spectroscopy tells us the chemistry; the microscopy tells us the physical changes,” Perrine says. “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—otherwise known as corrosion—which produce nucleation sites for the growth of minerals.
“We can watch the corrosion and film growth as a function of time,” says Perrine. “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.”
In a video recorded at Perrine’s lab, one can observe how a sodium chloride solution corrodes the surface of iron gradually and continues to form rust as the solution dries. According to Perrine, slowing down and closely observing mineral formation in iron 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.
Source: Michigan Tech News, www.mtu.edu/news.