Viewing Corrosion at the Nanoscale in Real Time

Corrosion affects almost everything made of metal, from cars to boats to underground pipes and even the fillings in teeth. Corrosion is a gradual decaying process, which carries a global cost of trillions of dollars annually. Not only does it have a hefty price tag, corrosion also poses potential safety, environmental and health hazards as well.

A rusty chain. Credit: © Daan / Fotolia

A rusty chain. Credit: © Daan / Fotolia

"Corrosion has been a major problem for a very long time," said UC Santa Barbara chemical

engineering professor Jacob Israelachvili. This is particularly true in confined spaces, such as thin gaps between machine parts, the contact area between hardware and metal plates, behind seals and under gaskets, any seams where two surfaces meet. Israelachvili adds that it has been a huge challenge to closely observe such electrochemical dissolution. However, this has now changed.

Novel Way to Observe Corrosion Process in Real Time

Using the SFA2000 Surface Forces Apparatus (SFA), an instrument developed by Israelachvili, he and his research team were able to get real-time observations of the corrosion process in crevice and pit corrosion and investigate corrosion in confined spaces.

The study is published in the Proceedings of the National Academy of Sciences, and was conducted with graduate student Howard Dobbs and project scientist Kai Kristiansen of UCSB, and colleagues at the Max-Planck-Institut für Eisenforschung in Düsseldorf.

With the SFA, we can accurately determine the thickness of our metal film of interest and follow the development over time as corrosion proceeds.

Kai Kristiansen, Project Scientist, UC Santa Barbara

This set up also allowed the researchers to have control of the salt composition of the solution, the temperature, as well as the electric potential of the nickel surface.

Effects of Crevice and Pitting Corrosion

Crevice and pitting corrosion isn't the kind of widespread surface rusting you may see on the hulls of old ships exposed to the ocean. Crevice and pitting corrosion is intense, localized attacks, in which the visible decay can look deceptively minor.

In fact, it is only when things fail completely that the damage is clear, whether it be machines breaking apart, bridges buckling, seafaring ship engines malfunctioning, or dental fillings falling out.

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The Experiment

For this experiment the researchers studied a nickel film against a mica surface. They focused on the starting point of corrosion, when the surface of the metal starts to dissolve, and saw that the degradation of the material didn’t occur homogenously.

Instead, certain locations where there were possible microscale cracks among other surface defects experienced intense local corrosion resulting in the rapid appearance of pits.

"It's very anisotropic," Israelachvili explained, stating that it is a very complex process, as even inside the crevices, different things occur near the opening versus deep inside. "Because you've got diffusion occurring, it affects the rate at which the metal dissolves both in and out of the crevice.”

The first step in the corrosion process is usually very important, since that tells you that any protective surface layer has broken down and that the underlying material is exposed to the solution.

Howard Dobbs, Graduate Student, UC Santa Barbara

According to the researchers, the corrosion then extends from the pits, often at rapid speeds, because the underlying material is not as resistant to the corrosive fluid.

One of the most important aspects of our finding is the significance of the electric potential difference between the film of interest and the apposing surface in initiating corrosion.

Kai Kristiansen, Project Scientist, UC Santa Barbara

It is more likely that corrosion will begin and spread quickly when the electric potential difference reaches a certain critical value. In this instance, the nickel film experienced corrosion while the mica remained whole, being more chemically inert.

We have seen this interesting effect before with other metal and non-metal materials. We have some pieces of the puzzle, but we are still seeking to unravel the full mechanism of this phenomenon.

Howard Dobbs, Graduate Student, UC Santa Barbara

Conclusion

Scientists can build upon this research into real-time, micro- and nanoscale mechanisms of corrosion. This may lead to models and predictions of when materials in confined spaces are likely to corrode, and the mechanisms by which they corrode.

"Basically, it's a matter of prolonging the lifetimes of metals and devices," Israelachvili said. Furthering his point, Israelachvili said it is particularly relevant now, when devices are often very small and can even be placed inside the body. Understanding how to adequately protect corrosion-prone surfaces will reduce the damage made to them and therefore reduce the need to repair and replace them.

Conversely, it would also be beneficial to understand how to accelerate dissolution in appropriate situations, such as with non-traditional cements, aluminosilica, for example, that produce less carbon dioxide.

An important step in the cement formation is the dissolution of cement's main ingredients, silica and alumina, which is very slow and requires highly caustic conditions unsafe for use in large-scale production. Improving the dissolution rate while avoiding the need for unsafe, caustic solutions would remove a technological barrier in the implementation of non-traditional cements.

Howard Dobbs, Graduate Student, UC Santa Barbara

This information has been sourced, reviewed and adapted from materials provided by SurForce, LLC.

For more information on this source, please visit SurForce, LLC.

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