Researchers at the University of California, Santa Barbara (UC Santa Barbara) (Santa Barbara, California) developed a device1 called a surface forces apparatus (SFA) to get a real-time look at the process of crevice and pitting corrosion on confined surfaces.
The apparatus, used with an electrochemical attachment, allows the researchers to directly visualize electrochemical reactions, dissolution, and pitting on surfaces confined in nanoscale gaps or crevices.
“With the SFA, we can accurately determine the thickness of our metal film of interest and follow the development over time as corrosion proceeds,” says UC Santa Barbara chemical engineering professor Jacob Israelachvili.
Limitations of Prior Methods
According to the team, the close observation of electrochemical dissolution has long been a problem in confined spaces—such as thin gaps between machine parts, the contact area between hardware and metal plates, behind seals and under gaskets, and seams where two surfaces meet.
In response, the researchers began trying to get a better understanding of the origins of localized corrosion, in which visible decay often looks deceptively minor and widespread surface rusting is absent.
“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,” says researcher Howard Dobbs, a UC Santa Barbara graduate student.
Apparatus Experiment
To learn more about the crevice corrosion process, the researchers launched an experiment by studying a nickel film against a mica surface in a confined area of ∼0.03 mm2 and focusing on the initiation point in which the metal surface would begin to dissolve. The team followed these crevice corrosion processes in real time while varying the pH-neutral sodium chloride (NaCl) solutions and applied surface potentials.
Using the apparatus to observe in situ nano- to microscale dissolution and pit formation, the researchers saw that the material’s degradation did not occur in a homogenous fashion. Rather, certain areas—locations where there were likely small cracks and other surface defects—would experience intense local corrosion, resulting in the sudden appearance of pits.
“It’s very anisotropic,” Israelachvili says, explaining that even within crevices, differences were observed near the surface vs. deep inside the crevice. “Because you’ve got diffusion occurring, it affects the rate at which the metal dissolves both in and out of the crevice. It’s a very complex process.”
The researchers found that once the protective surface layer breaks down, corrosion spreads from the pits, and often does so rapidly 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 opposing surface in initiating corrosion,” says Kai Kristiansen, a project scientist at UC Santa Barbara.
The initial corrosion proceeded as self-catalyzed pitting, visualized by the sudden appearance of circular pits with uniform diameters of 6 to 7 μm and depths of ∼2 to 3 nm, the researchers say. At concentrations above 10 M NaCl, pitting initiated at the outer rim of the confined zone, while concentrations below 10 M NaCl initiated pitting inside the confined zone.
When the electric potential difference reached a certain critical value, corrosion was more likely to begin and spread faster, the researchers found. In this case, the nickel film experienced corrosion, while the more chemically inert mica remained whole.
“We have seen this interesting effect before with other metal and non-metal materials,” Dobbs says. “We have some pieces of the puzzle, but we are still seeking to unravel the full mechanism of this phenomenon.”
Further Research Steps
Going forward, the researchers believe their research could lead to improved models and predictions on how and when materials in confined spaces are likely to corrode.
“It’s a matter of prolonging the lifetimes of metals and devices,” Israelachvili says, adding that an improved understanding of how to protect corrosion-prone surfaces on small devices would reduce the need to replace them due to damage.
Conversely, understanding how to accelerate dissolution could also be beneficial. One example of this, according to the researchers, includes the use of nontraditional cements—such as aluminosilica—that produce less carbon dioxide (CO2).
“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,” Dobbs says. “Improving the dissolution rate while avoiding the need for unsafe, caustic solutions would remove a technological barrier in the implementation of nontraditional cements.”
Research on the project was funded by the U.S. Department of Energy (DoE) (Washington, DC).
Source: UC Santa Barbara, www.ucsb.edu. Contact Jacob Israelachvili, UC Santa Barbara—email: jacob@engineering.ucsb.edu.
References
1 S. Fernandez, “Corrosion in Real Time,” The UC Santa Barbara Current, Sept. 13, 2017, http://www.news.ucsb.edu/2017/018262/corrosion-real-time (Oct. 11, 2017).