Crevice and pitting corrosion is a constant challenge in a variety of fluid handling applications, one which costs trillions of dollars each year as well as carrying a host of potential safety, environmental and health hazards.
Close observation of this kind of corrosion has long been a challenge, but now scientists from the University of California Santa Barbara are able to get a real-time look at the corrosion process in confined spaces.
Unlike surface rusting, which appears on metal objects and machinery after prolonged exposure to water, crevice and pitting corrosion is a series of intense, localised attacks where visible decay can look deceptively minor, until there is a catastrophic failure, which can see pumps break and pipes buckle. It is a particular problem in confined spaces, for example behind seals and under gaskets, and at seams where two surfaces meet.
Using a device called the Surface Forces Apparatus (SFA), UCB chemical engineering professor Jacob Israelachvili and a research team were able to get a real time look at the process of corrosion on confined surfaces. "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, a scientist on the project, said.
In the experiment, the researchers studied a nickel film against a mica surface. They focused on the initiation of corrosion, the point at which the metal surface begins to dissolve. They observed that the degradation of the material did not happen in a homogenous way. Instead, certain areas – likely locations where there were microscale cracks and other surface defects – would experience intense local corrosion resulting in the sudden appearance of pits.
"It's very anisotropic," Israelachvili said, explaining that even within the crevices, different things are happening near the opening versus 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 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," said Howard Dobbs, a graduate student involved in the research.
From the initial steps the corrosion spreads from the pits and often does so rapidly, according to the research, 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," Kristiansen added. When the electric potential difference reaches a certain critical value, the more likely corrosion will begin and the quicker it will spread. In the experiment, 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 said. "We have some pieces of the puzzle, but we are still seeking to unravel the full mechanism of this phenomenon."
The real time insights into the micro- and nanoscale mechanisms of corrosion provide valuable information that could allow the scientist to eventually develop models and predictions of how and when materials in confined spaces are likely to corrode.
“Basically it’s a matter of prolonging the lifetimes of metals and devices,” Israelachvili said. The research has the potential to further understanding on properly protecting corrosion prone surfaces, in turn reducing the need to replace them due to damage.
It will also improve understanding of how to accelerate dissolution, in applications where it could be beneficial, such as with non-traditional (e.g., aluminosilica) cements 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," Dobbs said. "Improving the dissolution rate while avoiding the need for unsafe, caustic solutions would remove a technological barrier in the implementation of non-traditional cements."
The team's research has been published in the Proceedings of the National Academy of Sciences.