Researchers at Sandia National Laboratories, the Department of Energy’s Center for Integrated Nanotechnologies and the Aramco Research Center in Boston have found that a particular form of nanoscale corrosion is responsible for unpredictably decreasing the working life of steel pipes, according to an open-access paper recently published in Nature’s Materials Degradation journal.
Using transmission electron microscopes, the researchers were able to pin the root of the problem on a triple junction formed by a grain of cementite—a compound of carbon and iron—and two grains of ferrite, a type of iron. This junction forms frequently during most methods of fashioning steel pipe.
A Sandia National Laboratories transmission electron microscope helped create this phase equilibrium map showing areas where corrosion of steel was observed at the triple junction formed where one cementite grain abuts two ferrite grains. (Micrograph courtesy of Katherine Jungjohann)
The researchers found that interfacial disorder in the atomic structure of those triple junctions made it easier for the corrosive solution to remove iron atoms along that interface.
In the experiment, the corrosive process stopped when the triple junction had been consumed by corrosion, but the crevice left behind allowed the corrosive solution to attack the interior of the steel.
We thought of a possible solution for forming new pipe, based on changing the microstructure of the steel surface during forging, but it still needs to be tested and have a patent filed if it works. But now we think we know where the major problem is.—Sandia’s principle investigator Katherine Jungjohann, co-author and lead microscopist
This was the world’s first real-time observation of nanoscale corrosion in a real-world material— steel—which is the most prevalent type of steel used in infrastructure worldwide. Through it, we identified the types of interfaces and mechanisms that play a role in the initiation and progression of localized steel corrosion. The work is already being translated into models used to prevent corrosion-related catastrophes like infrastructure collapse and pipeline breaks.—Aramco senior research scientist Steven Hayden
To mimic the chemical exposure of pipe in the field, where the expensive, delicate microscopes could not be moved, very thin pipe samples were exposed at Sandia to a variety of chemicals known to pass through oil pipelines.
Sandia researcher and paper author Khalid Hattar put a dry sample in a vacuum and used a transmission electron microscope to create maps of the steel grain types and their orientation, much as a pilot in a plane might use a camera to create area maps of farmland and roads, except that Hattar’s maps had approximately 6 nanometers resolution.
By comparing these maps before and after the liquid corrosion experiments, a direct identification of the first phase that fell out of the samples could be identified, essentially identifying the weakest link in the internal microstructure.—Khalid Hattar
The sample we analyzed was considered a low-carbon steel, but it has relatively high-carbon inclusions of cementite which are the sites of localized corrosion attacks.
Our transmission electron microscopes were a key piece of this work, allowing us to image the sample, observe the corrosion process, and do microanalysis before and after the corrosion occurred to identify the part played by the ferrite and cementite grains and the corrosion product.—Sandia researcher and co-author Paul Kotula
Localized corrosion is different from uniform corrosion. The latter occurs in bulk form and is highly predictable. The former is invisible, creating a pathway observable only at its endpoint and increasing bulk corrosion rates by making it easier for corrosion to spread.
With this analysis, we identified facetted pitting at the phase boundary, uniform corrosion rates from the steel surface, and data that suggest that a re-initiating galvanic corrosion mechanism is possible in this environment. These observations represent an important step toward atomically defining nanoscale corrosion mechanisms, enabling the informed development of next-generation inhibition technologies and the improvement of corrosion predictive models.—Hayden et al.
Other authors include Sandia researchers William Mook and Daniel Bufford and former Sandia researcher and experimental lead Claire Chisholm, who is now at the University of California at Santa Barbara.
The work was funded in part by Sandia’s Laboratory Directed Research and Development program. The Center for Integrated Nanotechnologies is a DOE Office of Science user facility operated by Sandia and Los Alamos national laboratories for university, industry and other national lab researchers.
Steven C. Hayden, Claire Chisholm, Rachael O. Grudt, Jeffery A. Aguiar, William M. Mook, Paul G. Kotula, Tatiana S. Pilyugina, Daniel C. Bufford, Khalid Hattar, Timothy J. Kucharski, Ihsan M. Taie, Michele L. Ostraat & Katherine L. Jungjohann (2019) “Localized corrosion of low-carbon steel at the nanoscale” npj Materials Degradation 3, Article number: 17 doi: 10.1038/s41529-019-0078-1