Researchers at Sandia National Laboratories (Albuquerque, New Mexico, USA), the Department of Energy’s Center for Integrated Nanotechnologies (Albuquerque, New Mexico, USA), and Aramco Research Center (Boston, Massachusetts, USA) have determined that a “triple junction” of three chemical elements is responsible for the sudden failure of steel-based oil pipelines. This triple junction consists of one grain of cementite, a compound of iron and carbon, and two grains of ferrite, an iron-based oxide, and often forms during most methods of fashioning steel pipe.
According to the paper published in Nature’s Materials Degradation, a primary goal of the research project was to gain “a better understanding of the mechanisms by which corrosion initiates and progresses at these types of interfaces in steel,” and in so doing, “mitigating corrosion-related losses.”
The research team performed an experiment in which corrosion consumed the triple junction, but which left a crevice through which corrosion could still attack the steel interior. The team was able to detect nanoscale corrosion in samples of steel pipe using transmission electron microscopy (TEM), a technique that transmits a beam of electrons through a specimen to form an image. Through the use of TEM, the researchers found that corrosion could be traced to an “interfacial disorder in the atomic structure” of triple junctions, according to Sandia.
“This was the world’s first real-time observation of nanoscale corrosion in a real-world material—carbon steel—which is the most prevalent type of steel used in infrastructure worldwide,” says Aramco senior research scientist Steven Hayden. “Through it, we identified the types of interfaces and mechanisms that play a role in the initiation and progression of localized steel corrosion.” Hayden added that these findings have already been used to create models that can prevent infrastructure collapse, pipeline breaks, and other problems caused by corrosion.
A member of the Sandia research team, Khalid Hattar, used TEM to create a map of the steel grain types and orientation contained in a dry sample of thin steel pipe in a vacuum. “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,” says Hattar.
As Paul Kotula, another Sandia researcher, adds, “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.” TEM technology enabled the researchers to observe the corrosion of the sample and to identify the roles played the component parts of the triple junction in the corrosion process, says Kotula.
When he began working in the field of corrosion research, Hayden admits feeling “daunted” by its complexities and the lack of knowledge surrounding it. But now he feels confident that the project he’s helped undertake “will allow us to rethink manufacturing processes to minimize the expression of the susceptible nanostructures that render the steel vulnerable to accelerated decay mechanisms.”
Source: Sandia National Laboratories, www.share-ng.sandia.gov.