Researchers at the U.S. National Institute of Standards and Technology (NIST) (Gaithersburg, Maryland) are developing a noninvasive method using THz radiation waves to detect iron corrosion at early stages.
NIST officials say the method, which utilizes x-ray diffraction (XRD) and magnetic characterization, can detect the initial stages of corrosion on steel rebar. The waves go directly through any concrete covering, seeking to reveal corrosion before it can cause any significant degradation of the structure.
The institute believes the method, referred to as a noninvasive, nondestructive “spectral fingerprint” technique, will be most useful on bridges, roads, and other aging physical infrastructure.
Iron Oxide Products Evaluated
When water and oxygen corrode iron, different iron oxide products are produced. The two most common are goethite [FeO(OH)] and hematite (Fe2O3) .
“The brown rust that forms when you leave a hammer out in the rain is mostly goethite, and when a steel reinforcing bar [rebar] corrodes inside a concrete bridge deck, that is mostly hematite,” says NIST physical chemist Dave Plusquellic. “We have shown in our new study with goethite, and our previous work with hematite, that terahertz radiation—electromagnetic waves with frequencies 10 to 100 times higher than the microwaves used to cook food—can detect both corrosion products in the early stages of formation.”
The NIST THz wave detection method works because goethite and hematite are antiferromagnetic, meaning they show an antiparallel alignment. The pairs of electrons sitting side-by-side within the iron atoms in these materials spin in opposite directions, leaving them unaffected by external magnetic fields. In contrast, the electrons in the iron atoms of a household magnet, which is ferromagnetic, spin in the same direction and are either attracted or repelled by external magnetic fields.
“Terahertz waves will flip the spin alignment of one of the electrons in a pair and get absorbed by hematite or goethite,” Plusquellic says. “Using a millimeter wave detector, we discovered that this antiferromagnetic absorption only occurs within narrow frequency ranges in the terahertz region of the electromagnetic spectrum—yielding ‘spectral fingerprints’ unique to goethite and hematite, and in turn, iron corrosion.”
The antiferromagnetic detection method was first conceived in 2009 by William Egelhoff, an NIST fellow and pioneer in the field of magnetic materials. The researchers describe the method in-depth in a new online paper.1
Considering modern advances in THz sources and detectors, the NIST believes the technique can detect tiny amounts of iron-bearing oxides from early-stage corrosion of steel that is surrounded by concrete, polymer composites (such as pipe insulation in a factory), paints, and other protective materials.
“In the laboratory, we have demonstrated that a 2-milliwatt terahertz source can produce waves that detect hematite through 25 millimeters of concrete,” Plusquellic says. “Using terahertz sources with powers in the hundreds of milliwatts and state-of-the-art receivers with unprecedented signal-to-noise ratios, we should be able to penetrate 50 millimeters, the thickness of the concrete covering the first layer of rebar used in most steel-reinforced concrete structures.”
Limitations of Current Methods
Current imaging methods for uncovering corrosion use microwaves to record changes in the physical state of the affected steel, such as changes in the thickness of a rebar within the concrete of a bridge or other structure, the NIST explains.
“Unfortunately, by the time such changes are detectable, the corrosive process is already well on its way toward causing cracks in the concrete,” says physicist and NIST Fellow Ed Garboczi.
The THz radiation method goes a step further than traditional detection methods, the researchers explain. Rather than simply reporting the presence of a material phase that denotes probable corrosion, they are seeking to identify its spectral signature to gain insight into the environment of its formation.
Additionally, Garboczi notes that most of the existing microwave imaging methods rely on comparisons with baseline measurements of the steel taken at the time of construction, a practice that only goes back about 25 years.
“That’s a real problem since the average age of the 400,000 steel-reinforced concrete bridges in the United States is 50 years, and there is no baseline data available for many of them,” Garboczi explains.
Going forward, the researchers are planning to move beyond goethite and hematite by expanding the study into other corrosive products.
Next on their list is finding a spectral fingerprint for akageneite [Fe3+O(OH,Cl)], an iron corrosion product formed in the presence of chloride ions that come from sources like seawater and road-deicing salt.
“Akageneite can cause problems in steel-reinforced concrete similar to those seen with goethite and hematite,” Garboczi says.
Source: NIST, nist.gov.
1 S.G. Chou, et al., “Using Terahertz Waves to Identify the Presence of Goethite via Antiferromagnetic Resonance,” Applied Magnetic Resonance, April 2017, https://link.springer.com/article/10.1007%2Fs00723-017-0884-y (May 15, 2017).