A new evaluation method using terahertz waves can detect the early stages of corrosion on steel rebars directly through their concrete covering. Developed by researchers at the National Institute of Standards and Technology (NIST), the noninvasive “spectral fingerprint” technique reveals the corrosion of concrete-encased steel before it can cause any significant degradation of the structure it supports.
When water and oxygen corrode iron, different iron oxide products are produced, with the two most common being goethite and hematite.
“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 Dave Plusquellic, a NIST physical chemist. “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.”
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.
“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.
Most microwave imaging methods rely on comparisons with baseline measurements of the steel taken at the time of construction, Garboczi says. The practice is only about 25 years old. “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,” he explains.
Because goethite and hematite are antiferromagnetic, the NIST terahertz wave detection works. The pairs of electrons sitting side-by-side within the iron atoms in these materials spin in opposite directions, which leaves 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 notes. Researchers used a millimeter wave detector to discover "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," he says.
The new technique has the potential to rapidly detect tiny amounts of iron-bearing oxides from early-stage corrosion of steel surrounded by concrete, polymer composites like pipe insulation in a factory, paints and other protective materials, according to NIST.
“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.”
A paper describing the evaluation method appears in Applied Magnetic Resonance.
BCC Research reports that the global market for terahertz radiation devices is expected to total $57.3 million in 2017, $335.1 million in 2022, and $1.3 billion in 2027, demonstrating a 42.4% CAGR from 2017-2022.