Radiation used to detect ‘spectral fingerprint’ of steel corrosion
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Researchers have developed a technique similar to X-rays that can see through concrete to detect early signs of corrosion. This could be helpful in detecting damage to steel reinforcements in concrete structures, such as bridges and roads.
Corrosion is a major threat to the safety of steel frameworks within buildings and other infrastructure. When water and oxygen corrode iron, iron oxide products are produced. The most common are goethite – brown rust – and haematite, a brittle mineral that forms when steel reinforcements corrode inside concrete.
During X-rays and CT scans, electromagnetic radiation is beamed towards a person or object to see inside. This new technique, developed by researchers at the US National Institute of Standards and Technology (NIST), uses similar principles, but at higher power.
Current imaging methods for uncovering corrosion in steel-reinforced structures use microwaves to monitor changes in the steel, such as changes in the thickness of a reinforcing bar encased in concrete.
"Unfortunately, by the time such changes are detectable, the corrosive process is already well on its way toward causing cracks in the concrete," said Dr Ed Garboczi, a fellow at NIST. By the time a significant change like this is detected, the structure could already be too dangerous to continue using.
“We have shown in our new study with goethite, and our previous work with haematite that terahertz radiation – electromagnetic waves with frequencies 10 to 100 times higher than microwaves used to cook food – can detect both corrosion products in the early stages of formation,” said Dr Dave Plusquellic, a physical chemist based at NIST.
This non-invasive technique could be valuable in industry, as it could reveal hints of corrosion before a structure undergoes a dangerous level of internal degradation.
The technique uses goethite and haematite as indicators of steel corrosion. These minerals are antiferromagnetic: pairs of electrons within them spin in opposite directions, unlike in ordinary household ferromagnets. High-energy terahertz waves flip the spin alignment of one of the electrons and are absorbed by the mineral.
“Using a millimetre 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 haematite, and in turn, iron corrosion,” said Dr Plusquellic.
By detecting these spectral fingerprints, early-stage corrosion could be identified in steel embedded in concrete, polymer composites such as pipe insulation, and other protective materials.
The NIST researchers have demonstrated that haematite can be detected through 25mm of concrete, and by increasing the power of the radiation source, they expect to be able to penetrate 50mm, the typical thickness of concrete covering reinforcing bars.
Next, they will attempt to find a spectral fingerprint for akaganeite, another iron corrosion product closely related to haematite, which is often formed in the presence of seawater and other substances containing chloride ions.