這是一個位于加利福尼亞北部的麥克艾比海灘附近的廢棄建筑，可以看出鋼筋混凝土的腐蝕十分厲害。利用太赫茲波NIST評估新方法可以直接穿過覆蓋的混凝土，檢測鋼筋在早期階段的腐蝕。
An
abandoned building on Northern California's McAbee Beach shows the
destructive power of corrosion on a steel-reinforced concrete structure.
A new NIST evaluation method using terahertz waves can detect the early
stages of corrosion on steel rebars directly through their concrete
covering. Credit: With Permission by Per Loll, Denmark
Read more at: https://phys.org/news/2017-04-superman-nist-method-concrete-early-stage.html#jCp
當你遇到摔傷、撞傷或其他打擊性創傷時，醫生首先要做的是做X光、CT掃描或核磁共振檢查，以確定內部是否有損傷。美國國家標準與技術研究所（NIST）利用與其相同的原理，在更大強度上來檢測腐蝕。主要檢測國家橋梁，道路和其他易老化的基礎設施。
他們開發的是一種非侵入性的“光譜指紋”技術，揭示了混凝土包裹鋼的結構可以引起任何明顯的退化。該檢測方法是在《磁共振應用》（Applied Magnetic Resonance）雜志中的一篇新論文中提出的。
當水和氧腐蝕鐵，產生不同的氧化鐵產品，其中最常見的是針鐵礦和赤鐵礦。當你敲開一塊廢棄面時，里面的褐色銹就是針鐵礦，腐蝕鋼筋混凝土橋面的主要是赤鐵礦。”NIST物理化學家Dave Plusquellic說“我們已經展示了我們對針鐵礦的新研究，我們以前的工作是研究赤鐵礦。太赫茲輻射電磁波頻率比用來做飯的微波爐電磁波頻率高出10至100倍，因此可以檢測到腐蝕產物早期的形成。”
目前，揭露腐蝕的成像方法是使用微波記錄受影響的鋼的物理狀態的變化，如在橋梁或其他結構的混凝土的鋼筋厚度的變化。
物理學家和NIST研究員Ed garboczi說“不幸的是，隨著可檢測時間的改變，腐蝕過程已經造成混凝土裂縫。”
此外,Garboczi說大部分的微波成像方法依賴于比較建筑時鋼的基線測量,這種做法只追溯到大約25年。他解釋說“這是一個真正的問題，因為在美國的40萬個鋼筋混凝土橋梁的平均年齡為50年，有許多他們沒有基線數據。”
NIST的太赫茲波檢測方法的工作基礎，是針鐵礦和赤鐵礦為反鐵磁。換句話說，在這些材料中的鐵原子并排的電子對反方向旋轉，不受外部磁場的影響。與此相反，一個家用磁鐵就是鐵磁，其鐵原子的電子自旋在相同的方向上，由外部磁場吸引或排斥。
“太赫茲波將翻轉一對電子的自旋排列，得到赤鐵礦和針鐵礦的吸收”，Plusquellic說到“我們發現，使用毫米波探測器，這種反鐵磁吸收只發生在獨特的針鐵礦和赤鐵礦身上，他們在狹窄的頻率范圍內產生“光譜指紋”，并且，鐵腐蝕的電磁頻譜在太赫茲區域。”
通過目前太赫茲源和探測器的進展，新的NIST測評技術能夠從早期腐蝕鋼材包裹的混凝土，聚合物復合材料 (如工廠的絕緣管)、顏料和其他防護材料中快速檢測出微小的含鐵氧化物。
“在實驗室中，我們已經證明，一個2mW太赫茲源可以產生波通過25毫米的混凝土檢測赤鐵礦。”Plusquellic說“利用毫瓦太赫茲源和最先進的接收器，我們應該能夠穿透50毫米的覆蓋混凝土，這個厚度是大多采用鋼筋混凝土結構的第一層厚度。”
接下來，NIST的研究小組將試圖找到一種akageneite光譜指紋圖譜，鐵腐蝕是氯離子的產物，氯離子來自于海水和道路除冰鹽。Garboczi說“akageneite光譜指紋圖譜可以導致鋼筋混凝土類似與針鐵礦和赤鐵礦。”
反鐵磁腐蝕檢測方法是由NIST磁性材料方面的研究員和開創者William Egelhoff在2009提出。
When
you suffer a fall, an on-the-field collision or some other traumatic
blow, the first thing the doctor will do is take an X-ray, CT scan or
MRI to determine if anything has been damaged internally. Researchers at
the National Institute of Standards and Technology (NIST) are using the
same principle, but in a more powerful form, to detect corrosion, the
primary danger threatening the health of the steel framework within the
nation's bridges, roads and other aging physical infrastructure.
What they have developed is a noninvasive "spectral fingerprint" technique that reveals the corrosion of concrete-encased steel before it can cause any significant degradation of the structure it supports. The detection method is described in a new paper in the journal Applied Magnetic Resonance.
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," said 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."
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," said physicist and NIST Fellow Ed Garboczi.
Additionally, Garboczi said most of the 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," he explained.
The NIST terahertz wave detection method works because goethite and hematite are antiferromagnetic. In other words, 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 said. "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."
With current advances in terahertz sources and detectors, the new NIST nondestructive evaluation technique has the potential to rapidly detect tiny amounts of iron-bearing oxides from early-stage corrosion of steel 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 said. "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."
Next up for the NIST team will be an attempt to find a spectral fingerprint for akageneite, an iron corrosion product formed in the presence of chloride ions, which come from sources such as seawater and road deicing salt.
"Akageneite can cause problems in steel-reinforced concrete similar to those seen with goethite and hematite," Garboczi said.
The antiferromagnetic corrosion detection method was first conceived in 2009 by the late William Egelhoff, a NIST fellow and pioneer in the field of magnetic materials.
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More information: S. G. Chou et al, Using Terahertz Waves to Identify the Presence of Goethite via Antiferromagnetic Resonance, Applied Magnetic Resonance (2017). DOI: 10.1007/s00723-017-0884-y